Thermal vs Mech. Efficiency of Engines: Petrol vs Diesel

[elixer akm]

What is the difference between thermal efficiency and mechanical efficiency of an
engine...and which engine is more efficient ..petrol or diesel

 

[Simon Bridge]

Mechanical efficiency is the work out divided by the energy in.
Thermal efficiency is the mechanical efficiency where the input energy is heat.
The efficiency of a particular engine depends on it's construction ... you can look up typical figures.

 

[jack action]

Thermal efficiency is how efficient the engine can transform the heat produced by the combustion into work moving the piston(s), i.e the work found by a PV diagram.

Mechanical efficiency of an engine is how efficient the engine can deliver the work moving the piston(s) into work output, i.e. work at the crankshaft. Friction is the main loss here, but it will also include losses due to systems necessary to run the engine (lubricating or cooling systems for example).

 

[poe]

What is the difference between thermal efficiency and mechanical efficiency of engine...and which engine is more efficient ..petrol or diesel

Thermal efficiency refers to thermal energy and thermal energy refers to the chemical energy of a fuel converted to thermal energy through combustion. Thermal efficiency is total available thermal energy as opposed to thermal energy used to perform work.

And mechanical efficiency refers to total available energy/force to perform work as opposed to actual work performed.

Diesels are more efficient, but do you know why?

 

[Shaunob]

Petrol engines do not have higher compression ratios than diesels, quite the opposite in fact.