Automotive Why engine efficiency drops for light loads?

[poe]

Design efficiency is: usable energy out divided by (thermodynamic and mechanical and thermal energy losses minus total energy put in).

Design efficiency has a theoretical limit of up to but not including %100. It sets the limits on thermal and mechanical efficiency and usable energy, which is total energy minus the thermodynamic limit. Nature is obviously way efficient. What is the efficiency of photosynthesis?

This is the best I can do from my phone... does it make sense?

mechanical efficiency is: total mechanical energy out divided by total mechanical energy created, created/generated being key here. So while fuel that didn't combust during the combustion/power stroke falls in the design efficiency category, fuel that did combust falls in the category of mechanical efficiency.

Mechanical energy here is the pressure on the piston head, I picture atoms bouncing off the piston head. During and at the end of the power stroke whatever pressure that could have done any kind of work, but didn't, is a mechanical loss. And remember, the shorter the time interval is for the power stroke, the more loss there is during the stroke. F1 cars have short strokes because the loss that occurs during the stroke gets worse at the end of the stroke.

Thermal inefficiency is the measure of loss of heat from one body or medium to another. It is about heat transfer. So heat ejected through exhaust does not count, because it's the medium itself being transferred.

 

[jim hardy]

Always test your concepts by seeing whether they lead you to the correct equation. Human mind is quite capable of believing things that Mother Nature disallows.
That's the trouble with words - what sounds plausible isn't necessarily so.

 

[berkeman]

Thread closed for Moderation...

 

[berkeman]

Thread re-opened.

 

[jack action]

mechanical efficiency is: total mechanical energy out divided by total mechanical energy created, created/generated being key here.

Where does the lost energy go? This definition is true if you think the lost energy linked to mechanical efficiency goes into friction and powering the accessories.

So while fuel that didn't combust during the combustion/power stroke falls in the design efficiency category, fuel that did combust falls in the category of mechanical efficiency.

The fuel that doesn't burn would go into the thermal efficiency (which you incorrectly refer to as «thermodynamic efficiency»), i.e. when you divide the amount of energy found in the PV diagram and divide it by the potential energy you can get out of the amount of fuel going into the engine.

Mechanical energy here is the pressure on the piston head, I picture atoms bouncing off the piston head. During and at the end of the power stroke whatever pressure that could have done any kind of work, but didn't, is a mechanical loss.

No, whatever pressure that could have done any kind of work but didn't, is included in the thermal efficiency found with the PV diagram.

F1 cars have short strokes because the loss that occurs during the stroke gets worse at the end of the stroke.

No. The only reason to have a shorter stroke is to have a smaller engine.

Thermal inefficiency is the measure of loss of heat from one body or medium to another. It is about heat transfer. So heat ejected through exhaust does not count, because it's the medium itself being transferred.

This loss is also included with the thermal efficiency, i.e. found with the PV diagram. A heat loss during the process will inevitably induce a pressure loss. It would be more evident looking as temperature-entropy (TS) diagram, which gives the same energy output as the PV diagram.

The heat ejected through the exhaust is certainly included into the thermal efficiency as well.

Design efficiency is: usable energy out divided by (thermodynamic and mechanical and thermal energy losses minus total energy put in).

So what you sum up as «thermodynamic and mechanical and thermal energy losses» is in fact officially thermal efficiency.

I think you think that a PV diagram is just for perfect, theoretical, processes and this is where you come up with «thermodynamic efficiency». It is not. The processes can be evaluated with their expected losses or the PV diagram can be measured on the engine itself, which will include all losses.

What you call «design efficiency» is what is officially called mechanical efficiency, i.e. the one determined by the energy at crankshaft output divided by the one found with the PV diagram.

So, unless you can find an official source for your efficiency definitions, you should stick with the traditional definitions to avoid confusion.