4cyl vs 3cyl

[Crash101]

I am wondering what the advantages and disadvantages are to diesel engines making the same horsepower, but with one more or less cylinder. Is there a mileage or efficiency advantage to one? More or less torque? Please clue me in.

This one is always sure to produce many responses. In reality the number of cylinders do little in providing more or less power, or efficiency. Fewer cylinders are cheaper, more are generally smoother, with some exceptions. A four stroke engine makes a complete cycle in two revolutions, or 720 degrees. This constant is what makes an inline four cylinder engine the worst for vibration. When two pistons are up, two are down (720 degrees divided by 4=180 degrees apart). With three cylinders, one is coming up and one is coming down while the other is up being fired. Like poker, three of a kind can be better than 2 pair.

 

[whistlepig]

I am wondering what the advantages and disadvantages are to diesel engines making the same horsepower, but with one more or less cylinder. Is there a mileage or efficiency advantage to one? More or less torque? Please clue me in.

The more cylinders the better on engines of equal size and horsepower. Each cylinder does less work thus less wear and tear. Also less load build up between the firing of the cylinders. Better power transfer. This was the demise of the old John Deere two cylinders.

 

[_RaT_]

Excellent comments from you, Ford and of course DickB. My smoothest diesel was a International 454 with 3 cylinders. My L48 and my L3830 shared the same rods, piston sizes but the L48 was 4 cylinder and the L3830 was 3. They were equally smooth. My L4850 is 5 cylinder and is no more or no less smooth than my previous 3 or 4 cylinder engines.

Two engines of the same size can have different bores and strokes. They can also be tuned differently. While what you say is very often true, but it's not gospel.

Most inline-three cylinder engines employ a crank angle of 120 degrees, and are rotationally balanced. However, since the three cylinders are offset from each other, the firing of the end cylinders induces a rocking motion from end to end, since there is no opposing cylinder moving in the opposite direction as in a inline six that is inherently balanced. In other words, the cylinder firings induce a vibration.

The inline-four layout is the simplest design that is in perfect primary balance. However, it suffers from a secondary imbalance that causes minor vibrations in smaller engines.

Primary balance and secondary balance refer to vibrations at the first and second harmonic of the crank's rotational frequency, respectively. Higher order harmonics also exist, but as the orders increase, the magnitudes decrease, thus orders higher than the second are typically neglected.

So all this crap means that neither is the winner, and it all depends on what the manufacturer creates. My two John Deere tractors are both three cylinder engines. The little 455 has to idle around around 1600 rpm to be smooth. The bigger 790 does nicely at 800-1000 rpm.




Agreed. If you see two models with the same hp ratings, look for the torque specs. Chances are that the four cylinder will have more cubes and more torque. FYI - turbos tend to give you more power toward the top of the rpm band.

 

[_RaT_]

If this is true it makes me wonder why the GM (Isuzu) and the Ford V8 diesels have always been less than they could be compared to the 6 cylinder from Cummins. Even today I would take the Cummins over the Ford or Isuzu but not by as much as I once would have. On the other hand, I would gladly take the Cummins but you can keep the Dodge body. :laughing:

The more cylinders the better on engines of equal size and horsepower. Each cylinder does less work thus less wear and tear. Also less load build up between the firing of the cylinders. Better power transfer. This was the demise of the old John Deere two cylinders.

 

[whistlepig]

If this is true it makes me wonder why the GM (Isuzu) and the Ford V8 diesels have always been less than they could be compared to the 6 cylinder from Cummins. Even today I would take the Cummins over the Ford or Isuzu but not by as much as I once would have. On the other hand, I would gladly take the Cummins but you can keep the Dodge body. :laughing:

We are getting off the subject but an inline six cylinder is the best motor ever made. Gas or diesel.

 

[SPYDERLK]

The more cylinders the better on engines of equal size and horsepower. Each cylinder does less work thus less wear and tear. Also less load build up between the firing of the cylinders. Better power transfer. This was the demise of the old John Deere two cylinders.

Hummm... Then you must be assuming that the pistons, pins, rods, crank and bearings are of equal strength and quality. My guess would be that a smaller piston will be coupled with smaller components because the loads would be less. Kind of blows the argument. It all depends on what the manufacturer designs.

Consider also the thermal detriment of going to too many cyls. ... In any given displacement, more cyls causes more combustion chamber area. This means more of the heat of combustion goes into the metal parts instead of producing mechanical power.
larry

 

[rScotty]

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'm not sure that helps. The whole object of a tractor is to run at slow speeds and slow RPM. In fact, the slower the better - as long as there is enough power available to get the work done. Torque is what we want available when reducing the RPM is more important than increasing it.
rScotty

 

[kuboman]

Consider also the thermal detriment of going to too many cyls. ... In any given displacement, more cyls causes more combustion chamber area. This means more of the heat of combustion goes into the metal parts instead of producing mechanical power.
larry

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

 

[Rob-D]

Nothing like the old Buick Straight 8 for smoothness or a modern BMW straight 6...

Interesting that you say that. BMW specifically states that the inline six is much smoother than the V6 which they believe has inherent balance issues.

The BMW 6 on the 325 I owned years ago was a sweet engine!

My friend has a 990 Deere with a 4 and it was much smoother than the 3 on my 2720. The 2720 may have been too light for the three, I'll see what happens when the 3320 gets here. The 2320 seemed smoother than the 2720. Less HP for the weight.

Rob

 

[LD1]

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

I agree.

Two engines of the same displacement and same storke length with the only difference being 3cyl vs 4cyl, they will have exactly the same piston surface area. And depending on combustion chamber type/shape, that will be the same also.

Some 3cyl engines may seem to operate smoother than some 4cyl's. This is all in what the MFG does to reduce the harmonics and vibrations. But the bottom line is, the more power strokes per revolution, the smoother the engine will generally run, due to less time in between "explosions". 12 and 16 cylinder engines run extreemly smooth compaired to 6 and 8 cylinder engines.

But there are more than just the # of cylinders that contribute to percieved vibrations. Its all in what the MFG does to curb the vibrations. More agressive cam profiles and timing can make an engine "seem" to run rougher or smoother. Hydraulic vs solid lifters. Roller vs solid cam.

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

To the OP: There are many different engine sizes and styles in the 3 and 4 cylinder tractor market. All MFG make them different. You have to test them for yourself and be the judge. Example:A JD 3cyl may be smoother than a Kubota but a kubota 4cyl may be smoother than a JD. Maybe even one JD 3cyl is smoother than another 3cyl JD of different size. There are way more variables other than just 3cyl vs 4cyl.