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Force pushing against piston in car engine

by daletaylor
Tags: engine, force, piston
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Apr2-06, 10:39 AM
P: 10

I've done a bit of research to find my answer, but I haven't been able to come up with anything.

Does anyone know what the average amount of force pushing against the piston in a car engine when the explosion occurs? I know it probably differs greatly with the car, but if anyone knows some average force needed for the average car, could you please post it?

Thank you very much,
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Apr2-06, 01:55 PM
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brewnog's Avatar
P: 2,793
Not to be rude or to brush you off, but if you fully understood the workings of an engine then you'd realise that it's not a very useful question to ask.

I'll try and talk you through the steps here.

Firstly, the force on the piston depends only upon the piston face area (which is fixed for a particular engine, just look at the bore dimension in a data sheet) and the amount of pressure acting thereon. You know that pressure = force / area, so you can work backwards from there.

The pressure side of things is the complicated one. The in-cylinder pressure itself varies drastically on a number of factors. Many of these vary from engine to engine (turbocharging, type of engine (SI/CI), intended use of engine, and so on). When you pin it down to an individual engine, there are still a wealth of factors which affect the pressure. Load and engine speed are probably the two you are concerning yourself with. At low load, or idle, the in-cylinder pressure (and therefore force on the piston) is only sufficient to overcome the parasitic losses in maintaining the engine's state of turning over (friction, pumping power, fans/alternators etc, and other resistive factors). When the demand on the engine is high, for acceleration or developing maximum power, this pressure has to be much higher, and is achieved by burning much more fuel with more air to get more expansion.

I could throw a figure like 2000kPa out there for a 'typical' in cylinder pressure, but it would be completely meaningless with no context.

Would you mind sharing what you're trying to work out so that we can help? Perhaps looking at BMEP would be a much more useful parameter for you.
Apr2-06, 11:42 PM
Sci Advisor
P: 789
Dale - brewnog already went into detail about how complex the calculation is and how the number doesn't have a direct bearning on output power or loads on the engine.

But for a ballpark, for a very high performance gasoline engine at wide open throttle, the data I've read says the pressure curve may reach a peak of near 1100psi for a naturally aspirated engine. For a forced induction, its going to increase based on the boost, and the same for the addition nitrous oxide. I guess the top fuel dragsters see peak pressures of around 5000psi, but that is a very isolated and extreme example.

You could simply take your average figures and work backwards. So if you had figures of like 10 HP to push the car down the road, engine spinning at 2000 RPM, drivetrain losses of 20%, maybe a cylinder that's 3" in diameter with a 3" stroke and assuming my back-of-napkin-calc is within an order of magnitude of being correct and I come up with 36psi average (with no respect to time). What I don't get is what does that mean? Nothing as far as I can tell, there's far too many other things to consider.

Apr3-06, 11:32 AM
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brewnog's Avatar
P: 2,793
Force pushing against piston in car engine

Just to reiterate, the pressures we've quoted are peaks, and by no means averages. Using them to calculate a piston force to try to work out engine torque would be completely useless, since these pressures are only attained for a fraction of a second.
Apr3-06, 06:28 PM
P: 10

Thanks a lot everyone.

The reason I was asking is just to have a ball park number for the force required to push the piston. I would use this number to compare other methods of creating that same amount of force.

Thanks alot,
May4-06, 08:12 AM
P: 156
cant you just find that easily from the torque output of the motor???
May4-06, 12:01 PM
P: 22,315
Not real easily, but yes. You need to know the positions of the pistons and the length of the lever arm (where the rod connects to the crankshaft).
lonewolf theo
Feb8-11, 11:57 AM
P: 2
would it be possible to achieve the same amount of force using an electromagnetic principle?? similar to a rail gun or a coil gun??

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