Better MPG in higher gears, help me understand

When traveling at the same speed, a vehicle will get better miles per gallon in a higher gear. Recently I rode a Ducati Diavel which reports an instantaneous fuel consumption reading in MPG. Going 55MPH in 5th gear it would report 45MPG, downshifting to 4th, and adding more throttle, but maintaining 55MPH, it would then report 36MPG. (Under full throttle acceleration in 2nd gear, it would report 6MPG, but that’s just boasting.)

My problem is that I’m having a hard time understanding exactly why that is so. Given the same road and the same wind, the amount of energy required to move the bike+rider through the air at 55MPH should be constant. There is energy needed to overcome wind resistance, rolling resistance, and friction in the drive train. Why is more energy consumed in the lower gear? Where is that energy going? Somewhat more work is being done to spin the engine and transmission faster, but the rear wheel is spinning at the same speed at 55MPH and drag coefficient is the same, regardless of gear. Is 9MPG really being lost just to spinning the engine at 4200RPM instead of 3500RPM?

Yes, your last sentence is basically it. It takes more energy for the engine to turn at those speeds, and there are more pumping losses. The pumping losses are due to the fact that the engine is trying to suck more air volume in due to the higher RPM, and it’s fighting against the resistance caused by the throttle plate. Other things start offering more resistance too, such as the water and oil pumps spinning faster and pumping harder. The pistons need to reciprocate quicker. There’s probably many other losses.

The engine is doing more work. Work requires/expends energy.

Imagine you as the human engine, pedaling a bike in 1st gear everywhere, even up to fastish speeds that see you pedaling like a crazy person. Yeah, I think you can see how you’d be wasting energy.’

The engine is doing more work at higher RPM.

  1. The engine is an air pump
  2. The more air you put into the engine the more fuel is required to maintain a 14.7:1 (by weight) air fuel ratio.
  3. The engine pumps more air at 4500 RPM than it does at 3500 RPM

Make sense?

Or to put it another way - every time a cylinder fires a certain amount of fuel is consumed. The higher the rpm the more times the cylinders will fire and therefore more fuel will be used in the same amount of time.

Not sure if this is true. Of course at higher RPM’s more volume of air is drawn into the cylinders during a specific time interval but I believe if you are comparing the same vehicle speed at different gears the amount of air drawn in by weight will be almost the same since at a lower gear you wont have the throttle open as wide causing higher vacuum in the intake manifold. Of course that is assuming a friction-less system. In reality in a lower gear you will be drawing more air (by weight) but this is because you need to have your throttle open slightly more due to pumping losses, friction at higher engine speed etc.

In other words given two different engine speeds (4500 rpm and 3500 rpm) you may be drawing more volume of air at 4500 rpm but if manifold vacuum were high enough at 4500 rpm then you could be drawing in as much air by weight as you would at 3500 rpm with less manifold vacuum.

I am still working on getting my Master Tech’s status so please correct me if I am wrong.

Not exactly. The amount of fuel is metered based on the mass of air entering the cylinder. And the cylinder may always draw in the same volume of air, but it doesn’t always draw the same mass. When the throttle is closed, the engine is sucking against a large restriction, and fills the cylinders with a low-pressure charge…hence, less mass. When the throttle is wide open, there is very little pressure loss through the intake train, and each cylinder pulls in a much greater mass of air during every stroke.

A turbocharged engine makes this difference in the mass of the charge even more significant.

So does this compound the effect even more? 4th gear = throttle wider = greater mass of air so even more fuel burnt?

A rider has to open the throttle more to make the engine turn 4200 rpm than they do to make it spin at 3500 rpm… hence more fuel consumption. Isn’t it that simple?

That does make a lot of sense. I guess I was only thinking about the added work from spinning things faster, not the volume of air being pumped around. So at the same speed, but more RPM, the engine is using extra gas to filter small particles out of the air, heat it up a bit, and shoot it out the tail pipe.

I guess the pumping air business also accounts for some of the difference in economy between different displacement engines. For example, a V6 3.7L Mustang is rated at 31MPG highway, and the V8 5.0L is rated at 25MPG highway. The power needed to move both cars at 55MPH is probably fairly similar, but the V8 is pumping a lot more air with it’s more cylinders and larger displacement, even though the road horsepower at a given speed is similar for both models.

This also makes a great deal of sense based on my empirical findings. I have a turbo charged car, the turbo doesn’t boost much until about 3000 RPM, so I get much better gas mileage around town if I shift around 2800 rather than 3200. I know once the boost comes on the computer is going to deliver even more fuel, even for a fraction of a second, and that does add up moving through all the gears. Yes, shifting sooner means slower starts means better mileage, but really a 500RPM consistent short shift can be the difference between 22MPG and 25MPG for me.

Backward. What you say is only true when reving the engine at rest. (no load) At a given road speed (power to wheel) wider throttle opening will be required to make the power at low rpm than at high rpm.

It is pumping loss, as aerodave explains. Throttling a spark ignition engine works partly by reducing the air/fuel present, but partly by loading the engine down via intake manifold vacuum.

In addition to taller gearing, you can reduce pumping loss by riding at higher altitude, or by replacing some of the intake air with exhaust gas. (Exhaust gas recirculation…EGR) EGR is done mainly to reduce NOx emissions, but has the happy side effect of improving fuel economy at mid power levels.

Yes, but my original question is, where is all of the extra energy going? The bike is still only going 55MPH, so no more work is obviously being done. It seems to be going to moving air through the engine, as well as the extra losses from friction for moving the engine and transmission faster. So the hidden work is mostly pumping air.

If an engine were running in a vacuum, either using a rocket fuel which contains oxygen, or with an O2 injector to go with the fuel injector, would it be closer in efficiency at different RPM, because there would only be friction losses and not pumping losses? I guess NASA should get to work on that internal combustion powered moon rover.

I am in Colorado. I always try to ride at higher altitude. I’ve yet to find a route that’s uphill both ways.

The Diavel’s trip computer is probably super dangerous, because the rider really needs to be watching the road, not a little blinking number. It was fascinating to play with though. The engine has 3 power settings, 100HP, 162HP slow response, and 162HP fast response. Unfortunately I wasn’t able to do any extensive testing for fuel economy between the different modes. I found I preferred 162HP fast response “sport” in all conditions, but if 100HP “urban” gives better economy it would probably be worth using for sedate riding, just to save a few dollars commuting in traffic.

The EGR does not improve gas mileage by reducing pumping loss. Instead what it does is it lets a little of the exhaust gas (which is now inert for the purposes of internal combustion) to take up some space in the cylinder so not as much “fresh air” (which has plenty of oxygen in it) can get in there. This means that since there is less oxygen in the cylinder than there would be under normal circumstances the ECU can dial back the fuel injector a little bit so you use less fuel.

What does sort of help reduce pumping loss is the PCV (Positive Crank Case Ventilation) valve. Although its main purpose is to reduce blow by gases from escaping the engine it does have the effect of putting a slight vacuum in the crankcase. This results in there being a slight vacuum on the underside of the piston which, in theory, would reduce pumping loss.

I don’t understand what you’re saying. Running a lower gear ratio means that your engine has to work harder as it takes more engine revolutions to spin the back wheel at the same speed. You’re trading torque for more top speed.

If you are a motorcyclist, then you are surely familiar with engine braking.

Imagine the difference in the force of the engine braking you experience when you completely let off the throttle at 4200 RPM vs 35000 RPM. Is it a difference of about 20%? That seems reasonable to me, on my BMW K1300S.

That corresponds exactly to your observed fuel economy numbers.

By the way, how did you like the Diavel? Is there any wind protection at speed to speak of?

Uh, 3500 RPM, rather.

I am, by the way, in Colorado too. Biggest thing that pisses me off about this state is that all engines without turbos are about 25% slower than at sea level.

In high school physics we learn that the drag force, resistance, of an object in motion in a medium increase proportionately to the SQUARE of the velocity.

For purposes of understanding why your fuel consumption went up, that would be the increased velocity of all the moving parts in your engine and transmission prior to the drive shaft.

I can’t help but notice that if I square the increase in RPM that you reported I would get 44% while the increase in fuel consumption is 53 %. Close enough for the explanation I’d say.

It’s clear to me that your reference to “bike” is a motorcycle. Anyone who ever rode a bicycle in varying gears would understand how much more energy goes into spinning the engine at high RPMs. That’s the whole point of offering you several gears.

I knew engine braking was related to pumping air, I guess I’d never put that together with higher RPM under power also being related to pumping air.

The bike was fantastic. Easy to handle at low speeds and a blast in the canyons. There was no wind protection to speak of. It’s not an upright riding position, so it’s not like a Triumph Scrambler, for example. They do have some little windshields available. At 60 it felt fine, but it was a calm day. And for something with a “touring” mode, the optional rear bags are about the size of pockets, in the salesman’s words. Best suggestion for touring: get a passenger with cargo pants.