So I am aware of the existence of high specific output engines like in the Honda S2000 or Ferraris, but one common criticism those cars tend to have is their lack of torque. Why does it seem so difficult for these engines to make more torque as well?
In: Engineering
In a word. Leverage.
High torque engines use longer throws on their crankshaft, which allows the pistons to exert more leverage on the crank as they fire.
High revving, short stroke engines such as those you mentioned have to keep everything more compact because at such high speeds everything is under a tremendous amount of stress, if you tried to make a high torque engine such as one out of a semi rev the same as a Ferrari, it would fly apart because the materials simply don’t have the strength to hold together.
So, because a short stroke engine can’t exert as much leverage on the crank, it cannot generate high torque.
The way I understand it is that imagine hitting a standard hammer against a wall. You can hit it either rapidly or hard. Any attempts to hit the wall rapidly and hard at the same time will require an immense amount of strength and energy. You’ll also get fatigued very quickly. The same principle applies to pistons inside the engine.
Less turning moment radius, smaller wire gauges, smaller magnets, smaller shafts, etc. all limit the maximum mechanical output of a motor. Now stick alot of gears on the end you can turn that high speed input into high torque output but will very high mechanical losses.
torque is how powerful something is in one revolution of the engine. hp is how powerful it is over time. so, a small engine spins really fast to make that hp.
you can get small engines to rev really high and make a ton of hp, but they feel like a limp noodle until you reach the powerband.
motorcycles hit this point hard. my 650cc bike is slower than a 600cc supersport. the supersport revs almost twice as high and makes about the same torque, just way up in the revs. at those same revs it makes about 40% more hp though.
Torque and horsepower are not two independent properties of an engine, they’re tied together via the RPM by a formula:
T = P * 9549 / r
Where T is torque in N*m, P is power in kW, and r is rotational speed in RPM. (for power in HP and torque in lb*ft the coefficient is 5252)
So if you have a 10 kW engine and connect it to a gearbox which outputs, say, 950 RPM, you will get around 100 N*m of torque regardless of the engine “size”.
Have you ever unscrewed a screw?
When you first have to un-tighten the screw out of the wood, have you ever noticed how hard it is to do that by the metal shaft of the screwdriver? It’s so much easier to unscrew by the handle. But once it’s loose, you may have noticed that it just takes forever to *keep* using the handle, so you use the metal shaft instead, and then it turns like lightning.
That’s what’s going on in cars. Small engines are like the metal shaft of the screwdriver. They’re crazy fast, but only for stuff that isn’t very demanding. Bigger engines are more like the handle. They’re great if you need some real work done, but they take forever to get anything done if there isn’t much work for them to do.
The reason ultimately boils down to the fact that, perhaps unintuitively, when you push on something, it actually *pushes back*. When a car’s gear tries to twist the axel, the axel tries to twist back. It doesn’t wanna be twisted. So it starts a little war between the gear and the axel, where both of them are trying their hardest to twist back against the forces causing them to fight each other.
Each of them brings their own set of allies to the fight. The gear is bringing the engine and the explosions in its pistons. The axel is bringing the weight of your car and the friction of the road beneath the wheels. If the gear and its allies are stronger, the gear wins out, and the car is forced to move.
But suppose the axel wins because the car is just too heavy, so the gear’s allies come to a stalemate. How should the gear proceed in order to get moving again?
Well, bringing more firepower to the fight usually helps. One way is to bring a bigger, badder gear. That can tip the scales in the gear’s favor and allow things to move again.
But large armies take a lot of time to move and coordinate. If your army is very wide, then the troops at the edge of your army will need to march a lot farther in order to turn around than the troops in the middle. If you want to have the ability to turn around quickly, you need a smaller army.
So if the axel isn’t putting up much of a fight and you just want to steamroll its allies and get the war over with, you’d be better served with a smaller, more nimble gear.
Hope that helps.
Although you can measure the horsepower of an engine, when you see horsepower stats for vehicles they are never measuring that. Rather, they’re measuring how much force can be used to spin the wheels when the car is in its highest gear at the engine’s optimal RPM. Because that’s how horsepower is measured in the real world, it makes more sense to think of torque as a measure of the raw power of the engine and horsepower as a measure of how efficiently the transmission can convert that power into work.
High performance cars tend to weigh as little as possible because weight is the biggest cost constraint in attaining good acceleration, top speed, and handling. IE, if Car A weights twice as much as Car B, it will cost a lot more to make Car A perform the same as Car B on a racetrack. So if you have a $150k budget for your new Ferrari, the way you make that Ferrari go as fast as possible is to make it as light as possible.
The two biggest sources of weight in a car are the engine and transmission. The more torque an engine produces, the heavier both it and its transmission have to be to survive the forces being applied to them. But like I said, weight is expensive and the goal of high performance cars is to weigh as little as possible. This means that you want the lightest possible engine and transmission for the performance that you can obtain.
The way that sports cars do this is by having *relatively* low powered engines (at least compared to heavier cars like trucks) that produce power through the use of a complex transmission that has a large number of gears. Adding more gears doesn’t add any more weight, but it does allow you to have a much higher gear ratio in the highest gear. That higher gear ratio allows you to very efficiently convert the engine’s torque into work when the car is already travelling at high speeds, producing more horsepower than the engine otherwise would with fewer gears.
This isn’t to say that a Ferrari doesn’t have a powerful engine – a typical Ferrari has an engine that produces about as much torque as the engine in an F-150. But when you start to get into more mid-range sports cars, like the Porsche Boxster, you also start to get engines that are producing 2/3 as much torque as a typical truck.
Car engine stats are as much marketingspeak as physics…..
Eg, actual practical torque at the wheels is a matter of the transmission as much as the engine…..
You can take a really high winding engine, hook it to a beast of a transmission, and get gobsmacking amounts of torque on the ground (see the M1 tank and it’s turboshaft engine – the transmission is a few times larger than the engine, and turns 1500 shaft HP at a-million-something-RPM into 2500ftlbs torque at the tracks & the ability to make 70 tons go remarkably fast)
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