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[ToxicPerformance]

Hi, does anyone know around how much pressure/force the combustion makes in the cylnders to shoot the pistons down? I know that different cars will have different numbers..but I just want a rough estimate of how much pressure, force, or whatever it's measured in.

Thanks a lot.

[ivymike1031]

Peak pressure levels in the cylinders of a highly rated diesel are in the neighborhood of 190bar (that's about 190*14.7psi); more common values are in the 165bar to 180bar range. Multiply that by the bore cross-sectional area (A.xs = pi * (bore^2) / 4 ) to get force.

High output naturally aspirated gasoline engines are generally in the neigborhood of 80bar - 90bar pcp, if memory serves.

[ToxicPerformance]

Originally posted by ivymike1031
Peak pressure levels in the cylinders of a highly rated diesel are in the neighborhood of 190bar (that's about 190*14.7psi); more common values are in the 165bar to 180bar range.

So only about 14.7 psi of pressure is used to push down the pistons in an engine?

[texan]

Huh? 1 Bar = approx. 14.7 psi, so 190 bar of peak cylinder pressure inside said diesel equals 2793 psi. Note this is a long ways of BMEP levels, showing just how little peak pressures matter. It's getting sustained pressures on the piston head that make real power.

[BeEfCaKe]

If it was 1 bar, then the piston wouldn't even move... cause that's simply normal air pressure...

[ivymike1031]

re peak pressure doesn't matter: tell that to a conrod.

Peak pressure isn't the only indicator of an engine's power output, but it is certainly a critical value to consider when designing the internals. Bearings, conrods, pistons, piston pins, etc. all feel PCP directly. BMEP is a calculated value that doesn't have a direct effect on the components.

You can increase BMEP (and thus power output) quite a bit without changing PCP, typically by changing the compression ratio, but there are limits. For a diesel, you'll eventually start to smoke and have trouble starting.

Example values for 2500psi PCP (170bar)
approx BMEP, corresponding compression ratio
150psi, 22:1
180psi, 20:1
210psi, 18:1
250psi, 16:1 (starting is getting more difficult)
340psi, 14:1 (requires manifold heat to keep running)
450psi, 12:1 (very difficult to run engine at all)

I know that you have some flexibility with a gasoline engine too, but I expect that it's far less than what you have for a diesel.

[texan]

My bad, that's what I get for not finishing sentences. What I meant to say is that peak cylinder pressure has far less meaning to overall power output than mean cylinder pressure. I of course agree completely with mike in that it's the cornerstone of parts strength design, thank for the clarification.

To illustrate my point further, take readings from a test NA engine. Then add a moderate amount of nitrous injection to the mix, and watch the peak cylinder pressure skyrocket for an attendent moderate increase in power output (if the engine is making 150hp, a 75hp shot of nitrous can nearly double peak cylinder pressure). Now instead add a turbocharger to the mix and double power output to 300hp, you'll notice that peak cylinder pressures only increase to about 150% of NA levels (assuming no detonation). The point being: nitrous accelerates burn rate drastically and has as such has a large effect on PCP, but a smaller comparative effect on mean pressures. Standard forced induction, however, does not drastically increase burn rate and therefore has more of an effect on mean pressures than peak pressures, by way of mainting high cylinder pressures further into the power cycle. Late cycle pressures are often nearly triple what they were in the NA engine when running significant amounts of boost, while peak pressures dont even double.

Ps- Mike, didn't you transpose those MEP values relative to the compression ratio? Simple Otto cycle thermodynamics indicate that higher CR also equals better pressure ratios. Or am I just that thick when it coms to diesel power tuning?

[ivymike1031]

no, I believe the numbers are correct. Perhaps what you missed is that all of the BMEPs are at a fixed PCP? To increase BMEP, more boost is used, but keep cylinder pressure from getting too high, CR is dropped (same as in a gas engine).

[ToxicPerformance]

hmmmmm....pretty confusing stuff. What I meant in my question was about how much pressure does the combustion of the gas/air make to push down on the pistons. In other words, around how much pressure (preferably in psi) is pushing down on the pistons to turn the crankshaft in normal engines? I know that different cars and different engines will have totally different numbers, I just want a rough estimate.
Thanks a lot for all your help

[ivymike1031]

yeah, really confusing isn't it?

80*14.7 = 1176psi.

[ToxicPerformance]

Thanks a lot

[texan]

Originally posted by ivymike1031
no, I believe the numbers are correct. Perhaps what you missed is that all of the BMEPs are at a fixed PCP? To increase BMEP, more boost is used, but keep cylinder pressure from getting too high, CR is dropped (same as in a gas engine).

Of course, again my bad. Perhaps I should either work or surf, not both.