Cooled EGR

[ConnClark]

Okay lets first get a few facts down

soot = incomplete combustion of fuel

higher intake temps = higher combustion temps = higher NOx emissions

higher combustion temps = higher heat loss to cooling system

To keep NOx emissions low in a diesel two main approaches are employed. Reducing peak combustion temps and reduce excess O2. Spreading the combustion evenly throughout the combustion chamber reduces hot spots where most NOx is produced. Reducing the O2 in the cylinder means the fuel spray has to travel farther to encounter enough O2 to burn thus spreading it out. The denser the intake charge the more mass the fuel has to heat and thus the lower the combustion temps. The colder the intake charge the more the fuel has to heat a given mass to get it into NOx production ranges.

The whole purpose of cooling the EGR on a diesel was so they could use it more effectively or use lessof it to control the NOx emissions and pack more excess air in the cylinder to reduce soot. The mixture of air and cooled EGR is more dense and thus more of it can be put into the cylinder on the intake stroke. This helps reduce the peak combustion temps and reduces NOx. This also reduces the heat lost to the cooling system and thus more power can be extracted from the fuel.

An ideal diesel optimized for efficiency would not uses EGR at all. EGR reduces the specific heat ratio of the combustion gasses and reduces the amount of power that can be extracted from the fuel. Instead of EGR spreading combustion throughout the cylinder evenly would be accomplished by controlling the velocity of the atomized fuel droplets so they would burn up just before they hit the edge of the combustion chamber. Additionally to maintain the ideal constant pressure combustion of the diesel cycle fuel would be injected in at a rate to ensure this. Since this is very hard and expensive to do it is not done.

In the case of a direct injection diesel engine EGR percentages are used to slow burn rate of the fuel. It aids in spreading out the combustion and thus heat evenly in the cylinder. The percentage can also be used to help approximate the constant pressure combustion cycle of the diesel cycle. IT reduces excess O2 content so there will be less for nitrogen to combine with to make NOx. It also increases charge mass that in turn reduces combustion temperature.

Increasing EGR percentages past a certain point are detrimental to efficiency however. It slows the burn rate of atomized fuel to the point it strikes the wall of the combustion chamber which in turn causes an incomplete burn. It slows the combustion rate to the point where the ideal diesel cycle constant pressure combustion isn't maintained. It reduces the amount of fuel that is burned by depriving it of O2. It also reduces the specific heat ratio of the combustion charge.

In conclusion More EGR in a diesel is not better. The optimum percentage used for efficiency will be determined by injector pop pressure, atomization of the fuel, droplet velocity, etc.... Even if you were to tune all your injectors to be exactly the same then you will be limited by the ability of your emissions control system's ability to control it accurately. Unless you really have studied, weighed and measured all factors involved and the trade offs, increasing your EGR flow is more likely to drop your mileage on a modern direct injection diesel.

Edit: I should add that I think changing your EGR system to a cooled EGR system will probably be beneficial to mileage.

[tasdrouille]

I'm picking on your words, but overall more heat is transfered to the cooling system with EGR than without. Not from the cylinders walls, but the EGR cooler itself. Quicker warmup times are becoming quite handy for some people this time of year.

The following is for a fairly typical diesel engine with rotary pump type injection system. It's possible to increase efficiency with EGR, it's just tough and it's not a panacea.



EGR will contribute to clog your intake manifold because it introduces soot back in the intake charge. This soot combines with crankcase vapors to form a thick paste in your intake manifold over the miles. Also, EGR should not be cooled below the condensation point as this would aggravate the clogging .

[Piwoslaw]

ConnClark, thanks for the comprehensive (not to say exhaustive) write up. The main reason I decided to go ahead with the EGR cooler is because later versions of my engine have it, so it is a no-fuss direct swap. Yes, I take into account that the newer engine's ECU compensates for cooled EGR and may keep the valve open for a shorter time. I'll see if I notice a difference in fuel economy and in power. If something goes wrong I can still tweek the EGR valve to open less.

[euromodder]

Originally Posted by UFO

EGR does help with NOx emissions, but it's a secondary pollution.

NOx hardly qualifies as secondary pollution, it's about the most harmfull thing to come out of the exhaust !

Biodiesel generates around 10% more NOx than regular diesel, so disabling the EGR while on biodiesel is not the best of options.


If your car has EGR, please keep it operational !
Better mileage at the expense of polluting MORE is not my idea of driving more environmentally friendly.

[Piwoslaw]

I picked up the car today. The EGR cooler is fitted, cam belt and pulleys replaced, coolant heater installed. It's hard to tell if there is any difference, since I only drove 8km, and the engine was warm when I got the car back. It did feel like it had more power, but this may only be a feeling - I spent half the day driving my Grandma's Corsa (1.0 liter engine), plus I had biking shoes on. My brother-in-law said that maybe the cam timing was (unintentionally) adjusted while replacing the cam belt and pulleys.

[COcyclist]

"EGR will contribute to clog your intake manifold because it introduces soot back in the intake charge. This soot combines with crankcase vapors to form a thick paste in your intake manifold over the miles". ---I have read that this should not be as much of a problem since low sulpher diesel has been adopted in the U.S. Does anyone have any experience with this?

So EGR reduces Nox and may help with faster warmup but what would make more power on say, an off-road racing diesel engine, EGR or no EGR?

[UFO]

Originally Posted by euromodder

NOx hardly qualifies as secondary pollution, it's about the most harmfull thing to come out of the exhaust !

Biodiesel generates around 10% more NOx than regular diesel, so disabling the EGR while on biodiesel is not the best of options.


If your car has EGR, please keep it operational !
Better mileage at the expense of polluting MORE is not my idea of driving more environmentally friendly.

Maybe you can support that claim. All that I've read about NOx is it requires hydrocarbons and UV to create smog. Modern pollution controls have reduced vehicle HC output to the point that additional NOx control is pointless.

Even so, if I net 10% better mileage with the disabled EGR, my total pollution output with biodiesel is WAY ahead. Additionally, I don't have to clean out my intakes anymore. Bonus.

[UFO]

Originally Posted by COcyclist

"EGR will contribute to clog your intake manifold because it introduces soot back in the intake charge. This soot combines with crankcase vapors to form a thick paste in your intake manifold over the miles". ---I have read that this should not be as much of a problem since low sulpher diesel has been adopted in the U.S. Does anyone have any experience with this?

So EGR reduces Nox and may help with faster warmup but what would make more power on say, an off-road racing diesel engine, EGR or no EGR?

That's easy. No EGR will allow more air. More air allows more fuel. More fuel equals more power. If EGT is an issue, use water injection.

[vtec-e]

I did read somewhere that water mist injection would lower combustion temps and therefore NOx.

[ConnClark]

Originally Posted by vtec-e

I did read somewhere that water mist injection would lower combustion temps and therefore NOx.

probably and it is true. Water injection has been successfully used on car ferries and large stationary diesel generators as a NOx pollution control. As a side effect maximum power and fuel efficiency goes up slightly. Note usually when used for pollution control water injection is not tuned to to improve power or fuel economy but is tuned for the maximum practical NOx control.