Make mine another vote for Caddyman. Larger engines are not more efficient, they simply have the potential to produce more power. Many do, some do not. There are countless other factors involved, such as reciprocating masses, bore vs. stroke, compression ratio, and on and on.
I might point out in response to the comments that larger engines have to work less that that statement is not necessarily true either. It takes a lot of energy to keep whipping those pistons up and down, and to compress all that fuel mix, thousands of times a minute. A well designed, well balanced 4-banger will typically get better mileage than a equally well designed well balanced larger engine in the same vehicle because the larger engine uses more power just running the engine. This is very easy to verify simply by looking at EPA ratings for cars with optional engines. The smaller engine will typically have higher EPA ratings, sometimes significantly.
This thread's title line makes the statement that large engines are more efficient than small engines. This is simply not a true statement, especially in the automotive world...
That’s absurd.
Brake HP Specific Fuel is a known metric for any well-known engine, and the winners are ALWAYS big lumbering beasts, that allow enough room to have a tea party inside a cylinder and redline at 150rpm.
As I’ve said before, a shipbuilder could get sufficient power from 100 SBCs bolted end-to-end…could free up some space and weight, too. They DON’T do it because it’s LESS efficient.
That inline-6, 250cc Honda engine? What kind of real-world MPG would it have gotten? Thumper 650-ish mills generally beat the 250cc sport bikes as the pump.
I dunno why it bothers me when folks put out factually incorrect data, but it does. Google BSFC, snd you’ll see what sort of engines REALLY rule.
When you move up into huge motor vehicles, fuel consumption is just one of many factors that need to be considered. Reliability and service life, overall operating cost along with manufacturing cost all get considered…
The huge diesels that power cargo ships are indeed very efficient but a 2 liter common rail turbocharged diesel optimized to run at 1800 RPM is even MORE efficient…It produces more WORK per gallon of fuel than the monster ship engine simply because it wastes less heat…
"You would not sneeze and steer with one of these."
I think that says it all. The Yaris is much more aerodynamic then the Tecel ? It was like driving a cardboard box on a windy day…and just as much insulation. We tried one out for my daughter. Three miles down the road at 65 and back to the dealer we went.
Exactly @same.
You will do better towing econonmy wise if you work them hard with the bigger motor. But, drive the vehicle within it’s limits and normal commuting ? Smaller motors have always been more economical for me. It all depends on how you drive and what you are asking of it.i can tell you this for experience. When I towed my pontoon boat with my four cylinder truck, it got twenty percent worse mileage then towing the same load with a 6cylinder SUV. We had both at the same time. Driving back and forth to work, the four cylinder truck got 24 mpg while the 6 cylinder SUV , 21 mpg. Not towing ? Advantage; smaller motor.towing; advantage larger motor.
“That inline-6, 250cc Honda engine? What kind of real-world MPG would it have gotten? Thumper 650-ish mills generally beat the 250cc sport bikes as the pump.”
My 250cc V-twin got 70+mpg all the time. Fuelly reports a 650cc Suzuki Savage thumper at 50 to 60 mpg. Both about 30hp, both weigh 380 lb. We can all find examples and parse words to prove our points.
“The huge diesels that power cargo ships are indeed very efficient but a 2 liter common rail turbocharged diesel optimized to run at 1800 RPM is even MORE efficient”
Um…no. The big diesel can exceed 50% thermal efficiency. No road-going engine can top that.
If we’re talking only about engines, the lion’s share of the energy loss is in the exhaust heat. If there was an economical and practical way to extract all that waste heat, the efficiency improvement would be huge, but no one has cracked that problem yet in a practical passenger car.
59% thermal efficiency…This is a long, long ways from a 454 Chevy or a 440 Dodge or 460 Ford, which on there BEST day can get maybe 22-23% thermal efficiency…
(Though, I must say, I’m surprised at how the 2.5L TDI from Audi ranks so high…only the mega-motors beat it! Also note how the Wright R-3350 (from 1945) noses out Ford’s 2011 EcoBoost engine!)
I think that the great equalizer between big and small engines is the fact that small engines run at higher rpm than large engines do. Large engines win hands down when it comes to volume to surface area ratios, meaning less heat escapes from the combustion chambers at a given engine speed. However, another way to prevent heat from escaping is to expand those hot gasses before they have time to cool off, which is why high rpm is good for thermodynamic efficiency.
Not only can small engines rev high, but because of their high surface area to volume relationship, they must rev high in order to be efficient. Large engines can run efficiently at lower rpm.
A John Deere model A tractor makes about the same horsepower with its 321 cubic inch engine that my 18 cubic inch Kawasaki Ninja 300 motorcycle makes. The John Deere being a two cylinder engine has a very low combustion chamber surface area for a 321 cubic inch engine, on the other hand, the Kawasaki Ninja 300 has a very low combustion chamber surface area for an engine that makes 35 horsepower. That big John Deere makes its rated horsepower at 975 rpm, the Ninja 300 can’t even idle that slow.
When you get down to model airplane engines, I have learned from experience that it’s hard to get a throttled Cox .049 to idle reliably below 5000 rpm, that’s close to redline for a typical automobile V-8.
@meanjoe, it would be interesting to see BSFC numbers for more recent TDI engines, like the current Passat that gets 50+ mpg highway. The Audi and VW TDI numbers from the link you posted earlier are from 1990 and 2000, and I expect today’s numbers are probably a little better.
^I think that’s fair. I think that a mad rush to maximize power per cc (like to meet a racing spec) can result in worse MPGs than a more modest engine of somewhat greater displcement and equal hp…but that’s more an issue for MCs, and perhaps exotics.
Actually, I’d like to see a “baby 8” in a sport compact: flat-plane, short stroke, maybe 3L. (Drop it in a MR2!) Of course, with all that surface area and bearings/journals, it’d be pretty inefficient…
Yea, but the “520” makes up for it with a 7:1 compression ratio and a 1350 rpm engine speed. The old “all fuel” “A"s only had a 3.91:1 compression ratio.
The real monster was the old “D” model, with a 501 cubic inch engine, bore 6.75”, stroke 7", pistons the size of 3 pound Folgers coffee cans.
Uncle had a “G” model that ran off of kerosene I think and the boss had a 620 diesel that was hard to start,the old 2 bangers didnt run generators exactly right(at least thats what the kids told me)-Kevin
The 2 cylinder diesels offered a pony motor starter. A gasoline engine was mounted to the diesel and shared the water jacket. The gasoline engine could be started and run up to operating temperature to get the diesel warm and then crank over the warm diesel. One hundred horse power in the field is considerably more than 100 horse power in an automobile commercial.
And, in those large displacement tractor motors, it’s all about horsepower, not just torque, the opposite of what some may think. The large displacement motors give you longevity and reliability at the expense of horsepower. 100 horses is a hundred horses but the tractor motor can give you those horses, all day, all week, month after month for a generation at much lower RPM. It’s the transmission that is the multiplier that supplies the torque. The higher the horsepower, the more appropriate gearing will supply more torque. Most tractor motors don’t even give you torque numbers…it’s superfluous in these large displacement motors. It’s all about horsepower and longevity. It IS more valuable in the field as you use nearly the maximum horsepower nearly all the time. Otherwise, the tractor becomes less efficient. In tractor use, it’s more about efficiency in work done then distance traveled in the use of fuel. If a larger displacement motor uses 15% more fuel per hour, who is to say it is LESS efficient if it does30% more work. Being able to throw more snow (or till a larger area) in one mile even using more fuel is more important then just going one mile with less fuel.
One motor with 1.4 L displacement has 35 HP. Another tractor motor has a larger 1.8L motor with the same 35 HP. Given the appropriate gearing to match where these HP ratings occurr, they can do exactly the same amount of work over the same time. The smaller motor "may be " a little more efficient. But, the life expectancy for the larger motor is potentially significantly more…another trade off.