Increasing EGR flow for better mileage

[Daox]

That was a great article. Lots of useful info! I especially like the NOx vs EGR dilution chart here. Makes me think that most mfgs just use ~10% to reduce NOx emissions where more can be useful for mileage gains.

[tasdrouille]

I was reading John B. Heywood's book Internal Combustion Engine Fundamentals last night. Here's an interesting paragraph relevant to EGR

At part-load operating conditions, it is advantageous to dilute the fuel-air mixture, either with excess air or with recycled exhaust gas. This dilution improves the fuel conversion efficiency for three reasons: (1) the expansion stroke work for a given expansion ratio is increased as a result of the change in thermodynamic properties of the burned gases; (2) for a given mean effective pressure, the intake pressure increases with increasing dilution, so pumping work decreases; (3) the heat losses to the walls are reduced because the burned gas temperature are lower.

[drbobwoolery]

I now plan to dig around pick yer part for big V6 and V8 EGR valves. Stepper motor or PWM driven have both been used, I understand, by GM and Ford, respectively. 10% flow from a 5 liter engine would be 50% in my 1 liter engine, assuming the wide open valve is the biggest restriction. The exhaust passage will have to be easily cleanable. What a shame to dump the exhaust heat into cooling water and get no work back for it. I have an Accel 300+ capacitive discharge multistrike ignition that I hope will help get/keep the fire going in hard to ignite conditions.

[user removed]

I read through this thread but did not delve into all the links. EGR in my understanding reduces peak combustion chamber temperatures, to avoid pre ignition, and allow more timing advance and higher compression, without exceeding NOX emission standards.

Adding EGR will decrease peak combustion chamber temperatures, which will directly affect the power produced.

Most EGR systems are designed to allow EGR introduction when manifold vacuum is low to non existent. They work with a combination of ported and manifold vacuum working against each other. Modern systems use computer controls to accomplish basically the same thing.

Peak combustion chamber temps only occur when timing is close to max and effective compression is also close to max. This is when NOX levels really jump.

EGR's purpose is to reduce peak temperatures from 3500 to about 3200 degrees. This is one of the main reasons why engine performance has increased so much since the first anemic emission choked engines of the early 1970s.

Think of it this way. If you eliminate EGR you would have to retard ignition timing or reduce engine compression to prevent pre ignition.

In my opinion adding significantly more EGR under conditions other than very low manifold vacuum and very advanced timing will reduce peak combustion chamber temperatures when high temperatures are necessary for best power for fuel consumed.

You may find a better balance, with a slight increase in efficiency, by tweaking the factory systems, but it will probably be insignificant. There may be a balance point that is more economical, but it would have to be where the factory specs were more performance based.

As always these are just my opinions, take them with that in mind.

regards
Mech

[Daox]

You are perhaps right Mech. However, I know the 2010 Prius uses up to a 30% EGR charge. Add that to the fact that it is an atkinson engine and you really seem to be running pretty high levels of EGR. There has to be some sort of gain to be had (at the cost of peak power).

[user removed]

Understand maximum effective compression. This is when you have almost no manifold vacuum. It means your engine is creating very close to the mechanical potential compression. Maximum effective compression is the main reason why you have best BSFC when the difference between the compression created and the pressure increase when the fuel is ignited you have the best "leverage" and creation of power. Maximum effective compression can occur at very low throttle positions, which a vacuum gauge will confirm. When you combine best compression and lower engine speeds, you have loaded the engine so it produces the best power for the least fuel.

The Prius runs close to 13.5 to 1 compression ratios, so that level of EGR would be possibly an essential component of that compression level. The same situation will not apply to any engine that has significantly lower compression. The reason why the early 70s engines were so gutless was they dropped the compression ratios and added EGR. When people tried to disconnect the EGR pre ignition was the consequence.

When oxygen sensors appeared and computer controls made engine management much more precise, compression ratios rose back to pre emission premium fuel levels. This happened beginning in the early 1980s. Nissan bumped the compression in the Z Car from 160 gauge to 190 gauge and power levels climbed.

My Echo runs 10.5 to 1, same engine as the Prius (up to 09). I doubt that you would see that percentage of EGR on the same engine with 10.5 to 1. Not sure if the Echo uses the Atkinson cycle. Atkinson developed his cycle to try to get around Otto's patents. (think that is right might be Benz).

regards
Mech

[Christ]

quote from Wiki:

The original Atkinson-cycle engine allows the intake, compression, power, and exhaust strokes of the four-stroke cycle to occur in a single turn of the crankshaft and was designed to bypass patents covering the existing Otto cycle engines.

[drbobwoolery]

A few pages back Heywood was quoted to the effect that pre-ignition, pumping loss, and the increased mass of working fluid at part throttle all contribute to increased efficiency in typical operation.
If 50% of the intake charge is cooled EGR at half power, there is nearly twice the total mass of gasses to heat with combustion and expand for power.
Putting large amounts of cEGR at low outputs presents an ignition problem. If the Accel multistrike box and the stock coil peter out, speedshops are full of hotter coils.

with the exhaust gasses around 1000 F, reforming ammonia for hydrogen would give an easily measurable amount. There are claims that small amounts of hydrogen help combustion, so maybe we will see. If we got really lucky, the refrigerant effect of expanding liquid ammonia would do much or all of our exhaust gas cooling. This would be dead loss refrigeration, and one must expect issues of suspicion of methamphetamine synthesis when seeking to occasionally buy a gallon of liquid ammonia. Still, it's the safest storage of hydrogen I know, as well as a topnotch refrigerant. The unreformed ammonia burns fine when there is enough hydrogen around to get it lit.

Bottom line, these very lean, or very diluted mixtures are hard to ignite. One possibility is hydrogen, possibly from catalytic reforming of ammonia in the mixed gas stream toward the intake.

To adapt this to a G10 Metro will require megasquirt control, and an appropriate MAF sensor, as the speed density calculation is thrown off by the dilution.
How will I ever get it back through smog after this?

[Daox]

Slowly progressing, I've come back to this project. I recently won an ebay auction for the chevy blazer 4.6L EGR valve shown below. I'm hoping to develop some sort of control for it. If anyone has any insight as to how these work I'm all ears. I'm hoping I can actuate it gradually instead of just on/off, but it looks like its more of an on/off deal.

I picked this valve because of the engine size. According to the chart a few posts ago, 10% EGR flow really knocks down on NOx. I am assuming that this valve will flow about 10% of a 4.6L engine's intake at cruise. An incredibly rough estimate shows that that will be ~30% flow on a 1.5L engine. We'll see if that is too much once coupled with the TDI EGR cooler.

[born2pdl]

In the textbook, Heywood also has a figure showing the effect of increaing EGR on BSFC for moderate burn rate and fast burn rate engines. For moderate burn rate engines, BSFC decreased (mpg improved) as EGR went from 0% to ~7% EGR, then reversed. For fast burn reate engines, BSFC decreased (mpg improved) as EGR went from 0% to ~18% EGR, then reversed.

How do you determine which burn rate your specific engine uses (moderate -vs- fast)?