diesel combustion questions

[crossbones]

Gentlemen, my questions are about a method that I use to "tune a older style IDI diesel with mechanical injection"

Basically, I utilizes the Seebeck Effect of the glow plug and monitor the millivolts produced............I monitor this under real driving conditions............I only use the millivolts to provide a "relationship of temperature in the cylinder" and the different tune of the engine.............

My objective is to have the lowest rate of change in the millivolt readings from idle to operating RPM at high way speeds........
Example only...........say at idle I have a 5 millivolt reading and then at high speeds I have a 15 millivolt reading .......this would then be a 10 millivolt rate of change.............the objective here is to reduce the rate of change for idle to highway speeds say to 2 millivolts.............

Where I have achieved a very low rate of change (say under 3 millivolts) by "tuning" temperatures and injector timing, I have noticed a increase in fuel mileage, decrease in water jacket temperature and decrease in exhaust temperatures...........

First Question: Can I assume peak cylinder temperatures has decreased or has peak cylinder temperature stayed the same and perhaps efficiency has increased?

Usually, to get these very low rates of change, I have to advance injector timing from OEM speck................everything I have read says advancing the injector timing increases NOx levels (time factor before autoignition and cylinder temperatures)

Second Question: with the above limited information, what might I assume about NOx levels with increasing injector timing and the decreases in millivolt readings....................

Third Question: Is there a mathematical relationship between peak cylinder temperatures and engine efficiency.........

thanks in advance for your time and knowledge,
James

 

[dgallup]

I have never used that method to tune an IDI engine but I can tell you that advancing the timing will improve efficiency up to a point. It will also increase the peak cylinder pressure, the rate of cylinder pressure rise and NOx. Generally peak cylinder temperature will increase at the same fueling level but if you are reducing fueling due to improved efficiency then I suppose peak cylinder temperature may go down. Exhaust gas temp will go down. I would be very concerned about large increases in timing as you may have disastrous effects on engine life. Some IDI engines made from gasoline blocks (the Olds diesel comes to mind) will break blocks, cranks & pretty much everything else with just a few degrees of timing advance.

 

[Fabrico]

Amazing that none of the Diesel engine manufacturers over the years noticed this phenomenon.

 

[crossbones]

A little more back ground: I stumbled on this with trying to tune IDI diesels for Vegetable Oil a few years back...........after doing a lot of research, I have found several patents from glow plug manufactures beginning the the 1980's which are very similar to the method I use, and I now know that Isuzu diesel engineers used something similar in the early 1980's when faced with a diesel that did not run well set at factory speck timing...........so far, I have found mention of something similar in testing cylinder balance in diesels as early as the 1960-1970's.............

I agree there is a point when advancing the timing increases cylinder temperatures as indicated with the millivolt readings.......I stop advancing the timing at the very first reversal of the decreasing millivolt readings.............when I reach this point, and I check the injector timing, mathematically I am fairly close to the timing for the "estimate" of the cetane value of the fuel I am using and the different temperatures involved...............

Am I wrong in thinking that if ignition is retarded and you advance the timing that peak cylinder pressure per degree of rotation goes down?

James

 

[Lou Scannon]

What do you mean by "peak cylinder pressure per degree of rotation ". I don't understand that. Do you mean rate of pressure rise? Rate of pressure rise will change with injection timing, but I think how it changes it depends a lot on the injection and combustion systems in question. With a mechanical IDI system I'd go along with dgallup that advancing timing will increase peak rate of pressure rise.

 

[crossbones]

I should have used the words "Rate of pressure rise per degree of rotation"...........thanks for bringing it to my attention .........

It is my understanding that the measurement of "Rate of pressure rise per degree of rotation" begins with any increase in pressure from the combustion process up to peak pressure divided by the timing advance in degrees................

Example only: 1000 PSI peak pressure with a injector timing of 10° would equate to a 100 PSI rate of rise per degree of rotation......and with the same peak pressure and a 15° injector timing would be 66.66 PSI rate of rise per degree of rotation.....

James