I don’t understand why large-displacement engines are more efficient than small-displacement ones. The gasoline contains the same amount of energy when it gets burned, after all, and neither engine is spewing loads of unburned hydrocarbons out of the exhaust ports.
Is it that the ratio of combustion chamber volume to the frictional surface area of the piston rings is smaller in a large-bore engine? (That is, more cubic inches to square inches.)
This has puzzled me for years. Any book recommendations would also be appreciated.
What is your definition of “large” and “small” displacement? A “small” (say, 1-liter) gas engine can be made extremely efficient.
Look up “Carnot efficiency” and you’ll find plenty of material to answer your question. The main factor, after making the combustion process itself as efficient as possible (burning the fuel completely) is thermal efficiency. More work can be extracted from the burned fuel if the engine operating temp can be made as high as possible. This is why insulating small engines improves their efficiency.
The ideal internal combustion engine would be made to run at temps that would melt metal. Unfortunately, it’s difficult to make an engine out of super-high-temp materials like ceramics. But a ceramic engine would be superior in terms of efficiency because it could be run at higher temps, and thus higher Carnot efficiency.
An engine is more efficient the more open the throttle is - and a small engine operating at wide open throttle would be more efficient than a large one throttled back to produce the same HP. It’s just that people don’t like the lack of power from small engines.
Less quench area per volume,higher torque at lower speeds to move the appropiate load.the manus have a time with this apparently,a Crown Vic can achieve close to 30 mpg with a mid size V8,while my v6 Dakota struggles to achieve 20 mpg at cruise,there are other factors,but straining an undersize engine to achieve effiecency rarely works(the results are dramatic on over the road trucks also)-Kevin
It’s simple: Larger displacement engines do not have to work as hard to pull the same load as a 4 cylinder or weak 6 cylinder engine. A 4 cylinder or weak 6 cylinder engine in a truck that has to work is a no-no for me.
Scale effect mostly.
When you double the dimensions of a cylinder, you increase the volume by a factor of eight but the surface area only quadruples.
An engine turns heat into mechanical energy by allowing the hot gasses to expand. This expansion lowers the temperature of the gasses, the heat being converted into mechanical energy. Heat that escapes from the combustion chambers to the engine parts to be removed by the cooling system is heat that the engine will never be able to convert into mechanical energy. The less surface area, the less heat escapes and more heat gets turned into mechanical energy.
Wartsila-Sulzer makes a ship engine with a bore of 38 inches and a stroke of 98 inches that achieves a brake specific fuel consumption of .278 pounds per horsepower-hour at full power and .260 pounds per horsepower hour at maximum economy. Most auto engines have a BSFC figure somewhere in the .4 to .6 lb./hp.-hr. range.
@ B.L.E.: So it comes down to the ratio of volume to surface area, then. That makes sense to me, as does your description of the “use” of waste heat.
But jesmed1’s assertion that “The ideal internal combustion engine would be made to run at temps that would melt metal.” does not seem consistent with this. Moreover, jesmed1’s assertion seems to imply that a hotter-running engine would be more efficient, yet the energy cost of raising the engine temp must come from somewhere.
Perhaps I’ve been misinformed all my days. [shrug] I was told that motorcycle engines are less efficient, as evidenced by how hot they run. But they rev WAY higher, and are burning the same petrol.
The 2008 Susuki GSXR 1300 makes 194 HP with a 1298CC engine, whereas the 2015 Civic makes 143 from a 1798CC engine. The F650 makes 362HP with 6,800CC, or 0.0532 HP per CC. The aforementioned 'busa yields 0.1515 HP/CC making it 300% more efficient. Curious.
Aside from the volume/surface area ratio being different, why would any given engine type capture more combustion energy?
The 2008 Susuki GSXR 1300 makes 194 HP with a 1298CC engine, whereas the 2015 Civic makes 143 from a 1798CC engine. The F650 makes 362HP with 6,800CC, or 0.0532 HP per CC. The aforementioned 'busa yields 0.1515 HP/CC making it 300% more efficient. Curious.
@Neutrino
Don’t confuse horsepower per cubic inch with efficiency.
How many gallons per hour is that GSXR burning while it’s producing 194 horsepower?
A top fuel dragster puts out around 19 horsepower per cubic inch but I can’t imagine a more inefficient engine with a fuel consumption measured in gallons per second.
Also don’t confuse gas mileage with engine efficiency.
Your engine will operate at maximum efficiency at wide open throttle. However, as you realize, that’s not where maximum gas mileage occurs.
Well, not all small-displacement engines are more efficient; generally, it works the other way around. It all comes down to how the engine is designed, though.
If the small-displacement engine produces high power through the use of a turbo, then it may be less efficient. Turbos (on gas engines especially) tend to burn more fuel at full power than NA. Turbos (like the Ecoboost) score decent MPGs IF they are used lightly; staying out of the boost, they’re essentially an unboosted small engine. Once you run them hard enough to stay boosted, efficiency drops.
You see this, especially in race-oriented engines, that a LOT of design compromises were made to make max HP/cc, at the cost of everything else, including economy. I’m talking about lumpy cams and similar. For instance, a sportbike might have a heavily-tweaked 400cc (to meet a racing spec), when a real-world 650cc might be more roadable, and more effecient.
I understand how MPG and engine efficiency are not the same thing and cannot be equated. But I don’t quite get how harnessing more power from the same displacement is not.
Would not these two (any two) engines be burning the same mix, thus have the same energy density, at whatever is the maximally efficient RPM? It seems that, if they’re both at a 17:1 ratio (air/gas), then netting more power per combustion unit volume would be more efficient.
But then, one is revving to 9,500 and the other to 2,600. Hmmm . . . Assuming a 17:1 ratio, the 1298 CC engine is burning 9,891,153 units of fuel per hour; the 6800 CC engine uses 1,040,000. Fascinating. The larger engine is making 1.86 times the HP of the smaller one, but is using close to 1/10 the fuel. Hmmmmm . . .
Yes, you’ve confused ‘specific output’ (hp/cc) with ‘efficiency’ (power x time per unit of fuel). Motorcycles can be very inefficient when tuned for high hp, just like a race car engine can be. You’ll want to go to wiki and read about efficiency.
I don’t get the comments on big engines getting better mpgs. At least in cars, with a given technology smaller engines always do better.
I can pretty much guarantee that the 1298 cc motorcycle engine is running considerably richer than 17:1 air/fuel ratio at full throttle, most likely close to 12.8:1. Most auto and motorcycle engines have fuel delivery calibrated to run rich at full throttle and lean at cruise, that’s why full throttle is not necessarily an engine’s most efficient operating point.
There are two reasons for the rich fuel mixture, one is cooling, the surplus fuel helps the engine survive when it’s burning close to 16 gallons/hour making 194 horsepower. The other is the fact that a rich fuel/air mixture has a higher octane rating than a lean one does. Engine designers take advantage of this. Since full throttle is something that most cars and motorcycle engines only briefly visit, it makes sense to optimize the compression ratio for part throttle cruise for maximum cruise fuel efficiency and then relying on a rich mixture to allow the engine to run full throttle without detonation.
Av-gas pumps even display the lean and rich octane numbers for the fuel for example 100/130. The lower number is the lean mixture octane rating and the high number is the rich mixture octane rating. At full power, the air fuel mixture needs to be rich but at cruise running at 70% full power, the air/fuel mixture can be leaned out. If the engine had the compression ratio lowered to run lean at full power, the engine would have a needlessly low compression ratio for high altitude cruising and it would result in higher overall fuel consumption.
“But jesmed1’s assertion that “The ideal internal combustion engine would be made to run at temps that would melt metal.” does not seem consistent with this. Moreover, jesmed1’s assertion seems to imply that a hotter-running engine would be more efficient, yet the energy cost of raising the engine temp must come from somewhere.”
It may seem paradoxical, but it is true that higher engine temps increase efficiency. This is why a stuck open thermostat that causes your engine to run cold will reduce efficiency and MPG. This is why truckers cover their radiator grills in winter. And if you research ceramic engine technology, you’ll find that, in fact, engine efficiency does improve at higher temps.
The Tour de Force of engine efficiency was Honda’s 250cc six-cylinder motorcycle racing engine. Honda wanted to prove his 4-stroke engines could outperform any 2-stroke design and he did…
To get a feel for the future, take a spin in a Chevy Cruze with the 1.4LT engine…Try the 2.0L turbo-diesel too…These cars get 45-50 mpg and their acceleration will put a big grin on your face…
Large displacement engines have a very high reciprocating mass which destroys efficiency. Their large surface area and exhaust gas volume pour the BTU’s out into the surrounding environment…Efficiency is Gone With The Wind…
The Tour de Force of engine efficiency was Honda's 250cc six-cylinder motorcycle racing engine. Honda wanted to prove his 4-stroke engines could outperform any 2-stroke design and he did....
I don't know, what kind of mpg did the Honda RC166 engine get, or how did the gallons per hour compare with that of an engine that used cubic inches rather than revving like a model airplane engine to achieve the same horsepower?
Back (many years ago) in my internal engines course, we compared an .049 2-stroke engine to an engine on a ship (where the stroke was 2 stories high and you could park a car on the piston).
We were given this exercise to see how close the efficiency the two engines were. And we were quite surprised to see similarities in both power output per unit of volume and brake specific fuel consumption when we normalized the engine sizes.
