Thread: Engine Braking
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Old 09-30-2012, 02:35 AM   #5 (permalink)
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Originally Posted by wobombat View Post
Okay, so considering that diesels are much more efficient at idle and low throttle speeds than gasoline engines, would their BFSC charts have the sweet spot at a lower throttle amount, like say 20 or 30% than gasoline engines and therefore would P&G not be as fuel efficient with diesels?
Because friction is usually the biggest force. Pumping can exceed it on diesel compression brakes but friction is pretty huge. For peak efficiency the balance becomes how much fuel energy do you want to blow out the exhaust vs. increasing the proportion of fuel energy that is *not* going towards friction. Higher load produces more power but is thermodynamically less efficient in the absence of pumping work, but less power produced means a greater proportion of power went towards overcoming friction.

You're correct about engines consuming a lot of power at idle. The fuel flow rates look low until you realize actually going somewhere at moderate speed takes only maybe 2 times the fuel it takes to idle. Diesels consume quite a bit of fuel idling too because of friction, but since they don't have pumping to worry about it's quite a bit less.

Diesels having a lower peak efficiency load means that P&G is less effective, but it still beats running at very low loads.

As for how pumping (throttling) relates to friction, there are no real reliable friction figures for engines but ballpark numbers I've seen give that throttling is around the same magnitude of energy loss as friction when engine braking.

Still, engine braking is just like using your friction brakes except it doesn't wear anything down and you get a bit of free alternator charging, so you want to use it only to come to a stop.
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