Why wouldn’t the car just decelerate smoothly when you take your foot off the gas? And why does it often continue even if you step on the gas again?
In: Engineering
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The engine has flexible mounts to absorb vibrations. You are flexing those mounts when you do this, and it gets lurchy. You can rip them if you keep doing that.
This also happens in an automatic when it’s configured to hold gears for longer (like a sport mode).
The engine is the single most inefficient part of your car. When it’s pushing the car, it is throwing away most of the energy it makes. If you stop giving it gas, it still wastes a lot of energy (though considerably less), but now that energy is coming from whatever else keeps the engine turning. If you’re in gear, then “whatever keeps the engine turning” is the car’s movement.
Low gears give the engine better leverage over the car, which is great when the engine needs to give the car a push, but when the engine is actively sucking energy away from the car then giving it “better leverage” means you slow the car down way more.
Acceleration turns into deceleration, and you feel it.
The change in acceleration is actually called ‘jerk’
In all transmissions, a clutch connects and disconnects the engine (power) from the gearbox (shifter) and drivetrain (wheels). In automatic transmissions, a computer makes that connection and disconnection very smooth when shifting into other gears, so it’s hard to notice any jerk because it won’t stay in the same gear longer than it should. If you put an automatic transmission into low gear (L or 2), you’ll feel similar jerkiness. If you press the clutch pedal in a manual transmission at the right times, it will feel smooth too.
if you are proficient you can minimize that shifting jerkiness to a smoother ride, especially on a nicer sedan. It’s all about shifting well before the top rpm range and quickly moving to the correct gear before reengaging the clutch with enough gas to spin up to match the speed you’re going.
It’s called backlash. It’s kind of abusive to the car but in racing it helps you slow down for the corners. It’s really not a great thing to do to your car it puts unneeded stress on many components. I’m not sure how deep or technical of an answer you want but an analogy is two friends pulling on a rope playing tug of war and one of them suddenly let’s go
It’s mainly due to compression in your engine. When you are on the gas, fuel is being injected into the cylinders and with air which explodes and expands giving your car power.
When you let off the gas, there is only air in the cylinders. But there is no explosion to expand and push the cylinders.
Instead the cylinders actually have to compress the gas, which takes work, and then that is ejected out of the cylinder. So that jerk you feel is the slow down caused by the wheels turning the cylinders of the engine compressing air.
If you push in the clutch that disconnects the wheels from the engine and that stops the compression braking.
Taking your foot off the gas starves the engine, so the combustion firing is no longer fast enough to keep up with the wheels’ rate of turn from the car’s inertia pushing it forward. Since, with the clutch engaged, the wheels are still directly connected to the engine, it’s being pushed faster than it naturally “wants” to go for this reduced amount of fuel, so you get “engine braking”. In a higher gear and/or higher speeds, the engine braking feels smoother, but at lower speeds it is jerky because you “feel” the cylinders individually resisting the turning and there may be some misfiring cylinders. If you disengage the clutch (push the clutch pedal) the engine and wheels can both turn smoothly again at different rates.
If you have a good “feel” for this, you absolutely *can* let off the gas to slow down, or first downshift and let off the gas to really slow down without braking. But you have to recognize that below a certain RPM (speed of rotation) the engine will be very unhappy with this.
Why does it still jerk if you push on the gas again? Because, after slowing down without shifting gears, the engine now has to overcome a much higher resistance, without mechanical advantage of a lower gear ratio, to accelerate the wheels again. So the engine strains and skips, or stalls completely. That’s why you would want to downshift before trying to accelerate.
I’ll often downshift to engine brake into a corner, use the real brakes to adjust the speed for best cornering path, and, since I’m already in a lower gear, accelerate out of the corner rapidly. But this is at a relatively high speed.
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