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I am trying to figure out how much gas an engine uses when engine braking in a four-stroke engine (motorcycle). I understand that the gas consumption should be close to zero because the wheels are responsible for moving the engine's pistol up and down and not the throttle (which is closed). But my question is, is it really zero?

Let me give you an example. Lets assume that an idling engine rotates at 1000rpm. The throttle is closed meaning the carburetor's valves are closed. But still in each Intake round a small amount of gas will be used in order to keep the engine running. Let's assume that this amount is 0,01mL per round. Since we are at 1000 rpm this means that we will use 1000*0,01 = 10 mL every minute. Now lets assume that we are driving and get off the throttle.

  • If an engine spins at 5000rpm will it consume 5000 * 0,01 = 50mL every minute?

I have also read that some modern engines are smart enough to completely stop providing gas to an engine when engine breaking leading to zero gas consumption.

  • If this is true, in which motorcycles does this apply? Do they need to have an electronic fuel injection or are more modern ride-by-wire systems required?
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  • Welcome to Motor Vehicle Maintenance & Repair! Pertaining to the second part of your question, it is absolutely correct modern fuel injected engines in automobiles turn fuel off between ~1000-2000 rpm during deceleration. I don't know if it is the same for motorcycles. Sep 7 at 11:01
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A carbureted engine will use a tiny amount of fuel in this situation. The throttle plate is closed which produces a high vacuum in the intake which means there is very little air flowing which therefore means there is little fuel.

A fuel-injected engine has the ability to completely shut off all the fuel anytime the FI ECU stops the injector(s). When they do this is dependent on the specific programming of the unit but generally when decelerating there is no need for any fuel so it shuts it off totally. There may be some variations of when to turn it on or off due to them trying make transitions smooth vs. jumpy.

But generally under a deceleration condition, little to no fuel is being used.

Motorcycles and other gasoline powered vehicles all behave similarly in this respect.

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  • Throttle plates are never fully closed otherwise the engine wouldn't run as a huge vacuum is generated as the engine runs. All throttles, when closed are not fully closed to allow the least amount of air that equates to idle rpm. Simple motorcycles close throttle with a small opening to allow the engine to run when releasing throttle when coasting, shifting gears, etc. This allows the engine to run. This small throttle opening still uses some fuel but is negligible in the overall running of engines.
    – F Dryer
    Sep 7 at 15:52

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