How does acceleration in car works?


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

Gears. Bikes can have multiple gears also. In a low gear, its very easy to pedal but you dont go very fast. In a high gear, its harder to pedal, but once you get moving you’re going faster. Gear ratios are what allows this to happen. Someone with more knowledge can explain in more detail.

The car is much much heavier than a person on a bike, so even though we might press on the gas and make the engine go “3 times faster “ it needs to do a lot more work to make the car actually start moving faster.

The gearbox.

It’s a train of gears which make it possible to choose between different levels of Speed and Force.

When you’re going up a steep hill, you want there to be a lot of force pushing, so you setting it into the first gear, will change the ratio of rotations between your engine and your drive wheels, to have very many rotations on the engine (input), while having very few rotations on the wheels (output).

Whereas when you’re driving a +100km/h on a highway, you don’t really need that much force, so you change to a higher gear, which will have a much smaller difference between the input & output gear.

Most cars in the US have automatic transmissions, and most of those are coupled to the engine by a torque converter. The torque converter is essentially two turbines. The turbine on the engine spins a fluid inside the torque converter, and that fluid spins the turbine coupled to the transmission. Due to that coupling, the input speed will not always match the output speed. On top of that, gears in the transmission and differential multiply the torque of the engine, so that even a thousand revolutions a minute might translate to only a few miles an hour

Sounds like you have an automatic gearbox, possibly even a CVT? If you had a manual car then it would work just the same as a motorbike does.

Your car has different gear ratios.

As an example, first gear might have a ratio of 3.1:1, which means for every 3.1 turns of the engine, you get 1 turn of the driveshaft. Second Gear will have a gear ratio around 1.9, so for every 1.9 turns of the engine, you get 1 turn of the drive shaft. Eventually you can get to something like 0.8:1 in 5th gear, where for every 0.8 turns of the engine, you get 1 turn of the driveshaft.

If you then look at what’s going to happen, if you hit 5,000 RPM in 1st gear, you need to keep a constant number of turns of the driveshaft as you gear up. Since that means you’ve got ~1600 RPM on the driveshaft, in second gear that needs ~3000 RPM, so (assuming an automatic transmission), your car will automatically lower the engine speed to match and then second gear goes in.

One major difference compared to a bike is that, if a cars wheels spins, they are directly connected to and will spin the engine. With a bike, the tires do not have the same effect, so the pedals don’t get turned by the tires.

While everyone’s answer of “gears” is close, I don’t think that’s the answer you are looking for. The motor in the car does not stay synchronized, as you describe it, with speed because of the torque converter.

The back of the motor has an output shaft. That output shaft is carrying all the “twisting” power the engine outputs. In a vehicle wtih an automatic transmission, it goes into a torque converter. It’s a donut shaped part has a fluid in it, and 2 “fans” facing each other. [Here]( is a diagram.

The “fans” spin the fluid, so the fan on the input side spins the fan on the output side (which is the side that goes to the transmission to power the wheels).

The reason the car doesn’t spring forward is because of this fluid coupling in the torque converter. It’s also why when you stop, the transmission can stay in gear.

The torque converter has a “stall” speed. This is the fastest the engine can turn, in RPM, before the torque converter can no longer just absorb the engine and MUST turn the output shaft or risk damage. For example, my Yukon stall speed is around 1600RPM. That means if that if I press the brakes and the gas, at around 1600RPM the torque converter is done, and I better take my foot off the brakes because something is going to break.

Torque converters do have a “lockup” that makes them 1:1, where clutches essentially make the output shaft match the input shaft speed. It’s generally part of the higher gears (my Yukon, for example, locks out in 5th and 6th).

Automatic transmissions are amazing pieces of engineering, what a GREAT question!

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.

Not it doesn’t.

Lol Murickans.

When you step on it in a regular, non-automatic car, the engine doesn’t rev into high gear. The rpm stays strictly proportional to the velocity. The exact proportion depending on which gear you’re in.

In an automatic car, who know what unholy shenanigans is going on in the “second engine”? Rethoric question. Don’t care.