Engine Size x MPH = Fuel Economy?


#1

I know this was discussed on the show, problem is like some of you, I listened for 30 years or so… I did try searching the forums, but maybe not searching right, came up empty. (pun acknowledged, not intended.) My Dad, (who turns 90 in three weeks) claims that with a bigger engine (a V8, vs, a nice little 4 cylinder) at a certain point you get better mileage going 70 MPH on the highway than 55.

To me, this doesn’t make sense–fuel burns to produce momentum… the more momentum, the more fuel–and in fact that bigger V8 engine come with a couple thousand pounds more vehicle to push along, so I think going 55 MPH will get me better mileage than going 75.

??

(for the record, new-used '02 Suburban Beast trying to eek every mile I can out of that gas tank… :smirk: )


#2

You are correct for 99.9% of cars. With modern computer controlled fuel injection, that number is more like 100%. The only exception might be an old, carburetor-equipped car that had a bad combination of carburetor jetting and transmission gearing. I’ve heard of this happening, never seen it.


#3

I remember having this discussion with my grandpa - he drove V8’s for 50 years. His thinking was that the engine was more efficient at faster speeds ‘cause it was doing the amount of work it was designed for and therefore running at its peak performance level, which he inferred would also be the best MPG. Of course, he never tried running 55 mph for a full tank to test it.
55 mph will be much better than 70, and in all honesty, with a big ole’ Texas station wagon like that, much, much safer.


#4

Someone buys a Suburban and is concerned about fuel mileage. OK fine.


#5

All I can say is…eek!
:wink:


#6

Hahaha!!! I actually need it to tow… TRUST ME, the goal is as soon as I can to get a nice little pregnant rollerskate for the daily stuff. I don’t think ANYONE gets a full sized Suburban if they don’t have to. Have you tried to park one these suckers lately? :grimacing:

So far, she seems to be averaging around 16.5-17mpg. I regularly (like 2-3x/mo) do a 300 mile highway trip though without the trailer, so…


#7

Much as I don’t like it, the upcoming fuel efficiency standards have all manufacturers downsizing their engine displacement. For the first time in history you can buy a full size pickup truck with a 4 cylinder engine.

It’s a trade off between internal friction and increasing the specific horsepower per cubic inch. Hyundai will not even offer a V6 in their non luxury autos!

However engine life is being sacrificed on the efficiency altar! I doubt if a turbo 4 in an F150 Ford truck will get to 300,000 miles without a complete overhaul!


#8

Actually, a large-displacement engnie that is appropriately cammed for low RPM torque, can get surprisingly good MPG numbers. The LS1 and the 3800 V6 come to mind as engines that out-MPG smaller DOHC engines, when moving a similar load.

Look at trucking, where MPG isn’t everything…it’s the only thing. LARGE displacement for the HP they make, even for Diesel engines. That’s because low RPM/large engine tends to outdo high RPM/small engine in fuel efficiency.


#9

Basically an engine is an air pump. With the air it sucks into the intake and out the exhaust, there is a percentage of fuel injected into that air stream. So, more air, more fuel. The amount of air pumped is a function of displacement X engine RPM X MAP (manifold air pressure) or throttle opening. A large engine geared for low RPM may not pump more air than a small engine geared for high RPM.

Now there are other factors such as efficiency. With mechanical valve actuation, i.e cam and lifters, there is a range of engine speed that is most efficient. Operating outside that range can reduce efficiency. Worse case could be operating a vehicle at a very low speed, i.e 20 mph where the vehicle has to either use a lower gear ratio or operate well off the cam profile. It would get lower fuel mileage under these conditions that at faster speeds, say 45 mph.

Another factor is drag. There is drag from internal friction which is pretty linear. Then there is aerodynamic drag which is non linear. The drag increases proportionally faster that the speed. The drag at 80 mph could be 4 times the drag at 40 mph for example. So going 75 mph is going to use much more fuel than 55 mph, all else being equal.

There was a case of one guy here who got better mileage at 70 mph than at 55, but he was driving a Lincoln Mk8 which lowered itself by 2" at 65 mph, thereby lowering its aerodynamic drag.


#10

Another factor that can lower engine efficiency at low rpms is greater heat loss to the combustion chamber walls during the power stroke.
There’s more time for that to happen at low engine speed.


#11

Gearing can make a difference of course. Some of the newer cars are geared so high that if you’re going 55 mph the trans downshifts on almost any significant hill. My mother-in-law’s Malibu turns 2K RPM at 70 mph, and I have to do at least 75 to avoid downshifts on the hills.


#12

Yeah, but balancing that out is the fact that “low RPM” engines tend to be big-cube engines, and by the “square/cube rule,” tend to have less surface area per unit of volume than a smaller, higher-RPM engine.

(“Square/cube rule” refers to the fact that, as you increase the size of an engine proportionally, the volume grows by the cube, but area only by the square. If you took a cylinder, and doubled all of its dimensions, it would have 8 times the volume, but only 4 times the surface area.)


#13

Going 75 may avoid downshifts, but I’d be very surprised if it saved gas.


#14

And all this time I thought that was what automatic transmissions were supposed to do.


#15

The guy with the Lincoln Mark which gets better mileage at 70 compared to 55 would be me; and it’s been checked repeatedly over the past 15 or so years.

I don’t consider the car lowering itself to be the main factor. I think it’s more a matter of the engine lugging at 55 compared to the RPMs at 70. At 55 MPH the engine is only turning about 1200 or 1300 RPMs and it feels doggy. I have to run 80 to hit 2000 RPM and the best part of the power band is still higher.

My old BMW R100/7 was the same way. At 55 it was a pig and it took 70 or 75 MPH before the engine started breathing better and eliminating the slightly sluggish feeling.


#16

Low volumetric efficiency doesn’t necessarily mean low thermodynamic efficiency. For example, the Prius reduces power by “mistiming” the intake cam rather than closing the throttle, the late closing of the intake valve allows the piston to blow excess fuel air mixture back into the intake manifold. Reducing volumetric efficiency this way rather than simply closing the throttle more is a less parasitic method of lowering the air the engine pumps for reduced power output.
The Prius still uses a butterfly throttle mostly because there is a limit to how much the power can be reduced by cam phasing. You reach a point where the spark plug fires while the intake valve is still open.


#17

With dual cams, 4 valves per cylinder and variable cam timing, the range of efficient operation has been widened quite a bit over the older 2 valve, fixed cam engines. But it still has its limits.


#18

The concept of throttle-less throttling of engines goes all the way back to 1849 when George Corliss patented the Corliss steam engine. This engine used variable intlet valve timing instead of a steam throttle to control the engine’s power.

Around 1:28 in this video, you get a closeup of the trip cams that let the inlet valves snap shut for steam shutoff. This also makes the characteristic “tock-tock” noise of a Corliss engine.


#19

This is Toyota’s approach. Sometimes you have to step back and look at things differently.


#20

Apples vs. oranges, but in 1999 we got plowed into by a man who was probably on his cell phone. We rented a big Buick le Sabre to the Snow zone, and at 70 mph it got over 33 mpg.