Why are big engines so horribly inefficient?

[brewnog]

Russ got this one covered in post 5. In thermodynamic terms, bigger engines are more efficient. But if too large an engine is specified for the average duty cycle being asked of it, you won't be running as close to peak efficiency as a smaller engine, and a smaller engine will outperform in fuel economy.

The 455 in my Camino is still drawing 455 cubic inches of air/fuel mix per cycle. At idle, of course, it doesn't cycle as many times per minute as it does at 140 kph on the highway. It idles at more or less the same speed as a 273 slant six, which at idle draws 273 cubic inches of mix per cycle.
Aside from variable displacement engines, the swept volume doesn't change.

I think the others have just about sorted this out with semantics about volume, but at idle, your engine will be drawing in less fuel and air than when it's running balls-to-the-wall. This isn't just because of the engine speed; your Camino could be doing (say) 50 down the motorway in top gear, still at engine idle speed, but it will be using more fuel (and air) than it was when idling at the traffic lights because the throttle will be further open. (I don't think you were disputing this, Danger, but I thought I'd make it clear for some others.)

By the way, the same goes for a Diesel engine guys; it doesn't have a throttle but the engine definitely isn't using as much fuel and air at idle as it is on load. Interestingly though, the air mass flow rate for a naturally aspirated Diesel engine is reasonably proportional with engine speed, regardless of load, unlike a gasoline/petrol engine, because it doesn't need to maintain an overall air/fuel ratio in the cylinder.

A quote from this Wiki article would seem to indicate that size doesn't make that much difference, as most of the power losses aren't friction related:

"Most internal combustion engines waste about 36% of the energy in gasoline as heat lost to the cooling system and another 38% through the exhaust. The rest, about 6%, is lost to friction."

The heat lost to the cooling system is highly dependant on engine size; largely because the surface area to volume ratio decreases with increasing cylinder displacement. The heat loss is (nominally) a factor of surface area, power (nominally) of displacement.

 

[Averagesupernova]

Thank you Brewnog!

 

[Aether]

Fuel consumed versus time yes, but not fuel consumed versus distance. If a vehicle has double the acceleration, it reaches cruise velocity in 1/2 the time. Take the simple case where acceleration is constant in both cases, and the average velocity is 1/2 the cruise velocity, which is the same in both cases. So if acceleration is doubled, then only 1/2 the distance is covered in that 1/2 of the time (with constant acceleration). Then more fuel is consumed in order to reach the same distance that the slower accelerating vechicle took to reach cruise speed.

I see your point, but this doesn't have anything to do with engine efficiency; it has to do with assumptions that you are making about what the drivers of these vehicles will do after the initial acceleration. For example, if in your example above both engines are switched off after attaining cruising speed, and then both vehicles coast to a simultaneous stop at the same finish line, then the vehicle with the faster acceleration will have used less fuel.

 

[Aether]

The heat lost to the cooling system is highly dependant on engine size; largely because the surface area to volume ratio decreases with increasing cylinder displacement. The heat loss is (nominally) a factor of surface area, power (nominally) of displacement.

At any given speed, a higher surface area to volume ratio for a lower displacement engine leads directly to more cylinder heat loss. However, if this lower displacement engine is operated at a proportionally higher speed (e.g., maintaining the same output power) then it isn't necessarily true that a higher fraction of the fuel energy will be lost to the cooling system.

 

[Danger]

I think the others have just about sorted this out with semantics about volume, but at idle, your engine will be drawing in less fuel and air than when it's running balls-to-the-wall. This isn't just because of the engine speed; your Camino could be doing (say) 50 down the motorway in top gear, still at engine idle speed, but it will be using more fuel (and air) than it was when idling at the traffic lights because the throttle will be further open. (I don't think you were disputing this, Danger, but I thought I'd make it clear for some others.)

Yeah, I think that there's a major communications failure here. Thanks for helping to sort it out. In the very simplest terms, I'm going to point out what I was talking about.
1) The swept-volume of an engine does not change unless it's a variable displacement unit such as some of the new hemis and other such-like; if it's 350 ci at idle or not running at all, it will still be 350 ci at 8,000 rpm. (This is assuming no rod stretch, for the nitpickers.)
2) The volumetric effectiveness does change with speed. If the valve train, intake runners, and exhaust system can't breathe as much as the cylinders want, you have to add more throttle to try and compensate. That's why the 60's hemis were blowing everything away--those man-hole size ports. Likewise why so many engines now have multiple valves per cylinder. Where I misinterpreted Average and Jeff was in thinking that they were trying to imply that small engines are not subject to that whereas large ones are. My fault for not reading more closely.
As for the thermodynamics information that Russ put up, I really don't understand anything with that many numbers in it. I know that he knows what he's talking about, though, so I'll just take it at face value.