Diesel Cycle - Heat Engines - Thermo.

[Nenad]

Hello,

If anybody could answer this question for me that would be great. I have been wondering about it all morning. In the Diesel cycle for heat engines, on the second stage of operation, the Volume increases while pressure stays the same. I understand how this can be accomplished by raising the temperature, but is it done by the injection of highly compressed gas in the piston at the top (combustion position). I understand the Otto cycle and how it raises the pressure while volume is constant but I don’t see how this can be accomplished the other way around. Are my assumptions correct? If somebody could fill me in on this information or give me a good site to look it up on, that would be great.

Regards,

Nenad Miljkovic

 

[brewnog]

No.

On the stage you're talking about, the air is already in the cylinder at its compressed state. Because it's compressed, the air is rather hot. Fuel is injected into the cylinder, and the heat present causes ignition. The mixture expands, and the ensuing stroke is when work is done.

 

[Nenad]

I understand this but then how does only Volume increase and pressure stays the same. By what you say, then the Otto Cycle and Diesel Cycle should be the same. I understand that in the Otto cycle the spark ignites the fuel air mixture and then the Pressure increases while volume stays constant because the piston is at the up most position. Then Pressure Decreases and Volume Increases for the next stage. This is how I see the Diesel cycle working as well, but the only difference is the Ignition stage, where as the Otto cycle the P increases while in the Diesel Cycle, the Volume increases. If you could explain this, that would be great. Thanks for the response by the way.

Regards

Nenad.

 

[minger]

I replied to your other thread.

 

[Clausius2]

I assume that Minger and you are talking about the combustion stage, where pressure remains constant and volume increases, aren't you?.

Well, whereas the Otto cycle represents better the behavior of gasoline engines, the Diesel cycle is representative of Diesel combustion type (obvious, isn't it?). The reason for the pressure remaining the same is due to the proper combustion mechanism, although I encourage you to know that in the real world, all engines (diesel or gasoline) are a mix of Otto+Diesel engines. The cycle you are referring to, is an "ideal" representation of a real Diesel engine, where there is a stage of pressure rising with the volume remaining constant at the TDC, and a stage of volume increasing with pressure constant, and it has to do with the two main phases of combustion inside a diesel engine: the premixed and the diffusion combustion. In your cycle, the premixed combustion has been deleted, and only the diffusion stage is represented.

The diffusion stage of combustion is characterized by a deflagrative flame. Once the fuel is injected at the top dead center, the pressure is so high that fuel and air burn like a premixed flame in a detonation mode (auto-ignition). That phase is represented by the segment of volume constant and pressure rising which I have said it has been erased in your "ideal" cycle. Once this occurs, the rest of the fuel burns in a diffusion flame mode, sustained by the former ignition. You should know that compressible effects in deflagrative flames are usually neglected, because the flame velocity is much smaller than the sound velocity. So that, as the flame advances inside the combustion chamber, there is no effect on the pressure. Therefore, in this stage the pressure remains constant and volume increases due to the crank movement.

As I have said before, real engines are a compound between pure Diesel and pure Otto engines.

Post any doubts you have about this stuff.

And please, DO NOT DOUBLE POST.

 

[Nenad]

thanks for the detailed description. It answered my question.

Regards,

Nenad