Increasing EGR flow for better mileage

[MetroMPG]

Just stoking the fire with more info:

This Toyota press release about the new hybrid system in the 3rd gen Prius & dedicated Lexus 450h hybrid discusses the 2 aims of their cooled EGR approach.

1) manage EGR to reduce exhaust gas temps in high-load situations (hills & freeway driving) to avoid needing to go to fuel enrichment (which also reduces EGT) to protect the catalysts; and

2) reduced pumping losses

The cooled EGR system allows precisely metered and spent exhaust gas to enter the intake system, lowering exhaust gas temperature while also reducing engine pumping losses via decreased intake vacuum. An EGR delivery manifold ensures even distribution of the exhaust gasses to all cylinders. The high-capacity EGR cooling system can reduce the gas temperatures from about 880˚ C to 150˚ C, improving volumetric efficiency. The cooled EGR also eliminated the need for exhaust VVT-i.

[Daox]

Alright, so from all that reading, it looks like we have two issues with running large amounts of EGR.

1) Fuel mixture dilution causing misfires and/or incomplete burn.
2) Hot EGR gas causing detonation.

#1 is going to be unavoidable. A high swirl/tumble head will probably help to some extent (or direct injection).

#2 is avoidable with some sort of EGR cooler. I know they use these on diesels. Can any TDI owners pipe up with any info on these systems? It looks like they run coolant through a heat exchanger to cool down the EGR charge.

[Daox]

Here is a picture of a TDI EGR cooler. It looks like they use engine coolant to cool the EGR. I'm guessing this is due to space constraints mainly.





I was also looking around at other EGR valves. It appears that the Accord engines use a similar EGR valve as the Insight in that its electronically actuated. This may make it easier to actuate and control accurately.

[wagonman76]

I have the EGR disconnected on my Celebrity. I did this back in 04. I did it because it is supposed to reduce or eliminate carbon buildup in the intake manifold (assuming it is clean to start with). I noticed absolutely no difference in performance, driveability, or mpg. So I left it disconnected.

[ConnClark]

EGR on diesels is done exclusively for NOx control. In general EGR on a diesel reduces power and efficiency because it drops the specific heat ratio of the combustion gasses. Cooled EGR on a diesel helps because it allows less EGR to be used to control NOx emissions. There is a limit to how much they can cool it before it starts robbing power. If cooled to the point that steam condenses back into water vapor energy must be expended to re boil the water.

Very small amounts of egr can be used to slightly increase power in a diesel. At lower rpms it lowers the peak pressure slows the combustion to better approximate the ideal diesel cycle. It also allows the fuel to spread farther in the combustion chamber and cause more uniform heating of the combustion gases. Unfortunately this low level does almost nothing to control NOx so it is not used in this way.

[Daox]

Thanks for the info ConnClark. Do you have any idea if the EGR systems on diesels are capable of flowing more than an average gas engine EGR system? It sounds like we'd need a larger valve to make a noticeable difference.

[tasdrouille]

Originally Posted by ConnClark

EGR on diesels is done exclusively for NOx control.

Maybe that is why it is done, but EGR on a diesel can have a positive impact on fuel economy when the car is tuned accordingly.

The following was for a single cylinder optical engine very similar in design, and in this case operating parameters, to a what you'd find in a recent diesel car.

[Daox]

Well, I picked up the TDI EGR cooler, so I guess this is now a project.

Now, I need an EGR valve. I need a large EGR valve to ensure it will flow as much as I want. I also need a way to control this EGR valve. There seems to be two kinds of EGR valves, vacuum actuated, and solenoid actuated. Suggestions are very welcome.

[Daox]

Bumping for suggestions and any other ideas.

[NiHaoMike]

Originally Posted by ConnClark

There is a limit to how much they can cool it before it starts robbing power. If cooled to the point that steam condenses back into water vapor energy must be expended to re boil the water.

Actually, the water expanding into steam would help with efficiency. That's the idea behind water injection.