No, it does not have to consume more air at 2400 RPM than at 2200 RPM. Assuming that would mean neglecting throttle position and the resulting manifold pressure. To maintain the same power at a higher RPM requires less manifold (and cylinder) pressure. In this case, taking the 55% power situation, the manifold pressure at 2200 RPM is 21.7 inches of mercury. The manifold pressure at 2400 RPM is 20.7 inches of mercury (these figures are measured). The net result is the same flow of the fuel/air mixture, the same power being developed, and the same fuel consumption, even though the RPM varies. The chart is accurate.
Now, please consider the following situation. I’ve said something like it before, but I’m going to put it in different terms. This is very important to the point I’m trying to make, so please humor me here.
You have an engine, running at 3000 RPM, with no load on it, no real power being developed. It doesn’t take much throttle to maintain this 3000 RPM, right? I mean, you can sit in your car in neutral and just a little throttle will bring it up to 3000 RPM. Let’s say it’s burning 2 gallons per hour of gasoline.
Now. You apply a load to the engine. Assuming that you don’t open the throttle, the RPM will drop under the load. But we want a constant RPM, so we open the throttle more to bring the engine back up to 3000 RPM. More throttle, more airflow, more fuel being consumed. SAME RPM. It natural to expect more fuel to be consumed, because this power we are now generating to oppose the load isn’t free. It has to come from somewhere. The fuel consumption goes up, the RPM is still 3000.
Consider those two situations. Both running at the same RPM, putting out different amounts of power and with different amounts of fuel being burned. You can see here that fuel burn will not be constant at any given RPM.
Your fuel consumption depends on how much power you are making. How much power you are making depends on two things:
- RPM.
- cylinder Brake Mean Effective Pressure (BMEP)
If you keep the power (and fuel burn) constant, BMEP will rise when RPM is reduced. BMEP will drop when RPM is raised. You more or less control the BMEP with the throttle.
So RPM alone is not the determining factor in fuel consumption. RPM and BMEP must be used together. You cannot make a blanket statement than an engine will use X amount of fuel at Y RPM. You can, however, make a blanket statement that an engine will use X amount of fuel at Y amount of power. Hence the term we use to rate the fuel efficiency of an engine, Brake Specific Fuel Consumption. BSFC, as you probably know, is how many pounds of fuel you burn per hour for a given power output. Power output, not RPM.
Now, where does the advantage of “gearing to reduce the RPM” come in? The BSFC of most engines is lower at a lower RPM. The engine is more efficient at a lower RPM. That does not mean it has a constant fuel consumption at a lower RPM. Power will still vary, but you will be producing that power more efficiently. Note that I don’t disagree with you on this point. You are correct about it being better to use a lower RPM. I haven’t said or meant otherwise.
But, a constant fuel consumption at a given RPM is just not true. I appreciate your last response being kinder than the first.