Why do larger engines get less MPG? Couldn’t a V8 be geared to a lower RPM and consume the same or less than an I4 at a given speed?

Question

I imagine some of the difference is due to weight (larger engines being in trucks) or performance (sports cars made to accelerate rather than cruise well), but it seems that almost without fail the bigger the engine the less MPG, I don’t understand where the extra energy goes if not to the wheels

In: Engineering

Answer 1

Engines aren’t particularly efficient. Gas escapes around the piston rings, heat escapes into the cylinder walls, friction in every sliding part. These losses are part of the engine and if it is running then it is wasting. A bigger engine has bigger losses.

Some of these inefficiencies also get *worse* at low RPM, while others get worse at low pressure, so no matter what if your engine is running at a power level outside of the sweet spot it was designed for it’s going to be wasteful, and the farther it is from the sweet spot the more wasteful.