Non-adiabatic expansion in Diesel and Otto cycles

[racctor]

TL;DR

How can the temperature in a non-adiabatic expansion stay high while the the pressure drops to a very low level?

Hello!

I have a question that has been bothering me for a while now. If we look at the expansion step of a real otto or diesel cycle, we see that while the pressure drops to near surrounding levels, the temperature remains relatively high ( high T of the exhaust gas). How is that possible? How can energy enter the system that increases the temperature, but not the pressure? Or in other words, how can energy dissipate into kinetic energy of the gas but not increase the pressure?

I think the same problem occurs for me in the compression step of real heat pumps: If i add work to a real system to reach a certain pressure, how come the temperature of real systems is higher than in the ideal isentropic compression?

If we take into account that in those examples we experience some heat loss to the surroundings, wouldn't that just decrease the mechanical Work i can get out of the system, but since heat is lost that way the temperature drop should also be the same as the pressure drop?

Your help is very much appreciated
kind regards

 

[Andrew Mason]

There are two expansions in the Diesel cycle. The first is a constant pressure expansion while the fuel burns followed by an adiabatic expansion. In the initial expansion part the temperature increases but pressure is fairly constant as heat is added. In the adiabatic expansion temperature and pressure both decrease as the volume expands.

In the Otto cycle heat flow in occurs at constant volume followed by adiabatic expansion. As in the diesel cycle, the pressure and temperature decrease during the adiabatic expansion.

AM