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
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Engines aren’t particularly efficient. Gas escapes around the piston rings, heat escapes into the cylinder walls, friction in every sliding part. These losses are part of the engine and if it is running then it is wasting. A bigger engine has bigger losses.
Some of these inefficiencies also get *worse* at low RPM, while others get worse at low pressure, so no matter what if your engine is running at a power level outside of the sweet spot it was designed for it’s going to be wasteful, and the farther it is from the sweet spot the more wasteful.
Larger engine means the engine is made of larger and heavier parts which means more fuel is required to move those parts.
To some extent, you can. But either way, you’re carrying around all that extra engine.
If you took a fuel efficient car and put 500 pounds of “unused engine” in the trunk, you’re going to get worse fuel efficiency.
> I don’t understand where the extra energy goes if not to the wheels
It’s lost through friction and heat
It is possible to engineer engines to be more efficient at all kinds of configurations, but it’s a numbers game
It’s not worth making an engine that gets 2 mpg more if it is ten times the price of the 2 mpg worse engine, which is kind of what F1 does with its 1.6L V6 engines
They have and had complex systems to recover energy from the engine that normally would be lost, but they are and were so complicated and costly to produce that teams want to get rid of some parts. You can read up on the MGU-K and MGU-H for more information on those, but I digress
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.
This was done in practice by WW2 pilots flying long range missions in certain aircraft. For instance, the F4 Corsair, normally with a 2800 cu.in. 18 cylinder radial engine. Dial the rpm way down, like 1800, and the boost up to maintain power. This was pretty hard on the engine, though.
Source: a close relative was a Corsair pilot for USMC.
There is a lot of energy wasted moving pistons up and down and turning a heavy crank, extra cans, valves, timing chain, etc.
Just look at the improvements in efficiency eliminating pushrods, just a small rod that goes up and down… millions of times.
Smaller, lighter, fewer moving parts is more efficient.
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)
It really just depends on the engine design, vehicle’s gearing, and vehicle weight.
My work truck is a 2023 Chevrolet Silverado with a 5.3L V8. I average roughly 19 mpg in it. One of my peer’s just got a 2023 Ford F150 with a 2.7L V6. He averages roughly 19 mpg in it. Our trucks are fairly comprable. Both four door with 6 ft beds, exact same 10 speed transmission and two speed automatic transfer-case and are both geared similarly on the ring gears. The primary differences, he has a heavier frame, I have heavier duty rear axle, and mine has a mild-steel cab/bed/fenders as opposed to his full aluminum body. The two trucks are with 600 lb of one anther in terms of weight.
Another great comparison, I used to have a 2006 Dodge Ram 2500 MegaCab with a 5.9L Cummins turbo diesel (last fully production year for the common rail). 4 speed automatic and a fixed geometry turbo big, heavy (7600 lb was my day-to-day unloaded weight) pickup. If I could run 65-70 mph unloaded…26 mpg pretty normal. Back it down to 55 mph and I could break 30 mpg with cruise control set (but I live in Texas and no one has time for 55 mph) but if I would bump it up 75 mph I’d see a pretty significant drop to 19 mpg. The way that truck was geared, at 55 mph you just happened to be seeing 1600 rpm (peak torque for a 5.9 Cummins diesel) and at 70 mph I was sitting about 1900 rpm. Break 2000 rpm and the fuel economy quickly began to go downhill and at 75 mph the engine ran at 2100 rpm.
It’s not always a fact of larger engines make worse fuel mileage…but it has a lot to do with engine’s design and the gearing of the vehicle and at which speeds the engine is most efficient.
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