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With engines such as in lawnmowers or motorboats (or even manual gearbox cars when the clutch is fully released), what does typically happen when there's a sudden increase in resistance during or immediately after the air-fuel mixture ignites in a cylinder (like the propeller gets stuck, or the blade hits on something, or car brakes are suddenly applied with no clutch)?

What happens with the pressure inside the cylinder, does it just blow past the piston rings if it can't push down on the piston? How do engines compensate for such occurrences?

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  • Welcome to Motor Vehicle Maintenance & Repair! May 7, 2023 at 23:50

1 Answer 1

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I coped this text from this website

This stroke is where the spark plug ignites the air/fuel mixture, creating very high cylinder pressure which rise very quickly. Peak cylinder pressures near TDC (where spark occurs) will be in the range of 300 psi for engine's at light loads, to 1000 psi for production engines at full power to 1500 psi or greater for race engines. This is where the engine's power comes from, as it forces the piston down. As the piston goes down, the cylinder volume increases which reduces the cylinder pressure. When the piston gets to the bottom on the cylinder (BDC) there may only be 100 to 500 psi in the cylinder.

So the engine is designed to take pressures around 1000 psi which will be at its highest around TDC.

If as you question, the piston couldn’t move anymore, I see the following things happening -

  1. The hot gasses will start to cool causing a pressure drop.
  2. The piston rings have a small gap around their circumference and do not perfectly seal against the piston, so gasses will slowly escape passed the rings.
  3. The inlet and exhaust valves will likely not provide a perfect gas tight seal, so gasses will slowly escape passed the valves.

The engine won’t explode due to it suddenly stopping, since it is designed to take the maximum possible pressure that occurs every revolution.

The pressure will slowly drop due to cooling and gas leaking away.

What is more likely to cause damage in a situation where the engine suddenly stops moving, is the rapid change in momentum. The crankshaft for example can snap or become bent due to rapid deceleration. Here I am talking about suddenly stopping like in your example of a lawnmower stopping after hitting a rock for example, not a car engine stalling when pressing the brakes.

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  • It might be of use to mention a hydrolock event. While it could be considered a "worst case scenario", it's what can happen when an engine suffers a complete dead stop. It wouldn't happen to a lawnmower, and probably not to a boat motor, but it is a good example of "what happens" during an event like what the OP is stating. May 8, 2023 at 11:07
  • Also a consideration, and likely more significant to the situation being asked about, is the flywheel. There is a lot more energy stored in the rotating flywheel than in the current cylinder in its power stroke. So things like broken crankshafts and the like are usually the result of the output shaft being obstructed and the flywheel's energy breaking the weakest link in the engine. Same with hydrolock, the flywheel does the damage and in that case the connecting rod takes the abuse.
    – jwh20
    May 8, 2023 at 12:07

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