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

Let’s assume the V8 is essentially two I4 strapped together. They have the same stroke and bore, and all the internal components require the same force to move. Both engines require some fuel to push the pistons, and that force is generated by detonating fuel. For each power cycle of the engine, the I4 will detonate fuel in 4 cylinders. The V8 will detonate fuel in 8 cylinders. It won’t be double the fuel, since each cylinder is doing less work. But it will be more fuel than the I4.

Someone else mentioned cylinder deactivation, where the V8 only detonates fuel in 4 cylinders. In this case the other 4 cylinders have weight and friction that must be overcome, so the V8 will still need to use more fuel than the equivalent I4.