Reviewing Efficiency of Otto & Diesel Cycle Heat Engines

[lee123456789]

Homework Statement

Review the relative efficiency of ideal heat engines working on the Otto and Diesel cycles.

Relevant Equations

none

Hi, i have this question and don't understand it. can somebody explain what i have to do.
i know a ideal engine is a engine running on carnot cycle
i what the cycles are
i don't know what relative efficiency is and what there looking for
(Q) Review the relative efficiency of ideal heat engines working on the Otto and Diesel cycles.

 

[berkeman]

I Googled Otto versus Diesel Cycle and got lots of good hits. Here is the first one on the hit list in case it helps...

https://vivadifferences.com/difference-between-diesel-cycle-and-otto-cycle/

 

[russ_watters]

You've been given the cycle names to review (neither was Carnot). It's easy enough to Google the equations for efficiency...or look them up in your textbook.

 

[lee123456789]

You've been given the cycle names to review (neither was Carnot). It's easy enough to Google the equations for efficiency...or look them up in your textbook.

ive written about mechanical and thermal effiency for both. the ideal engine bit and relative efficency got me. every other question specifically said mechanical and thermal efficency

 

[lee123456789]

What is ideal heat engine. The ideal bit got me.
In the question am I just comparing mechanical efficency and thermal efficiency.

 

[russ_watters]

ive written about mechanical and thermal effiency for both. the ideal engine bit and relative efficency got me. every other question specifically said mechanical and thermal efficency

You didn't ask us about "mechanical and thermal efficiency". What, exactly, did these other questions ask?

"Ideal" here generally just means no other losses.

P.S., the first hit on Google for my guess at that follow-up question is a 5 year old PF thread.

 

[lee123456789]

You didn't ask us about "mechanical and thermal efficiency". What, exactly, did these other questions ask?

"Ideal" here generally just means no other losses.

P.S., the first hit on Google for my guess at that follow-up question is a 5 year old PF thread.

comparing otto vs diesel in heat engine, if deisel has higher thermal efficiency and work done would that mean it has high mechanical efficiency.

 

[lee123456789]

I just want to check my comments i made, if it makes sense before i submit

Summery statement I've made

• Otto and Diesel both have practical and theory cycles whereas carnot has only theoretical cycle.
• If heat engines where running the theory cycles of Otto and Diesel, there would higher effiency than their practical cycles.
• Diesel Theory cycle has greater output and effiency than Otto cycle by looking at PV digrams.
• the Carnot cycle would have the highest efficiency, it determines the maximum possible efficiency in heat engines.

Theory assumptions for carnot cycle. (do these assumptions apply to diesel and otto cycle aswell?)

• There is zero wasted energy as it is because assumed there is no friction and output work is maximised.
• There is a cold and heated reservoir, greater the difference in temperatures greater the efficiency. As thermal energy isn't lost, no energy is wasted so temperature difference doesn't decrease so efficiency remains high.

 

[Lnewqban]

comparing otto vs diesel in heat engine, if deisel has higher thermal efficiency and work done would that mean it has high mechanical efficiency.

If you are comparing two real engines, thermal and mechanical efficiencies are not directly related.
Thermal efficiency means that the machine produces more mechanical work from the same thermal potential of the fuel, or that it is able to obtain more energy from it by inducing higher differentials of pressure and temperature (that is the Diesel type of engine in this comparison).

How much of that generated mechanical work is useful and how much is wasted in form of friction, etc., is the meaning of mechanical efficiency.
At least in theory, since the Diesel engine is subjected to higher combustion forces and its more robust reciprocating parts have more inertia to the cycling accelerations, losses due to friction should be higher than those in an Otto engine of identical dimmensions and rpm's.