Quote:
Originally Posted by 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.
|
I was a little surprised also, but if you think about it, it kind of makes sense. There is an inherent ignition delay. While the start of ignition is dependent on the pressure in the cylinder to heat the air and fire it off, its also a function of time for air/fuel to mix and get burning.
At low engine speeds, there is more time per degree of crank angle revolution. Thus in the same amount of crank rotation, more time has passed for the combustion to begin. At higher speeds, the time is much shorter, so you must inject earlier to ensure combustion has begun at the proper crank angle. Any cylinder pressure rise before top dead center is only working against the piston anyway, and lowering the mean effective pressure (usable power).
We also know EGR increases ignition delay. This would allow you to run more injection advance, but how can we be sure that we are getting down to the target O2 concentrations on the figures provided? The air we are pulling into the engine starts at around 21% O2. Assuming that re-circulated exhaust is now nearly 0% O2, and that the TDI engine can re-direct up to, say, 30% EGR rate, you would get down to ~15% O2 conc. My numbers could be way off though.
Ernie, I don't think you can draw any conclusive evidence of either ignition delay
or rate of burn directly from these graphs since they only show the end result of total combustion efficiency. They probably correlate though.
Those graphs are great, btw, and thank you for posting them again Tas. But I find a couple points troubling:
1) If you notice on figure 28 "work conversion efficiency", there is a dark red area up in the >20% O2 concentration, slanting back from 5 deg SOI towards even earlier injection. On the first plot, figure 27, "combustion efficiency", this is also the highest % area. Why then, on the final "fuel conversion efficiency" plot does this obviously overlapping area not produce good results? This means that the fuel conversion efficiency is not so simple as overlap between the first two figures. I'll have to read the whole paper and learn up.
2) The experimental engine used is quite similar to a modern diesel, EXCEPT:
- It only has 800 bar of injection pressure. Modern common rail engines run up to 3 times that amount of pressure, greatly affecting fuel atomization and injection control.
- It was running 1.2 bar of intake pressure (is this absolute or ABOVE ambient?)
- The intake temperature is 90 degrees Celsius! That is very, very hot. My TDI typically runs more like 30-50 C. That is going to have a huge effect upon the combustion process.
- Is also has a swirl ratio of 3.77. I'm not too sure, but I think this is a lot.
Anyway, this is a terrific discussion, lets keep it going.
Good call on the fluids Vekke, I have some new synth trans fluid waiting to go in, and definitely plan on using 5w30 oil during the winter from now on!