Hard-core CEG\'er
Joined: Aug 2003
Posts: 4,140 |
Originally posted by Christian:
Originally posted by Big Jim:
You are almost there. If it is producing less power than required to propel the car it is engine braking and that engine braking is assisting in stopping the car.
Okay, that I get. However for maximum braking would the power not be cumulative to the momentum as it is still creating some momentum ... and if you are braking hard, once the velocity drops (if dropping quickly) to match the velocity that the engine is trying to produce, further hard braking is going to be working against the engine too, as the engine is not dropping quick enough to match the drop in velocity, so you're (from that 'sweet spot') braking against both the momentum of the car and the engine's power output (even though it is dropping) ?
Yes. The engine is creating resistance to the car's motion above idle speed, since the air in the cylinders is being compressed. As demon stated, there is no fuel coming out of the injectors when coasting. No fuel means no power. None. You are compressing air in the cylinders, and letting it out the exhaust. This take a lot of energy from the kinetic energy of the car, slowing you down. Even in a car with the injectors spitting fuel at idle speed, the force it takes to compress the non-firing cylinder that revolution is less that the force created by the firing cylinder. Thus, the engine produces DRAG on the trans. You can feel this when the transaxle clunks upon letting off the gas. This is the gears locking in the opposite direction: the wheels are now pushing the engine. Once you get down to idle speed, the amount of energy created by the firing cylinder is exactly equal to the energy expended compressing the non-firing cylinder, so it's a wash. Coast lower than idle, and you're going to be hurting your deceleration, as well as probably stall the engine, unless you hit the clutch soon!
Hope that helps. The key is: the engine takes power too, it doesn't just make it. Whether the TOTAL amount of power coming from the engine is positive or negative depends on whether it is above or below the speed it would natually spin with a given amount of throttle.
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