Quote:
Originally Posted by drmiller100
throttle pumping loss is apparently measurable, and noticable. If you have a manual transmission, you can feel it yourself - coasting down the road, turn off the ignition and note your rate of deceleration. Then floor the throttle - you should feel deceleration lessoning.
This is the reason diesel engines have such crappy compression braking with out "jake brakes" and the like.
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Actually if you are coasting with the engine engaged to the power train, and you floor it (engine off) you will increase the resistance.
Why?
Because when you have the throttle closed the in cylinder compression is very low since the air flow is restricted by the closed throttle (high manifold vacuum). With the throttle wide open you have 100% of the available compression, which in most engines is about 10 times atmospheric pressure.
You can see it with a vacuum gauge. 0 measured vacuum means highest possible compression while highest vacuum reading means lowest possible compression. Obviously it takes more energy to compress 10 atmospheres into one than it does to suck 1 atmosphere into an almost perfect vacuum (emphasis on almost). This is why most engines shut the fuel off during deceleration. The compression is so low the fuel will not burn anyway and you have huge amounts of unburned hydrocarbons coming out of the engine.
regards
Mech