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4 cyl vs 3 cyl diesel

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E. WOZ
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2002-09-07          42136

I'm looking at buying a tractor in the 30hp range. I've noticed that the Kubota B2910 has a 4 cyl. diesel when the New Holland and the Deere have only 3 cyl. engines!! Is this a big issue that I should pay attention to?? Does the 4 cyl. have more torque than the 3 cyl.?? They never show torque specs in their brochures. Does this really matter??

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TomG
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2002-09-08          42139

I don't think it's much of a practical issue. It eventually does become an issue if engine work is required since 4-cylinder engines are more expensive to rebuild. But few compact owners are ever going to need a rebuild. I wish I could recall an explanation I heard about 3-cylinder diesel engines having some advantages over 4-cylinder ones. The explanation had something to do with firing and degree spacing.

Torque and HP can get to be pretty complex subjects. I think several long discussions are in the archives. Basically, HP and torque are closely related and can be calculated from each other at a given rpm. Torque basically is HP divided by RPM times a constant to get from HP to foot pounds through minutes. I think the formula is in the archives.

The formula would give torque from HP at the rpm speced for the HP rating. That rpm probably is the max HP point and probably isn't the max torque point. However, two points are pretty close in most diesel engines.

In practice it's a little tough to judge performance from HP or torque specs. Analyzing specs is a good way to learn about tractors and that has value in itself. However, my notion is that all the major manufacturers make good machines and the best way to find the best tractor is to try a few around dealers' yards. The best tractor for a person often is the one that feels best. Never the less, keep asking the technical questions here since most people here used this board to start learning about tractors. I imagine you’ll get some useful responses, but talk to a few dealers as well.
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EdC
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2002-09-08          42142

I believe the unknown advantage mentioned that is related to firing sequence and degree spacing is that 3 cyl engines have a natural vibration reduction advantage (as do 6 cyl engines) over 4 or 8 cylinder engines and thus require less vibration dampening enhancements such as heavy flywheels and counter balances. ....

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jeff r
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2002-09-08          42153

Let's look at it the dollars and cents point of view. A 3 cylinder engine is way cheaper to build than a 4 cylinder engine. One less piston, one less connecting rod, one less set of valves,shorter crankcase, less machining on the crank journals and engine block/heads, LESS material to buy, 3 injector set rather than 4. A manufacturer puts LESS money in manufacturing costs with a 3 cylinder than a 4. Personally I think internal cumbustion engines should come with an even numbers of cylinders. With a 4 cylinder you have a cylinder firing every 90 degree turn of the crank not 120 degrees as with 3 cylinder. I challenge the "smoothness" theory with odd numder cylinders. How many 3 and 5 and 7 cylinder cars you see on the roads today???? Makes one wonder doesn't it???? ....

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DRankin
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2002-09-08          42159

Audi used to make a 5 cylinder and for all I know maybe still does.
Three's and fives are the basic engines and I see the sixes, especially the V-6 as a "doubled" three and the same for the latest craze.... V-10's, which are just "doubled" five cylinder engines. Conversely, some years back, Pontiac made a slant four that was no more and no less than a half bank V-8. So I think the three and five cylinder technology is alive and well out there.
Note on Kubota’s: The B 7500 engine (21HP) is a three-cylinder version of the B 2710’s 4- cylinder (27 horse) power plant. The bore and stroke dimensions are identical.
The same relationship exists with the B2410’s 3 cylinder (24HP) and the B2910’s 4-cylinder (rated @30 HP), again, same bore and stroke numbers.
Also reading between the lines, the four cylinder models have uniformly higher top speeds for the same max RPM's, a bit more speed than you can account for by considering their slightly larger tires.
Translation: the three cylinder versions are geared a bit lower overall to compensate for their reduced horsepower and torque. So, not to worry, pick the HP range you need and trust that the tractor, engine and drive train are configured correctly to do the job.
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Peters
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2002-09-09          42164

Dynamic balancing is natural for a 3 and 5 cylinder diesel engine. 4 and 6 for a gas engine.
There are a number of 5 cylinder diesels made as well as three cylinder.
Volvo also sells a 5 cylinder gas engine, but has counter balance shafts to enhance dynamic balance. ....

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jeff r
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2002-09-09          42167

The reason guys, mfg's use the same pistons as the 3 to 4 cylinder counterpart is DOLLARS guys. WHY SHOULD A MFG make 2 different pistons between 3 and 4 cylinder engines? That's right he doesn't. Just because a small car companies like AUDI and Volvo make a 5 cylinder doesn't make them a good design. I stand by what I said. A 3 cylinder is far less expensive to manufacture than a 4 cylinder. Combined with the same piston size between 3 and 4 cylinder blocks and you have a very economical parts manufacturing process. IT's ALL ABOUT DOLLARS GUYS. FIgure out the dollars and cents equation and the rest is easy. Even numbered cylindered internal cumbustion engines manufacrured since 1940 out number odd numbered cylindered engines a billion to 1. How many people reading the posts on this site have an odd numbered cylinder car or truck? How many pieces of HEAVY diesel construction equipment have 3 and 5 cylinder diesel engines? What do farm tractors have? I rest my case. ....

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TomG
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2002-09-09          42173

My sort of subject, but I'll add a voice of moderation for a change and say that maybe we should keep a balance of following our interests and helping Woz's tractor buying decision. I'm interested in specs and design issues, but I really do believe specs can get in the way of buying a tractor. To me, going through specs is a way to learn about tractors, and specs act sort of like a bookmarker for a person while waiting for the 'Ahh this is the one' experience.

Vibration damping is what I remember about 3 vs. 4-cylinder design comment I mentioned. I'm also certain that fewer cylinders does reduce manufacturing costs. There's a good chance that everything said here is true. How manufacturers saw off design vs. cost issues would be very complex and beyond me but I'm usually among the first to think that in recent years cost is dominant. However, my Ford 1710 has a 3-cyl diesel. I think that NH continued to use the 3-cyl engine when they first took over Ford but later used a 4-cyl engine in a redesigned 1710. If true, I wouldn't know how to explain that in terms of costs.

Mark: I'm not sure if your engine comparisons have the same displacements or not. There are various reasons for differences in speed other than gearing even in the same displacement engine. I heard a story about some NH models I think where an expensive and an economy version of about the same tractor were produced. The expensive version was rated for higher HP but the tractors used the same engine. The explanation was that the injector pump was turned up on the expensive model. The story didn't note if the rad also was larger to keep the engine cool.

Other than design, on a level the max load-load rpm adjustment on the throttle linkage is what determines top-speed. If it’s turned up the tractor will go faster, but maybe that’s not so good for the engine.
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Peters
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2002-09-09          42182

Hello Jeff;
I repeat. The 5 cylinder does not make sense for a gasoline engine. Audi and Volve make the engine for space considerations.
For a diesel this is different a number of companies make 5 cylinder diesels. Mercedes make a 5 cylinder for their cars. Volkswagon just made a V10, the highest power for a auto. Deere make 5 cylinders.
I am not sure why the balance is good in a diesel vs the gasoline engine. I would assume it has some thing to do with the point of ignition with in the cylinder.
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DRankin
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2002-09-09          42183

Tom, I am getting the bore and stroke specs right out of the Kubota sales literature. ....

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Mr Ed
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2002-09-09          42190

I am not sure if diesel engines have the same characteristics as gas. I once had a 3 cylinder Yamaha motorcycle. The advantage of the 3 vs 2 and 4s was that it had almost the same HP as a four but was able to spool up more quickly like a twin, giving the torque characteristics of those engines. I would assume the same would apply to diesels but, again, I am not expert on diesel engines.

I do know the Yamaha triple ran a bit rougher than a four. In my opinion, engine smoothness is not as musch an issue on a tractor given how it is used. ....

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EdC
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2002-09-09          42213

We have spun into engine technology which may be irrelavent in the tractor purchase decision, but it is fun. The vibration characteristics should be the same for diesel or gas -- the important thing is 2 vs 4 cycle. In a 4 cycle 4 cylinder engine there is a power stroke every 180 degress of rotation. This divides evenly into the 360 degrees of rotation so the power stroke impulses continue to enhance the vibration and have to somehow be compensated for. In a 3 cylinder engine there is a power stroke every 240 degrees and being it does not divide evenly into the 360 degrees of rotation it happens at a different rotational point each time and does not reinforce the vibration impulses -- it smoothes them.
All aircraft radial engines were an odd number of cylinders for this reason as vibration caused metal fatigue. Some that seem to to have an even number (like 18) were actually two "odd number radials" attached together. However, most of these Kubota's are land locked.
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E. WOZ
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2002-09-10          42219

WOW! More info than what I expected. Thanks! Sorry Kubota guys, but I think I'm going with a New Holland. I got a killer deal on a TC33D w/ 7308 loader ($15,500). Helps to have a family member to get that extra Ford Employee discount. Thanks for all the help in my first tractor buying experience! This website was a huge help! ....

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TomG
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2002-09-10          42223

I relate to the vibration discussion from my days in the sound biz. Vibration and sound have a lot in common. Engines have vibrations (sound) and they produce standing waves. I’ve used phase reversal and delay tricks to fix many feedback problems since particular frequencies can be reinforced or cancelled at the microphones.

I imagine the design goals in engines for undesired vibration are similar to sound buz: If you can't prevent them, cancel them; if you can't cancel them, then try to sop them up; and if you can't sop them up, then reduce the power. I imagine that sopping and power reductions are the least desirable in both engines and sound. In sound, sopping means compromised mixes and dead sounding rooms. In engines sopping means adding mass to the engine. Reducing power as a solution raises the question of why bother to build it in the first place.

In addition to the comments that the degree spacing affects how vibration reinforces or cancels there would seem to be many other factors. The physical size of an engine affects the distance vibration travels and therefore the frequencies that cancel or reinforce. Things like displacement and design (eg. in-line vs. v) would affect the size and resonate frequencies. RPM would affect the frequencies produced. Material and size of crankshaft components would affect how vibration propagates.

I suspect that differences between the cylinder number and arrangements that work best for gas and for diesel engines result from differences in combustion and characteristic rpm. Diesels in vehicles tend to be high compression-low rpm engines compared to gas engines in the same application. I think combustion in diesels also occurs over a longer part of the power stroke. It would be interesting to know if the designs for high-torque-low rpm industrial gas engines are more similar to gas or diesel engines in equipment.

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TomG
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2002-09-10          42224

Mark: The thought I had is that equal bore and stroke in two engines with a different number of cylinders would have different displacements. Since displacement is a a big factor in HP, I didn't know if the engines being compared have the same displacement or not. Engines of different displacement that have the same HP, almost by definition, would have different tunings. ....

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DRankin
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2002-09-10          42232

I probably was not clear enough. The examples I cited have identical individual cylinder displacements but differing numbers of cylinders.

Example: Both of the following have a 3.0 inch bore and a 2.9 inch stroke,
B2710, 4 cylinder, 1335 cc’s, 27 HP @ 2600 RPM.
B7500, 3 cylinder, 1001 cc’s, 21 HP @ 2600 RPM

Example: Both of the following have a 3.1 inch bore and a 3.1 inch stroke,
B2910, 4 cylinder, 1498 cc’s, 30 HP @ 2600 RPM.
B2410, 3 cylinder, 1123 cc’s, 24 HP @ 2600 RPM

I think the math and the conclusions speak for themselves.
....

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DRankin
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2002-09-10          42233

E.WOZ, Blue is good. Very sturdy reliable machines. If you go the backhoe route, go with blue there too. ....

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Peters
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2002-09-10          42234

Ok E.Woz is there anyway I can be your cousin twice removed and get the discount? The price I was quoted, which was good was higher than that without the FEL.
Ed you explained to me why gas tend to be 4 6 and 8 and why a diesel can be 3 and 5 and this can be a good arrangement for vibration.
Tom the key is as you state RPM, but the reason is the difference in dependance on balanced exhaust flow. High RPM gas engines require a balanced exhaust flow to provide proper exhaust from the cylinders. High compression engines such as diesels and some race engines do not need the balance flow to exhaust efficiently. Efficiency is not that critical on a race engine. ....

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TomG
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2002-09-11          42268

I'm happy that Woz felt he got some good from this discussion. I did as well, but I hope we're not quite ready to wrap it yet.

I'm still curious about the possible differences between the best number of cylinders for gas and for diesel engines. I'm starting from wondering if the characteristic knock of diesels makes any difference, or why diesels have problems with coolant cavitation while I haven't heard of the problem in gas engines.

So, I'm wondering if there's something about diesels that produces characteristic vibration that makes for example 3-cyl good for diesels but not for gas?

I posted a basic sound spectrum curve I measured on my Ford at PTO rpm awhile back. I recall thinking that I expected to find the ' diesel knock' in several spikes on the curve but was surprised to find a fairly flat (around 90 or 95db at the seat and under a canopy).

I guess I’d expect gas engines to have a similar curve, but maybe not as noisy, so maybe 'characteristic vibrations' isn't a good explanation for why 3 cylinders may be good for diesels. Of course, there may be spikes in the sound that my fairly broad 2/3rd-octave spectrum analyzer on slow wouldn't pick up. Suppose I could measure my Ford ½-ton as well and maybe carry on to my chain-saw for a 2-cycle comparison., but then I’ve already trashed my measurement as possible inadequate so why bother? I’m still curious about what it is about a diesel that might make a difference.
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Gregory Lee
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2002-09-19          42653

What implements does Kubota say can be used with the:
3 cyl
4 cyl? ....

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DRankin
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2002-09-19          42657

More horsepower, not more pistons, will accommodate larger blades, wider mowers and more powerful backhoes. ....

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Tommy Smith
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2002-09-30          43131

Just to throw in my two cents on the "odd number cylinder" debate. I have a Winibago that sits on a VW chasis and it has a 5 cylinder emgine. It weighs 7000 lbs and the motor ( gas ) pulls it just fine, even in the mountains. ....

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