4cyl vs 3cyl

[SPYDERLK]

I think displacement determines combustion area and fuel used not the number of cylinders

I am referring to the total of the surface areas of the combustion chambers. The engine with more cylinders will have more internal surface area exposed to combustion. This causes more heat loss to the metal resulting in reduced expansion. An efficiency issue - lower mechanical hp per fuel consumption rate.
larry

 

[Egon]

In general however, two engines constructed exactally alike, the 4cyl will typically be smoother.

Are you sure?

 

[LD1]

Are you sure?

As sure as the earth is flat and also at the center of the universe

 

[kuboman]

I am referring to the total of the surface areas of the combustion chambers. The engine with more cylinders will have more internal surface area exposed to combustion. This causes more heat loss to the metal resulting in reduced expansion. An efficiency issue - lower mechanical hp per fuel consumption rate.
larry

Hmmm....I am going to have to do some calculations on this one. Volume is volume no matter how many cylinders you have. Volume dictates surface area I believe. Also as long as you have a properly working thermostat the engine will maintain a specific temperature no matter how many cylinders are involved.

 

[Duffster]

Volume dictates surface area I believe.

Nope.

Picture a 1' x1' x 1' Cube, it has 1 cubic foot of volume and 6 sduare feet of surface area. 1/6

Now a 2 x 2 x2 Cube has 8 cubic feet of volume and 24 square feet of surface area. 1/3

 

[LD1]

Well I was going to figure the surface area of like sized 3cyl vs a 4cyl but I can find two. The biggest "current production" 3cyl I could find was kubotas 111 CI engine. And the smallest 4cyl was 122CI by various MFG's.


But I will point out that there is a lot more to thermal efficency than just surface area of the cylinder. And even two 4cyl may not have the same surface area. It just depends on the bore and stroke.

But thermal efficiency also has to do with thermal mass, how thick the casting is in certain areas, the coolant jacket placement, efficency of the cooling system. Theres just a lot more to it than surface area.

If you took 2 engines of the same HP rating, with the same CI, both naturally aspirated, and ran them at the same RPM, I doubt you would even notice a ounce per hour fuel difference. Theres just way to much other things that effect how a tractor uses fuel.

 

[EE_Bota]

Are you sure?

I am sure that member Egon knows quite a bit more than he is saying!

 

[EE_Bota]

It's not that simple. Torque does not always rule. Torque is best for motors that pull loads and do work. At the drag strip horsepower will win the race.

I agree with you whistlepig, torque does NOT rule.

Tractors do work. HP is the rate of work per unit time. I can use a gearbox to divide or multiply torque. I can use a gearbox to divide or multiply rpm. I can NOT use a gearbox to divide / multiply horsepower. HP rules.

 

[SPYDERLK]

Well I was going to figure the surface area of like sized 3cyl vs a 4cyl but I can find two. The biggest "current production" 3cyl I could find was kubotas 111 CI engine. And the smallest 4cyl was 122CI by various MFG's.


But I will point out that there is a lot more to thermal efficency than just surface area of the cylinder. And even two 4cyl may not have the same surface area. It just depends on the bore and stroke. Yes, shorter stroke will minimize the increase in surface area shown by greater #s of cylinders.

But thermal efficiency also has to do with thermal mass, how thick the casting is in certain areas, the coolant jacket placement, efficency of the cooling system. Theres just a lot more to it than surface area.Yes, lots of tuning tricks, but thermal efficiency is a hard one to keep as good with low displacement cyls. Think of the larger proportional area of "cold" cylinder wall. Where you get real benefits is better breathing to higher rpm. Less air pumping losses both above and below the pistons.

If you took 2 engines of the same HP rating, with the same CI, both naturally aspirated, and ran them at the same RPM, I doubt you would even notice a ounce per hour fuel difference. Theres just way to much other things that effect how a tractor uses fuel. I know you mean 2 differently configured rather than identical engines. You would want to be outputting equal HP so rpm would probably be different. Im thinking the engine with the hotter exhaust would use a little less fuel.

Absolutely, theres more to thermal efficiency than surface area exposed to the burn. I just introduced it as a factor to realistically moderate any tendency to think that the more cyls the better.
larry

 

[LD1]

Absolutely, theres more to thermal efficiency than surface area exposed to the burn. I just introduced it as a factor to realistically moderate any tendency to think that the more cyls the better.
larry

You are correct that more cylinders is not always better.

But the whole theme of this thread is what engines are "smoother".

And the short answer to that question (but not always the case) is the engine with more cylinders.

The bottom line is that the vibrations come from the "explosions" on the power stroke. There are very minute surges in the RPM of the crank for each one of these. Followed by a minute deceleration of RPM. The more frequent the explosions per revolution, the smaller these accels and decells are. Which minimizes felt vibration and harmonics. Especally at lower RPM or under deceleration. Everything is smoother at a higher RPM because of the momentum of the crank/flywheel.

Which is another thing I forgot to add earlier, using a heavier flywheel and heavier crank will also limit the vibrations due to the stored inertia in the mass. But it will also make it rev slower, but it's a tractor and fast reving engines aren't really what you want.