Optimised Diesel Combustion and SCR Exhaust … · 25th Aachen Colloquium Automobile and Engine ......

25th Aachen Colloquium Automobile and Engine Technology 2016 1273

Optimised Diesel Combustion and SCR Exhaust Aftertreatment Combined with a 48 V System for Lowest Emissions and Fuel Consumption in RDE Oliver Maiwald, Rolf Brück, Stefan Rohrer, Michael Zaki, Axel Schatz, Frank Atzler

Continental Emitec GmbH, Lohmar, Germany

Summary

Modern passenger car engines need to fulfil demanding targets. For the Diesel engine there is a particular focus on the trade off between fuel consumption and the emissions of nitrous oxides, NOx, in real driving.

Best fuel efficiency is achieved when the engine is operated at the optimum position of heat release, HR50. However, this coincides with the highest engine out NOx emissions. To avoid the deterioration of fuel efficiency, engine internal NOx suppression was reduced as much as possible in favour of a highly efficient aftertreatment system. This employs a low pressure drop design and electric heating, serving to shorten the light off time upon cold start.

Additionally moderate 48 Volt electrification was implemented. This provides energy recuperation during vehicle deceleration as well as torque support for the combustion engine during acceleration. This not only reduces the peak load but also can decrease the dynamic demands on the “slower” engine controls, like e.g. the air path.

This paper presents the status of work in progress of this systemic assessment of the interaction of the three ingredients "engine optimisation", "highly efficient aftertreatment" and "mild hybridisation", to yield a Super Clean yet cost effective Diesel car, fit for the future requirements of CO2 and NOx reduction.

1 Introduction

Modern vehicles need to fulfil challenging requirements with respect to emissions, noise and fuel consumption. For Diesel engines there is a particular focus on the trade off between fuel consumption and the emissions of nitrous oxides, NOx, in real driving. On the one hand the Diesel engine offers excellent fuel efficiency, and this will be a main contributor to achieve the EU target of 95g CO2/km. On the other hand some of this fuel consumption must be sacrificed to curb the NOx emission. To keep the effects of this dilemma as small as possible there are several solutions:

Best fuel efficiency is achieved when the engine is operated at the optimum position of heat release, HR50. In order to warrant a stable HR50 also in severe transients of

1274 25th Aachen Colloquium Automobile and Engine Technology 2016

future test cycles and real driving, a package of model based air path and combustion control will be implemented on a test vehicle.

However, best fuel efficiency usually coincides with the highest engine out NOx emissions. Engine internal NOx reduction usually incurs a penalty in fuel consumption. This is true for both strategies, late combustion as well as the use of EGR. Therefore engine internal NOx reduction measures were decreased as much as possible in favour of a highly efficient aftertreatment system. The effects of EGR and the corresponding operating strategies will be discussed in Section 3. The use of Continentals Compact Combustion will only briefly be discussed. An overview of best practise injection patterns will be given in Section 2. The full treatise on heat release rate shaping, using discrete pilot injections was published in [1].

From the above said it is clear, that the aftertreatment system needs to have a much higher NOx conversion efficiency than achieved hitherto in the New European Driving Cycle, NEDC, if engine thermodynamics, and hence fuel consumption, are to be improved at the same time significantly. Own measurements have shown, that typically the NOx conversion in this cycle is not higher than app. 70%. This is due to the low exhaust gas temperatures, which are not suitable to properly run aftertreatment systems. Although this situation improves with the advent of higher load cycles, like the World Harmonised Light Duty Cycle, WLTC, and the introduction of Real World Driving Emissions, RDE, such systems also need to provide sufficient NOx conversion, when the vehicle is run in stop-and-go traffic or low load city driving ("shopping cycle") or after cold start during engine warm up. This will be aggravated in the midterm by the introduction of cold start emissions testing from as low as -7°C. For these reasons the city section of the NEDC was explicitly selected for testing, since it reflects well the problems at low exhaust gas temperatures and long warm-up phases.

For the current assessment the Continental "CompactCat" was employed. It features a low pressure drop design, and avoids a dedicated Ad-Blue mixer through the patented flow routing. It was equipped with electric heating, serving to shorten the light off time upon cold start. Sections 3 will discuss several heating and warm keeping strategies.

Additional to measures on the combustion engine and the exhaust gas aftertreatment, moderate electrification for energy recuperation as well as the supply of electrically generated torque to the drive train was implemented. It has been shown, that a large part of the advantages of hybridisation can be harvested with a very cost effective 48 Volt side attached arrangement [2] and such a system was used for the here presented investigation. Additional to the features of start-stop and engine off coasting, the feature of “phlegmatisation” of the combustion engine was implemented. Phlegmatisation means the replacement of torque from the combustion engine by torque from the electric machine during transients. It not only reduces the peak load but can also decrease the dynamic demands on “slower” engine controls, for example the air path. The impact of this system will be discussed in Section 4.


For the current project a demanding set of targets was devised. A C-Class vehicle weighs in at about 1400 to 1500 kg. The World Light Duty Test Protocol, WLTP, demands 90% payload, bringing the effective vehicle inertia to app. 1800 kg. For such a vehicle the first targeted NOx limit would be the original from the NEDC, 80 mg/km, reduced by a reasonable engineering margin to app. 65 mg/km with a conformity factor, CF, of 1. In a second step an assumed future EU target of 40 mg/km would be attempted. For this the permissible engine out NOx emission would be of the order of 500 mg/km, if an average conversion efficiency of the DeNOx system of > 93% is assumed. The corresponding CO2 target of the full system including the 48V mild hybridisation should be app. 80 g/km for the NEDC and 90 g/km for the WLTC.

This paper presents a report of the current state of this systemic assessment of the interaction of the three ingredients "engine optimisation", "highly efficient aftertreatment" and "mild electrification", to yield a Super Clean yet cost effective Diesel car, fit for the future requirements of CO2 and NOx reduction.

2 Diesel engine optimisation

For modern Diesel engines the future development focus will be on simultaneous fuel consumption and NOx reduction. In the past, nitrous oxides have been reduced engine internally by high EGR rates and late shift of combustion. Both measures incur a significant penalty in fuel consumption. Therefore, exhaust gas aftertreatment will not only replace the engine internal NOx reduction but will also be the enabler of savings in fuel consumption.

Fig. 1: Typical soot NOx trade off for a Diesel engine. For low EGR / high NOx (engine out) strategies fuel bound emissions (PM, HC, CO) will be less relevant, as shown by the window "future optimisation".


2.1 Fuel path

A "high engine out NOx" calibration, which yields considerable CO2 savings, is based on several ingredients, shown in Fig. 2.

Fig. 2: Typical injection pattern, rail pressures and EGR rates in an engine map [1]

2.1.1 Rail pressure

The level of rail pressure is adapted to the amount of EGR in any one operating condition. This means, that for operating conditions with reduced EGR rates, lower rail pressures may be possible, saving some high pressure pump work.

2.1.2 Injection patterns

Generally it would be desirable to use up to three pilot injections in the operating range of low engine speed and load. These can be premixed and only burn very shortly before the main combustion, giving an ideal boot to ramp shaped heat release rate. At high load and engine speed, a single pilot is sufficient to suppress excessive combustion noise. Also in these conditions a small fuel quantity and short distance to the main injection serve to achieve the ideal heat release rate. In both cases as little of the combustion should happen to early before the main combustion, since this contradicts the requirements for low fuel consumption, i.e. to unnecessarily pressurise the charge against the piston moving towards TDC. A general physics based "recipe" for the application of pilot and post injections was published by Continental at the Stuttgart Symposium in 2011 [1]. This type of injection strategy was termed "Continental Compact Combustion". The Continental Servo Injector PCRs5 can achieve the ideal small pilot fuel mass of 0,8 mg/shot as well as a dwell time of 150 µs [4, 5]. However, in order to avoid changes in combustion noise when


switching between numbers of pilots usually a compromise with two rather than three pilots at low load and one pilot at high load is applied. A change in combustion noise upon changes in the number of pilots can only be avoided with extremely short dwell times of 100 µs and less, decreasing with increasing engine load and speed. Also a very high robustness in metering small fuel quantities would be required. These prerequisites for an “ideal” heat release shaping so far can only be achieved with direct drive injectors, due to the time limitations of servo hydraulic switching mechanisms. The penalty in fuel consumption found between the "ideal" and the "compromise" injection pattern was found to be of the order of 2% in both, NEDC and WLTC, confirmed in various independent testing series with different engines between 1.6 and 2.2 litre displacement. Another indispensable prerequisite for an ideal heat release shape and position is the accurate control of the cylinder charge with respect to oxygen mass and concentration. Only then advantages in CO2, NOx emissions and combustion noise can be achieved simultaneously. This is discussed in the next Section.

2.2 Accurate metering of air and recirculated exhaust gas, cylinder pressure indication

Ideal heat release shapes, as discussed in the previous Section, can only be achieved in transients, if the cylinder charge is well controlled with respect to oxygen mass and concentration. This is particularly true in real driving or in aggressive test cycles like the WLTC. Conventional map based controls of boosting and EGR cannot sufficiently satisfy this requirement. Therefore the ignition delay and, hence, the position of the combustion event with respect to TDC can vary excessively.

Fig. 3: Improved soot-NOx trade off, with the Continental Model Base Airpath “GasX” on WLTC. Each square represents the summary result of one WLTC, with the variation of EGR rates for each square.


Continental has developed an airpath control, based on physical models, to keep the deviations at bay. Results are shown in Fig. 3. The solid line with squares denotes the soot-NOx trade off achievable in WLTC with a variation of EGR settings, using a conventional air path, based on the knowledge of boost pressure and air mass at the metering device. Point 0 is the NOx/PM emission performance achieved with the conventional map based airpath and was used as a typical emission performance target for serial introduction (app. 150 mg NOx / 35 mg PM). Point No. 1 is the emission performance achieved after changing from the map based to the model based airpath, with the same EGR setting, translated into oxygen setpoints. There was no deterioration in emissions performance. Point No. 2 is the emission performance achieved with the new airpath after an optimisation of the oxygen & air massflow setpoints for the engine hot phase of the WLTC. In this optimisation the NOx peaks were removed by a suitable application of the O2 precontrol. Point No.3 is the emission performance achieved when this exercise was repeated in the engine cold start and warm up phase of the WLTC. The simultaneous decrease of both, PM & NOx is achieved by an efficient model based pre-control of the EGR valve and turbine vane position (VTG). With this precontrol the actuators virtually always reach their setpoints, and the EGR rate is automatically adapted corresponding to the oxygen target and the turbo charger limitation. This feature is very important for future highly transient emissions test cycles. The accuracy of air and EGR metering can be further improved by adaptation of the manifold air flow sensor MAF through the use of in-cylinder pressure indication. The indication also facilitates the adaptation of the fuel mass and the HR50 to their setpoints. All of these adaptations serve to reduce the emission dispersion by reducing individual contributions to the system dispersion.

Fig. 4: Benefits of on board in-cylinder indication


Shown in Fig. 4 are two ICPS applications: the Closed Loop Combustion Control, CLCC, and the ICPS based air mass flow adaptation. The objective of CLCC is to control the combustion dispersion by controlling the torque via the fuel mass and the centre of combustion via the phasing of the start of injection. This algorithm can also compensate for life time dispersion of the injector, of the fuel pressure sensor and the crank signal dispersion. However, the position of HR50 is not only affected by fuel path and mechanical issues, but also by the inaccuracies of the air path. A deviation of HR50 due to e.g. EGR drift cannot be compensated for by CLCC, i.e. by a fuel metering correction, without introducing a huge shift in NOx/PM emissions. In order to solve this discrepancy the ICPS based air mass flow adaptation was introduced.

Fig. 5: Reduction of NOx dispersion (left) and PM dispersion (right) though the use of CLCC and ICPS based MAF adaptation; (sqrt = square root of....)

Fig. 6: Reduction of NOx dispersion and CO2 advantage in WLTC by activation of CLCC and air mass flow adaptation.

This algorithm facilitates the reduction of fresh air life time dispersion to 2% by adapting the MAF sensor with the trapped air mass flow calculated from in-cylinder pressure data. These are measured when the engine is in overrun and EGR is switched off. These two strategies decreased the NOx dispersion by 70% and the PM


dispersion by 50% in WLTC, as shown in Fig. 5. Fig. 6 shows the effect of the reduction of NOx dispersion from ±20 to ±10 mg/km. This facilitated the decrease of the NOx engineering margin and, hence, leads to a fuel saving of 1 g/km in WLTC. The variables included tolerances in fuel mass, air mass, rail pressure and crank sensor errors.

The ICPS not only provides the data for these algorithms but most notably can compensate for the variation in Cetane Number of different fuel qualities (country, climatic zone, etc). Additionally, the ICPS offers an adaptation of conventional physical NOx models. This yields an improvement of the robustness and the accuracy of the NOx, which is compatible to control a DeNOx system. A more detailed report on the topics in this Section is found in [6].

2.3 EGR for NOx suppression

The use of EGR cannot be avoided in future engines. Since all aftertreatment systems rely on a suitable temperature level for proper functioning, it is inevitable to especially consider the operating conditions during which the engine warms up, here in particular the temperatures from as low as -7°C, and operating regimes where sufficiently high temperatures in the exhaust system are never achieved, so-called "shopping" or "taxi" cycles. Here the use of EGR is an appropriate means of curbing NOx. Fig. 7 shows, among other parameters, a schematic depiction of EGR rates for different operating conditions. While EGR rates can be kept low when the engine and aftertreatment system are sufficiently warm, during engine warm up EGR needs to be increased to EU6a levels to compensate for the lack of aftertreatment. At low load and engine speed this can be done with a relatively small penalty in fuel consumption, and this is shown in Fig. 7.

Fig. 7: Fuel consumption over centre of combustion for 12%EGR (triangles) 22% (squares) and 27% (diamonds) at n = 1500rpm, IMEP = 6.5bar

It shows the trade off of fuel consumption over HR50 for three different EGR rates, 12, 22 and 27%. The large diamond symbol denotes a typical EU6 application point without SCR (only engine internal DeNOx measures) with the HR50 at 15°crk aTDC. Contrary to the higher load application, shown below, the optimum HR50 for lowest CO2 only varies only from 6° to 7°crk aTDC. In this range the fuel consumption


increases by less than 1% with the addition of EGR. Yet, between the application point and the best point there is a difference of 3%. For the application of the best point a denoxation of nearly 90% is necessary from the aftertreatment system (centre diagram). Measures to achieve this will be discussed in Section 3. Combustion noise shows a relatively wide spread between the different EGR rates, due to the change in premixedness of the fuel in this operating condition (right diagram). Here it is necessary to closely control the EGR rate (refer to last Section), to avoid unpleasant changes in CN.

Fig. 8 shows the EGR application at a high load of IMEP = 14 bar and an engine speed of 2280 rpm. The EGR rates were 6, 10 and 15%. An injection pattern consisting of a single pilot, a main and a post injection, optimised for the individual EGR rate, was employed.

Fig. 8: Fuel consumption over centre of combustion for 6%EGR (triangles) 10% (squares) and 15% (diamonds) at n = 2280rpm, IMEP = 14bar

The large diamond symbol again denotes a typical EU6 application point without SCR with the HR50 at 18°crk aTDC. With rising EGR rate the optimum HR50 for lowest CO2 moves from 8° to app. 12°crk aTDC. Also, the fuel consumption increases significantly with the addition of EGR, from 205 to 213 g/kWh, i.e. by 4%. However, between the application point and the best point there is a difference of 5.4%! To harvest this, a denoxation of some 80% is necessary from the aftertreatment system (centre diagram). Combustion noise was only mildly affected by app. 0.2 dBA (right diagram). This clearly shows that EGR at high load should be avoided with respect to low fuel consumption. This combines well with the fact that at high load the aftertreatment system will reach, respectively maintain, light off temperature much easier, i.e. engine internal measures can be avoided with a highly efficient and working SCR system.

Fig. 9 shows a schematic explanation, as to why fuel consumption is more affected by the introduction of EGR at higher engine loads and speeds. The dominating variables are time and fuel mass. For a large injected fuel mass also the combustion duration is long. Here "long" refers to a longer time expressed in degrees crank, while the time per revolution has decreased at the same time. As a consequence some part of the combustion will increase the in-cylinder pressure while the piston still


moves towards TDC. Also the end of combustion will move to later positions (in °crk) with increasing engine speed, wasting more heat when the exhaust valve opens, than an earlier end of combustion. An elongation of the combustion process due to the use of EGR aggravates these effects and additionally shifts the centre of combustion to thermodynamically unfavourable later positions as shown in Fig. 8.

Fig. 9: Schematic depiction of the effect of short and long combustion duration

In conclusion this means that at cold start and engine warm up, as well as low load driving at exhaust temperatures below the light off temperature of the aftertreatment system, EGR is an adequate means of NOx suppression without a large penalty in fuel consumption. At higher load, EGR should only be used for transient correction. Usually at this load, the light off temperature is reached much quicker and the aftertreatment system can be applied. This will be discussed further in the aftertreatment Section.

3 Exhaust Gas Aftertreatment

3.1 General Considerations

Exhaust gas aftertreatment, EAT, by Selective Catalytic Reduction, SCR, has been a proven technology since the introduction of the EU4 emission legislation in the truck industry. In long haul applications NOx conversion efficiencies of above 98% have been reported. However, the load collective for passenger cars is much different, with large shares of low load driving, e.g. in cities. Additionally the current legislation requires emission conformity in test procedures including cold start at 20°C, and in the future down to -7°C. From these boundary conditions it is clear that for the proper functioning of any catalyst system it is necessary to minimises heat loss from the catalyst system, if necessary to provide additional heat and/or to lower the reaction temperature at for the NOx reduction. This Section will mainly be concerned with the heating of the aftertreatment system, and here in particular the electric heating in the system context with a 48V hybridisation of the drive train.


There are a number of different methods of warming up a catalyst system. In 2013 the project No. 1027 [7] of the Research Association Combustion Engines (german: Forschungsvereinigung Verbrennungskraftmaschinen, FVV) looked at variabilities in the gas exchange by variable valve timing and lift. It was found, that early blow down (early exhaust valve opening) offered the greatest heat flux onto the EAT, but also the highest penalty in fuel consumption. Retaining rest gas was found to yield a smaller temperature increase but also a much smaller deterioration in fuel consumption. The full effect on fuel consumption only becomes visible, when the time is considered for which any one such measure is needed. However, fully variable valve trains are expensive, compared to other catalyst heating technologies (see next paragraph). For Diesel engines so far no particular application has been found, that affects combustion and thermodynamics in the same fundamental manner as it does in gasoline engines, and would justify such an expense. In gasoline engines VVT and VVA help to reduce knock tendency and, hence, facilitate the optimisation of the centre of combustion and therefore the fuel consumption. Mazda presented in 2011 the Skyactive Diesel engine [8], which uses a comparatively simple system with a second exhaust valve lift, to re-induce exhaust gas and thus accelerate engine warm up and increase the exhaust temperature for the heating of the aftertreatment system.

Another method is the use of a so-called "5th injector", which has been employed on a number of engines in the past. Here an additional (low pressure) injector, placed in the exhaust line before the Diesel oxidation catalyst, DOC, sprays fuel directly onto this catalyst, where the fuel is oxidised. This reaction provides energy for either catalyst heating or particle filter regeneration at good efficiency, i.e. conversion of chemical energy into heat. However, the DOC temperature needs to be above the light off temperature of some 250°C to be able to convert the hydro-carbons. Therefore this method is unsuitable for catalyst heating after cold start. However, it can be employed to prevent the catalyst temperature to drop below the light off temperature, when it has been reached once. For cold start and warm up, late combustion is one method to divert some of the heat produced in the cylinder onto the EAT, and to maybe facilitate the application of the “5th injector”. If the cost for the 5th injector is to be avoided, then the engine injection system has to be capable of performing post injections at late crank positions, e.g. between 90° and 120° aTDC. There the cylinder back pressure is low, which necessitates the split of the post injection quantity into several small fuel portions. Otherwise, the spray penetrates through to the wall and washes off the engine oil. Also, the fuel being bound in the oil film does not contribute to the desired cat heating. Multiple post injection capability makes the ECU and software more expensive and expose the injector to higher wear.

The Lean NOx Trap, LNT, has a light off temperature – better: adsorption temperature – of app. 150°C, which is well below the hydrolysis temperature of the SCR system at app. 180 to 190°C. In order to achieve early denoxation upon cold start a closely coupled LNT was employed e.g. by Daimler [2] combined with an underfloor SCR system for the medium and high loads, for the very strict US emissions limits. However, like the VVT mentioned above, the LNT, due to its high


loading with precious metals, also is a relatively expensive measure to solve the cold start/warm up and city cycle problem.

Continental has developed an electric catalyst heater EMICAT®, based on their METALIT® substrate. This was presented in Aachen in 2010 [10] for 12V boardnets. This paper also included a system price assessment. Here, the added cost for the electrification was offset by the possible reduction of precious metal loading in the wash coat. The latest development is a 48V version, which can readily be combined with the 48V hybrid systems also developed by Continental. This combination allows e.g. for a more efficient use of recuperated braking energy for the electric heating of the aftertreatment system, than the 12V system of 2010.

3.2 Continental Emitec CompactCat® with electric catalyst heating EMICAT®

Fig. 10 shows the schematic of a Continental Emitec CompactCat design, including the possible locations for the EMICAT® heated disc. The CompactCat provides good heating up of the DOC or LNT by guiding the exhaust flow first through the substrate and then back through a ring cross section around its outside. This also provides good mixing and evaporation of the urea spray without the necessity for an additional mixer. Good mixers usually increase the back pressure on the engine which deteriorates the fuel consumption. From the CompactCat the exhaust flow enters into the succeeding closely coupled section, which may contain an SCR or SDPF.

Fig. 10: Continental Emitec CompactCat design with three possible positions for electrically heated discs of metallic substrate

Position one offers direct heating of the DOC, which would improve the NO to NO2 ratio quickly. However, the surface at the rear end of the substrate, which has a hydrolysis coating, onto which the Reactant Dosing Unit, RDU, sprays the urea solution, will only achieve a suitable temperature after the substrate has warmed through. Here an additional heated disc can be positioned. The same is true for the SCR/SDPF section, albeit some additional heat is lost between the DOC exit and the


SCR entrance through the canning. Therefore position three would be the favourable position to locally and quickly reach the light off temperature of the DeNOx section, without much heat loss.

3.3 Catalyst heating strategies

In order to investigate the pros and cons of the various heating disc arrangements computer simulations were done by means of a finite element program, where the cell size reflects the cell density of the real catalyst brick. The program allows for the introduction and dissipation of heat to and from the brick(s). Also considered is the exothermic energy from HC and CO. In the current investigation the heated discs were assumed to have a nominal power of 2.8 kW at 48V. All investigations were performed using the NEDC as a base cycle, since the city section of the NEDC well reflects worst case low load driving.

Fig. 11: Simplified catalyst model for the assessment of heating strategies

Fig. 11 shows the simplified model of the CompactCat shown in Fig. 10 for the assessment of heating strategies. It neglects the heat loss between the heated hydrolysis disk and the SDFP entry and combines heated disks 2 and 3 into one. However, the model does consider the heat loss to the environment, which makes the necessary amount of heating energy very large! The effect of heating of the DOC alone is shown in Fig. 12 for the DOC brick and in Fig. 13 for the SDPF brick. The DOC heater was run for 230 s in total over a time span of 600 s. It was switched in PWM made such that a temperature of 200°C was maintained, which gives a reasonable margin versus the minimum hydrolysis temperature of 180°C, the hysteresis was 20°C. In this case the SDPF was not heated additionally. The T30mm indicates, that 26% (30mm of 115mm total length) of the SDPF volume was active, i.e. above 200°C after 165 s, which at these low mass fluxes should be sufficient for good NOx conversion. 50% of the volume was active after 260s (T57mm).

Fig. 14 and Fig. 18 show the temperatures when both heaters, that for the DOC and that for the SDFP, are used simultaneously. The SDPF heater was only switched on for a total of 44 s to achieve the 200°C threshold after the heated disc (T3mm). One


quarter of the catalyst volume was active after 115 s, i.e. 180°C were reached at T30mm, compared to the 165 s when only the DOC heater was used. 50% were active after 260 s (T57mm at 180°C). With 50% of the SDPF volume active, also larger mass flows, like those occurring at a sudden load increase, can be cleaned of NOx.

Fig. 12: Temperatures in DOC due to the heating of the DOC with 2.8 kW power from the heating disc.

Fig. 13: Temperatures in SDPF due to the heating of the DOC with 2.8kW power from the heating disc.


Fig. 14: T in DOC, both heaters before DOC and SDPF active

Fig. 15: T in SDPF, both heaters before DOC and SDPF active


Fig. 16: T in DOC, heater before DOC active until ignition temperature for HC dosing reached, then HC dosing of 6 mg/cycle for the full duration of 600 s

Fig. 17: T in SDPF, heater before DOC active until ignition temperature for HC dosing reached, then HC dosing of 6 mg/cycle for the full duration of 600 s; Heater before SDPF active to accelerate NOx conversion

Fig. 16 and Fig. 17 show the case where the DOC heater was switched on until ignition condition for HC dosing were reached. This was assumed to be 200°C at T3mm, which was achieved after 43s. Although T3mm fell back to 100°C after the end of electric heating, the oxidation of fuel in the catalyst downstream of the 3 mm position


was maintained, increasing the DOC temperature to 330°C at T30mm after 105 s and maintaining above 220°C at this position at all times afterwards. A T30mm of 180°C in the SDPF was reached after 90 s, with the combination of DOC HC heating and 43 s of additional electric SDPF heating. For this case the DOC heater was limited to the first 40 s, based on the knowledge from case one (electric DOC heating only). For this case 40 s in total were sufficient to heat up the first quarter of the brick (T30mm) to above 200°C, of which only 24 s were on time. Independently, the SDPF heater was switched on for fast light off, i.e. early NOx conversion, for 43 s. This maintained the SDPF temperature above 200°C after app. 40 s.

Hydro carbon injection for catalyst heating appears to be a good measure to produce high amounts of heat. However, it bears two disadvantages: HC should not slip out of the tailpipe. Also, the oxidation of HC on the DOC is the preferred reaction, before the conversion of NO to NO2. However, the conversion of NO2 is important to facilitate the fast reduction reaction of NOx on the SCR catalyst. Both, HC oxidation and NO to NO2 conversion, can be facilitated, if the DOC has sufficient brick length. Considering a Tout of the DOC brick of 400°C (Fig. 16) it can safely be assumed, that the HC were all oxidized on the DOC. After 90 s T30mm was still rising steeply and it was concluded, that a further optimization would be necessary, to reduce the HC dosing to a value of 2 – 3 mg/cycle, as well as the additional electric SDPF heating.

An additional source of heat loss is the convective cooling of the catalyst when the vehicle is in overrun, i.e. when air/cool gas is pumped through the catalyst system at fuel cut off. Measures and effects for this case will be discussed in Section 3.4.

In conclusion it can be stated, that electric catalyst heating positively helps to achieve a very short light off time for the closely coupled SDPF or SCR system, which is of great importance for future cold start and low load driving requirements, and these will be included in the upcoming RDE requirements

3.4 Keeping the catalyst warm

Once the aftertreatment system has been warmed up suitably for high NOx conversion, it is necessary to keep it in this state. Stop and go traffic or long stretches of engine overrun make it necessary to implement temperature management mechanisms. While the engine is running only additive heating - electric or exothermic heating from the DOC, engine internal measures – can be used to maintain the catalyst temperatures. During engine overrun there is an additional option. Since the engine does not need to fire in this condition, the inlet and exhaust throttle valves, as well as the EGR valves and turbo VTG can be applied to minimise convective cooling of the aftertreatment system by large amount of fresh air being pumped through the system during overrun. Fig. 18 shows a schematic of the engine and aftertreatment system and two strategies, called overrun 1 and 2. Overrun 1 uses the low pressure EGR path, which keeps the turbo charger spinning, while incurring a penalty in heat loss from the surface of the LP EGR system. Overrun 2 uses the high pressure EGR path, which prevents cooling of the catalysts better than Overrun 1, but increases the engine response time upon tip-in (load take up, when the driver pedal is activated after the overrun phase), because the turbo charger is


not kept at speed. Both strategies have the disadvantage, that upon tip-in the pipe work of the EGR system(s) needs to be scavenged, to remove surplus exhaust gas. This elongates the response time and may lead to notable jerking of the vehicle. Additional to the overrun strategies two electric heating strategies, EHC1 and 2 were tried. These are presented in the top two diagrams as well as the resulting temperature curves in Fig. 19. The heating power was 900 W at 12 V supply voltage. Essential difference between EHC1 and 2 is the longer on-time of EHC1 at the start of the cycle. While none of the electric heating strategies was able to lift the temperature after DOC sustainably over the 180°C hydrolysis threshold during the city driving section of the cycle, the combination of EHC1 and Overrun 1 achieved 180°C after only 70 s ! The on time of EHC1 was app. 33%, that of EHC2 app. 30%. The difference between EHC1 and EHC2, which is the longer initial heating period, exemplarily shows, that it is important to quickly introduce a sufficiently high amount of heat. If the target temperature is not achieved, the whole energy expense was in vain! The drop in temperature below the 180°C threshold at the third and fourth repetition of the city driving section indicates the need for an improved application of on-time of the electric heater. However, the data clearly show that the omission of convective cooling is one important constituent of an overall strategy, in particular if the focus is on minimizing the additional heating energy input into the system.

Fig. 18: Warm keeping strategies using inlet and exhaust throttle valves, EGR valves and turbo VTG to minimize convective cooling upon engine overrun


Fig. 19: Electric heating, 900 W, here on a conventional 12 Volt net, combined with and without overrun strategies (overrun strategies no gas flow through the EAT during overrun)

Due to the potential problems in drivability when such throttle valve strategies are applied, it is now suggested to use the synergies of the 48V hybridisation. With such a hybrid system the combustion engine is shut off completely during coasting and electric braking, avoiding completely the convective cooling of the aftertreatment system. Such the 48V system not only facilitates the employment of a 48V heating system but also offers a synergy with the aftertreatment system, keeping it at temperature.

An additional source of heat loss from the EAT is the radiation and convection from the outer casing.

Fig. 20: Enthalpy fluxes measured on engine test bed on a non insulated catalyst system


Typical values for a non insulated catalyst system, measured on an engine test bed, are shown in Fig. 20. This suggests a heat loss of 1,5 to 2kW for the NEDC city section, i.e. for the engine operating range 50 to100Nm. Compared to a heater power of 2.8kW this means that app. 50% of the heat is lost to the environment. Therefore an energy saving of at least 50% would be possible through suitable insulation, especially taking into account, that the heat loss from a moving vehicle is even greater than that measured on a test bed with blowers.

In conclusion this Section has shown, that proper insulation of the exhaust system and the omission of gas flow through the aftertreatment system during overrun both do have a significant effect on keeping the aftertreatment in a suitable temperature window and on saving energy for heating measures. The overall energy balance has to be validated for each system configuration individually, including improvements in engine efficiency due to the load point shift when driving the electric heaters.

4 Mild Hybridisation using a 48 Volt System

4.1 Introduction

Mild hybridisation with 48V has been found to provide many of the advantages of much more expensive high voltage systems [2]. Especially the basic functions “start-stop”, “engine off coasting” and some degree of electric torque assist or very low speed electric driving (parking manoeuvres or stop-and-go traffic) can readily be implemented with such systems. The system presented in this paper is one with 48V P0 Belt Starter Generator, BSG. In “P0 position” the BSG is located in the conventional Front End Accessories Drive, FEAD, as shown in Fig. 22.

A particular focus of this report is on the effect of engine “phlegmatisation”, meaning the replacement of torque from the combustion engine by torque from the electric machine during transients. This is done mostly during vehicle/engine accelerations, to reduce the peak load of the combustion engine. This in turn can reduce NOx and soot emissions significantly. Also, the control of the engine is particularly difficult in transients compared to quasi steady state conditions. This refers mainly to the inherently “slow” air path, i.e. the air and EGR handling system. The handling of gaseous media is implicitly slower and more difficult, compared to the control of liquids (fuel) and the ignition, which can be done true to the cycle. The improvements in air path accuracy were explained in Section 2.2. The functioning of “Phlegmatisation is depicted in Fig. 21.


Fig. 21: Recharging, phlegmatisation and recuperation phases in a section of the WLTC. Legend: ICE, internal combustion engine; BSG, belt starter generator; SOC, state of charge of traction battery, Torque = crank orque;

During phlegmatisation the upper envelope of the torque curve is the total torque demand of combustion engine plus electric machine, which initially was the demand for the combustion engine before phlegmatisation. The lower envelope is that of the phlegmatised combustion engine. During recharging the lower envelope is the initial demand on the combustion engine and the upper curve depicts the traction torque including the toque necessary for recharging. During recuperation the lower envelope is that for friction plus electric braking, the upper envelope that for electric braking alone.

4.2 Drive train architecture

For the current investigation a 1.6 litre 4 cylinder Diesel engine was at first thermodynamically optimised, with a focus on low fuel consumption, before the hybridisation was introduced. Fig. 21 gives an overview of the Continental 48V mild hybrid system and more details are presented in [11]. Basic technical specifications of the drive train are given in Fig. 23. It consists of a belt starter generator, driven via the belt from ContiTech, and Schaeffler decoupling belt tensioner, which reduces the tensioning forces in the belt drive while maintaining the full capability to transmit power. The energy produced by the BSG, either from conventional alternator operation or from brake energy recuperation is fed into the 48 V traction battery and via a DC/DC converter to the 12 V board net battery. The clutch-by-wire from LUK enables automatic sailing/coasting, without the necessity of the driver to intercept. This setup is simple and cost effective, and provides the most important hybrid functions to save fuel. It includes features like start stop with comfort high speed start, coasting (including engine off coasting), sailing (maintaining velocity), low speed engine off electric driving (creeping) in stop and go traffic. This architecture was shown for a gasoline powered car at the Vienna Symposium in 2014 [12].


Fig. 22: Continental system architecture and components for electrification: Belt starter generator 47Nm, DC/DC converter to supply the 12V, 48V Li-Ion battery, belt from ContiTech, belt tensioner from Schaeffler

Fig. 23: Technical specification for vehicle and 48V components

4.3 Phlegmatisation strategy

In 2015 an extensive study on the subject of phlegmatisation was published at the SAE World Congress [13], where the combustion engine was run in automated mode on an engine test bed in WLTC cycles. The corresponding torque traces were derived from the simulation of the electric system. The resulting changes in NOx emission and fuel consumption from the test bed measurements were validated for the current paper. For this the vehicle was run on a chassis dyno. The experiments were then repeated on the engine testbed, again in automated mode, several times in order to lend some statistical relevance to the data, without the spread of a test bed driver. Fig. 24 shows the torque split between the electric machine and the combustion engine exemplarily for a part load point at 100Nm. On the right side of the scale the combustion engine provides all of the torque, the BSG none. Here the degree of phlegmatisation is zero. On the left side, 30% of the torque is supplied by the BSG, the rest by the combustion engine.

Component Technical Data

10 Ah, 13 cells

460 Wh

3 kW, ƞ > 96% 12V/48V DC/DC Converter

Continuous max power: 11 kWMax torque: 47 Nm

Vehicle VW Golf 7 (MQB)

Engine : 4 cyl., 1.6 l TDI, 77 kW250 Nm, 1500-2750 1/minGearbox: 5 speeed manual+ hydrostatic clutch (LUK)

48V BSG

48V Battery


Fig. 24: Torque split between ICE and BSG, exemplarily shown at a steady state operating point at 100Nm total torque.

However, the degree of phlegmatisation is not a function of a steady state torque request, but of the dynamics of a load request itself. This is depicted in Fig. 25. When the driver request is infinitely fast, the limitation is the drive train torque available. Here the guiding variable is the torque available from the BSG. The solid curve is the torque provided from the combustion engine alone and is calculated by the ECU taking into account a number of important engine functions, like the smoke map, drivability factors etc.

Fig. 25: Phlegmatisation as function of a dynamic torque request

The dashed line depicts a moderate degree of phlegmatisation, with the sum of the two torque sources, combustion engine + BSG, remaining equal to the request of the solid line. This is also true for the dotted line. But in this case the maximum phlegmatisation is applied and there is not only the upper limit of the combustion engine (solid line) but also that of the maximum capability of the BSG (dotted line). More phlegmatisation would not be possible, since the BSG would not be able to supply the corresponding torque. The degree of phlegmatisation is expressed as ratio of the actual rate of change of torque from the BSG to the maximum possible rate of change, Equation 1.


Equation 1:

The process of calculating the degree of phlegmatisation for any step in time is shown in Fig. 26. In the first step the requested torque gradient is compared to the threshold value. Is it above the threshold then phlegmatisation is initialised.

Fig. 26: Schematic of the phlegmatisation software, simplified and linearised curves: Decision for or against phlegmatisation and reaction of ICE (left) and BSG (right).

The lower half of the image shows exemplarily for the case of an overall torque of 100Nm the lowering of the torque gradient for the ICE (left diagram) and the contribution of the BSG (right diagram), ramping in and out of its torque support. The effect of phlegmatisation on NOx and CO2 was schematically shown as depicted in Fig. 27 [14]. The CO2 emissions will reach a minimum when the state of charge of the battery is at the same level at the start and at the end of a test cycle. If recharging becomes necessary, fuel consumption will increase again. The right border was defined to be the state where fuel consumption reaches its initial value. Irrespective of this border, phlegmatisation can be continued further in order to reduce NOx, but at the expense of fuel consumption. This schematic is now reflected in the measurements shown in Fig. 28. The minimum fuel consumption is reached near the maximally possible phlegmatisation of the current BSG. At 100% phlegmatisation the rising slope of the curve indicates, that additional recharging now becomes necessary. This is also indicated by the SOC in the upper diagram. For clarity it has to be stated, that for the current investigation, the boardnet energy consumption was not considered. Usually with small mild hybrid systems, the amount of recuperated energy is just about sufficient to cover the demand of the boardnet. This was also


found in the previous investigation [13]. However, it was also stated in [14], that in increase in the capacity of the hybrid system, including the battery capacity, or a change in the test cycle as well as a heavier vehicle with more kinetic energy to be recuperated or the change from a P0 to a P2 system will significantly affect the trade off shown in Fig. 28. This means, that these considerations need to be performed for each system individually.

Fig. 27: Effect of phlegmatisation on CO2 and NOx emissions

Fig. 28: Baseline measurement at zero phlegmatisation and various degrees of phlegmatisation from left to right in WLTC.

As stated above, the NOx emissions can be reduced further with the degree of phlegmatisation, irrespective of the effect on fuel consumption. Fig. 29 shows the specific NOx emissions in [g/kWh] and the NOx mass flux in [g/h]. While the specific NOx are a fair measure to compare different engines, the NOx mass flux holds the information on the absolute NOx output of the ICE. Comparing the two diagrams, it is an obvious statement that at low loads the NOx mass flux must be low and at high


loads it must be high. This is not reflected in the diagram of specific NOx on the left, where NOx emissions rise for low loads, with respect to each kW produced. Using the dashed lines for constant engine speed in those diagrams, one arrives at the load sections shown in Fig. 30.

Fig. 29: Specific NOx emissions in g/kWh and NOx mass flux in g/h

The greyed boxes show the relevant areas where NOx reduction by phlegmatisation is possible. This is approximately always the range from full to half load, with most of the effect in the upper third.

In conclusion it is stated, that phlegmatisation helps to reduce fuel consumption and NOx emissions, in particular during strong acceleration. It can be tuned for the best effect of either NOx or CO2 reduction, and this depends strongly on the system setup, i.e. BSG and battery capacity, P0 or P2 hybrid configuration, vehicle inertia and test cycle. Therefore, phlegmatisation needs to be optimised for each system individually.


Fig. 30: NOx emissions in load sections for 5 engine speeds, see also Fig. 29

5 Conclusions and Outlook: Overall System Optimisation, Exchange of Energies "PowerX"

The main conclusion from the current paper is, that an overall system approach is needed for best results.

Efficiency increases in combustion engines, and in particular in Diesel engines lead to ever lower exhaust gas temperatures. This makes the use of exhaust aftertreatment systems more difficult.

Downsizing and the subsequent use of turbo charging lead to slower response of combustion engines, compared to naturally aspirated ones. Here the use of 48V mild hybrid systems with electric torque support or phlegmatisation as well as electric super charging help to overcome turbo lag as well as the need for scavenging in gasoline engines [15].

Optimisation of the Diesel engine towards higher engine out NOx through the optimisation of the centre of combustion and the reduction of EGR rates will improve fuel consumption, however with the consequence of lower engine out temperatures.


Electric heating for early aftertreatment light off, combined with HC heating for lowest fuel consumption during catalyst heating will help to achieve very stringent NOx limits. The increase in fuel consumption for these heating measures can be offset through HR50 optimisation and reduction of EGR rates in the ICE.

Electric + HC heating are an attractive competition to variable valve actuation systems, even to simple VVA systems, like e.g. the Mazda double exhaust valve actuation. Electric “ignition” heating by the electric heater will be necessary for early light off of the DOC for HC heating.

Engine off coasting (mild hybrid system) not only reduces fuel consumption and NOx emissions, but also prevents convective cooling of the aftertreatment system during vehicle braking/vehicle overrun. This greatly reduces the energy consumption of catalyst heating measures

Insulation of the EAT will be necessary to sensibly limit the energy consumption of any such heating system.

All forms of energy, mechanical, electrical, heat and chemical, being interchanged in a system comprising a combustion engine, a hybrid drive and an aftertreatment system, need to be managed in an optimal manner. The Conti approach is to use a “cost function”, to assess the availability of energies and the effort to efficiently use them.

Fig. 31 gives a system overview of energy consumers and providers in the vehicle. This serves as a basis for the energy management software PowerX (X for exchange).

Fig. 31: “PowerX” power exchange management, Block diagram, the software for the powertrain section “combustion engine + electric machine + battery” is currently undergoing tests in vehicles.


6 References

[1] ATZLER, F. et al. Engine map of smart Diesel injection strategies, 11. International Stuttgart Symposium, February 2011

[2] WAGNER, U.; SCHAMEL, A.; MAIWALD, O. et al. 48 V P2 Hybridisierung mit optimiertem Motorkonzept – bestmögliche Fahrbarkeit bei exzellenter Verbrauchs- und Kosteneffizienz, Wiener Motorensymposium 2016

[3] ATZLER, F.; ROHRER, S.; WEGERER, M.; MAIWALD, O.; FROMM, R.; SCHATZ, A. Fuel Consumption and Emissions of a “phlegmatised” Passenger Car Diesel Engine, FEV Diesel Powertrain 3.0, Leipzig, June 2016

[4] Technical Customer Specification PCRs5, Continental 2016

[5] DIAN, V.; KLÜGL, W.; KRÜGER, G.; AVOLIO, G.; NIGRIN, U.; KAPPHAN, F.; BOLL, C. Continental´s new piezo common rail system for efficient and clean diesel engines, SIA Conference, Rouen, June 2016

[6] VAROQUIE, B.; HELLEMANS, A.; POURNAIN, A.; HEINITZ, D.; BOUQUEY, C. ICPS based Combustion Control: An efficient way to reduce engineering margin for Diesel engines, SIA Conference, Rouen, June 2016

[7] HOHNARDAR, S.; DEPPENKEMPER, K. et al. Potentiale von Ladungswechselvariabilitäten im Hinblick auf Emission, Dynamik und Abgastemperaturverhalten beim Pkw-Dieselmotor, Research Association Combustion Engines / Forschungsvereinigung Verbrennungskraftmaschinen, Project No.1027, Book No, 1034, 2013

[8] TERAZAWA, Y.; NAKAI, E.; KATAOKA, M.; SAKONO, T. Der neue Vierzylinder Dieselmotor von MAZDA, Motortechnische Zeitschrift MTZ9 - 2011

[9] SCHOMMERS et. al. Bluetec – Das Konzept für Dieselmotoren mit niedrigsten Emissionen, Motortechnische Zeitschrift, 05-2008

[10] BRÜCK, R.; KONIECZNY, R. Thermomanagement für Niedrigstemissionskonzepte moderner Antriebe; Der elektrisch heizbare Katalysator; 19. Aachener Kolloquium Fahrzeug- und Motorentechnik 2010


[11] GRAF, F.; KNORR, T.; JEHLE, M; HUBER, T. Antriebsmanagement in Hybridfahrzeugen mit 48V Bordnetz, Motortechnische Zeitschrift MTZ 9 – 2014

[12] SCHÖPPE, D.; KNORR, T.; GRAF, F.; KLINGSEIS, B.; BEER, J.; GUTZMER, P.; HAGER, S.; SCHATZ, A. Downsized Gasoline Engine and 48 V Eco Drive – An Integrated Approach to Improve the Overall Propulsion System Efficiency, Wiener Motoren Symposium 2014

[13] ATZLER, F.; WEGERER, M.; MEHNE, F.; ROHRER, S. Continental Automotive Regensburg, C. Rathgeber, S. Fischer, VKM Technische Universität Darmstadt, Fuel consumption and emissions effects in passenger car Diesel engines through the use of a belt starter generator, SAE Paper No 2015-01-1162, SAE World Congress, Detroit, April 2015

[14] ATZLER, F.; WEGERER, M.; ROHRER, S.; ZAKI, M. Fuel Consumption and Emissions of a downsized Passenger Car Diesel Engine with 48V Phlegmatisation, Emission Control Conference, Dresden, June 2016

[15] ATZLER, F. Electric charging to avoid scavenging in future gasoline engines, Aufladetechnische Konferenz ATK, Dresden, 2015

7 Abbreviations

Ad-Blue Aqueous Urea Solution (32,5%), AUS

BSG, BAS Belt Starter Generator = Belt Alternator Starter

CF Conformity factor

CFD Computational Fluid Dynamic

CLCC Closed loop Combustion Control

CN calculated combustion noise from cylinder pressure [dB(A)]

CO carbon monoxide

CO2 carbon dioxide

crk crank angle [°]

DC/DC During currant converter, here from 48 to 12V

DOC Diesel Oxidation Catalyst


EAT Exhaust Aftertreatment System

EGR, LP, HP exhaust gas recirculation, low pressure circuit, high pressure circuit

EHC Electrically Heated Catalyst

HC hydro carbons, here: unburnt fuel

HR50 crank angle at which 50% of combustion heat is released

[°crk]

ICPS In Cylinder Pressure Sensor

ICE Internal Combustion Engine

IMEP indicated mean effective pressure [bar]

LNT Lean NOx Trap, also: NOx Storage Catalyst

MAF Manifold air flow (sensor)

mF mass of fuel [mg/stk]

MI multiple injection

MinDwell minimum dwell injection pattern, MinDwell

n engine speed [rpm]

NEDC New European Driving Cycle

NOx nitrous oxides

P0 (hybrid) Electric machine in conventional belt drive at engine front

P2 (hybrid) Electric machine between combustion engine and gearbox, with two clutches can be separated both sides from Drivetrain

pboost boost pressure [bar]

pcyl in-cylinder pressure [bar]

Pi pilot injection

PM particulate matter (often represented by “smoke number”)


Po post injection

prail fuel pressure in rail [bar]

RDE Real Driving Emissions

RDU Reactant Dosing Unit, Injector for urea solution

RoHR rate of heat release [kJ/°crk]

SCR Selective catalytic reduction (NOx exhaust gas aftertreatment)

SDPF SCR on Diesel Particulate Filter

Smoke filter smoke number, representing smoke or PM emissions

[FSN]

SOC start of combustion [°crk]

SOI start of injection (electric start of energising of the injector)

[°crk]

Tbm temperature before the onset of main combustion [K]

TCO coolant temperature [°C ]

TDC Top dead centre

TGas mean in-cylinder gas temperature (from CFD simulation) [K]

VTG Variable Turbine Geometry, variable turbine inlet stator

VVT, VVA Variable Valve Timing, Actuation

WLTP, WLTC World harmonised light duty test protocol / cycle

λ Lambda, Normalised Air - fuel ratio [-]

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