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

There’s some parts nobody has touched on yet, that I’ll try to explain simply:

1) Pumping losses. It takes energy to suck the air and fuel into the cylinder, energy to compress the piston, and energy to push to exhaust air out. A larger displacement engine has to push more air per revolution, and a larger engine with more cylinders will have more internal surface area that creates drag and friction

2) Fuel burn. The faster the air enters the cylinder, it has more energy and mixed with the atomized fuel better. This results in a more efficient burn. (There is a point where this turn to a hindrance, but that’s usually well above typical car engine rpm). A small engine turning faster will be flowing more air, allowing for better air velocity

If you want to see the results of this yourself, look for an engine efficiency map. This diagram shows where engines operate most efficiently (efficiency measured as most work done per fuel). Engines are most efficient under load, and around their peak torque value (usually around the middle of the rpm range). This efficiency doesn’t mean it uses the least fuel, it just uses the fuel most efficiently. If you are buying say a generator, it try to will run at this efficiency point to maximize energy produced per litre of fuel

To better take advantage of this principle for cars, the best way would be to use a small engine to charge a battery, and then drive the wheels with electric motors. Most car engines only use their advertised power when accelerating hard (e.g. passing on the highway, climbing a steep hill), and spend most of their life only needing a small fraction of their advertised power. For most driving, you could get away with a small (~40hp) engine at peak efficiency to charge a battery, and then use say 300hp electric motor that can cruise at 40hp without the inefficiency of an engine, but still put power down from the battery reserves when it’s needed in rare occasions. Unfortunately this is the most complex type of car layout, which is why it’s not frequently used (it has all the complexity of a ICE, plus all the complexity of an EV, and typically desired by very price conscious buyers looking for maximum fuel economy)