I read that a massive portion of usable energy is wasted and turned to heat instead of being used to turn the crankshaft — would there be like any way of reducing the heat/cooling the engine so you could get 50-70% thermal efficiency?
In: 407
I will try to actually explain this like you are (twenty)five:
A car engine is a heat engine. It simply has one job: Convert the heat from the combustion of the fuel into mechanical work; aka spinning a shaft. In a car engine, this heat makes the combusted exhaust gas expand and drive a piston. How efficienctly it can convert this heat into work is fundamentally limited by the fact that for heat to drive an engine, it needs to flow from where its hot towards where its cold.
The bigger the difference between the hot part (inside the cylinders of your engine where fuel is combusting) and the cold part (the atmosphere) the more efficient your engine becomes. This is fundamentally described by the Carnot equation: 1-Cold temp/Hot temp (both in Kelvin).
This simply shows that any heat engine can’t be 100 percent efficient unless the temperature of the cold part is at absolute zero. It also shows that if the efficiency of any combustion engine is limited to around 86 percent if the combustion temperature is around 2000 Kelvin.
So why is a car engine still so far from this limit? Mostly because the heat wants to go lots of other places. It will get absorbed by the engine block and get lost in the radiator, it will be retained in the exhaust and get lost in the atmosphere, etc. This is why so much engine development during the last 100 years has been about reusing this waste heat, or conversely, increasing combustion temperature.
Tldr; Temperature difference is the driving force of heat engines, but in real life the temperature difference cannot be high enough to reach 100 percent efficiency, and so much heat is lost everywhere else that even lower efficiency is normally achieved.
Latest Answers