RPM and MPG

[crokett]

I was out and about this morning and noticed the new van runs ~800 RPM less than the old at the same speed. That got me wondering, assuming the two engines were equally efficient and the vans weighed the same, would the different gearing mean a better MPG? I remember doing drills in cross country to lengthen my stride and therefore make me more efficient. I would think it would always take the same energy to move a given mass a given distance in a given time, no matter how fast the engine is spinning.

[LCHIEN]

assuming they are both the same number of cylinders....
the lower RPM for cruising should be better mileage.
Of course if the new van is 6 cyl and the old one 4 cyl then you might have a wash.

PS The output of gas motors is not flat torque, it increases with engine speed to a point then falls off drastically.
Theat's why gas autos and trucks have multispeed transmissions (trucks as many as 20-gear speeds) to operate atthe highest torque point.
If the same sized engine was propelling the same sized vehicle at the same speed comfortably (i.e. the motor not lugging-jerking from under torque) then the lower speed is more efficient.

With a limited number of speeds (usually four plus overdrive on modern cards), however the choice of fial axle ratio and tramnsmission gearing will set the RPMs for crusing at a different peed. As I said lower RPMs will mean lower fuel usage but the higher RPMs will give you more torque for quick acceleration and passing and uphil climbs. You may find the lower RPM vehicle downshifting to much higehr RPMs frequently on minor hills, passing etc. which will use more gas at that time. Choice of appropriate gearing of a vehicle is a tradeoff of driveability properties and mileage. For your new vehicle they were perhaps trying to get more mileage with gas prices at high levels.

[JimD]

Lower rpm = lower frictional losses if the size of the engine and the number of cylinders is the same. There are lots of other factors influencing mileage, however. The best mileage car I've ever owned was a little Ford Festiva - made by Kia. It has a 1200cc 4 cylinder and turned quite a few rpm on the highway. It was light, however, and fairly aerodynamic so it would regularly get over 40 mpg on the highway. The engine was little so the higher rpm was offset by the less contact area in the engine.

Jim

[crokett]

Both vans are 6cyl. Not sure the engine displacement on the new one. I should look that up. Old was a 3.3L. It seems counterintuitive that lower RPM would mean less gas used. It seems to me that a given mass should always take the same amount of energy to move a given distance in a given time.

[RayintheUK]

The difference is likely to be in the final drive ratio, so if at a given RPM you're getting a higher MPH, the engine is propelling the same vehicle faster for a given output. On long (mostly flat) journeys, that must equate to higher MPG. Downside is a possible (very slight) loss of torque, although I doubt thatyou'd notice it.

Ray

[cabinetman]

The difference is likely to be in the final drive ratio, so if at a given RPM you're getting a higher MPH, the engine is propelling the same vehicle faster for a given output. On long (mostly flat) journeys, that must equate to higher MPG. Downside is a possible (very slight) loss of torque, although I doubt thatyou'd notice it.

Ray

In addition to a difference in final drive ratios (axle), there may be a difference in the ratios of the equivalent gear ratios in the transmission for what gear is used for the test. For example, if the driven gear is the highest (like 5th on a 5 speed transmission), the rear axle ratios could be the same, but the vehicle getting lower RPM's might have a lower (smaller) transmission ratio in relation to the crankshaft speed.

Lower reciprocating engine RPM's in relation to the number of revolutions the rear wheels turn can be accomplished those ways. Another possibility for a difference in RPM's can be the rear wheel/tire diameter. A smaller diameter will yield higher engine RPM's.

What may play a part in the engines' efficiency as to MPG is when that difference allows the engine to move the vehicle the same speed at a much lower RPM it may be under a load which causes the engine to perform short of it's torque band, which could make it use more fuel. To exemplify this would be to say that around town in a standard shift vehicle if made to cruise at 15 MPH in high gear would stress the engine, whereas if shifted to a lower gear the engine would be operating at a higher RPM, and likely more efficient.

I stayed in a Holiday Inn Express earlier this year.
.

[mpc]

It seems counterintuitive that lower RPM would mean less gas used. It seems to me that a given mass should always take the same amount of energy to move a given distance in a given time.

Ignoring friction effects, mass determines how much power it takes to accelerate the vehicle at some rate, not the power to maintain a given speed. Friction just means it takes even more power to accelerate at that rate, and then it takes some power to maintain a constant speed due to friction losses.

Rotating engine, transmission, and driveline parts all have some friction. Friction losses generally build up with speed or RPM... so selecting the gear ratios in the tranny and final drive (differential) to reduce RPMs through the system will reduce friction and thus increase MPG. Aerodynamic drag (which goes up as the square of vehicle speed) of course plays a huge role too - more than mechanical friction typically at highway speeds. The side effect of gearing to get engine RPMs low at cruise speeds is usually less crisp acceleration. Most gas engines don't make a whole lot of power at low RPMs so, when you mash the gas to accelerate and pass... not much happens. If the transmission is programmed to downshift quickly - letting the engine RPMs quickly climb to the "meat of the powerband" then the vehicle will accelerate crisply.

Friction sources:
* drag of any bearing inside the engine: crankshaft support bearings, connecting rod bearings, wrist pins, friction on the camshaft to rocker arms or valve tappets, piston rings, etc.

* Drag of oil seals in the engine (rubber bits around the crankshaft, camshaft, etc to keep oil "inside" the engine... these rubber bits rub the rotating parts creating friction.

* All of the bearings inside the transmission and differential. And the oil seals in those as well.

* On auto tranny cars, the hydraulic pump of the transmission sucks up some engine power.

* "parasitic drag" from things bolted to the engine: alternator, water pump, power steering pump, air conditioning compressor, etc. Some auto shops sell "underdrive" pullies to replace the V-belt or serpentine belt pully on the crankshaft with a smaller diameter one; this leads to the water pump, power steering pump, etc. running at lower RPMs than the original design to reduce this parasitic drag. Of course there's the risk of turning them too slowly so they can't do their jobs!

Reducing engine RPMs reduces the friction of everything listed above. Other friction sources that don't vary with engine RPM:

* Aerodynamic drag - pushing the air out of the way (front of vehicle) and then dealing with the "wake" behind the vehicle since the air can't quickly recover.

* Flexing of the tires --> rolling resistance. This is why underinflated tires cost MPG. As the sidewalls flex, they absorb energy which manifests itself as friction. Of course there is the plain friction of the rubber on the road too - it takes some power to "lift" the rubber off the road at the backside of the contact patch. High MPG cars typically have skinny tires to minimize the contact patch size... and run high air pressures to minimize flexing.

Vehicle weight increases the friction of the suspension bearings and the tire flexing... so sticking a small engine in a heavy vehicle will get you better MPG but still not as good as that same engine in a lighter vehicle - even if the aerodynamic friction is identical.

mpc

[crokett]

Ignoring friction effects, mass determines how much power it takes to accelerate the vehicle at some rate, not the power to maintain a given speed. Friction just means it takes even more power to accelerate at that rate, and then it takes some power to maintain a constant speed due to friction losses.

Ahhh... so assuming no losses due to friction (or gravity) once the mass is accelerated to speed it will stay there forever with no addtional thrust? That makes sense.

[LCHIEN]

Ahhh... so assuming no losses due to friction (or gravity) once the mass is accelerated to speed it will stay there forever with no addtional thrust? That makes sense.

That's Newton's First law, also called the Law of Intertia.