Piston engine question

[Firewalled]

Hello all,

I was doing a bit of revising recently and instead of making things clearer, it made them muddier. What I can't understand is why a piston engine is said to be a constant volume engine. When the piston moves, dosn't volume vary? I tried reasoning it out with the gas laws, but that confused me even more

Cheers

[Abagnale]

Maybe because the combustion takes place at constant volume...

[helimutt]

It's a kind of play on words. The Otto Cycle (by Nicolaus Otto) engine has heat addition, Heat rejection , along with Adiabatic Expansion and Compression, all at Constant Volume.

I believe Turbines are known as constant pressure engines.

[TSIO540]

I think you'll find that when one piston compresses, another decompresses, thus making the total engine internal volume a constant value - regardless of the engine position on its 720 degree cycle

[Whirlygig]

Agreed TSIO540 but what about a single piston engine like on a moped?

Cheers

Whirls

[toolowtoofast]

apparantley it's about the volume of the gas at the time the energy is asked to do its work (ie at TDC)

http://www.grc.nasa.gov/WWW/K-12/airplane/otto.html

[Keith.Williams.]

In a piston engine the combustion starts just before TDC and ends just after TDC. Between these two points there is very little movement of the psiton, so the volume of the gas is virtually constant. The addition of heat energy at constant volume produces a large increase in pressure. It is this high pressure which pushes the piston down the cylinderduring the power stroke.

In a gas turbine engine the situation is totally different. If we were to produce a big pressure rise in the combustion chamber it would stop the air from flowing out of the back of the compressor. This would cause the compressor to stall/surge.

So in a gas turbine engine we burn the mixture at constant pressure. This means that we allow the gas to expand as we heat it. This causes it to accelerate towards the turbines.

So we say that combustion in a piston engine is at constant volume, while combustion in a gas turbine engine is at constant pressure.

[Firewalled]

Ah makes sense now. Thanks!