modifying the diesel for more economy

[ConnClark]

Originally Posted by Ernie Rogers

The textbooks say that EGR shortens ignition delay.

I would like to know what text books you read. All the books and papers I have read say say egr increases ignition delay.

[diesel_john]

Originally Posted by ConnClark

I would like to know what text books you read. All the books and papers I have read say say egr increases ignition delay.

we're just talkin' diesel, is there a dif. flame travel between gas and diesel? The hotter the cylinder before the fuel is injected the higher the initial pressure, the faster the burn? May depend on EGR cooler or not. Ignition starts when the fuel is introduced to red hot air. I could see how EGR would lower the peak pressure. So maybe EGR slows down the burn but not the start not so much, giving the appearance of ignition delay.

[tasdrouille]

Ernie, the papers I've read mention increased EGR increases the ignition delay. You might want to double check your textbook.

You might find the following paper interesting KAIST Open Access Self-Archiving System: The influence of Charge Dilution and Injection Timing on Low-Temperature Diesel Combustion and Emissions (if you can download it, the connection seem unreliable, but I have a copy I'll post tonight)

here's the interesting information anyway:

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.

[tasdrouille]

Regarding pavement surface, there can be a fair difference in rolling resistance depending on the material you're rolling on.

From Williams, A.R. The Influence of Tyres and Road Surface Design on Tyre Rolling Resistance. Institute of Petroleum. London 1981. 30 p IP 81-003.

[diesel_john]

tas, Which efficiency most relates to fuel mileage? combustion, work conversion, fuel conversion. is the -30, 30 crank degrees ATDC, am i reading the map wrong? wow, it would take a huge amount of EGR to get the O2 down to 14%.

[tasdrouille]

John, the last one, fuel conversion efficiency, is the most relevant to us. It is dependent on combustion efficiency and work conversion efficiency so we can say the last map is more or less a combination of the first two.

-30 after is like 30 before. So in the maps - is BTDC and positive numbers are ATDC.

It does indeed take a great amount of EGR to get down to 14% especially since there is still a lot of O2 left in the exhaust gasses of a diesel. I don't know how much the TDI EGR system flows at max duty cycle, but seeing how the pipe diameter is fairly bigger than EGR systems on gassers, and the pressure differential can be quite high, it must flow quite a bit.

[COcyclist]

Hi Tas,

So if I'm reading the last map correctly, in a modern diesel we want to lower O2 levels by adding more exhaust gas, and injection timing just a little before TDC for best fuel mileage right??

Many TDI owners eliminate the EGR with a block-off plate to reduce soot buildup. I have also read that some have suffered mpg losses in the process. Is it your opinion that a fully functioning EGR improves highway mpg?

If so, I may have to figure out an alternate location for a future EGT probe I'm hoping to install. (there is a nice kit out there with the thermocouple probe mounted in the block-off plate)

[Ernie Rogers]

Hello, Tas,

I am greatly impressed by your access to information. I just checked the book I have handy (Pulkrabek). He didn't bring up the subject, although he states that EGR slows the rate of burn.

That means I probably got the information from a paper. My recollection is that the explanation for shortened delay was that transient species in the exhaust catalyzed ignition. Also, EGR raises gas temperature and that is expected to shorten ignition delay. I trust that your information is correct, and that will cause me to favor avoidance of EGR in favor of higher O2 partial pressure.

As for my car, I did not notice a change in fuel economy as I went from factory EGR to miniimum approved EGR and finally to removal of EGR. In any case, that makes me quite happy that EGR is seen to have little or no benefit for engine efficiency.

And, thanks for the rolling resistance table. It seems to match experience.

Ernie Rogers

[Ernie Rogers]

I forgot to mention that I can't see a clear indication of delayed ignition for moderate EGR in the first graph. Although there is ample evidence for slowed rate of burn, especially at high EGR concentration.

Ernie

[diesel_john]

I am surprised that the sweet spot is located at so little advance for 1500 RPM.

So we can advance the timing because the EGR slows the combustion rate.

roflwaffle, on the mechanical NA rabbit pump, when the fuel is increased rapidly at low speed, does the injection duration increase? If so, does it start the injection earlier or end it later? Can the mechanism retard the timing under heavy load at low RPM? To ask another way, how does the mechanical pump adjust timing based on RPM and load?