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Just a strange question. Does each cycle of a 4 stroke engine use the same amount of fuel regardless of the RPm's? Another way of stating the question. Will the same amount of fuel be used for 100,000,000 cycles at 1000 RPMs, vs 3000 RPMs, vs 10000 RPMs? This is just a theoretical question, I am not interested in the practical aspect (for example fuel consumption per distance. This is the type of thing I think about when waiting for the light to turn green in my commute.

No, because volumetric efficiency varies with rpm. The ports, valves, and cam timing generally work best at a particular rpm and throttle opening, often the torque peak. Let's say that at torque peak, the cylinder of your R100 is getting a full 500ccs of air/fuel mix, or 100% volumetric efficiency. Away from that torque peak, the volumetric efficiency may be very small, as little as 10% at small throttle openings, so the cylinder won't draw in nearly as much fuel and air.

And, Melville's comments might be the end of the story if everything else was perfectly identical. Carburetors have different (overlapping) systems for different openings and different volumes of air passing through, so they don't give the same mixture at every rpm, as well as enrichening the mixture for acceleration with things like accelerator pumps, and emission controlled bikes being tuned to run very lean at low rpms, whether carbureted or injected.

Major Softie wrote:And, Melville's comments might be the end of the story if everything else was perfectly identical. Carburetors have different (overlapping) systems for different openings and different volumes of air passing through, so they don't give the same mixture at every rpm, as well as enrichening the mixture for acceleration with things like accelerator pumps, and emission controlled bikes being tuned to run very lean at low rpms, whether carbureted or injected.

Gaahhh! More variables!!

So another stupid question, is it the increase in volumetric efficiency that causes the rpm to increase in the first place? Am I just asking my first question backwards and upside down?! Thanks

JJH wrote:So another stupid question, is it the increase in volumetric efficiency that causes the rpm to increase in the first place? Am I just asking my first question backwards and upside down?! Thanks

If you give an engine access to the fuel and air it wants, rpm will increase until valve float or things go boom (assuming there's not an increasing load to limit the engine's rpm). Carburetors and other intake systems are then, not designed as devices to make the rpm increase when desired, but devices to reduce the rpm when desired. A "throttle" is so named because it "throttles" or restricts the intake, just as "throttling" a person restricts their ability to intake air. Opening a throttle is removing the restriction.

I think going down hill a a fixed speed and RPM will burn less fuel then going up hill at the same speed and RPM.

bbelk wrote:I think going down hill a a fixed speed and RPM will burn less fuel then going up hill at the same speed and RPM.

No question, but I've found that my injected vehicles don't seem to save nearly as much fuel under light or no-load (like going downhill for many miles) as my carbureted vehicles do. OTOH, the injected vehicles tend to get better mileage overall - but less variance according to load.

Don't know all the why's, but that's been my experience.