Homogeneous Charge Compression Ignition (HCCI)

Engines

Contents

HCCI Engine Importance Working principle Starting of HCCI engines Control methods of HCCI Dual mode transitions Performance & Emission Characteristics Recent developments HCCI prototypes Advantages Disadvantages

What is an HCCI Engine?

• HCCI is a form of internal combustion in which the fuel and air are compressed to the point of auto ignition.

• That means no spark is required to ignite the fuel/air mixture.

• Creates the same amount of power as a traditional engine, but uses less fuel.

• The exothermic reaction releases chemical energy into a sensible form that can be transformed into work and heat.

Traditional combustion (left) uses a spark to ignite the mixture. HCCI (right) uses piston compression for a more complete ignition.

IMPORTANCE

SI engines have very low NOx and PM emissions CI engines have high efficiency. Homogeneous Charge Compression Ignition

(HCCI) is a promising alternative combustion technology with high efficiency and lower NOx and particulate matter emissions.

HCCI engines can achieve gasoline engine-like emissions along with diesel engine-like efficiency.

Ignition occurs at several places in case of HCCI engines. This makes the A/F mixture burn nearly simultaneously.

PRINCIPLE

HCCI is characterized by the fact that the fuel and air are mixed before combustion starts and the mixture auto-ignites as a result of the temperature increase in the compression stroke.

Optical diagnostics research shows that HCCI

combustion initiates simultaneously at multiple sites within the combustion chamber and that there is no discernable flame propagation.

Comparison of SI and HCCI combustion

Spark Ignition HCCI

HCCI Concept

Starting HCCI engines Charge does not readily auto ignite cold engines. Early proposal was to start in SI mode and run in

HCCI mode. It involves the risk of knocking and cylinder

failure at high compression ratios. Now intake air pre-heating with HE and burner

system allows startup in HCCI mode with conventional starter.

Starting HCCI engines

Control methods of HCCI combustion

The spontaneous and simultaneous combustion of fuel-air mixture need to be controlled.

No direct control methods possible as in SI or CI engines.

Various control methods are:

Variable compression ratio. Variable induction temperature. Variable valve actuation. Variable exhaust gas percentage. Variable fuel ignition quality.

Variable compression ratio method:

The geometric compression ratio can be changed with a movable plunger at the top of the cylinder head. This concept used in “diesel” model aircraft engine.

Variable induction temperature:

The simplest method uses resistance heater to vary inlet temperature. But this method is slow.

Now FTM (Fast Thermal Management) is used. It is accomplished by rapidly varying the cycle to cycle intake charge temperature by rapid mixing.

FTM system

Rapid mixing of cool and hot intake air takes place achieving optimal temperature as demanded and hence better control.

FTM Control method

Combustion timing can be controlled by adjusting balance of hot and cold flow

Variable exhaust gas percentage:

1. Combustion timing can be controlled by using EGR in HCCI engines.

2. Both Hot EGR and Cool EGR can be used.

3. Hot EGR will increase the temperature of the gases in the cylinder and will advance ignition.

4. Cool EGR dilutes the fresh charge, delay in ignition and reduces the chemical energy and engine work.

5. Reduce HC and CO emissions.

Variable valve actuation (VVA):

This method gives finer control within combustion chamber.

Involves controlling the effective pressure ratio. It controls the point at which the intake valve closes. If the closure is after BDC, the effective volume and hence compression ratio changes.

Variable Fuel ignition quality:

By adopting dual fuel concept and blending we can control the onset of ignition and the heat release rate in the HCCI engines.

Control methods of HCCI combustion

Dual mode transitions

Need:

When auto-ignition occurs too early or with too much chemical energy, combustion is too fast and high in-cylinder pressures can destroy an engine. For this reason, HCCI is typically operated at lean overall fuel mixtures

This restricts engine operation at high loads.

Dual mode transitions

Practical HCCI engines will need to switch to a conventional SI or diesel mode at very low and high load conditions due to dilution limits

Two modes: HCCI-DI dual mode (Stratified charge

compression ignition)

HCCI-SI dual mode (Homogenous charge

spark ignition)

SI mode transitions

It equips VVA and spark ignition system.

Operates in HCCI mode at low to medium loads

and switches into SI mode at higher loads.

Transition is not very stable and smooth.

DI-HCCI Long ignition delay and rapid mixing are required

to achieve diluted homogeneous mixture. Combustion noise and NOx emissions were

reduced substantially without an increase in PM. Combustion phasing is controlled by injection

timing. Thus DI-HCCI proves to be promising alternative

for conventional HCCI with good range of operation.

Engine performance

Indicated Mean effective pressure

The IMEP increases with the increase of premixed ratio at low to medium loads.

The indicated thermal efficiency shows deterioration at high load with large premixed ratios.

Emission characteristics

Nox emission

NOx emissions decrease firstly at low premixed ratios and exhibit a trend of increasing at higher premixed ratios.

Pre-mixed ratio has no significant effect on soot emission and the soot emission could remain at the same level but then have a peak value with a certain higher premixed ratio relating to the equivalence ratio.

Emission characteristics

Emission characteristics

CO & HC Emission

• The change of CO with premixed ratio is mainly depending on whether the premixed equivalence ratio exceeds the critical value.

• UHC increases almost linearly with the premixed ratio mainly due to the incomplete oxidation in the boundary layer and the crevices.

Recent developments in HCCI

Turbo charging initially proposed to increase power. But there are certain challenges also. They are:

1. Exhaust gas temperatures low (300 to 350 °c) because of high compression ratio.

2. Post turbine exhaust gas temperature must be high enough to preheat intake fuel-air mixture in HE.

3. Low available compressor pressure ratio.

Solutions to turbo charging

1. Use VGT (Variable Geometry Turbine) which allows for a greater range of turbine nozzle area, better chance to achieve high boost.

2. Combining turbo charging and super charging may be beneficial.

EGR (Exhaust Gas Re-circulation) Can be adopted for higher efficiencies and lower NOx emissions.

HCCI prototypes General Motors has demonstrated Opel Vectra and Saturn

Aura with modified HCCI engines.

Prototype HCCI car Saturn from GM

Mercedes-Benz has developed a prototype engine called Dies Otto, with controlled auto ignition. It was displayed in its F 700 concept car at the 2007 Frankfurt Auto Show

HCCI Prototypes

• Volkswagen are developing two types of engine for HCCI operation. The first, called Combined Combustion System or CCS, is based on the VW Group 2.0-litre diesel engine but uses homogenous intake charge rather than traditional diesel injection. It requires the use of synthetic fuel to achieve maximum benefit. The second is called Gasoline Compression Ignition or GCI; it uses HCCI when cruising and spark ignition when accelerating. Both engines have been demonstrated in Touran prototypes, and the company expects them to be ready for production in about 2015.

HCCI prototypes

In May 2008, General Motors gave Auto Express access to a Vauxhall Insignia prototype fitted with a 2.2-litre HCCI engine, which will be offered alongside their ecoFLEX range of small-capacity, turbocharged petrol and diesel engines when the car goes into production. Official figures are not yet available, but fuel economy is expected to be in the region of 43mpg with carbon dioxide emissions of about 150 grams per kilometre, improving on the 37mpg and 180g/km produced by the current 2.2-litre petrol engine. The new engine operates in HCCI mode at low speeds or when cruising, switching to conventional spark-ignition when the throttle is opened

Advantages

1. High efficiency, no knock limit on compression ratio.

2. Low NOx and no NOx after treatment systems required.

3. Low PM emissions, no need for PM filter.

4. HCCI provides up to a 15% fuel savings, while meeting current emissions standards.

5. HCCI engines can operate on gasoline, diesel fuel, and most alternative fuels.

6. In regards to CI engines, the omission of throttle losses improves HCCI efficiency.

HCCI automobiles could reduce greenhouse gas emissions

Disadvantages

1. The auto-ignition event is difficult to control, unlike the ignition event in spark -ignition(SI) and diesel engines which are controlled by spark plugs and in-cylinder fuel injectors, respectively.

2. HCCI engines have a small power range, constrained at low loads by lean flammability limits and high loads by in-cylinder pressure restrictions.

3. High HC and CO emissions.

References• “Homogeneous Charge Compression Ignition”

http://en.wikipedia.org/wiki/Homogeneous_Charge_Compression_Ignition November, 2008.

• “New HCCI Engine”

http://videos.howstuffworks.com/multivu/3284-new-hcci-engine-video.htm November, 2008.

• “GM Takes New Combustion Technology Out of the Lab and Onto the Road”

http://www.gm.com/experience/fuel_economy/news/2007/adv_engines/new-combustion-technology-082707.jsp