4cyl vs 3cyl

[Pushing_Tin]

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.

 

[Monster5601]

Good question, wondering this myself.

 

[KennyV]

the more cylinders the smoother it runs,,,
more uniform power and torque...

 

[KennyV]

With all other things being equal...
The more cylinders, the smaller each cylinder will be and the better it will breathe.
You will have a power stroke more frequently with more cylinders...
Efficiency, performance will go up...

 

[Egon]

If they are both inline engines the three cylinder requires fewer internal components for balancing.:thumbsup:

 

[KennyV]

From a component stand point alone the fewer the cylinders ... the fewer the parts.. .
less machining sometimes less cost, on fewer cylinders.

 

[magicheater]

the more cylinders the smoother it runs,,,
more uniform power and torque...

I thought this too but there seems to be a lot of complaints about the Kubota B7800 and B3200 being vibrators when compared to 3 CYL versions of similar sized tractors. I'm wondering if there is not some sort of harmonics taking place in the B's that adds to the vibes. Just one of those unanswered questions in the back of my mind.

 

[Ford tractor]

the more cylinders the smoother it runs,,,
more uniform power and torque...

Uhhhh, not exactly...

A 4 cylinder engine's crankshaft has 2 pistons going up and 2 going down on every revolution. They typically need a counter balancer to minimize vibrations. Some tractors have them, some don't. Obviously, the smoother ones have them!

A 3 cylinder crankshaft has equal spread across 120 degrees. When one is at TDC, there is one going up and one going down. It works out that the 3 cylinder is typically as smooth as a 4 cylinder. They are also simpler and cheaper to manufacture.

I would look at the capabilities of the overall package, and not so much at how it is attained. The 3 cylinder tractor will typically be a little shorter. Might be a little lighter, but that depends on the casting material (Al vs iron) the thickness and other factors.

 

[LD1]

A four cylinder will typically run smoother overall.

While you do have two going up and two going down @ the same time, They are on opposite ends of the 4-stroke cylce. One is on compression and the other on exhaust. One will be on power strole while the other on intake. Most 4-strokes the cylinders will be paired 1-4 and 2-3.

On gas engines, they use 2 coils. Each coil feeding a "pair" of cyls. The coil fires eache time the piston is @ top dead. Even if it is on exhaust stroke, cause its "partner cylinder" is on compression.

All engines are balanced according the the reciprocating and rotating weight they are slinging around. If you hooked an external power source to a 3cyl and a 4cyl, there would be no difference in vibration since they are both balanced.

The more vibration on fewer cylinders come from fewer power strokes per revolution. On a 3cylinder, it is 120* between fires. On a 4cyl it is 90*. Each time you have a power stroke, the gases srive the piston down, and then it requires the stored momentum of everything to continue spinning to the next power cycle. A 3cyl has to use more of this stored momentum. It is too much to detect by us, but the motor will actually slow down between fires, and then speed back up once fired.

Since most engines open the exhaust valve (signaling the end of power stroke) at about 5-10* BTDC the engine is really only making power for about 80* of the power stroke. On a 4cyl, that leaves 10* up to the momentum and 40* on a 3cyl. These very high frequency speed-ups and slow-downs happen at a higher magnitude in a 3cyl. That is why the seem to vibrate/run a little rougher.


A perect example of this is the Motorcycles of the 70-80's. Most were either single cylinders, or two cylinders in V or in paralle. While they ran good, the inline-4's that were comming into play were night and day smoother.

 

[Hawghauler]

If they are both inline engines the three cylinder requires fewer internal components for balancing.:thumbsup:

Very true

 

[rbargeron]

From the buyer's standpoint, there really isn't a compelling advantage of 4 vs 3 cylinders. From the viewpoint of a manufacturer, a full line of tractors will need a range of power output. It makes economic sense to produce engines with 2,3, 4 or more cylinders to cover that range. Tooling costs, common parts, etc. all help simplify the operation. Balance shafts and other tricks to reduce vibration are not needed for tractors - where owners are less concerned than a Lexus customer might be. The differences in actual thermal efficiency of the different configurations is not large. Over the years, tractor engines have had 1 to 6 pistons ranging from tea-cups to gallon paint cans - they all work.

 

[Monster5601]

I found another thread about this subject on this site from several years ago and after reading the posts, there is still no answer.

http://www.tractorbynet.com/forums/owning-operating/37654-4-cylinder-vs-3-cylinder.html

The math says that the 4 cylinder will produce smother torque. On a 4 stroke engine, the crank shaft has to turn two time for one power stroke or 720 degrees. Divide the number of cylinders in to 720 degrees to see how ofter, in degrees, you get a power stroke. Also, a power stoke last for 180 degrees of crank shaft rotation.

I wish I still worked in powertrain (GM) then I could go ask the design engineers this question.

 

[rbargeron]

........after reading the posts, there is still no answer............

If you ask the design engineers, they'll cite other factors too, like bore, stroke, flywheel weight, aspiration, and method of fueling, not to mention characterizing the load. Like anything else there's a lot to it.

 

[LD1]

Just for kicks.....off the top of my head.


2 main bearings
1 main cap
2 main cap bolts
2 rod bearings
1 rod cap
2 rod cap bolts
1 rod
1 wrist pin bushing
2 wrist pin keepers
1 wrist pin
1 piston
3 oil rings
2 compression rings
2 lifters
2 pushrods
2 rocker arms
2 rocker arm studs
2 rocker arm nuts
2 rocker arm pivot washers
2 valves
2 valve stem seals
4 valve locks
2 valve springs
2 valve spring retainers

45 fewer parts INSIDE the engine. This is not counting if the bolts have washers. And I could progagally break some things down farther, such as rod studs. This is just a rough Idea. Not counting outside the engine either, sucj as injectors and glowplugs.

 

[DavidAubke]

I had always thought that between two engines of the same overall size, the one with fewer cylinders would have more low-end torque due to the larger size of its cylinders.

I used to have two 550cc motorcycles. One a single-cylinder and the other a four-cylinder. The "thumper" felt like it really pulled at the low end but once the four-cylinder got its RPMs up, it felt faster.

 

[Egon]

The inherent smoothness of an engine depends on the number of cylinders and their orientation.


There are a few configurations that are inherently balanced. Others require additional mechanical items to balance them.:thumbsup:

There is no secret to it and it's well analyzed. Just try a Google search. It surely beats guessing!:thumbsup:

 

[Pushing_Tin]

Thanks for the replies, I was just wondering because I am looking at moving up in tractor size and all the JDs have 3 cyl and the Kubotas have 4 cylinders for about the same HP.

 

[PHILIP8N]

Thanks for the replies, I was just wondering because I am looking at moving up in tractor size and all the JDs have 3 cyl and the Kubotas have 4 cylinders for about the same HP.

I was looking at a JD3720 with a 3 cylinder before I bought my Kubota L3940. Both similar HP (Deere had more), 3 cylinder Deere smaller (92 cubes) engine than 4 cylinder Kubota (122 cubes). 3 cylinder had a turbo on it. Is this the same case as you??? The 3 cyl turbo probably has a better low end torque curve, but in this case I would rather have the larger 4 cylinder natually aspirated engine. FYI, all 4 cylinder engines in the Kubota Grand LXX40 line have dual balancer shafts. I once worked on a JD3029 3 cylinder in an air compressor and that was the worst enigne I have ever seen vibration wise. Philip.

 

[beppington]

I don't think the # of cylinders tells you much.

40 HP is 40 HP.

I think you need to go drive whichever 3 or 4 cylinder model you're curious about to know whether it vibrates more or less than any others, or more or less than you're expecting.

 

[Dr. Bob]

I have a B7800 ('03, purchased new) and I have never felt that it was too rough. Come on man, its a tractor not a Caddie! My 7800 has 400 hrs on it and has never failed to start nor had anything break (and it lives outside). I love this tractor. I agree with the others: get the tractor that can do the work you need (and then some just for "insurance").