Is gas consumption determined by RPM?

If I'm running my bike at 3000RPM will it consume the same amount of gas in a fixed length of time regardless of how fast I'm going? 120 mph in 5th gear and 30 mph in 1st gear both at 6000RPM. Will both situations burn through 1 gallon of gas at the same rate (not the same distance)?

These are the things I think about as I ride to work in the morning with gas rolling past $4/gallon.

(I also think about the fact that my bike's clock is off by 4 minutes but I always forget to change it when I've stopped. Been bugging me for like a week....)

This question is like one of those Algebra 1 questions about two trains leaving a station, one going twenty mles an hour faster than the other........ arrrrrgh

I imagine that you'll burn more gas at 6K rpms at 120mph because there's more load on the engine and you'll need to open the throttle more to maintain that speed.

Gas consumption directly relates to HP generated. If you're using 20hp you're burning X amount of gas roughly. I've got the numbers/formulas written down somewhere. So if you're at 10k RPM on a 600 but only generating 20hp due to a closed throttle you're burning the same amount of gas per second as my GS500E at 4k RPM WFO praying it'll build more revs and get out of it's own way.

Yep...what he said.

Load is also a factor.

If you're at a fixed speed doing a fixed rpm, then changes in load would require changes in throttle position (level ground, climbing, descending). So, to maintain constant fuel consumption, you'd need constant speed, constant rpm, *and* constant load.

Welcome Chris, nice to see you here, long time no see (in person)
JohnnyB (John Branson)

Ok, dug out my notes...

To generate 1HP you need to burn 1.62 cubic feet of fuel/air mix per minute, using a stoichiomatic ratio. For boring old gas that should be roughly 14.7:1.

@ STP 1cu/ft of air weighs approx 0.0807lb.

( 0.0807 x 1.62 ) / 15.7 = 0.008327lb of fuel per minute per HP.

Gasoline is roughly 6.073 lb per gallon so...
0.008327 / 6.073 = 0.00137gal per minute per HP?

Err...does that take combustion efficiency into account? a typical gas engine puts less than 20% of the energy it consumes out the driveshaft.

Sam

http://en.wikipedia.org/wiki/Engine_efficiency

It doesn't take overall efficiency, volumetric efficiency, or ideal A/F ratio for whatever gas you're running (like say modern pump ethanolated garbage) into effect at all. Figured I'd just toss out the 'on a blackboard only' numbers just because that's what I had on that page of my notebook instead of piling on all the other details that VERY quickly complicate the mess. I've got a couple perl scripts that jumble the numbers for me, as I learn new math I add to the sim. Haven't made much progress lately though, seems buying a house slowed me down.

All of the various efficiency losses can be combined with the energy contained in the fuel to give you a number called the brake specific fuel consumption (bsfc). It is the rate of fuel consumption divided by the power output. It can be written as (lbs/hr)/hp or it can Kg/KWh or any other way you can relate the quantity of fuel to the energy produced.

Bsfc is much higher (less efficient) when the engine is lightly loaded, the throttle is mostly closed and the engine is spinning rapidly. It goes down greatly when the engine is fully loaded. At lower rpm the most efficient point is somewhat less than full throttle and at higher rpm it is basically full throttle.

So your engine is producing the lowest bsfc when it is maxed out. However, aerodynamic drag increases by the square of the velocity so when you you are maxed out you are using all that efficiency to stir up the air around you, very little is used to move you forward, so your fuel consumption per hour goes up a lot. The most efficient point is probably found by going 40 or 50 mph or so in as high a gear as possible. The single reason a small motorcycle has such great fuel consumption is because it is producing the most power it can at a speed where the drag is still reasonable.

Car and motorcycle gear trains are usually designed to have a little or a LOT of extra power on tap at cruising speed and they never reach the most efficient combination of load and speed for their engine. Truck, boat and airplane engines are designed to produce the lowest bsfc at their designed cruising speed. This is why you pass trucks going up hills. One the the main reasons for hybrid fuel efficiency is that they can operate the engine at optimum bsfc to charge the battery and then draw on the battery when extra power is needed.

I don't know if that answers the question, but it will give you more to think about! I think the real answer is "It depends..."

My source is "Engines, an Introduction" by John L Lumley of Cornell University