How is fuel consumption related to rpm?

Question

I know consumption increases with rpm, but in what proportions does it increase? Is it linear, e.g 2000 rpm is double the consumption of 1000. Or is it in the degree of 2 or something, like 4000rpm uses 4 times the petrol of 2000 beacuse it doubled. I know its a bit of a random question but there you go...

Answer 1

fuel consumption is related to rpm but not in a linear or transparently assocaitive fashion

rpm is the number of revolutions per minute the engine (crank) is doing... so it reflects the throttle setting. however fuel consumption is proprotionate tothe amount of load on the engine and the amount of acceleration or deceleration.

an engine at a constant RPM with the same load will (should) use the same amount of fuel. however as the load characteristics change that fuel consumption will also change. as an engine accelerates it will use more fuel, unless the acceleration is held at a fixed rate.
as the air / fuel mixture changes (due to either a change in altitude or pressure or humidity or oxygen content so to will the consumption
if you decelerate ther engine RPM may drop slightly but the fuel consumption may drop dramatically. an example of this go out in a car with a trip / fuel consumption meter. on my car it averages around 35 MPG at 70..90mph, if a decrease the throttle setting (eg when going down a hil the momentary fueld consumption may drop to give an MPG equivalent of 100+MPG, if I accelerate to maintian speed going up a hil the load has increased the revs have stayed the same (or at least similar) and the fuel consumption increases leading to an equivalent MPG of anywhere between 10..20MPG.

Answer 2

Fuel consumption has MANY variables. The biggest is the load on the engine. If you are going up hill with all your friends in the car, but keeping around 2000 rpm and under 70 mph you are going to be using more fuel than if you were going that 2500 rpm mark and 70mph alone on a flat road. Yes speed and rpm are a part of engine load, but don't forget about weight. It also plays a major part.

Answer 3

Fuel consumption is related primarily to how hard the engine is working, which is the product of torque and rpm. You would use much more fuel going up a hill than down hill with the same engine speed.

Engines tend to have maximum torque at lower revs than maximum power; this will depend on the design.

One engine test that used to be demonstrated to engineering freshmen at Bristol University was to run an engine against a constant resistance (=torque), and then measure fuel consumption as speed is increased. Fuel consumption was plotted against power output, which at constant torque was the same as plotting against speed

Any given engine has first to overcome its own internal friction, then it will have a more or less linear region where fuel consumption is proportional to power, and at higher revs the fuel consumption increases more rapidly.

The characteristic is different for 2 stroke and 4 stroke engines - with 2 stroke engines porting played a big part;
A carburated engine will be different from a fuel injected engine, and a diesel will be different from a petrol (Gasoline) engine.

Answer 4

Interesting Q, I'll come back later to see if any proper egg-heads give you a real answer.
What I would say is that fuel consumption would have more to do with the load being put on an engine, rather than its speed. I think it would take less fuel to turn an engine at say, 4000rpm while stationary than it would to actually drive a car forwards while doing 2000rpm, since the engine would have only its own inertia to overcome rather than the weight of the car.
I could be wrong, though!!!

Answer 5

Fuel consumption is related to RPM in that
RPM determines the amount of mixture burned
over time. This relationship is almost linear.
This relationship is not linear in that the mixture
changes with load, (BMEP), and volumetric
efficiency, (which decreases with increase in RPM).

Answer 6

consumption versus rpm depends on your engine meaning , its efficiency and power developed.

The easiest way to know, would be to look at the output torque curves of your engine as the Power delivered P=TΩ (T being the torque) so consumption and Torque are directly linked