W4S1.2 Engine Optimization

7/23/2019 W4S1.2 Engine Optimization

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© IFPEN / IFP School 2014

Sustainable MobilityTechnical and environmental challenges for the automotive sector

Week 4 – Session 1 & 2 – Engine Optimization

Maria Thirouard



W4 – S1&S2 – Engine Optimization p. 1


This section will provide the necessary information for you to succeed in this week’s serious game.

In this game you will be asked to optimize an engine. In other words, you will need to find the right

settings of the engine to optimize fuel consumption, NOx, particulates and noise.

Some of the subjects have already been discussed in previous lessons. Today we will go further into

the effect of the engine settings on combustion to improve fuel consumption or reduce pollutant

emissions.

First, I will give you the general framework of engine optimization and then we will talk about the

selected settings -EGR, Fuel injection pressure and Start of Injection- and their effect on the

engine’s results.

Engine Optimization FrameworkThis is a simplified exercise that is meant to give you the general trends for diesel engine

calibration.

What is engine calibration?

It’s the adjustment of the engine settings to yield optimal results from the engine. The optimal

results that we look for might be different: increase power output, reduce fuel consumption or

reduce pollutant emissions. Usually these goals are mutually exclusive. For example, the engine

might be optimized with respect to power output but in exchange, fuel consumption will not be the

lowest possible. Of course, we would like to find the best result in everything but it’s just not

possible. We can add technology to boost results… but then it always comes with a price.

Actually, engineers are continuously looking for ways to improve the engine results. In general a

trade-off is found and we accept that if you win some kW of power here, you lose some fuel

consumption there.

Why a diesel and not a gasoline engine?

Present day diesel calibration is more challenging since the number of variables to optimize

combustion and pollutant emissions is significantly higher.





How is the calibration done?

In real life a lot of tests are performed. We test all the variables, in different combinations, we look

at the results and we try to reach the targets. In this serious game, you will only get an insight of

this process. My goal is to show you the challenges that calibration engineers are facing nowadays.

Let me introduce the problem to you, a problem that has been simplified for teaching purposes.

The results that we want to optimize are: fuel consumption, NOx emissions, particulates emissions

and noise. As explained previously, you will only work with 3 variables: EGR rate , Fuel injection

pressure and Start of Injection.

EGR rateEGR or “Exhaust Gas Recirculation” is a technology that reintroduces a part of the exhaust gases at

the intake of the engine. The goal of EGR is to reduce the production of NOx by reducing the

amount of O2 available and by lowering combustion temperature. You also saw that the main

drawback is the resulting increase in soot emissions.





Let’s look again at the graph of Equivalence Ratio against Temperature presented by Prakash. You

remember that diesel combustion is essentially heterogeneous. Well, let’s see how this really

happens in a diesel engine. The results that I’m about to show you were obtained by 3-D modeling

so each dot represents a little cube inside the combustion chamber. At the beginning there is only

air in the chamber so the equivalence ratio is equal to 0. As the piston goes up the pressure and

temperature inside the cylinder rises. Then, the injection of fuel starts. Inside the combustion

chamber you now have fuel that is not mixed but also fuel that is partially mixed with the

surrounding gases so the equivalence ratio varies. This explains the dots all along the axis of the

equivalence ratio. As the piston goes up and injection goes on, the fuel and the air mix further

together and when the right conditions of pressure and temperature are met, combustion occurs

by auto ignition. The time between the start of injection and the start of combustion is called the

auto ignition delay. Combustion is represented in the video by a rapid increase in temperature.

Combustion continues after the end of injection as the remaining unburned fuel mixes with the

remaining air available in the combustion chamber. At the end, the temperature goes down as the

gases expand with the piston’s movement. This is the path of a classic diesel combustion.

Let’s now see the path of a diesel combustion with EGR. As explained by Prakash, there is lessoxygen so, globally, equivalence ratio is higher, temperature is lower because CO2 gases introduced

with EGR act as a heat sink during combustion so we fall directly into the zone where soot is

produced. Here is an important trade off that you need to keep in mind: when we use EGR, NOx is

reduced but soot is increased.

Regarding efficiency, the use of unreactive gases slows down the combustion process. In terms of

efficiency, combustion is less instantaneous, and might not be phased at the right moment either.

Usually the use of EGR reduces efficiency and therefore, increases fuel consumption.





Let’s now see what happens to noise. Noise in diesel engines is produced by the rapid combustion

of fuel accumulated during the auto ignition delay. Remember that the auto ignition delay is thetime between the start of injection and the start of combustion. The quick combustion of the fuel

that is premixed during the auto ignition delay is called premixed combustion. When using EGR, the

auto ignition delay is longer so a larger amount of fuel is accumulated during the auto ignition

delay. When this fuel burns in premixed combustion, the noise produced is usually higher.

And why is noise is SO important? It’s actually just important for the comfort of the passengers. But

yes, it is taken into account in the optimization procedure.

Summarizing the general trends: EGR will reduce NOx, increase PM, reduce efficiency and increase

noise.





Fuel Injection PressureYou remember from the previous lessons the Common Rail Direct Injection system? As we

explained, the idea is to inject fuel at a very high pressure. Fuel injected at high pressure atomizes

rapidly giving very small droplets which mix quickly with the air surrounding the jet. And you will

probably remember, it’s usually combined with swirl to enhance mixing and avoid pockets of fuel-rich zones. Consequently, local richness decreases, and this reduces soot emissions.

The use of a higher injection pressure increases the rate at which the fuel enters the combustion

chamber: for a given injection duration, the amount of fuel introduce at high fuel pressure will be

higher than at low fuel pressure. As a consequence, when using high injection pressure, the

amount of fuel introduced during the auto ignition delay increases. A larger amount of fuel burnsvery quickly in premixed combustion and as a consequence, the noise increases. This is another

important trade off: when using high fuel injection pressure, smoke is reduced but noise is

increased.





You can see an example in this video. By avoiding the pockets of fuel-rich zones, smoke is reduced.

Since the amount of fuel burned during the premixed combustion is higher, the in-cylinder pressure

rises very quickly and so does the temperature.

Let’s look back at the classic path of diesel combustion. By increasing the injection pressure, the

equivalence ratio distribution is more on the lower richness side, and the temperature distribution

goes more to the higher temperatures side. As you can see, increasing fuel injection pressure

increases NOx emissions. Please keep in mind that this illustration is just representative of what

happens. The combustion process is very complex.

What happens to efficiency?

Efficiency is generally improved with higher injection pressures. This is because combustion is faster

and usually at a better phasing. We will explain what combustion phasing means in a few moments.





Before that, let’s summarize the general effect of fuel injection pressure: high injection pressure

reduces soot emissions, increases noise and NOx, and reduces fuel consumption.

Start of Injection or SOIThe start of injection is one of the main engine parameters to set when doing engine calibration. As

its name indicates, it is the moment in the engine cycle where the start of injection is triggered. By

convention, the reference point is the combustion’s top dead center so the 0 is placed at TDC. SOI

is measured in crank angle degrees. If the start of injection is before TDC, the values are positive. If

the start of injection is after TDC, the values are negative.





An ideal engine would burn instantaneously at TDC. In reality, injection and combustion take some

time. Placing the injection at the right moment will phase the combustion at the right place in the

cycle to optimize the cycle efficiency. If the injection is too early, combustion will take place when

the piston is moving up and a part of the energy will be lost by the opposite piston motion.

Additionally, heat transfer to the cylinder walls is usually higher in that case (because temperature

is higher), so the cycle efficiency will be low. On the other hand, if the injection is too late, we will

not recover much energy during the power or expansion stroke and a lot of energy will be wasted

to the exhaust.

Generally, if there is only one injection, the start of injection is placed before the TDC. Then, with

the auto ignition delay, combustion starts around the TDC and goes on while the piston moves

down. Most of the time a sequence of 2, 3 or 4 injections is used.

Let’s now talk about noise. When SOI is early in the cycle, the pressure and temperature conditions

inside the combustion chamber are not favorable to auto ignition. This means that the auto ignition

delay is longer and the amount of fuel cumulated during the auto ignition delay is high.

Consequently a big premixed combustion is produced with high levels of noise. So increasing the

SOI will generally produce high levels of noise





There is also a direct effect of SOI on NOx emission. As has been shown with the in-cylinder

pressure signal: when SOI is early, the big premixed combustion produces a rapid rise in pressure

and temperature. In these conditions, NOx emissions increase. On the other side, late SOI will

produce low NOx emissions.

I will not talk about the effect of start of injection on soot emissions since there is no clear link

between these two parameters.

Summarizing, it’s important to place the injection at the right time to improve efficiency. If it is too

early or too late, fuel consumption will increase. Finally, increasing SOI increases NOx and noise.




Now, you can see that the difficulty is to optimize all parameters together when the effects are

crossed-linked. And don’t forget that there are more parameters to optimize than what I’ve shown

you here. As I told you at the beginning, we try to maximize the benefits and reduce the drawbacks.

That’s it for this lesson. Have fun with the serious game

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W4S1.2 Engine Optimization

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