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
The extra energy will go to the fact that for the same sized cylinders the V8 will have double the friction, double the valvetrain mass and friction etc.
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.
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.
Modern engines does that already by turning off some of the cylinders when the power is not needed. My new v8 6.2l gets better mileage than my 20y old v6 3l did.
Between my vehicles the 6.2L v8 gets about 5mpg better than the 3.7L i5. I’m guessing aerodynamics and wheel/tire weight are the biggest factors here.
Its not always the case this happens, on some cars, the smaller engines are no better for fuel economy then the larger ones.
I remember years ago, my dad had a 1.8 honda civic, and when it was in for service got given a 1.4, the car was basically the same weight, but the 1.4 was worse for economy then his 1.8 was. The cars themselves where a very similar weight, but the 1.4 had to work harder then the 1.8 did to travel at the same speeds, and therefore used more fuel.
From this I learnt to never buy the smallest engine in a particular car as the economy from a larger engine would be better.
It also depends on the weight of the vehicle and the transmission. For example, my wife’s 2008 Ford Edge has a V6 with a 6spd auto transmission and weighs 5300 lbs. My 2005 Mercury Grand Marquis has a V8 with a 4spd automatic transmission and weighs 4100 lbs. My MGM gets better MPG than the Edge.
Steady-state(I.e. highway driving) Fuel economy is heavily affected by internal friction. Even if the I4 and V8 are the same displacement the V8 will have more internal friction. It has 2x the cylinders, 2x the cams, 2x the rods, all of that causes extra friction, which means you need to burn more fuel to overcome the friction, which means you can’t have better overall fuel economy.
They have more displacement and use more fuel for the same number of turns of the crankshaft.
If you want to get more kinetic energy out of it, you need to put more chemical energy into it.
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