3 cylinder vs 4 cylinder debate

mcfarmall · Oct 4, 2020

shooterdon said:
There is a reason the industrial gen sets run at 1800 RPM.

The other reason is that the way industrial gensets are wound, you can get 60hZ at 1800 rpm versus 3600 rpm from your single cylinder "screamer" generator. 60 revolutions per second = 3600 revolutions per minute. 60hZ = 60 cycle per second. On your screamer, the voltage cycles one time per second. So if you wind a generator so that the output cycles twice as fast, (two cycles per second) you can run it at half the rpm.

Sorry to stray off-topic.

LouNY · Oct 4, 2020

mcfarmall said:
The other reason is that the way industrial gensets are wound, you can get 60hZ at 1800 rpm versus 3600 rpm from your single cylinder "screamer" generator. 60 revolutions per second = 3600 revolutions per minute. 60hZ = 60 cycle per second. On your screamer, the voltage cycles one time per second. So if you wind a generator so that the output cycles twice as fast, (two cycles per second) you can run it at half the rpm.

Sorry to stray off-topic.

2 pole generator head requires 3600 rpm for 60 Hz power,
4 pole generator head requires 1800 rpm for 60 Hz power,
6 pole generator head requires 1200 rpm for 60 Hz power,
8 pole generator head requires 900 rpm for 60 Hz power,
12 pole generator head requires 600 rpm for 60 Hz power,
I have never seen a 60 Hz generator with more then 12 poles,
many of the older piston driven natural gas generators for power generation were 12 pole on massive stationary engines.

mo1 · Oct 4, 2020

mcfarmall said:
The other reason is that the way industrial gensets are wound, you can get 60hZ at 1800 rpm versus 3600 rpm from your single cylinder "screamer" generator. 60 revolutions per second = 3600 revolutions per minute. 60hZ = 60 cycle per second. On your screamer, the voltage cycles one time per second. So if you wind a generator so that the output cycles twice as fast, (two cycles per second) you can run it at half the rpm.

Sorry to stray off-topic.

That is not correct.

First, the synchronous speed of a motor is RPM = (60*Hz)/(number of poles/2) as LouNY's table shows.

Secondly, you have the concept of AC frequency and number of poles in a generator confused. AC at two cycles per second = 120 cycles per minute (120 Hz.) You would get 120 Hz by driving the generator head at twice the speed listed in Lou's table, so if you had an 1800 RPM engine, you would use it to drive an 8 pole head instead of a 4 pole head OR take the four pole head and increase the engine speed from 1800 to 3600 RPM. Things like this were done "back in the day" to run certain high-speed motorized equipment using AC induction motors; I have an old direct drive wood shaper with a two-pole motor designed to run at 120 Hz off of a motor-generator frequency changer to give a nominal 7200 rpm spindle speed. The motor-generator set had a four-pole 1800 rpm motor driving an 8-pole wound-rotor motor to give 120 Hz.

Third, 3600 rpm is a common speed for industrial spark-ignition (gasoline/natural gas/propane) engines, not just smaller single-cylinder ones as gasoline engines' powerbands are typically much higher than 1800 rpm. Generac uses Ford's 6.8 L V10 in many of its LPG/natural gas gen sets and this engine is rated to run at 3600 rpm. (Ford LSG875 & WSG1068 460 and 6.8 liter engines for dry fuel applications from Powertech Engines Inc.)

Fourth, the generator head may very well run at a different RPM than the engine driving it due to speed-change gears being present between the engine and the generator head. This includes pretty much every PTO generator made.

Texasmark · Oct 16, 2020

The sales brochure I got with my 3910 has a blerb from Ford about why they build their engines in that HP range with 3 low RPM (1800 PTO), either square, or over square engines vs old school diesels that had lots of cubes, long stroke, and low RPMs. Ford talks about piston speeds per rpm and flaunts the fact that theirs is low, reducing friction. 3 cyl is half a 6 cyl one of the most naturally balanced, smooth running engines (Ford flaunts the fact that their 3s don't have a crankshaft counter balance as a result) in use....just check out how many on and off the road large machines use 6 cylinder engines. My 3 Fords are extremely fuel efficient......but not as fuel efficient as my Newer Bransons with the Cummins licensed A1100 and B3.3NA engines. The 6530 run (per data sheet) 0.046 gallon per HP (being used at the time) per hour. My 3910 is close at 0.065 best I can measure and the 3000 is close behind. Never tested the 2000....figured it was a wast of time.....refueling is so seldom you forget where to find the fuel cap....well, almost. Grin.

5030 · Oct 16, 2020

Industrial Toys said:
Interesting. I never knew that. Why is that? It's actually worse than that. They run at 3600 to make 60 hz.

Depends entirely on how many poles the generator head has. a 2 pole head to make 60hz has to turn at 3600 rpm. A 4 pole head, 1800 rpm. My standby diesel is a 4 pole with a JD turbo diesel, 1800 rpm.

5030 · Oct 16, 2020

While on the subject of 3 or 4 cylinder diesels, Kubota made a 2 cylinder sub cut at one time... It was a shaker.

Egon · Oct 17, 2020

BrokeFarmerJohn said:
So I have been doing research on 3 cylinder vs 4 cylinder Diesel engines.

Has anyone actually tested if a 3 cylinder is more fuel efficient? Or tested which one is smoother, quieter etc?

For example I have a Mahindra 5555 with a 2.7L 3 cylinder engine (55hp) I believe it痴 same engine in 5570 (70hp). A friend has a LS 6168 with a 2.5L 4 cylinder engine (68hp).

It got me thinking which one was best. Not a tractor to tractor comparison but 3 cylinder to 4 cylinder comparison.

Note* (fun fact) my 3 cylinder rated 540 PTO RPM is 2059 engine RPM while the LS is 2500 engine RPM.

I have browsed and read a lot on this subject from multiple forums but most info seems to be opinion not factual testing.

It all depends on the engine design regardless of the number of cylinders. They can all be made to be well balanced. Fuel efficiency is again dependent on the design Of the systems involved and the situation used in.

There is no better or worse based on the number of cylinders.

MHarryE · Oct 18, 2020

When designing for Tier 4, a challenge was holding down costs. Improvements like common rail injection, variable geometry turbos, after treatments to control pollutants enabled us to downsize from a 6 to a 4 in the most popular model for which I was in charge. Same bore diameter, same stroke resulted in reduced swept area, reduced friction. When I moved to the division in 1989 one of the first projects was putting that 4 into a machine, 107 Max HP rating. Same 4, with tweaks like larger bearing journals and everything else beefed up, same engine had ratings up to 225 HP. Reducing cylinders does reduce parasitic losses; however, there are a lot of other parameters to consider. The 3 cylinder in my mini-excavator needs to be running around 1400 rpm before it smooths out. We had that same problem putting the same 3 into a small asphalt compactor but nothing compared to the 2 cylinder Hatz diesel the 3 replaced.

mcfarmall · Oct 19, 2020

mo1 said:
That is not correct.

First, the synchronous speed of a motor is RPM = (60*Hz)/(number of poles/2) as LouNY's table shows.

Secondly, you have the concept of AC frequency and number of poles in a generator confused. AC at two cycles per second = 120 cycles per minute (120 Hz.) You would get 120 Hz by driving the generator head at twice the speed listed in Lou's table, so if you had an 1800 RPM engine, you would use it to drive an 8 pole head instead of a 4 pole head OR take the four pole head and increase the engine speed from 1800 to 3600 RPM. Things like this were done "back in the day" to run certain high-speed motorized equipment using AC induction motors; I have an old direct drive wood shaper with a two-pole motor designed to run at 120 Hz off of a motor-generator frequency changer to give a nominal 7200 rpm spindle speed. The motor-generator set had a four-pole 1800 rpm motor driving an 8-pole wound-rotor motor to give 120 Hz.

Third, 3600 rpm is a common speed for industrial spark-ignition (gasoline/natural gas/propane) engines, not just smaller single-cylinder ones as gasoline engines' powerbands are typically much higher than 1800 rpm. Generac uses Ford's 6.8 L V10 in many of its LPG/natural gas gen sets and this engine is rated to run at 3600 rpm. (Ford LSG875 & WSG1068 460 and 6.8 liter engines for dry fuel applications from Powertech Engines Inc.)

Fourth, the generator head may very well run at a different RPM than the engine driving it due to speed-change gears being present between the engine and the generator head. This includes pretty much every PTO generator made.

I thought hZ was cycles per second (1hZ=1 cycle per second, 60 hZ=60 cycles per second and so on) at least that's what I found on the internet. AC at 2 cycles per second would be 2hZ. At any rate, I have a meter that plugs into my wall at home and it reads 60hZ all the time. I would think that if the sine wave of AC changed 60 times per MINUTE (1 cycle per second), my lights would be flashing. Apologies for straying further off topic!