How engine capacity affects fuel consumption?

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Hi.
If both engines are identical in desing, how difference in CC would affect fuel consumption?
The only information i have found out is that, every engine needs to run under specific load for the most efficiency. Higher and less load would decrease the efficiency.

Most of the korean and japanese cars in my country, such as hyundai, nissan, kia and so on have 1.6L engines. Ä°s it possible to guess if they had, forexample: 3.2L Engines, with same design, how much more fuel would they consume ?
 
If you double the engine size, as in your hypothetical example, without changing the gear ratios, the engine would spend more time at loads too light (relative to its capability) for it to be as efficient, because friction and throttling losses would increase vs. the original smaller engine. Therefore fuel consumption would increase considerably (although less than double). In practice, the larger engine is likely to be geared to run more slowly, which would partially ameliorate the loss of efficiency.
 
Good question. With so many new engines having fuel saving tech (cylinder shut down, auto start stop etc) and more of it coming every day the consumption/ fuel use equation is hard to get a grip on.
 
Originally Posted by NICAT

Most of the korean and japanese cars in my country, such as hyundai, nissan, kia and so on have 1.6L engines. Ä°s it possible to guess if they had, forexample: 3.2L Engines, with same design, how much more fuel would they consume ?

My guess is 20% more.
 
Gasoline powered engines have an ideal air-to-fuel ratio (i.e. Stoichiometric AFR). Larger displacement engines will require more fuel to reach that ratio. The ideal A:F ratio for gasoline is 14.7:1 so you can do the math.
 
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Originally Posted by BMWTurboDzl
Gasoline powered engines have an ideal air-to-fuel ratio (i.e. Stoichiometric AFR). Larger displacement engines will require more fuel to reach that ratio. The ideal A:F ratio for gasoline is 14.7:1 so you can do the math.

Yes, But it can make more power per revolution, means it can let you drive relatively higher gears. thus more efficiently use that power.
 
Originally Posted by double vanos
Good question. With so many new engines having fuel saving tech (cylinder shut down, auto start stop etc) and more of it coming every day the consumption/ fuel use equation is hard to get a grip on.


IMO it'll require standardized but realistic FE tests. I mean everyone and their grandmother knows that the switch to small displacement turbocharged engines was a way to game the FE test because these engines would operate off boost which in real life is almost never.
 
Originally Posted by NICAT
Originally Posted by BMWTurboDzl
Gasoline powered engines have an ideal air-to-fuel ratio (i.e. Stoichiometric AFR). Larger displacement engines will require more fuel to reach that ratio. The ideal A:F ratio for gasoline is 14.7:1 so you can do the math.

Yes, But it can make more power per revolution, means it can let you drive relatively higher gears. thus more efficiently use that power.


Of course but higher gearing has compromises of its own (packaging restraints and intended use of the vehicle). I'm assuming the OP is thinking "all else being equal". Basically comparing a 1.6 vs 3.2 liter I4 at idle speed. The 3.2 is going to burn more fuel per stroke.. The OP can do that math on that if he want's to find out how much more fuel.
 
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Originally Posted by BMWTurboDzl
Originally Posted by double vanos
Good question. With so many new engines having fuel saving tech (cylinder shut down, auto start stop etc) and more of it coming every day the consumption/ fuel use equation is hard to get a grip on.


IMO it'll require standardized but realistic FE tests. I mean everyone and their grandmother knows that the switch to small displacement turbocharged engines was a way to game the FE test because these engines would operate off boost which in real life is almost never.
Engine size is also taxed in other markets. But I assume you are correct especially as the size of the engine goes down, I wonder how often the 1.0 EB is into the boost?
 
Making a larger engine with the same design (power density) and larger displacement will cause the engine to operate in a different RPM band for the same power demand, and the answer to your question depends on the details of the execution.

One example of the hypothetical engine is where every part grows proportionally to, and around the increase in cylinder displacement, meaning literally everything else grows; valve-, port-, head-size, cams, bearings... and the mass of each individual component. Theoretically, so too does the mechanical friction loss.

Another example is when using the same engine series, but different displacement. Say a K20 vs a K24 with the exact same tuning characteristics. While we have a lot of shared specs like journal area, port size and valve size, which while being equal lend themselves to inequity given the displacement difference. Then we have glaring inequalities like rod/stroke ratio, affecting the piston movement characteristic at every stroke. K24 will have increased piston sidewall thrust (friction) and more erratic accel/decel.

But the larger displacement engine can come in handy for tuning out pumping losses, by operating it part or full time with Miller valve timing, controlling cylinder charge and thus torque output, slashing pumping losses thereby easily overcoming any mechanical disadvantage. This is seen on hybrid ICE engines and high efficiency NA engines. A Prius went up to 2.0L for a reason.
Another advantage of larger displacement, is the ability to control emissions by controlling combustion temperatures for a given power demand. Separate topic.

The recent fad of choosing small, high-power-density ICEs is a balance of economics. Higher strength, and inevitably more expensive parts are required.
-smaller expensive parts means savings.
-keeps weight down; economics again, plus regulations satisfying.
-small displacement is a HARD MECHANICAL LIMIT on potential power output (and theoretically lifespan) before the engine goes boom.
-use turbo pressure and valve timing to control cylinder charge, not so much displacement- wider ability to autonomously limit net power output

Basically exploiting untapped volumetric efficiency is a low hanging fruit and and companies are more emboldened to employ them because of recent advances in component tech allowing them to satisfy warranty. Thats why so many have taken the downsize route. OTOH a Corvette can get 30s MPG on the highway. So the answer to your question: it totally depends.
 
This is a great question without an simple or easy answer.

https://en.wikipedia.org/wiki/Brake-specific_fuel_consumption

Remember BSFC, or Brake Specific Fuel Consumption, is a measure of an engines efficiency.

One key issue is simple geometry. A cylinder and it's head transfers heat. Heat being the product that does the work.

Sooooo, the larger the bore, the smaller the surface area with regard to displacement, and therefore less heat is wasted. This is one reason why larger engines are more efficient. The Prius engine peaks at 41% thermal efficiency. The very best ship engines are near 55% (or about 34% more efficient than the Prius engine, by weight of fuel)


Put another way, your 1.6L engine could be made with just 2 cylinders, and all other things being equal, it would be slightly more efficient.


Another factor is internal engine friction. larger displacement cylinders have less piston ring swept area per CC. And piston rings are a major contributor to internal friction.

While Toyota has done remarkable things with modern automotive engine efficiency (ie small engines) , they've finally just matched what we achieved 60+ years ago in certain (big bore) piston aircraft engines. Today, many experimental aircraft engines reach 40+ % thermal efficiency.


https://www.treehugger.com/aviation...100-mpg-thats-better-than-most-cars.html
 
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As mentioned above, a larger engine can be run at a lower RPM for the same output. Consider the Corvette with it's 6.2L engine. Many 'vette owners report remarkable highway MPG with certain models. This is because the engine can be as low as 1200 RPM at 60mph. Even with cylinder deactivation disabled, it's very common to see 'vettes returning 32MPG highway at 65.
 
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The advantage of a forced induction engine is that it creates sufficient torque to pull the gearing of a much larger engine. The smaller capacity engine will reduce frictional losses but as already pointed out the overall impact on fuel economy will depend on how much time the engine spends on boost.

I can make a specific comparison between a 2 litre Mercedes 190e which I ran for 21 years and it's equivalent replacement, a W204 C class with a 1.6 litre supercharged engine which I've run for 4 years. The newer car has overall proved to use 30 % less fuel. despite being 20% heavier. In highway use when the engine is off boost fuel economy is really exceptional. I'll be doing a 100 mile trip to my sons for Christmas and I know it will return 50 MPG (imp) with ease. Show it a lot of acceleration though and some steep hills which use boost and MPG plummets to the low 30's.which is what the 190e returned. Overall I vastly prefer the newer engine because of the torque it can produce at low revs when needed, combined with the exceptional MPG on longer trips which pushes up the average MPG.
 
Originally Posted by BMWTurboDzl

Of course but higher gearing has compromises of its own (packaging restraints and intended use of the vehicle). I'm assuming the OP is thinking "all else being equal". Basically comparing a 1.6 vs 3.2 liter I4 at idle speed. The 3.2 is going to burn more fuel per stroke.. The OP can do that math on that if he want's to find out how much more fuel.


You keep saying that the OP can do the math, but it's not clear to me what the math is to do. Why don't you do the math here and show precisely what you mean?
 
Originally Posted by brages
You keep saying that the OP can do the math, but it's not clear to me what the math is to do. Why don't you do the math here and show precisely what you mean?


There is NO EASY math.

My 1993 Ford ranger, with a 2.3 liter 4-cyl and 3.73 rear gears never, ever got better than 25 miles per gallon of petrol on a very, very good day.

I removed the 2.3 liter and installed a 5.0 liter (over 2X the CC capacity!) V8 with 3.55 rear gears, and got 23-24 miles per gallon.

The math just doesn't compute, when there are so many variables.
 
I'm probably the only one here who has had the following in their fleet over the years
The Holden Torana was an Opel based car of the early 1970s. Available in a number of engines. The ones that I have had personally
202C.I. straight 6
1.9L 4 cylinder (the shorster stroke 6 with 2 cylinders lopped off)
253 c.i. V-8
308 V-8 - this was geared for nearly 3,000 RPM at 65MPH...very quick car, a bit rowdy on cruise.

All of them got within 1MPG on the highway.

It only take so much power to move the frontal area, CD, and weight through the air.

Around town, the V-8 used nearly 50% more fuel than the 4.
 
Originally Posted by Linctex


I removed the 2.3 liter and installed a 5.0 liter (over 2X the CC capacity!) V8 with 3.55 rear gears, and got 23-24 miles per gallon.

The math just doesn't compute, when there are so many variables.

I've heard about the same thing with the 2nd gen Explorers, the V8 on paper is identical to the V6 but in reality the 302 does better on MPG than the 4.0L SOHC V6. Even though the 302 is bolted to a 4-speed tranny and the V6 has 5 speeds behind it.

Toyota did a similar stunt with the 3rd and 4th gen Prius, but on a smaller scale with a .3L bump to a tweaked version of the Corolla's engine instead of the 1.5L Echo/Yaris/earlier Scion mill. The theory is that a bigger engine turns less to make the same power to improve efficiency.
 
I have a good comparison with this. I had a 1985 Bronco XLT with a tired 351w with 297k miles. It ran okay for what it was, just sluggish, and would average 12.0-12.5 mpg cruising at 65 mph. I considered that acceptable for that old tank. I rebuilt the carburetor and tuned it a little leaner at part throttle, combined with adjusting the spark curve, and managed to get it around 13.5 mpg. It was still sluggish.

I pulled the 351w because I had a fresh 466ci big block waiting to swap in. I swapped transmissions of the same design, just swapped to one that matched the big block's bell housing. Still used a C6 3-speed auto. The 351w was completely stock from carburetor to muffler. The 466ci was anything but stock. The pistons were 200 grams lighter than stock, I ported and machined the heads to induce more swirl, and bumped the compression up from stock 7.5:1 to 8.7:1. I tightened the quench distance from .070" down to just .034". The cam was a custom grind that I spec'd for strong dynamic pressure at low rpm. The intake was a dual plane that I did some slight grinding on to help equalize air/fuel distribution across the cylinders. The carb was a Quickfuel 750 DP, headers were 1-3/4" longtubes to 3" collectors and dual 3" Borla XS Pro mufflers (no cats), and run on cheap 87 octane. I also replaced the torque converter with a Hughes unit spec'd to provide the best fluid coupling efficiency possible.

That big block, with the same gearing, same size tires, moving the same heavy brick of a truck, averaged around 16.5 mpg at 65 mph. I later decided to lean on it a little more. I got the timing curve with combined mechanical and vacuum advance at part throttle at 42-46 degrees advanced. The air/fuel ratio I leaned out until it gave me a lean misfire and then went 1 jet size richer which ended up bouncing around 15.5-16.0:1 AFR. I also advanced the cam a few degrees as well. It got a best of 19.2 mpg and averaged around 18.0 mpg.

That's one vehicle I wish I'd never sold.
 
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