When I push the gas paddle, the engine speed is immediately sped up like 3 time but the car is only accelerating slowly, what mechanism is it that make cars acceleration not like the ones like bikes where the “engine” speed is the synced with the wheel speed?
In: Engineering
In an automatic car, you have a torque converter. This is a drum of oil bolted to the back of your engine. The engine spins, the drum spins, this imparts spin on the oil inside the drum.
Sticking out of the drum is a shaft. This shaft is connected to a turbine. As the oil spins, it imparts force upon the turbine, also causing it to spin. The shaft attached to the turbine is connected to the input shaft of your transmission.
So there’s a physical disconnect between the crankshaft of the engine and the drivetrain, and the energy is transferred across that gap through the oil. Why?
Because piston engines suck. No really. They’re not very energy efficient at all, they produce miserable torque, they have slow speeds, their torque and horsepower output varies over their range of engine speed, and worst of all, they suck so bad at producing torque, they have a minimum engine speed before they stall. Once an engine is running, it has to keep itself running, and it can’t do that below stall speed.
So if your engine and drivetrain were mechanically connected all the time, then how can your engine produce torque when you’re at a stop? The drivetrain isn’t rotating, meaning the engine wouldn’t be able to rotate, meaning it’s not producing torque to get the car moving from a standstill.
You need the ability to impart torque without stalling, you need slip. And that’s what the torque converter does. You have to sacrifice efficiency in order to accelerate the drivetrain, and thus the car, in order to prevent the engine from stalling.
Modern torque converters also have clutches. These are friction devices that clamp together, mechanically interlocking the engine and drivetrain. This is how manual transmissions work – you operate the clutch with your left foot, gradually introducing torque without stalling the engine, until the drivetrain is moving at sufficient speed that the engine won’t stall. Once you’re up to a sufficient speed, you don’t need slip. Even with a manual transmission, you can shift gears without the clutch pedal. The only time you need to introduce slip again is when braking to a stop.
Automatics will disengage their clutch, presuming they have one, when you accelerate hard enough in order to allow slip – a sacrifice made to reduce vibration, people don’t like that. Cheaper or less sophisticated automatics will always suffer slip. A manual transmission, if you try to accelerate too hard, or in the wrong gear, would just cause the car to vibrate like crazy.
The problem with this approach is that slip causes a delay – the engine revs high and the car isn’t synchronized with it yet, which causes the car to lurch. Screwing around with an automatic car like it’s a sports car can make it unpredictable. This is exactly how teenagers wrap their mom’s soccer van around light poles, like idiots – that lurch is actually very hard to control, even in a straight line.
Electric vehicles, by comparison, are some 98% energy efficient, and regenerative braking certainly captures the pure loss of energy that you suffer with a piston engine. The sacrifice is that batteries have absolutely nothing near the energy density of gasoline. Also about electric motors, they produce 100% of their torque at 0 RPM, which means they don’t have to idle. Transmissions are torque multipliers because piston engines produce so very little of it at low engine speeds, electric motors don’t need them. They can spin to higher speeds, and produce almost no vibration, and almost no noise.
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