Efficiency and engine RPM are not directly related, meaning that a 25% RPM reduction does NOT predict an efficiency improvement of “x%”, nor does it predict any improvement at all – although RPM reduction will generally improve efficiency.
Darin’s data proves this out. At first you get a 25% increase in RPM, but only a 15% MPG change (about 50% of the RPM change percentage). Later he tests with a 48% RPM increase, but sees a 60% reduction in MPG (150% of the RPM change percentage).
The efficiency of an internal combustion engine is largely infludnced by operating RPM, BUT there is one “sweet spot”, typically at or near “Peak Tq” that will return the best efficiency.
Incidentally, I’ve seen evidence of the “sweet spot” in regular driving (without attempting to test for it properly – though it’s on my list): when i switch into top gear, my “instant” fuel economy often continues to improve for a short while as I gradually increase RPM and build up speed.
Not only does that show MPG is not directly related to RPM, as you point out, it’s also evidence that fuel efficiency will be worse if RPM is kept too low.
You really have done a fine job of documenting everyting as you “test”, and I enjoy reading your updates.
I had a Bronco that got better mileage at 60 MPH than it ever did at 55 MPH, which certainly does not make sense based on the fact that the air drag at 60 is 112% of that at 55 mph, and the RPM is 10% higher as well.
Hi JohnG. Maybe the extra speed makes up for sundry losses like internal engine and g/box friction, A/C and alternator which are less car speed dependent. There should be a single speed which is most efficient in a given gear against a given incline, and for small inclines it probably is around 55 or 60 MPH, I suppose that’s why manufacturers choose this speed for fuel efficiency measurements
Engine friction is proportional to engine speed (RPM), so a 20% increase in RPM will result in more friction, the same goes for the drivetrain – anything with rotating mass really. That does not factor in the additional friction needed to overcome increased aerodynamic drag (the more Tq you need, the more friction in the trans. rear-end gears etc).
The EPA sets the test criterian for city and highway economy, and they do not do a very good job compared to Eastern European countries either.
Re: Darin’s MPG experiments….
Efficiency and engine RPM are not directly related, meaning that a 25% RPM reduction does NOT predict an efficiency improvement of “x%”, nor does it predict any improvement at all – although RPM reduction will generally improve efficiency.
Darin’s data proves this out. At first you get a 25% increase in RPM, but only a 15% MPG change (about 50% of the RPM change percentage). Later he tests with a 48% RPM increase, but sees a 60% reduction in MPG (150% of the RPM change percentage).
The efficiency of an internal combustion engine is largely infludnced by operating RPM, BUT there is one “sweet spot”, typically at or near “Peak Tq” that will return the best efficiency.
Thanks for the comments, JohnG.
Incidentally, I’ve seen evidence of the “sweet spot” in regular driving (without attempting to test for it properly – though it’s on my list): when i switch into top gear, my “instant” fuel economy often continues to improve for a short while as I gradually increase RPM and build up speed.
Not only does that show MPG is not directly related to RPM, as you point out, it’s also evidence that fuel efficiency will be worse if RPM is kept too low.
I’ll update the RPM/MPG page. Thanks – Darin.
Darin,
You really have done a fine job of documenting everyting as you “test”, and I enjoy reading your updates.
I had a Bronco that got better mileage at 60 MPH than it ever did at 55 MPH, which certainly does not make sense based on the fact that the air drag at 60 is 112% of that at 55 mph, and the RPM is 10% higher as well.
Hi JohnG. Maybe the extra speed makes up for sundry losses like internal engine and g/box friction, A/C and alternator which are less car speed dependent. There should be a single speed which is most efficient in a given gear against a given incline, and for small inclines it probably is around 55 or 60 MPH, I suppose that’s why manufacturers choose this speed for fuel efficiency measurements
James,
Engine friction is proportional to engine speed (RPM), so a 20% increase in RPM will result in more friction, the same goes for the drivetrain – anything with rotating mass really. That does not factor in the additional friction needed to overcome increased aerodynamic drag (the more Tq you need, the more friction in the trans. rear-end gears etc).
The EPA sets the test criterian for city and highway economy, and they do not do a very good job compared to Eastern European countries either.