So trying to figure out why electrical engine only needs a single gear while a combustion engines needs multiple gears. Cant wrap my head around it for some reason
EDIT: Thanks for all the explanation, but now another question popped up in my head. Would there ever be a point of having a manual electric car? I’ve heard rumors of Toyota registering a patent for a system which would mimic a manual transmission, but through all this conversation I assume there’s really no point?
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Internal combustion engines have extremely RPM dependent efficiency. If you try to run a car engine at 10000 rpm it will give you **awful** efficiency. It will also degrade the engine faster and require more repairs.
A transmission is a way to change the RPM from the engine to a different RPM of your tires. This allows you to drive any speed from 0 to well over 100 km/h in a similar RPM range, and thus hugely improves efficiency and reduces the maintenance required on the engine. It also increases the top speed of your car, since running a combustion engine at the speeds required to go to a cars top speed would simply not be possible without destroying a conventional engine.
A transmission also allows you to trade on a constant engine RPM to lower Tire RPM but higher torque, or vice versa, depending on what you need.
Electric motors have non of those disadvantages. Their efficiency (and their torque) is almost completely independent of their rotational speed, so there’s no need for a transmission
Electric engines are very torquey, especially at low rpms, so you don’t need low gears to get moving. And they can rev very high while remaining very smooth and putting relatively little additional strain on the motor, so you don’t need high gears to keep the revs down on the highway.
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A combustion engine only works in a fairly narrow range of rpm. They usually need at least 1000rpm to be able to generate enough power to propel a car.
The reason is that piston movement is directly proportional to rpm, and you can only fit a certain amount fuel+oxygen in each cylinder. So the amount of fuel you can burn, and the amount of power you generate is limited by rpm. There are ways to push that limit (e.g. by compressing and cramming more fuel+oxygen in), but that only goes so far. For more power, your engine needs to turn faster.
An electrical engine does not have that limit. You can supply more or less as much current as you want (until your wires start melting), regardless of whether the engine is turning or not.
So electrical engines work at lower rpm.
It also goes into the other direction though. Electrical engines have far less moving parts (no piston, valves, no mechanisms that convert piston movement to rotation, …), and thus can potentially work at higher rpm before falling apart.
Electric motors generally have good torque and excellent efficiency over a wide range of rotational speeds. Having a single set of gears or none at all generally doesn’t effect performance. In addition transmission gearboxes are heavy. Typically the added weight of having a transmission with several gears has been considered extra weight which carries a major penalty on electric cars, while not providing much benefit. By removing such weight one could add extra batteries for example, at a similar cost which boosts range.
Internal combustion engines like Diesels or Petrol engines have poor power and low torque at low speed, and they have very poor efficiency and reduced reliability at high speed. They are generally most efficient and produce the most torque at a relatively narrow range of speeds. Therefore it’s proven to give better performance by having an adjustable transmission with several different gear ratios between the engine and the wheels.
It’s to do with the torque curve. Internal combustion has a small range of rotation speeds where they develop good power/fuel efficiency and so on. Gears are needed to match the wheel speed to the good power zone of the engine. There’s also a limit to how fast one can spin a piston engine before the linkages and rods and valves and such destroy themselves. Remember anything going up and down needs to accelerate and stop and reverse direction insanely quickly.
Electric motors have a waaaay bigger torque curve. Almost from stall to max rpm an electric motor is giving you full power. Electric motors can also spin way faster than a piston engine because they’re just a rotating mass controlled by a computer. The faster a computer can flip switches, the faster you can make it rotate… And computers are really, really fast. The only physical limit to rotation speed is the strength of the metal itself (assuming magnetic bearings). Rotation generates centrifical force and so your limit on speed is the point where the metal windings start to stretch from that outward force and contacts the outer case. This is way faster than a piston engine can go. Even more crazy, this limit has now also been overcome. Tesla decided this wasn’t good enough in their drag monster and decided to wrap their motor with high strength carbon fiber to literally hold the copper in compression, getting their motor a rev range of 0 to ~20,000 rpm.
Electric cars do tend to use a reduction gear though. So a single, permanent reduction which gives electric cars even more power at the wheels.
Some electric cars do have a transmission. The Porsche Taycan has a simple 2 speed. The regular gear for everyday driving and “overdrive” for the Autobahn.
I know that ELI5 isn’t supposed to be about a literal 5-year-old but:
The thing that spins and goes bang a lot can only go bang at a certain speed or it’ll go BANG and/or overheat.
And if it goes bang too slowly, it won’t have the power from the bang to move around to bang again.
So it has a limited range (so 800rpm -> 6000 rpm, for instance) where it’s safe to bang that quickly, but where it’s not going to come to a halt (stall).
Because that range is no good for direct motion, you have to gear it up and down. Otherwise either you’re SCREAMING the engine along when you’re on the motorway, or you’re not able to pull away because the engine is trying to move the wheels too fast from a standing stop and instead the wheels stop the engine moving (again, a stall).
The range of rpm values of the typical ICE engine do not match the range of rpm values that you need the wheels to go through (from, basically, 0rpm up to 700/800 rpm or beyond).
Electric vehicles don’t have a spinning engine that can stall. They just put power into an electric motor (different to an engine) and whatever speed the motor is doing, adding power to it will make it go faster.
Combustion engines are like your legs in a bicycle. If you use a super tall gear you can’t crank the pedals because the effort needed is too high for you (engine stalls). On the other side there’s a point where you can’t pedal fast enough for the speed you are going. Your feet can’t keep up and you give up and take your feet off the pedals (engine over revs and throws a rod through the block)
Electric motors are like supernatural legs that don’t care about any of that. Huge amounts of torque from a dead stop in a tall gear (like standing on that pedal with your whole body) and also they can spin fast without losing power.
A lot of good responses concerning torque and rpm curves.
One thing being left out is: Springs.
Internal combustion engines have valves that permit the gas and exhaust exchanges to happen in the correct sequence. These valves have springs to push them closed when they are not being pushed open.
Springs are not infinitely fast. At some point the valves will not be closing fast enough. Gas exchange and compression can no longer happen once the valves can’t keep up.
At low rpm the valves are closing so fast that it is basically instantaneous when compared to movement in the rest of the engine. When rpm gets past 6000, the parts are all moving really fast, and then, depending on the motor, the parts are going faster than the springs.
That’s why torque and/or horsepower has an absolute limit. A limit that requires tremendous engineering to push rpms higher amd higher. It’ not at all worth the investment. Engineers can make combustion engines that go into the 20k rpm plus range. It’s just not worth it for a commercial car.
An electric motor can easily turn at 30k rpm. Requires no complicated engineering or ultra high tech, exotic materials.
A combustion engine can go from BRR to BRRRR.
An electrical engine can go from BRR to BRRRRRRRRRRRRRRRRRRRRRRRR.
The combustion engine needs the gears to multiply the few Rs it has so that it can power the wheels at higher speeds, but since the electrical engine can produce alot of Rs on its own, it can keep up with higher speeds without the need of complicated gears.
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