eli5 What’s happening when an engine stalls from low RPM?
In: 21
It doesn’t stall from low RPM, it is stalled therefore the RPM falls.
There are probably two basic reasons
a) Fuel/air issues. So if there is insufficient air or fuel entering the engine, it will stall.
b) Loading. For a given amount of fuel/air and engine RPM, the engine can only output a certain amount of torque to the crankshaft. If the load on that shaft is more than the engine can produce, it will stall the engine.
It doesn’t stall from low RPM, it is stalled therefore the RPM falls.
There are probably two basic reasons
a) Fuel/air issues. So if there is insufficient air or fuel entering the engine, it will stall.
b) Loading. For a given amount of fuel/air and engine RPM, the engine can only output a certain amount of torque to the crankshaft. If the load on that shaft is more than the engine can produce, it will stall the engine.
“Lugging it”. Internal combustion engines only work in motion. You can’t just halt the engine half-way between engine strokes and then start it up again. (Without invoking the starter.) The combustion only happens at certain points of a revolution. When you let the RPM fall too low and the engine dies, it lost too much momentum to keep the crankshaft spinning and the pistons moving. It takes energy on the compression stroke where it squeezes the fuel and air. If the energy of the momentum of the engine is less than the energy needed to compress the gasses, rotation stops, and the engine dies / stalls.
“Lugging it”. Internal combustion engines only work in motion. You can’t just halt the engine half-way between engine strokes and then start it up again. (Without invoking the starter.) The combustion only happens at certain points of a revolution. When you let the RPM fall too low and the engine dies, it lost too much momentum to keep the crankshaft spinning and the pistons moving. It takes energy on the compression stroke where it squeezes the fuel and air. If the energy of the momentum of the engine is less than the energy needed to compress the gasses, rotation stops, and the engine dies / stalls.
Engine compress then burn the fuel air mix. The resulting gases push the piston down again, and that’s your power from combustion transferred to the crankshaft.
It need to have the momentum to end the next compression, then ignite the mix, then it gets the energy to keep going until the next compression.
If you release the clutch too sharp with too much load for example, you steal that momentum, the next compression is not achieved the engine dies.
Engine compress then burn the fuel air mix. The resulting gases push the piston down again, and that’s your power from combustion transferred to the crankshaft.
It need to have the momentum to end the next compression, then ignite the mix, then it gets the energy to keep going until the next compression.
If you release the clutch too sharp with too much load for example, you steal that momentum, the next compression is not achieved the engine dies.
A single piston is relatively simple. It’s a airtight tube, with a bottom that moves like the plunger in a syringe. A valve opens and the piston moves down, and that pulls fuel and air into the piston. Then the piston moves up and compresses this mixture and a spark ignites it. The explosion pushes the piston down, which is where the power comes from, and then it comes back up to push the exhaust gas out and get ready for the next cycle. So that’s the 4 strokes of a 4 stroke engine. I’ll call them inhale, compression, expansion, exhale.
But you may have noticed something… The piston is moving on all of those, but only one of them creates power. It creates a lot of power, but only on one of the strokes. So to solve this you can basically do two things. The first is simple, you use a flywheel, which is basically a big heavy spinning disk, which keeps momentum. So the expansion pushes the flywheel around and the momentum is enough to move the piston through the other 3 strokes to get back to the next explosion.
The second is you can get multiple pistons and attach them to the same shaft out of sync with each other so piston 1 is currently exploding, and that’s moving piston 2 through exhale, piston 3 through inhale, and piston 4 through compression, and then piston 4 will go off moving the others through their strokes, etc.
But the important part of both of these is that the movement of the engine sets up the engine for the next power stroke. So hopefully by now you can see how it would be disruptive to stop the engine in between one power stroke and the next, or otherwise siphon off enough power that there isn’t enough left to make it back to the top. There would be nothing there to go through the other 3 strokes that are essential to setting things up for the next explosion.
A single piston is relatively simple. It’s a airtight tube, with a bottom that moves like the plunger in a syringe. A valve opens and the piston moves down, and that pulls fuel and air into the piston. Then the piston moves up and compresses this mixture and a spark ignites it. The explosion pushes the piston down, which is where the power comes from, and then it comes back up to push the exhaust gas out and get ready for the next cycle. So that’s the 4 strokes of a 4 stroke engine. I’ll call them inhale, compression, expansion, exhale.
But you may have noticed something… The piston is moving on all of those, but only one of them creates power. It creates a lot of power, but only on one of the strokes. So to solve this you can basically do two things. The first is simple, you use a flywheel, which is basically a big heavy spinning disk, which keeps momentum. So the expansion pushes the flywheel around and the momentum is enough to move the piston through the other 3 strokes to get back to the next explosion.
The second is you can get multiple pistons and attach them to the same shaft out of sync with each other so piston 1 is currently exploding, and that’s moving piston 2 through exhale, piston 3 through inhale, and piston 4 through compression, and then piston 4 will go off moving the others through their strokes, etc.
But the important part of both of these is that the movement of the engine sets up the engine for the next power stroke. So hopefully by now you can see how it would be disruptive to stop the engine in between one power stroke and the next, or otherwise siphon off enough power that there isn’t enough left to make it back to the top. There would be nothing there to go through the other 3 strokes that are essential to setting things up for the next explosion.
If you think about peddling a bike, you push your foot down on the peddle on one side, it brings the peddle around on the other side, so you’re always able to push: either with one foot or the other.
Now let’s say you’re peddling up hill, and it’s getting harder and harder to push, and at a certain point, although you manage to get the peddle part way around, you don’t quite clear it so that you can push with the other foot.
Because you’ve lost the momentum between the peddles, you can’t peddle anymore, and your bike stops.
If you think about peddling a bike, you push your foot down on the peddle on one side, it brings the peddle around on the other side, so you’re always able to push: either with one foot or the other.
Now let’s say you’re peddling up hill, and it’s getting harder and harder to push, and at a certain point, although you manage to get the peddle part way around, you don’t quite clear it so that you can push with the other foot.
Because you’ve lost the momentum between the peddles, you can’t peddle anymore, and your bike stops.
Latest Answers