Hello, my friend and I got into quite a heated discussion about engines and turbochargers. I argued that a an internal combustion engine is quite wasteful as much of the energy ( heat ) is wasted out the exhaust. This is where it gets interesting. I told him that a turbocharger essentially takes the " wasted " heat and turns it into boost. He contends that, the combustion cycle has nothing to do with making the turbocharger make boost. He claims that if you could spin an engine at whatever speed to make 20# boost, introducing fuel to that engine and it igniting would not affect the turbo. the #20 boost would be the same.

 

I do not know what kind of engine your friend is talking about.expanding exhaust gases has everything to do about boost. And once you start creating boost it has a domino effect, because now you get increasingly famous of expanding gas.

 

How does "spinning an engine " create boost?

 

Yes, ICE's are thermally inefficient. Yes, the heat from the exhaust is what is converted back into mechanical energy that drives a compressor to move more air into the engine.

 

Your friend is better off sticking to being a YouTube mechanic lol . JK maybe he’s huffing too much NOS that is skewing his thinking.

 

If this was true, than rolling at 3000 rpm down the highway a light load should make 20 psi as well as WOT at 3000.
Reality is, at light load, air density is low, at WOT air density increases, with boost, air density increases in multiples.
When air density increase, the same volume of air and fuel is burned, but the air and fuel density is much higher causing more mass flow thru the engine , creating pressure and velocity , making the tubine wheel spin. More load on the engine, more density, more turbo spinny spin to the point the wastegate as to vent exhaust to control turbo speed.

 

Heat delta also helps the spool.

 

Driving a turbine with room temperature air at 2 bar absolute generates only 1/3 the turbine power as the same massflow and backpressure but with typical exhaust gas temperature (600 C).