ELI5: why do gas cars need a gear box and electric cars do not?

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Internal combustion engines have a lot of moving parts. There’s the spinning crankshaft, the pistons going up and down, the valve cams that open and close the intake and exhaust valves for each cylinder, the timing chain or gears that keep those in sync with the crankshaft, the water pump, alternator, oil pump, fuel pump, and probably more stuff I’m not thinking of at the moment. Because of all those moving parts, the engine has limits for how fast it can spin. If it spins too fast, the physical limits of the metal all those parts are made of is passed, and something will break. But at the same time, internal combustion engines also don’t work very well at low RPM’s. Too low and they’ll completely stall. Because of that and the aforementioned limits, they can only work between about 1000 RPM and 4000 RPM for average cars. So, in order to be able to go 15 MPH sometimes and 80 MPH other times, there needs to be a gearbox to allow the engine to maintain that operating RPM range despite how fast the car is going. **Electric:** Electric motors don’t need to maintain a certain RPM in order to deliver power. They can exert full power from a dead stop and also exert the same power at full speed at 100+ MPH. So there’s no need to have a gearbox at all. An electric car’s motor can be rotating at 10 RPM while crawling down a driveway without issue.

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Combustion engines have to run at a particular set of speeds (RPM) such that they can sustain their momentum. The pistons roll round from the previous explosion and get into place for the next, and if they fail to get there in time, the next piston won’t be ready for the next explosion and you’ll get a stall. Alternatively, they can only move so fast and do so many explosions in a certain time (heat, strain on the components, lubrication, etc.) Thus a combustion engine has a range of RPM that it can run at before it either stalls or becomes dangerous. This RPM range does not generally match the speeds that you want the tyres to go round at, thus you have to gear it right so that the best RPM (efficiency, power, etc.) matches with a good road speed. The problem is that the range of RPMs isn’t enough to just link it to the wheels directly. You’d be very limited in road speeds and extremely inefficient. And there’s no one fixed gearing that would be suitable – you’d either have trouble pulling away from a standing start, or the engine would be SCREAMING along trying to get you to a reasonable speed. So you use gearing to select different gears depending on what you’re currently doing. An automatic will select from those gears for you, in a manual you have to know to pull off in first, corner in second, and get up to 5th/6th gradually on a motorway, and to know when to change between gears so that the engine doesn’t hit unnecessary strain and is able to change smoothly between gearings without the RPM going too low or too high. In an electric car, it’s just an magnetic electric motor. As you apply voltage to it, it turns. You connect it direct to the wheels. There’s almost no lubrication, there’s no mismatch in speeds to compensate for, it’s just directly driving the wheels. The more voltage you give it, the faster it goes, and it can turn slowly or very, very fast without hindrance. It literally fits all the speeds you want to do, directly, without having to match speeds between engine and wheel.

ledow

Electric motors can deliver full torque at any speed, while engines have a torque to rpm curve where they produce maximum torque at certain speeds. A gear box will allow the driver to allocate the optimal torque to different speeds.

garlopf

IC (Internal combustion) engines have a narrow range of engine revolutions per minute they work well at, if too low they stall, too high damages the engine, you then need the gear box to get either higher torque and lower speed, low gears, or lower torque and higher speed, high gears, or reverse (an idler gear is engaged), depending on the road. An electric motor works well at a large range of revs so you can just choose a single gear that gives decent torque and always use that, for reverse you simply switch the current around.

MansfromDaVinci

Gas has a range of spin where power is made. To stay in that range, a gearbox is used to change how fast the engine spins the wheels. Electric also has a range of spin that makes power, but it’s a lot bigger. Because of that, electric cars don’t need gearboxes, but some do have a second gear. Gas sounds better, though.

-King_Slacker