Philip Stevens wrote: I'm trying to understand fuel economy and engine efficiency. Does an engine run most efficiently at maximum torque, or maximum horsepower, or where? Also, is the point at which an engine runs most efficiently also the point of maximum fuel economy? I've also been told that manifold vacuum is related in some way to fuel economy. Anyone care to elaborate?
Rad Davis answers: Ah, a technical question! Just what I like.
An engine (no accessories), running at full throttle (we're ignoring power valves and other enrichment gadgets) has its lowest fuel consumption/unit power produced at the torque peak. The horsepower peak is a mathematical artifact of increasing RPM coupled with decreasing efficiency, and is not usually a good place to run for max. economy. Unfortunately, unless you're running a generator, or pump, or something like that, it's not a particularly relevant value, because automobile drivers *do* have engine accessories and power valves (late corvairs, anyway) and *don't* spend much time at full throttle at the torque peak.
OK, now a little quick theory: The reason that economy (efficiency) maximizes at the torque peak is because this is the place where the engine is inhaling the greatest amount of fuel and air into the cylinder which, when burned, makes the peak amount of cylinder pressure and thus the maximum torque.
So the instant you close the throttle some, you're losing efficiency and your fuel economy (expressed per unit power output) goes down. Again, if you're generating electricity or pumping water, this is a bad thing.
But cars are in a variable load environment. You may be going downhill at 60 mph, and only need 1/2 hp to maintain speed. So the engine is horribly inefficient in producing this 1/2 hp, but you're demanding so little compared to what the engine can make that your economy (expressed per unit distance traveled (like MPG)) is extremely high.
If you then complicate things by including parasitic power losses to the cooling fan, vacuum advance, power enrichment, and aerodynamic drag, things get pretty murky, at least from a theoretical standpoint. The good news is that we can make some emperical generalizations:
Engine vacuum is a good approximation of just how much fuel and air is going into the engine. Vacuum is near zero at full throttle. Higher vacuum=less fuel. So if you use the vacuum gauge as a guide, less is better.
Where this falls down is when it runs into situations when the engine *can* be steady-state loaded near full throttle and the torque peak. In our Pontiac, this would be pulling a trailer climbing Mount Everest at 60 mph in top gear. Fortunately, Corvairs have smaller engines relative to their weight, and weigh less than that Tin Indian did, too.
So if you take something like my Greenbrier (Corvair minivan, if you don't know), and drive down the road with a low rearend ratio (3.27:1) and tall van tires at 65 MPH, you're either at about 60% throttle or you're slowing down. Climbing even a 6% hill on an interstate you get up to 75% throttle or so. So you're in a situation where the engine (as a black box) is starting to be very efficient. Sure enough, the Greenbrier never gets less than 20 MPG on interstate trips, and set a record of 28 MPG driving through the mountains of Tennessee in heavy traffic. By the vacuum gauge, the fuel economy should suck, but in practice, it's better than most turbo corvairs can manage, even though the van is heavier and less aerodynamic.
This is the secret of cars like the Geo (now Chevy) Metro--it doesn't weigh anything, has no frontal area, and has a tiny engine (1 litre). So the little engine still has to work quite hard to drag even such a light car around, and is working very efficiently most of the time. Result: 54 MPG.