> Do you have a source for that?

You can work it out from first principles. The loud noise is a form of energy - therefore all of the sound reaching your ears is energy which was produced by the engine but did not contribute towards propelling the car.

> If it was true race cars would be a lot quieter

only if there was some way to recover the energy from sound or prevent it from being emitted in the first place

since race cars are loud we have to assume that losses from noise are an unavoidable side-effect of the workings of internal combustion engines

We can see here an article from 2017 celebrating a Formula 1 racing engine reaching >50% efficiency for the first time: https://www.britishgas.co.uk/business/blog/mercedes-formula-... "The F1 cars are now closing in on levels of thermal efficiency reached by diesel engines used in large container ships..."

so half the energy from combustion is lost as heat, noise etc

Most of that would be heat, didn't find any sources but noise is probably rounding errors. Think how many watts you need in a stereo to get the same noise level as a car, compared to how many kilowatts the engine has

I don't have a source for that, no - it's common sense. I imagine the reasons race cars are loud is simply that it's not worth the extra weight or complexity to ensure the system doesn't waste so much energy as noise.

The sci-fi angle I alluded to relied on hyper-science as the solution for ensuring that all energy within the system was used beneficially.

First law of thermodynamics.

A few points:

1)

Internal Combustion engines work by harnessing explosions. Each explosion pushes a piston, which is then harnessed to do useful work rotating the wheels.

The Exhaust note is caused by agressive expansion of gasses rushing into the exhaust. That expansion _could_ have been used to push the piston more (in practise you'd need a secondary expansion unit, which isn't all that practical .)

2)

The exhaust system is there to reduce (and modulate) noise levels (and emissions). It weighs a not insignificant sum, causes back pressure, which affects the efficiency of the overall system. (although now that heavily mitigated, even more so with a turbo.)

Granted, most of the energy is wasted through heat, but thats a different matter.

That logic doesn't hold. It is not a given that current engine technology makes it practical to reduce the noise in a way that saves energy. That does not imply the energy is not wasted, just that our technology is still crude.

Source: physics