On cars with manual transmissions, when in low gear (typically 1 or 2), why does accelerating and then taking your foot off the gas make the car lurch forward with that uneven, jerking motion?

Question

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

Answer 1

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.