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
A bigger engine with more cylinders inherently means more weight, inertia, and friction, which means more parasitic losses. It also costs more to build and takes up more space which means a bigger vehicle. When designing cars they always have to balance desired performance vs efficiency vs cost vs size/shape. I4s are generally the best combination for small vehicles.
There are *certain* situations you can get more mileage with a bigger engine though. For example, my Plymouth came with a 360/5.9L and very very high gears, so on the highway it runs at like 2300RPM. That’s below the power band of the 360 so it’s not running efficiently, it requires more throttle and more fuel to maintain speed. I replaced that with a 440/7.2L. With the bigger displacement and longer stroke it makes it’s power much lower in the RPM range, so now on the highway it’s operating more efficiently and requires almost no throttle, getting like 5mpg better.
However, even better mileage could be had in the same car with a change in gearing, and/or a straight six that’s 250 lbs lighter and has less friction, at the cost of performance. It’s always a balancing act.
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