I stood next to a Geo Metro the other day and it didn't look as small as it did from a distance. This got me to thinkin. I can see how you could get great gas mileage with a 3 cyl engine.... if it was in a really small, light weight car, but how true would this be with it having to move a heavier car with several people in it.... especially if you are having to floor it to keep up with freeway traffic? Just seems like it would have to use more gas to get the job done, not less. What are ya alls thoughts on this?

Gottaford.

 

Yes, a heavier load would require more power to move, which would take more fuel into the motor, which would decrease mileage.

 

BUT a heavier car/truck would require more fuel to move the same load. If your Metro seems small and under powered. It'll still use less fuel to move a load, than a truck or a larger car. An extra 1000lbs is still an extra 1000lbs regardless of the base weight of the vehicle being used.

Seeing as your not always carrying all that weight in the Metro, the end justifies the means.

Plus, I never had to floor it to keep up with traffic. my 88 Suzuki Forsa (1L carb) kept up with the flow of traffic just as good if not better than the bigger vehicles on the road @ 75/80 mph

 

I don't think the number of cylinders have as much to do with it as the size of the cylinders. All else being equal, I would think a 4-cyl engine of, say, 2500cc wouldn't vary much in fuel usage than a 3-cyl of the same total displacement. HP and torque curves may vary as far as comparative rpm ranges, but fuel usage shouldn't be all that different. Just my thinking...

 

I disagree. For a 4-cylinder engine to have the same displacement as a 3-cylinder engine, the 4-cylinder engine would have to have a shorter stroke, but the effect is countered by the fact that there are more cylinders, which compensates. This greatly affects torque. It's like comparing apples to oranges.

For example, take the 300 straight-six and the 302 V8. 300 cubic inches and 302 cubic inches are essentially the same displacement. However, the 300 only has 6 cylinders, and the 302 has 8 cylinders. To make the math work, the 300 has a longer stroke (4") than the 302 does (3"). The 300 has higher torque and typically is best suited for trucks pulling moderate loads across the farm. The 302, however, isn't considered a torque monster, but has better horsepower numbers, which is why it is typically best suited for cars cruising around at highway speeds. Why does this matter? Your 302, which has less torque, is going to take more pedal to get the same load moving than a 300 pulling the same load. That's where the difference in mileage comes from.

 

That is stroke though, you can play with bore sizes too. But a 2.5 L 3 cyl and a 2.5 L 4 cyl should get similar MPG, the 4cyl probably getting slightly less because there is more friction and rotating mass

 

fmc,

I have to disagree,...
Yes, for a given bore dimension you'd have a longer stroke.
But theoretically you could have the same displacement cylinder with many B/S ratios, although at some point it would become impractical.

With a long enough crankshaft you could have a very short stroke inline 6 engine for that matter.

I'm thinking that torque doesn't only come from bore to stroke ratio, but from rod length to stroke ratio too. *

Stroke length, or rather piston speed (distance traveled) sets a limit on rpm's, which is the primary reason that a short stroke motor will have a higher redline. All else being equal.
(look at the bore to stroke ratios of Formula One cars or MotoGP bikes)

Crankpin angle and firing pulses are another big factor in tractability.

An engine is just a positive displacement pump moving fuel/air mixture.
For a given volume, if you're revving higher you have the potential to make more power. BUT you're burning more fuel and have greater frictional and windage losses as well, so efficiency has to drop at some point.

 

You make very good points. You can have many bore\stroke ratios that would alter the results. For my example, though, I was holding the bore constant since both motors have a 4" bore. I should have been more specific with my example, stating that my explanation holds true for a constant bore size, hence the 300/302 example. You are right; if the bore\stroke ratio changes in both ways then there are many different torque and horsepower trends one can come up with.

 

302s post higher torque numbers than 300s, just at higher rpm.

300s in the 250-260 range, 302s in the 275-280....

A big bore short stroke GMC V6 posts numbers similar to the 300.

Displacement, cam, manifolding, carb, valve size etc.

GMC Big Block V6, V8, & V12 Engine Data

Back to the OP.

Small(er) engines to do bigger jobs:

Ford Motor Company - Press Release - FORD TO EQUIP HALF A MILLION VEHICLES WITH ECOBOOST ENGINE TECHNOLOGY FOR UP TO 20% BETTER FUEL ECONOMY

 

What great answers. Thanks ya all. I do need to ask another question though. Wouldn't flooring a three cyl a lot cause you to have to use more gas than if you didn't have to floor it thus making it not as economical as one would think? I think I'm having trouble wording what I'm trying to say. Sorry.

Michelle

 

Michelle,

The little 3 banger is designed to work with that little car. If you designed a bigger car you need to either spin it faster, or make it bigger to get more work out of it.
Either way you're burning more fuel. Otto-cycle engines are so inefficient the difference is likely to be very small.

I think what we are all saying, (how presumptive of me ) is that it doesn't matter as much how many cylinders an engine has. More is dependent on how the complete engine was designed (tuning).

The true measure of "economy".....
If we accept that there is a fixed amount of potential energy in a gallon of gas it is a matter of how well the engine converts that energy into work, and how much goes to waste. (Heat rejected to the environment, either out the exhaust or from the radiator.)

Leadhead is right in that potentially there would be less frictional losses in a 3 cylinder engine, and that's why these little econoboxes are fitted with them.

Does this better answer your question?


Howstuffworks "How Force, Power, Torque and Energy Work"

 

Frankly, it seems to me that it will vary from car to car.

My buddy bought one of those Metros in 1990 something...three cylinder engine, did fine around town, would not climb a hill if it's life depended on it. His mileage was mid 30's.

I do better than that with my four cylinder Escort.

Both five speed manual transmissions...you're gonna have to drive it to find out.

 

I live on the side of a Mountain (litterly on the side of a mountain range). My 3cyl did just fine. I use to drive it up to the local ski hill, now I wouldn't win any speed races up the hill, but it was faster than walking.

I'm getting my 4cyl 2.2L Mazda pickup back in about a week, and it has about the same horsepower as the 3cyl, and it does just fine up the hills too...

 

Thanks so much ya all. Things are now more clear. I really missed coming to these rooms. There are so many nice people here ready to help.

Gottaford.

 

Basically, here is what we have learned.

There are WAY too many variables for a shallow comparison between load, acceleration, bore, stroke, # of cylinders, compression rations for us to get into about with respect to the gas mileage.

One of the MAJOR things to consider though in ALL of this, is the efficiency of the motor at certain RPMs. the 300 was built to be efficient at a very low RPM, most straights are. The V configuration motors are more efficient at higher RPMs, this is all mostly due to balancing the engine. Then there comes gear ratios, etc. etc.

Way too complicated, however, it is possible, with some work, that I could contrive a formula for est. power and fuel consumption with respect to bore, stroke, #cylinders at given RPMs and compression rations using my MathCad program. (if you ask REALLY REALLY nice, I may try and work on it this summer while I am out of school)

One thing that was mentioned that I would like to argue right now though, the stroke to rod ratio. the ratio of the stroke length to the length of the rod has no 'direct' effect on the power of the engine. I say direct because the only real effect it has is that the longer the rod is, the more stress it is going to absorb when stroking, mainly in the compression stroke and the combustion stroke, when it is being loaded on. The forces on it are going to cause it to twist ever so slightly, and attempt to buckle. These actions do cause a slight loss in power of the engine, i.e. they are forces that are NOT applied to the crankshaft, instead absorbed by the rod, and some of that transfered to the cylinder walls and the piston and wrist pin. The longer the rod, the more energy it is going to absorb trying to twist and compess, so the greater power loss, the shorter the rod, the lower the power loss, up to a point, because at a certain shortness, you start running into tighter angles between the wrist pin and the crank/rod journal causing a loss of power as well. I believe that the ideal rod length is 1.5-2 times the stroke length? someone correct me if I am wrong in that. This is also the reason that aftermarket rods are designed how they are, to decrease the twisting force, allowing for them to apply more force directly to the crankshaft.