With electronic fuel injection on cars, the ECU may elect to run a lower air to fuel ratio, that is, inject more fuel, than 14.7:1 under high speed, high load operation, and run “closed loop”, that is closer to 14.7:1 using input from the oxygen sensor, under low RPM and low load. That will introduce another difference in fuel consumption in the real world. Is the bike fuel injected or carbed? I don’t know how carburators work but it must be something similar.
It goes into heat, which is why your vehicle would overheat sooner than later if you went pedal to metal in a low gear.
If you’re a visual person, this graph may help. It’s a fuel consumption map, and will tell you how much *fuel *a particular engine uses to produce a certain amount of energy.
In this case, the graph gives fuel consumption in grams per kW-h as a function of engine speed (rpm) and torque (BMEP) for a VW TDI. The TDI is a diesel engine, not a gasoline engine, so there are some differences between it and your engine, but the basic shape of the map is similar enough for this excercise.
If you shift gears while maintaining a constant speed, you keep the power delivered to the wheels constant. What that means on the fuel consumption map is that you’re moving the engine operation from one point to another along on of the blue constant-HP lines. Shifting to a lower numerical gear will increase engine speed and reduce engine torque (keeping output power roughly the same), so downshifting moves you down and to the right on the fuel consumption map.
As you can see, moving down and to the right increases fuel comsumption for the same output energy. Moving down, in general, increases the amount of energy lost to friction. Moving right, in general, increases the amount of energy lost to pumping air into the engine. Both of these contribute to increased fuel requirement and lower mpg.
It really has to do with the efficiency of the engine at different RPM. If it was a simple matter of less work at lower engine speeds, they would make the gear ratio so low you would reach the maximum speed the engine could push the car at a much lower speed. In fact millage starts to fall off at some RPM. I have heard anecdotes of a 5 speed car getting better millage in fourth gear.
I’m surprised no one has brought up piston and other frictional losses, which increase somewhat non-linearly as a function of engine speed. Especially as this subject has probably been covered about a score of times prior.
Excellent analogy, I may have to borrow that in the future.
Which helps explain where some energy is going: wasteful turning of the gears.
Sitting parked with the engine running yields terrible MPG.
Another thing to consider is gearbox efficiency. High gears have more efficiency than low ones.
Torque as well, lower gears have much higher torque for extra acceleration.
This covers it pretty well. For those not familiar with the fundamentals of IC engines, a couple more details:
-engine power output is the product of torque and RPM. On zut’s plot, torque is on the vertical axis (as BMEP), and RPM is on the horizontal. Engines tend to have better efficiency at lower RPM due to reduced mechanical friction. They also tend to have better efficiency at high torque output because mechanical friction doesn’t increase linearly with torque output, and also (on gasoline engines) because there is reduced pumping loss due to a more open throttle plate. For a given steady vehicle speed (and therefore a constant power requirement), upshifting does both: it moves you to lower RPM (decreased engine friction) and it moves you to higher torque output, a win on both parameters. Same power output, better efficiency, ergo better MPG.
-In addition to cutting NO[sub]X[/sub] emissions, EGR improves engine efficiency by two mechanisms. One is reduced pumping losses: since your incoming mixture is now diluted by exhaust gas, you have to open the throttle plate more to get the same amount of oxygen and fuel in as you did before EGR. The other mechanism is reduced heat loss to the combustion chamber walls: the exhaust gas blended with the incoming fuel and air actually absorbs some of the heat from the burning fuel/air mix, reducing peak temperatures. Instead of warming up the bore and head, that thermal energy remains available for conversion to mechanical work during the expansion stroke.