3 cylinder engine vs 4 cylinder engine

[Egon]

As has been stated it’s all about engineering design, materials used and machine tolerances.

The number and arrangement of cylinders has a lot of effect on engine balance. The number of power strokes per minute as well as crankshaft alignment also effects the smoothness and power delivery of an engine.

A one cylinder engine would, of course, have fewer parts but there are other factors that must be considered when using one.

My preference would be the straight six.

 

[s219]

My preference would be the straight six.

Ditto!

 

[Midniteoyl]

An inline 3-cylinder is much smoother than an inline 4-cylinder due to the firing pattern and inherent balance of the engine.

Not sure thats true. A power stroke every 90* in a four is smoother, inherently, than one every 120* in a three.

 

[ericm979]

Neither a three or an in line four cylinder engine has perfect balance. Both have secondary imbalances. The three is worse, so triples are more likely to get a balancer driven off the crank to counteract the imbalance.

Since all CUTS have solld mounted engines, balancer shaft(s) will have more of an effect on engine vibration than anything else. Of course the balancer(s) add more stuff to go wrong but they're well known tech at this point so they're unlikely to be an issue. The balancer shaft four in my Branson is very smooth and quiet.

 

[s219]

Not sure thats true. A power stroke every 90* in a four is smoother, inherently, than one every 120* in a three.

It's not about firing patterns fundamentally, though that has its own influence. It's more about balance. An inline three cylinder has perfect primary and secondary balance. A four only has primary balance, and the secondary forces will lead to shakes and a noticeable loss in smoothness. Large fours in automotive applications use balance shafts to deal with that problem but that adds a lot of complexity that would not always be suitable for a tractor engine.

 

[s219]

Neither a three or an in line four cylinder engine has perfect balance. Both have secondary imbalances. The three is worse, so triples are more likely to get a balancer driven off the crank to counteract the imbalance.

Since all CUTS have solld mounted engines, balancer shaft(s) will have more of an effect on engine vibration than anything else. Of course the balancer(s) add more stuff to go wrong but they're well known tech at this point so they're unlikely to be an issue. The balancer shaft four in my Branson is very smooth and quiet.

A three has perfect primary and secondary balance, related to forces, which is most of what we feel/experience/detect as engine smoothness. A three is imbalanced rotationally, which results in a back and forth torque about the middle cylinder. This is normally not a big deal. It can also be cured with a balance shaft, but it's not as big of a deal as the force imbalances on large four cylinders that really require a balance shaft to be acceptable. In the absence of balance shafts, the three will feel a lot smoother. I would bet that's why they are frequently used for smaller motors.

 

[ruffdog]

There is more to it than just 3 or 4 cylinder. With engine design, there is a sweet spot for stroke/bore ratio, operating RPM range, piston speeds at stated max RPM, combustion efficiency, etc...

 

[ruffdog]

There is 2 different types of "smoothness" being discussed here. One is power delivery. More cylinders will smooth the delivery of power out the end of the crankshaft. True. Most times a flywheel will smooth this out easily.

Then we have the "seat of the pants" smoothness. That is where you discuss primary/secondary balance, balance shafts, firing order, V angles, etc. that cause shaking and rocking motions. Two different things.

Most have been engineered to be smooth at operating speed like PTO RPMs.

 

[3gunr]

Not sure about all this tech stuff but my 4 cyl just sounds better than my 3s .

 

[Slowpoke Slim]

OP,

Either one would be fine as long as you match it's hp and torque outputs to whatever you need powered. The tractor is just a power delivery platform. It's the attachments and tools you use with it that matter.

Kind of like buying an air compressor or a power generator. You have to pick one with enough capacity to power the tool, or do the job.