Infinitesimally. Consider that an engine brake is on a diesel engine, and that it only comes on when the engine is on over-run (truck pushing the engine, basically), so the engine is in a no-fuel condition (unlike gas engines, diesel engines inject no fuel when on over-run). That means there is no fuel being consumed while the engine brake is on, or off when coasting down a hill. The only reason is will consume a truly tiny bit more fuel is if you coast down the hill you'll be going a whole lot faster at the bottom, and be able to coast farther up the other side of the valley... In the real world if you do over-use the engine brake when you could be quite safely coasting at a slightly higher speed, you will use a bit more fuel. If you were going to use the service brakes anyway, no difference.
I would say it depends on how you use it. The exhaust brake itself will not use any fuel per say as it is not fueling when the throttle is released. However, many times engaging it unnecessarily and not allowing the coach to "coast" can use more energy (fuel) than really needed to get it going again. I like to use my "coast" gear as much as possible. This involves looking quite a distance ahead and timing stops/starts as efficiently as possible and not slowing too soon or too abruptly just to have to get all of that weight moving again.
First question that comes to mind is are you asking about an engine brake or an exhaust brake...two very different animals. Depending on the usage the engine brake will consume a slight amount of fuel as it re-establishes engine power. However, that consumption is more than overcome by the savings realized in brake usage and slowing power. Those that use the engine brake, and the exhaust brake for that matter, tend to alter their driving habits significantly so the fuel savings does add up. Either way, unless you were doing a very controlled experiment, you would not notice any change, either up or down.....Dennis
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I would think the type of engine per the question would be critical. Is it gas or diesel? My gas v-10 has an engine brake but I imagine it's a glorified way of forcing it to stay in a lower gear which results in higher rpm but the lower gear applies drag rather than free falling down a hill. Much like downshifting a manual trans in a car to slow you down. But since the rpm is not caused by my manual induction of more gas but rather more by way of gravity applied force to "push" the engine I doubt it uses more fuel. And if it did I am sure it's immeasurable. Frankly I don't care. It's keeping my brakes from overheating.
zmotorsports wrote: I would say it depends on how you use it. The exhaust brake itself will not use any fuel per say as it is not fueling when the throttle is released. However, many times engaging it unnecessarily and not allowing the coach to "coast" can use more energy (fuel) than really needed to get it going again. I like to use my "coast" gear as much as possible. This involves looking quite a distance ahead and timing stops/starts as efficiently as possible and not slowing too soon or too abruptly just to have to get all of that weight moving again.
Hope that makes sense.
Mike.
YUP. Not coasting WILL cost you MPG!
Said another way, if you leave it on, you will either be applying accelerator or brakes. If you are interested in MPG, you need to add COASTING to the equation.
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