Why does the engine shake when in low RPM and high gear? Why does the engine stall if you don’t have enough speed?
In: Engineering
When the piston is going up it is compressing the air in the cylinder which slows down the engine. Then when the piston gets to the top it ignites the fuel and the piston is pushed back down. This constant slowing down and speeding up is felt as vibration. At higher speeds you do not notice it as it comes much faster and the engine have components balancing the vibrations to reduce them. But at slow speeds the engine can almost stop as it is compressing and then speeding up when the fuel detonates so you get a huge difference in speed through the stroke.
The engine stops when it no longer have enough speed to compress the air in the cylinder. When the air provides too much resistance it can stop the engine before it gets to the top. When the car is in gear and you are driving along the engine can get much slower without stalling because the entire car is pushing the piston up. It is not just the flywheel that have to stop but the entire car. But on the other hand the engine is not as powerful at lower speeds and the car provides a lot of friction for the engine to overcome. So when the engine speeds gets too slow the engine will just go slower and slower as the car slows down even with full throttle.
Basically, your trying to make the engine do too much work for it to keep running.
The engine needs to use some of the energy it produces to keep the engine turning (literally), so if you put too much workload onto it, the engine fails to turn properly, the next cylinder doesn’t fire properly, and the whole engine stops because theirs no power being produced.
This can happen any time the RPM get too low, but its easier to get that effect with a high gear and low speed, as the gears are physical connection to the wheels, which means the engine speed and wheel speed are directly linked. so if your wheels are turning slow, they will physically slow down the engine (and be sped up by the engine in turn), which with a high gear setting at low speed, will force the engine to run to slow to keep working and cause a stall.
Engines make power by making little explosions of fuel that push on the pistons.
These pistons are connected to a rotating shaft that drives the transmission.
One explosion can only push so hard on the pistons. Lots of explosions per minute (high rpm) makes lots of power and causes your car to speed up.
If you shift into too high of a gear, the engine slows down and the number of explosions go down. This makes less power and your car can get even slower. You can feel the shaking because you can start feeling every individual explosion instead of a smooth engine because it’s turning too slowly. It’s like the difference between a drumroll and someone hitting the drum one stick at a time.
Eventually the engine gets so slow that each little explosion stops pushing before the engine can prepare the next explosion. This is when the engine stalls.
Internal combustion engines don’t run smoothly, it just seems like they do. Each combustion drives a piston down with force creating a “pulse” in the entire drive train. When this is happening fast and without much resistance, you just feel a mild vibration through the car.
The engine also relies on mechanical advantage to move the vehicle. Low gears give the engine a lot of advantage, so it can turn the wheels easily. Just not very fast. High gears remove that advantage, but allow the engine to turn the wheels fast.
A manual transmission vehicle has a physical link from the engine to the wheels. The gears change how many times the wheels will turn for each turn of the motor, but if the wheels slow down, the engine WILL slow down to match. It is physically connected. And automatic transmission uses fluid pressure to drive the wheels and can spin independently from the engine’s rotations.
Let’s tie all this together. The engine is pulsing with each combustion stroke. It’s in high gear, so the wheels are very hard to turn. The engine is directly connected to the wheels, so it can’t freely turn faster without spinning the wheels faster. This means that when the pulse happens the engine can’t really turn the wheels. Instead, the engine gets shoved by its own power and tries to rotate instead, but it’s held in place by the engine mounts. This is the shake that happens at low speed.
The engine stalls at low speeds because it has very little power at low speed. That means it can’t turn the wheels enough to complete a full stroke and compress another cylinder enough for the engine to fire it and drive the piston down.
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