So a couple friends and I are trying to settle something.

Does a combustion engine use more gas at high RPM than at low RPM given that the throttle position is the same? So at WOT in 2nd gear, will an engine burn more gas at 6K RPM than at 2K RPM? What about if the gears are different (2nd gear at 2K vs 6th gear at 6K)? Does the gear position even matter?

I used to think the answer was YES, the engine will obviously use more gas because it is taking in more air/fuel mixture at higher RPMs. But one of the guys I'm arguing with (a pretty smart and knowledgeable fellow) has me doubting myself. I'm starting to think the answer is no: it uses approximately the same amount of gas at a given throttle position, regardless of the RPM.

I tried to use a fuel mapping chart to prove my now-doubted point, such as this one:

http://www.rbracing-rsr.com/fuelmapl.htm

... since it shows fuel usage (injector valve timing) increasing with RPM... But then I realized the fuel usage also increases with manifold pressure. And given a fixed throttle position, I think manifold pressure would decrease as RPM increased. But even so, I'm not sure that decrease is enough to offset the increase in RPM. It also makes sense since as RPM increases, the engine would be operating closer to its optimal range, which seems like it would decrease load... I think.

So anyways, can anyone definitively answer this question for me? If you could provide a reputable link, too, that would great. Not that I won't trust your word.

 

This is great to read, as I started the same thread in an Audi forum a few years ago. I've had many discussions with people about this and the consensus I get is the less you are on the gas pedal, the better the fuel economy. Yet it's weird at times. I'll be cruising in a certain gear, then downshift into the next lower gear when suddenly I see the MPG's drop in the display. This is without any change in pressure to the gas pedal. Just a change in gears. So, that tells me something about the engine rpm makes a difference. Still haven't been able to prove anything further and would love to hear some more comments about this topic. I drive to maximize fuel economy.

 

That would depend on the engine.



I've always assumed that a lower RPM is more ideal, however I did have a friend drive my car (which is a 5MT) and he did so in a manner in which he didnt downshift from 5th when accelerating and highway mileage went from 32 MPG from one tank to 21 MPG for his tank. Thus what you describe is plausible, though I realize that scenario is somewhat different. However, if you've ever driven an automatic in 2,3, etc. for an extended amount of time the RPMs tend to stay high and you use alot of fuel.

 

obviously it would inject more fuel per minute because the revolutions per minute went up when the throttle position is kept the same. Unless the fuel system is skipping cycles like in those multi displacement Hemis, which sounds unlikely. That is how you are getting more power out of an engine at higher rpms anyways, you are burning more fuel per second. That power aint free.

 

Many different factors, of course, affect fuel usage. But, in general, regardless of vehicle speed or RPM, fuel usage goes up with pressure on the gas pedal and decreases when you let up on it. This was true in the old days with carburators and still is, to a slightly lesser extent, with electronic fuel-injection and Drive-by-Wire throttles. The old adage is to pretend there's an egg between your foot and the gas pedal, and accelerate to keep it from breaking. Computer-mapping of EFI systems (and fuel cutoff systems on deceleration) puts some complexities into it, but, in general, that is still true......and you can verify this by watching the MPG/fuel-consumption rate on the trip computer or vaccum/fuel-economy gauge, if your car has one.

You don't want to let RPM, though, get too low at any given speed, particularly with a manual transmission that won't automatically downshift to compensate. Driving at too low an RPM for the appropriate road speed will cause the engine to lug, using more fuel and, in severe cases, possibly causing damage to the crankshaft.

 

If you ride at constant speed at low rpm/high gear and switch to high rpm/lower gear without changing the gas pedal position, the car will slow down. I.e. you need to press the gas pedal a bit harder in order to keep the speed constant at high rpm/low gear.
At high rpm bigger part of the fuel is wasted for just keeping the high engine revs. There are more losses in the drivetrain too.
On the other hand, the engine's torque curve is also important, since it shows how efficient is the engine at different rpm. Usually the torque curve drops quickly below 2000 rpm, so the engine is not very efficient there.
So, finally it all depends on the engine, but I would say in most of the cases riding at 2000 rpm will be more efficient than at 6000 rpm. However if you compare riding at 1500 rpm vs 4500 rpm, the things may be the other way around.

 

of course you use more gas, you can't have an engine run 3times faster on the same fuel, your fuel pump is just pumping faster

gears of course matter, the whole point of gears is to improve fuel economy at higher speeds, 6th gear at 2000 rpms is cruising at 65, 6th gear at 6000rpm, your trying to make a 10 hour drive in 6 once again more fuel is used

 

I think todays cars are tuned to have the optimal fuel economy at RPM and throttle levels that are tested by the EPA.

 

Which is precisely what happened in the situation with my buddy...he no longer gets to drive my Subaru.

 

Yeah, so the easiest way to "test" things is to see how the fuel economy is tested as you hold the accelerator to a fixed position as the car accelerates in a given gear.

I've tried this very un-scientific test in highway acceleration in my car from about 40-70 in 3rd gear while watching the MPG gauge, and I see it stay pretty much constant, in spite of the rising RPM.

 

To the OP's original question, the answer is dependent on how much horsepower your engine is generating at each RPM, but most likely yes it uses much more gas at WOT when doing 6k RPM than 2k RPM.

Let's assume the following scenario - your car is strapped to a hub brake dyno and you want to measure fuel consumption at 2k RPM in 2nd gear at WOT and 6k RPM in 2nd gear at WOT. The brake dyno will hold your engine RPM at the specified value and allow it to produce the maximum amount of power/torque possible.

Let's also assume that your car has a completely flat torque curve of 100 ft-lbs all the way from idle to the 6k RPM redline. The amount of fuel used directly corresponds to the amount of horsepower your car is generating at that point in time. At 2K RPM, your engine is generating a maximum of 38 horsepower. At 6K RPM, your engine is generating a maximum of 114 horsepower. Thus, at maximum efficiency, your car will be using 3x more fuel when generating maximum power at 6K RPM than at 2K RPM if assuming a completely flat torque curve.

Of course most engines torque curves will not be completely flat, but the same basic principle applies.

 

Manifold pressure is not constant at a constant throttle position. As the engine rpm increases, the cylinders will create a lower pressure. Greater pressure differential across the fixed throttle plate will mean the airflow will be greater across the throttle plate (unless you've reached maximum flow for the opening, then the air speed will try to go supersonic which opens a whole different can of worms).

If your throttle plate opening is fixed, the engine will use more fuel as rpm increases until you hit maximum air flow.

If the LOAD is static, then no, it will not use more fuel (well, your frictional losses will increase so there will be a slight decrease in efficiency), but cam timing and port shape will influence the actual numbers. An engine is most thermally efficient at peak torque, and torque peak is where the best economy lives. This is why a small engine running at peak torque will net better fuel mileage than a larger engine at lower rpm off peak torque.

To settle this - consider WOT as a fixed throttle setting, look at your fuel maps, and you'll see as rpm increases fuel consumption increases. At half throttle, fuel increases until flow is ultimately restricted by the throttle plate, then fuel remains constant because the ratio of fuel to air is also constant (in a relative sense, again depending on engine design, cooling system efficiency, port efficiency, cam timing, etc...)

So the real answer is, it all depends on a number of factors, and there isn't a simple generic answer unless and until you consider WOT as a fixed throttle position.

There are LOTS of games you can play with intake design, exhaust design, cam timing, and fueling to create special circumstances, and if you look at diesel engines - especially diesels designed to run generators - you'll learn a lot about how efficiency can be optimized for specific conditions.

 

That makes some sense, thanks Lance.

I guess, though, assuming a fixed throttle position, and even assuming you were in the same gear while you were accelerating, even though the engine would be using more gas as it revved up, mileage wouldn't be decreasing, would it? I mean the engine would be using more energy per unit of time, but would it also be using more energy per unit of distance? What about if you allowed the gear to increase as you accelerated? Would that make any difference? I wouldn't think so since the load would remain constant.

 

Efficiency is going to follow the torque curve. Peak efficiency is at peak torque, so your Brake Specific Fuel Consumption (BSFC) is going to change over the rpm range. If there are unharmonious elements - cam timing and port shaping creating reversion at either the intake or exhaust over a specific rpm range - efficiency will be very poor. If the cam timing is biased toward high rpm torque - lots of overlap - the engine will be horribly inefficient until the overlap timing starts working. If the exhaust port and exhaust manifold designs do not work well together there will be peaks and valleys in torque and peaks and valleys in efficiency mapping directly to the torque curve.

There are also volumetric efficiency concerns related to basic pumping losses which impact efficiency (and torque) which again map to port design (size and shape), cam timing, intake and exhaust design.

So the short answer is efficiency is going to change over the rpm range for sure even with everything else held constant.