JMLBlues@aol.com wrote:
In a message dated 2/2/02 3:43:46 PM Eastern Standard Time, rotaryeng@earthlink.net writes:
No, 30:1 could be correct at rated power, I don't really recall for sure. At high speed, light load, the AF ratio could be 50:1 or even 100:1. I'm talking from memory here. A quick read of Taylor will shed more authoritative light on the subject. One of the reasons NA diesels have poor power density is that they cannot utilize all the air they breath. They run much leaner "mixture strengths" than gasoline engines. I put mixture strength into quotes because a diesel is kind of a stratified charge engine -- there isn't a uniform mixture strength, at least at the start of combustion. Very rich mixture in the center of the injector spray, very lean near the cylinder walls. To keep 'em from smoking, the fuel delivery is restricted by the injection pump calibration. Tractor pull engines make a lot of smoke because they run a lot more fuel to get more power. It is entirely possible that even the tractor pull engines are leaner overall than gasoline engines.
Jon Lauter
I just looked it up in Taylor. Still looking for the definitive chart.
30:1 is definitely down on power. Page 102 volume 2 has a pressure chart
but one must integrate the pressure envelope get the torque.
BTW Taylor talks about fuel air ratio in terms of 1 through .04
so I am not sure what he means by that. 14:1 is by weight as I recall
so that would be .07 right? Perhaps he is saying the 14:1 is unity
and everything else is relative to that.
I assume Jon, you are using 30:1 in the weight sense as in 14:1 for
a gas engine right? That would make the pipe about 3.3 inches in dia.
or so for relative area (base is seven square inches), equivalent runner
air velocity and equivalent 200 HP power. For 350 HP base area would be
about 12 square inches or about a four inches dia. pipe.
I suspect the diesels are looking for lower runner velocities for
lower breathing losses and higher BSFCs which is what we should be doing.
Which was what I intended in my original response.
Paul Lamar
I just looked in Heywood, and he says 20% or more lean of stoichiometric at rated power. I don't recall the theoretical stoichiometric AF ratio for diesel fuel, it is probably a bit different than for gasoline. My own experience (Heywood notwithstanding) is that modern turbodiesels run high boost pressures and leaner fuel:air ratios to improve thermal efficiency (bsfc). It is very common for engineers to discuss fuel:air ratio in terms of equivalency ratios rather than absolute AF or F:A ratios. This eliminates changes in specific gravity. An equivalence ratio of 1 is stoichiometric, and that is probably what Taylor shows in the charts. Europeans talk about "Lambda," which is pretty much the same thing as equivalence ratio (maybe the reciprocal? I don't recall). Lambda of 1.0 = stoichiometric -- no excess air. Bosch called the O2 sensor a "Lambda Sond," or "Lambda Sensor."
Incidentally, when I left Cummins in 1994, the intake manifolds across the board were non-tuned, "log" manifolds. Just a big open box screwed onto the heads. These engines achieved .30 bsfc. 40000 lb trucks were getting 8 mpg. The advanced product development group was working on the next generation of engines, and they were looking at tuned manifolds. May now be in production, I haven't looked at a Cummins diesel in some time.
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