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

I’m not an engine expert, but thermal efficiency tends to increase with higher loading. A smaller engine under heavier load will be more efficient than a larger engine under less load.

I don’t know all the factors that play into this, but one is throttle. The more closed your throttle is, the more energy you are losing pumping air through the throttle. The larger engine will have more pumping losses through the throttle.