Heat engines: cooling the gas

By heating a gas inside a cylinder, you allow it to expand against a piston while maintaining a constant temperature.
The energy that you add as heat goes into the PV work of the gas against the piston.

To start again, you have to push the piston back to the start point, compress the gas.

My book says that "we must cool the gas first before it can be compressed to its original state with less work than was gained"

Why must the gas be cooled first?

It's slightly confusing..

As the process is not irreversible so we have to cooled down the gas in order to get the energy cosnt.

Well, think about it, if you do not cool it down and only attempt to compress it, you will end up at a different state...at the initial volume but hotter gas (and higher pressure) and so, you would have done more work...if you want to go through the same process but in reverse, then you need to take heat out of the gas, since what you did before was to put heat into the gas

sophiecentaur
Gold Member
2020 Award
In a car engine (internal combustion), you put fresh, cool air in every time. In a Stirling Engine (External combustion), the same air is used repeatedly and needs to be cooled to repeat the cycle.

Drakkith
Staff Emeritus
You NEED to cool the gas because the initial work was done by the gas itself. It was heated up and gained kinetic energy which forced the piston to move. Imagine an internal combustion engine in your car. The hot air is forced out through the exhaust valve and cool air is drawn in from the intake valve. The total amount of air doesn't change between the hot and the cold air, but the hot air would take much more work to compress again, as you would have to do work against the higher energy of the hot gas. (Plus it doesn't have any more O2 anyways)

It was heated up and gained kinetic energy which forced the piston to move.

When you explain it like that, it makes sense, but doesn't all the heat we put in the gas go into the work of moving the piston.

I thought the success in a heat engine was adding heat and allowing the piston to move so that all the heat goes into doing work, meaning the temperature stays constant....

Drakkith
Staff Emeritus
When you explain it like that, it makes sense, but doesn't all the heat we put in the gas go into the work of moving the piston.

I thought the success in a heat engine was adding heat and allowing the piston to move so that all the heat goes into doing work, meaning the temperature stays constant....

No, a large portion of the energy is simply lost as hot gas out of your exhaust. If the gas did not retain heat then we wouldn't need cooling for the engine and your exhaust would never be hot!

Realize that the piston and walls contain the gas as it heats up. So when the gas is heated by the burning of the fuel alot of this energy is given to the walls of the cylinder and the piston itself as heat.

sophiecentaur