Lets get a little technical: Compression ratios

[HappyHappy]

Ok you gurus of airplanes and all things that use heat engines for propulsion.

Why does a higher compression ratio deliver greater specific power (efficiency).

Piston engines are the object of the question, but the same goes for gas turbines.

This could be fun

One hint. 100% burn can be achieved in a gasoline engine at as little as 6:1
So a complete burn is not the reason.

HH

[Reservoir_Dog]

I'll assume you're referring to thermal efficiency.
High compression ratios = more time for the expansion cycle, less fuel can be used at the same combustion temperature, more mechanical power is generated, less heat loss through the exhaust system.
It's also why diesel engines are so efficient.

[HappyHappy]

Very close.
Not quite.
The remaiming item you missed is what time are you referring to?

HH

[fatman]

I was waiting on the CIA report but furloughs abound

[Pudfark]

Fats....you slay/sleigh me....that was funny....

[Juha]

Carnot cycle gives the answer:



Efficiency can be seen as a ratio between High and Low temperatures in the cycle -> higher compression allows higher temperature difference in the work cycle.
(Because compressing gas makes it hotter)

Happy, HH?

[HappyHappy]

Carnot cycle gives the answer:



Efficiency can be seen as a ratio between High and Low temperatures in the cycle -> higher compression allows higher temperature difference in the work cycle.
(Because compressing gas makes it hotter)

Happy, HH?

nailed it

[Juha]

Counter question: Why the compression ratio is usually lowered, when an engine is rebuilt with a turbo charger?

[HappyHappy]

Counter question: Why the compression ratio is usually lowered, when an engine is rebuilt with a turbo charger?

Pressure ratio (effective compression) must reflect the octane or performance
number (over 100 Octane) of the fuels in use. There is a loss in efficiency at lower power settings.
Modern automotive thinking is to use a very small highly boosted engine
to counter this by having a small engine at low power "traffic" situations
and running nearly constant boost in high power usage scenarios.

Aircraft engines use turbocharging mostly as altitude compensation
mantaining sea level boost at the carb inlet. Compression remains unchanged
but usually lower than in automotive. WWII era engines are typically 6 to 7:1

HH

[Juha]

Yep.