The Stovebolt Site Forums The Shop Area Engine Shop Engine pressure

Has anyone ever heard of what pressure is developed in the cylinders during the power stroke? Dave

Just a guess: 500-700 psi?

What brought that question on? Wouldn't it be the same as the compression reading plus or minus?

No. You have to remeber, the power stroke is after the compressed air/feul mix has been ignited, and as the mix burns, it expands greatly. Since the only part that's movable at that point is the piston, all the force goes to pushing it downwards.
I'd say panic is pretty close for a street type gas engine.

combustion pressure is the question, different than and quite a bit above static compression

Bill

Interesting article, next question, how much higher is it in a diesel.

Dave

In my old motor books, they refer to pressure as high as 400 psi. I think we are talking 1930s to 40s designs.

Nix - I think that has to do with compression ratio, about 18:1 in diesels [around 600psi nowadays I believe] and the fuel charge pressure

still hafta do the math Dave, and experimental observations as in the l1nk above to figure the "push" of the explosion, it's a thermodynamics thing that's part of investigations for diesels by the makers, to improve fuel efficiency and reduce emmisions, especially Nox, which increases with temperature

Bill

I was talking about them explaining the power stroke in a gasoline engine. I was surprised at such a low figure.

when exactly are you referring to, at idle or full throttle with a heavy load, it isn't going to be the same. Not that I know off hand, but it will vary quite a bit with load.

If I understand the question, you are refering to the pressure developed, in the cylinder, from the fuel charge burning. That is as varied as the engine, the rpm, the degree of tune, fuel, load etc, etc, etc. That pressure translates to horse power/torque. I didn't see that the linked article addressed this question. I would like to see some definitive figures.

Seems to me that if one had an engine with two plugs per cylinder, a compression gauge could be installed in place of one of the plugs and the engine run at various speeds to, at least, get some idea.

Looking on-line, I found an article dealing with the development of a high tem micro sensor to monitor ICE combustion pressure. They were looking for a 200 bar pressure rating which converted to psi is 2,900 psi. This was, supposedly, the upper limit.

I still want to hook up a gauge on a two plug engine and just see.

why?

Will, I'd like to see what kind of gauge you plan to use. An engine running at 2,000 RPM is getting 1,000 power strokes in a 60-second time frame. That's nearly 17 times per second. What sort of reading do you expect a mechanical device like a Bourdon-tube pressure gauge to give? It might be possible to mount a piezo-electric crystal in the combustion chamber and monitor the electrical signal it generates with a computer operating at a sufficiently high frequency to graph some sort of pressure curve, but where in the power stroke do you intend to take your pressure reading? The majority of the usable power is developed in the first inch of piston travel after top dead center. Beyond that point, the acceleration of the piston is running away from the pressure impulse, and the thrust transmitted to the crankshaft and the corresponding cylinder pressure is diminishing.

With a sufficiently sophisticated set of mathematic formulas and a powerful computer, it might be possible to produce a set of hypothetical pressure figures. The burning question would be WHY BOTHER?
Jerry

Yes I agree with Jerry, I have a friend that if given horsepower, and torqe, piston diameter and probably some other numbers at any given point in time could figure that stuff out easily, well maybe not easily, I have seen him doing 1 math problem that fills an entire page, but this is basic thermodynamic stuff. Which I guess is kind of like rocket science. But why do you want to know this? WE all want to know.

Just curious, I guess; no need to turn the dogs on me.

My brother could probably discuss this situation in more detail. When he's doing contract consulting work, computer modeling new parts designs for any of the major automotive manufacturers, he bills out his labor time at the rate of $1,000.00 per day.

Currently, he's heading up an engineering research team at Vanderbilt University that is redesigning the guidance system for several guided missiles deployed by the Air Force and the Navy. The big joke among the team members is "Well, yes this IS rocket science!"

He's also an ASE-certified Master Mechanic who is in the process of restoring a 69 Roadrunner with his son.
Jerry

this has been an interesting conversation so far, still haven't heard anything firm on what the pressures actually are. Measureing would have to be a bear.

I'm not sure measuring would be complicated at all. If one had access to a dual spark plug engine; substitue a gauge in one plug hole; run the engine. It might not be that simple; maybe getting the right gauge; something fluid damped and three thousand psi; a gauge such as used to check the pressures on tractor hydraulics.

you have to realize that in addition to the firing pulses 17 times a second you have 17 vacuum phases as well. You would have to use a check valve in the gauge like a compression testing gauge to eliminate the effect of the vacuum I don't know where you would get a dual spark plug engine. You still haven't said at what point in the power curve you are trying to measure. Like I said anyone who has studied thermodynamics can figure this stuff out without ever running the engine. You must know someone who has studied thermodynamics, or go to a college many professors would be happy to show off their knowledge. It is beyond my education, but lots of people can figure it out it is fairly straightforward physics. It is the same math they use to design engines, maybe call the design dept at general motors, they could probably use some positive PR about now.

You seem to think that, comparing cubic inch to cubic inch, a Briggs Stratton lawn mower and a nitro burning fuel dragster would be equal; I just don't think so.

I had a dual plug Nissan pickup a few years ago so I know they are around; for that matter, all airplane engines are dual plug and a check valve shouldn't be too much of a problem. If you get a math whiz to figure it out, how do you know it's right; sounds like a lot more fun to actually do it than to get someone else to figure it out.

oh it will be right, thats how they design engines. I'll say it one more time it is BASIC PHYSICS. All the stuff has been figured out long ago.
Its not cubic inch to cubic inch it is power per cubic inch. The more power per cubic inch the more pressure. volume times rpm divided by torque and horsepower or some nonsense like that. (it isn't even right, but it has some formula, something to do with boyles law or something)

If it's as easy as you seem to believe, go ahead and run the tests and let us know what your findings are. Problem solved!
Jerry

so what is the pressure? maybe you could butcher a spark plug some how to gain access to the cylinder pressure, I've seen them in magazine articles.

I never did hear anything for sure, either calculated or actually measured, gas or diesel.