3 cyl. vs 4 cyl.

[RobJ]

Now for the subtle details. Let's make an assumption that there are 2 engines that each have 200 cubic inches of displacement (3.3 liter). They will both have about the same Hp. Let's give them the same diameter cylinders. The 3 cylinder will have a longer stroke. If the cylinder diameter is 4 inches, the strokes will be 4 inches for the 4 cyl and 5.3 inches for the 3 cylinder. Operational effeciency and low speed torque are typically greater with a longer stroke as there is more time at the top of the stroke with the combustion pressure the highest. More chemical energy is turned into mechanical energy. This means more torque to the wheels for the 3 cylinder across all RPM's. Score one for the 3 cyl!

The 3 cylinder will also have less friction. It has 1 fewer set of piston rings to drag up and down the cylinder, 1 fewer set of wrist pin, rod and main bearings. It also has 1 fewer set of lifters, pushrods, cam lobes, rocker arms and valves to operate. Less friction is less wasted energy and less fuel used for the same power output. Score one more for the 3 cylinder. As the failure rate of engine parts is pretty constant (a rocker arm will fail every 30 billion actuations for example) across all engine types, fewer parts means fewer parts that can fail. That means longer times between failures.

If the engines have the same size oil pumps, the 3 cylinder will have more oil flow on each bearing surface (because it has fewer bearings to share the oil). That should make the friction surfaces run cooler and wear at a lower rate making the 3 cylinder even more reliable.



So, if you want an engine that has more torque to pull a plow, uses less fuel and is more reliable --> get the 3 cylinder.


So why are there 4 cylinder engines? You can get more displacement from more cylinders and that makes more power. At some point, the benefits of 3 vs 4 (or more) change where the stroke gets too long to be practical or the rotating parts get to heavy etc. About the biggest 3 cyl's made are in the 200 cu inch range (3.3 L)

Hope this helps you with your 4 cylinder buddies! Personally, I think they are just envious of your long stroke.

jb

I agree... I think a lot of this is bench racing.

 

[IH3444]

It's a toss up, if you got a good tractor with either a 3 or 4 banger. The only thing I didn't read mentioned is that a 3 cylinder will have a longer stroke, that was mentioned, what wasn't mentioned is the angle forces placed onto the cylinder wall with the increased stroke. These forces, and sharper angle do have a tendency over engine life time to wear the cylinder a bit more rapidly. As was stated, this is all bench racing, and the differences are very almost unmeasureably small.

 

[davemhughes]

IMHO I think 3 cylinders in tractors has more to do with manufacturing cost than anything else. With one less cylinder/piston/valves/injector/ ect....the 3 cylinders are cheaper to produce. I have owned both and 4 cylinder is smoother than the 3 cylinder I owned. But that again could be better engine mounts.

 

[Farmwithjunk]

I agree... I think a lot of this is bench racing.

Maybe.... But consider this. I know of no cases where a 3-cylinder was dropped from a tractor model in favor of a 4 cylinder, but can account for several cases where 4 cylinders were dropped in favor of 3. (Ford 2000,3000,4000 series- Deere 40 to 65 hp range utilities of mid 1980's vintage- Massey Ferguson diesels AND gassers in the early 1960's- ect) I doubt anyone will ever be able to convince me, or any other person of rational judgement that major manufacturers would INTENTIONALLY backslide with their technology AND SUCCESSFULLY STAY THAT WAY.

It may be a subject for debate amongst us "bench racers" but things seem to be less of an issue for the biggest names in tractor manufacturing.

 

[machmeter62]

Welcome to the TBN community!

First off, you do know the difference between a 3cyl and a 4 cyl, right?

One has an additional cylinder.

That's about it.



Now for the subtle details. Let's make an assumption that there are 2 engines that each have 200 cubic inches of displacement (3.3 liter). They will both have about the same Hp. Let's give them the same diameter cylinders. The 3 cylinder will have a longer stroke. If the cylinder diameter is 4 inches, the strokes will be 4 inches for the 4 cyl and 5.3 inches for the 3 cylinder. Operational effeciency and low speed torque are typically greater with a longer stroke as there is more time at the top of the stroke with the combustion pressure the highest. More chemical energy is turned into mechanical energy. This means more torque to the wheels for the 3 cylinder across all RPM's. Score one for the 3 cyl!

The 3 cylinder will also have less friction. It has 1 fewer set of piston rings to drag up and down the cylinder, 1 fewer set of wrist pin, rod and main bearings. It also has 1 fewer set of lifters, pushrods, cam lobes, rocker arms and valves to operate. Less friction is less wasted energy and less fuel used for the same power output. Score one more for the 3 cylinder. As the failure rate of engine parts is pretty constant (a rocker arm will fail every 30 billion actuations for example) across all engine types, fewer parts means fewer parts that can fail. That means longer times between failures.

If the engines have the same size oil pumps, the 3 cylinder will have more oil flow on each bearing surface (because it has fewer bearings to share the oil). That should make the friction surfaces run cooler and wear at a lower rate making the 3 cylinder even more reliable.



So, if you want an engine that has more torque to pull a plow, uses less fuel and is more reliable --> get the 3 cylinder.


So why are there 4 cylinder engines? You can get more displacement from more cylinders and that makes more power. At some point, the benefits of 3 vs 4 (or more) change where the stroke gets too long to be practical or the rotating parts get to heavy etc. About the biggest 3 cyl's made are in the 200 cu inch range (3.3 L)

Hope this helps you with your 4 cylinder buddies! Personally, I think they are just envious of your long stroke.

jb

An EXCELLENT post John!

 

[Egon]

a single plane crank

I would assume 4 cylinder engines can come in different configurations other than inline which is a basically unbalanced design.

 

[Joe1]

John Bud:

A good post

One thing I would add for your example is the 5.3" stroke 3 will have accelerated piston ring wear simply because for each revolution the piston drags 10.6 inches vs 8 inches on the 4 inch stroke 4 cylinder. Diesels last so long, this is usually not an issue.

Secondly, the three cylinder may be slightly less responsive to the throttle due to the heavier flywheel and/or counterbalance shafts to smooth out the engine. Also not an issue for a tractor.

The biggest advantage of a three over a four is cost savings in the terms of initial cost since the manufacturer can provide a tractor with the same HP cheaper than for an equivalent 4 cylinder and also less maintenance due to one fewer cylinder to maintain.

It's all academic anyway as they both work in the real world very successfully. An interesting food for thought discussion that has no right answer.

 

[_RaT_]

I have had 3, 4, 5 and 8 cylinder diesels. All worked very, very well, all were very smooth. My 3 cylinder L3830 on my Kubota is every bit as smooth as my previous 4 cylinder on my L48. Both had exactly the same piston size and stroke. I would not let a 3 cylinder vs 4 cylinder be a criteria for judging a diesels worthiness. I'll buy my next tractor based on the tractors abilities, not on the number of cylinders.

 

[BTDT]

Prior to the overseas invasion by tractors, I would say that the majority of tractors had 4 cyl or more, and 4 or 5 speeds. Then, tractors started becoming smaller by demand, so it's easier to fit a 3 cyl engine into a smaller frame, and you add more gears to get about the same performance. Most of the smaller tractors have 4 ranges and 4 gears plus reverse. Look at what used to be accomplished with lawnmower engine powered machines (Cub Cadets, etc.).

 

[dgl24087]

Prior to the overseas invasion by tractors, I would say that the majority of tractors had 4 cyl or more, and 4 or 5 speeds. Then, tractors started becoming smaller by demand, so it's easier to fit a 3 cyl engine into a smaller frame, and you add more gears to get about the same performance. Most of the smaller tractors have 4 ranges and 4 gears plus reverse. Look at what used to be accomplished with lawnmower engine powered machines (Cub Cadets, etc.).

Ford switched from the 4 cyl. in the 2000 and 4000 series to the 3 cyl. in 1965. My 1975 JD 1530 is a 45 PTO HP 3 cyl. diesel, 8x4 transmission. This was an evolution of the 1520, also 3 cyl., which replaced 4 cyl. engines in comparable HP model JD's in the late 60's. The whole point of this is that the change from 4 to 3 cyl. wasn't due to an overseas invasion of 3 cyl. tractors, just my personal observation.