Thermal vs Mech. Efficiency of Engines: Petrol vs Diesel

In summary, thermal efficiency is the ability of an engine to convert heat produced by combustion into work, while mechanical efficiency is the ability of the engine to deliver the work output from the piston(s) to the crankshaft. The efficiency of an engine depends on its construction and can be measured in terms of thermal or mechanical efficiency. Diesel engines are typically more efficient than petrol engines due to their ability to convert more of the available thermal energy into work output.
  • #1
elixer akm
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What is the difference between thermal efficiency and mechanical efficiency of an
engine...and which engine is more efficient ..petrol or diesel
 
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  • #2
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.
 
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  • #3
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).
 
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  • #4
elixer akm said:
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?
 
  • #5
Petrol engines do not have higher compression ratios than diesels, quite the opposite in fact.
 

FAQ: Thermal vs Mech. Efficiency of Engines: Petrol vs Diesel

What is the difference between thermal and mechanical efficiency of engines?

Thermal efficiency refers to the amount of heat energy that is converted into mechanical work, while mechanical efficiency is the measure of how well an engine can convert mechanical work into output power.

Why are petrol engines generally more efficient than diesel engines?

Petrol engines have a higher thermal efficiency due to their higher compression ratio, which allows for more complete combustion of fuel. Additionally, petrol engines have lower friction losses, resulting in higher mechanical efficiency.

Why are diesel engines often preferred for heavy-duty vehicles?

Diesel engines have a higher torque output at lower RPMs, making them better suited for heavy loads. They also have a longer lifespan and require less maintenance compared to petrol engines.

How do thermal and mechanical efficiency affect fuel consumption?

A higher thermal efficiency means that less fuel is needed to produce the same amount of power, resulting in lower fuel consumption. Similarly, a higher mechanical efficiency means that less energy is lost to friction, also leading to lower fuel consumption.

What advancements in technology have improved the efficiency of both petrol and diesel engines?

Advancements in fuel injection, turbocharging, and hybrid technology have all contributed to improving the efficiency of both petrol and diesel engines. Additionally, improvements in engine design and materials have also played a role in increasing efficiency.

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