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Tractor Talk Discussion Forum

HP-RPM-Diesel Fuel economy question

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Blue_Tractor_Man

10-19-2010 14:25:19
66.193.226.74



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I have 3 Kubota ZB600 2 cylinder diesel engines that I am using to drive centrifugal agricultural pumps. They are mounted on a truck and drive the pumps via a belt. (One pump per engine). I have the option of running the engine at any speed I want by changing pulley sizes. The pumps require about 5 HP to operate in the range that we use them. The engine is rated at 12 HP at 3200 RPM, but in a Kubota generator application they operate at 3000 while belt driving the generator at 3600 RPM.

The engine sounds like it hits a sweet spot at about 3000 RPM.

My questions: By looking at the attached performance chart, what would be the best RPM to operate the engine to achieve maximum engine life, fuel efficiency, sound reduction? Are those goals mutually exclusive?

The engines run 8-12 hours per day from March 1 -Sept 15.

These are the same engines I asked yesterday about intake valve wear. Thanks for the answers!

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RodInNS

10-20-2010 08:36:46
216.118.158.123



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 Re: HP-RPM-Diesel Fuel economy question in reply to Blue_Tractor_Man, 10-19-2010 14:25:19  
I think you better calculate your pump power requirements VERY carefully. When you rejig the drives you will increase the torque requirement, possibly to the point that the engine can not produce enough. Those engines have very flat torque curves... because it basically takes a fairly constant amount of torque to run a pump for a given pressure... but flow is increased by running it faster. Basically, you may run out of power... so calculate carefully.
It might just be easier to leave it alone.

Rod

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MarkB_MI

10-20-2010 17:29:58
166.217.128.237



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 Re: HP-RPM-Diesel Fuel economy question in reply to RodInNS, 10-20-2010 08:36:46  
Best answer so far, Rod.



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jdemaris

10-20-2010 07:52:59
67.142.130.47



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 Re: RPMs and fuel use . .1100 RPM Detroit wins in reply to Blue_Tractor_Man, 10-19-2010 14:25:19  
What MarkB_MI stated about gensets and RPMs is not universally correct.

The better higher priced and more efficient units run low RPMs, not high.

One sidenote - high 3600 RPM gensets often do not make perfect AC power either. More often, the lower RPM , higher priced gensets are the ones certified for perfect voltage and sine-wave.

I checked some test specs on small Kubota diesels. On all, 1800 is the opitmal RPMs as long as the horsepower need is satisfied.

Some quick fuel-use specs on gensets. Note the old-tech, 1100 RPM two-stroke-cycle Detroit 2-71 on a Delco genest beats them all. All figures have been adjusted to 1 gallon per hour, to make things easy to read. All amps at 120 VAC.

Isuzu indirect-injected diesel 1800 RPM - 120 amps at 1 gallon per hour

Deere direct-injected diesel 1800 RPM - 110 amps at 1 gallon per hour

Delco-Detroit two-stroke diesel 1100 RPM - 166 amps at 1 gallon per hour

Kubota in-direct injected diesel 1800 RPM - 97 amps at 1 gallon per hour

Perkins in-direct injected diesel 1800 RPM - 108 amps at 1 gallon per hour

Cummins direct-injected diesel 1800 RPM - 119 amps at 1 gallon per hour

Fast run:
Air cooled 3600 RPM diesel - 73.5 amps at 1 gallon per hour

Air cooled 3600 RPM gasoline (Honda) - 71 amps at 1 gallon per hour

Air cooled 3600 RPM Natural Gas (Honda) - 82 amps at 1 gallon per hour

Medium run for RV use:
Air cooled propane at 2400 RPM - 54 amps at 120 VAC uses 1 gallon per hour

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MarkB_MI

10-20-2010 03:41:09
166.217.170.212



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 Re: HP-RPM-Diesel Fuel economy question in reply to Blue_Tractor_Man, 10-19-2010 14:25:19  
It's hard to tell, but it looks like the specific fuel consumption is lowest around 2400 rpm. Clearly it rises sharply above 2800 rpm, so you want to keep it below that. I'd say anywhere from 2200 to 2600 rpm will give you good results. Better to have some horsepower in the bank in case you underestimated your requirements and to allow for engine wear.

The reason engines in gensets run so fast is because they need to maintain a fairly constant rpm regardless of load. In order to do that, they are set up to run close to the speed that gives maximum rated horsepower, even though they spend most of the time producing only a small percentage of rated power.

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jdemaris

10-20-2010 06:02:16
67.142.130.37



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 Re: No, not true . . . in reply to MarkB_MI, 10-20-2010 03:41:09  
Not near true, about gensets. The reason why many run so fast is because it is the cheapest way to build. That is, running a smaller engine near it's max.

The most pricey and fuel efficient standby gensets run very slow, at 1100 or 1300 RPM. The cheaper ones buy cheaper engines and run that at their near max.

HD Delco gensets with Detroit Diesels run at 1100 RPM. Newer four-sroke diesels powered by Cummins, Isuzu, etc. tend to run 1800 RPM.

The most efficient smaller portable gensets run even slower and match RPM to load. That is done by making DC instead of AC, running it through a full wave inverter to make AC at 60 cycles at any engine RPM.

I don't believe you're going to find test specs anywhere on any diesel engine the runs most efficient when out of it's peak torque-curve. And with most - that is 1800-2200 RPM.

A small cheap engine can be made to behave like one twice it's size - simply by runnig twice as fast. That's the cheap-way out for lower priced gensets.

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MarkB_MI

10-20-2010 17:28:05
166.217.128.237



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 Re: No, not true . . . in reply to jdemaris, 10-20-2010 06:02:16  
JD, re-read Blue Man's original post:

"they operate at 3000 while belt driving the generator at 3600 RPM"

Now what you're saying may be true of direct-drive alternators, it's certainly not the case here. Since they're using a belt-drive, Kubota has the option of running the engine at any speed they choose, and they chose to run it at 3000 rpm. And I was specifically addressing Blue's point about why THESE engines are run at 3000 rpm in a belt-drive generator application, nothing else. My positions still stands: they run them at 3000 rpm to ensure they have plenty of extra power so the engine doesn't droop under heavy load.

As for fuel consumption, there's no need to speculate as to what rpm gives best efficiency and how that relates to the engine's torque curve. Just look at the graph provided by Blue-Man: The specific fuel consumption is shown as the bottom line on the graph in the customary units of lbs per horsepower-hour. It's hard say exactly where it is lowest, but it's nearly flat from 2000 to 2800 rpm. With that in mind, it only makes sense to run the engine at a speed where it has plenty of horsepower on tap; better to burn a little extra fuel than to not have enough power to pull the load at all.

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jdemaris

10-20-2010 17:50:39
72.171.0.148



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 Re: No, not true . . . in reply to MarkB_MI, 10-20-2010 17:28:05  
I doubt "Kubota" has anything to do with it, unless the complete genset is actually made by Kubota. It's probably a unit just using a Kubota engine, and why it's hooked that way, I can't tell without seeing complete specs.

I do know this. I've been researching (and installing) gensets for solar apps for quite a while. Efficiency has been a big issue along with perfect sine-wave power, which many faster rigs do not make. I've yet to see any fast running gensets that run as efficient as the slow ones. I posted specific fuel consumptions specs on those. I'd like to see the specs on whatever this Genset is with the Kubota engine turning 3000 RPM against the belt-drive 3600 RPM head. The only time (I'm aware of) that an engine is not most efficient within its torque curve, is when it is not sized for the job properly. Peak efficiency calls for matching the desired load and running within the torque-curve at the same time. That's why big farm tractors are dogs on fuel when used for small jobs with low power demands.

My main comment was about the reason why some gensets have fast running engines. I still maintain it is because it's the cheapest way to build, and not for more consisitent power. It's also why RV gensets are so expensive. They have to use more engine, running slower, instead of less engine, running faster.

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MarkB_MI

10-21-2010 03:04:04
166.203.42.166



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 Re: No, not true . . . in reply to jdemaris, 10-20-2010 17:50:39  
JD, there's no such thing as an engineering decision that isn't a compromise. Sure, you can run this particular engine slower than 3000 rpm and get better specific fuel consumption. The chart says so. But then the genset builder would have had to derate the generator, since less horsepower is on tap at the lower speed. To get the same horsepower at the lower speed would require going to a bigger engine, which adds both weight and cost.

Taking your argument to its extreme, we should still be running Fairbanks-Morse generator sets from the 1920s, since they are very slow, reliable and efficient. But you can't put one in the back of your pickup and you can't buy one at Home Depot for 500 bucks.

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jdemaris

10-21-2010 05:33:51
67.142.130.21



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 Re: No, not true . . . in reply to MarkB_MI, 10-21-2010 03:04:04  
True, but I guess if we are going to desscribe "gensets" and engineering, the discussion has to be narrowed a bit. Yes, a smaller engine running fast (often air-cooled), that makes imperfect AC power, makes a unit much easier to haul around. It's a compromise, but as I've maintained - the fast running engine has nothing to do with making better AC power.

There's a big difference in the purpose and engineering between "portable" or "jobsite" gensets, as compared to little used "home backup gensets", compared to constant duty gensets designed to work a lot and make near perfect AC power.

Portable AC gensets rarely make correct AC power, but yes - are handy and easy to move around. They also tend not to be very efficient. Imperfect AC power rarely matters with power tools and not-electronic chip-controlled appliances. On the other hand, the more pricey portable DC to AC gensets are pretty efficient (often called Inverter series). Note they also run very slow much of the time.

Standyby gensets primarily designed for home backup come in many flavors, but many are cheap crap NOT designed to be used constant duty. In fact, many void the warranty if they catch you hooking one up to a house that lacks grid power. Read the small print in their warrantees.

The most expensive, slow-running, constant duty gensets that replicate grid AC power are the most expensive, and the most efficient have the slowest running engines.

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Sparktrician

10-19-2010 18:09:12
173.110.48.109



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 Re: HP-RPM-Diesel Fuel economy question in reply to Blue_Tractor_Man, 10-19-2010 14:25:19  
To me....3000 RPM on a 2 cyl diesel is FAST

unless you are trying to blow or pull something..



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buickanddeere

10-19-2010 18:28:41
209.240.125.206



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 Re: HP-RPM-Diesel Fuel economy question in reply to Sparktrician, 10-19-2010 18:09:12  
Calculate the piston speed in ft per second. The wee Kobota with maybe a 2-3/4" stroke at 3000rpm is running slower than a 6" stroke diesel at 1500rpm.



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jdemaris

10-19-2010 18:57:16
72.171.0.138



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 Re: HP-RPM-Diesel Fuel economy question in reply to buickanddeere, 10-19-2010 18:28:41  
That makes me think of Ford tractors in the 50s-60s. Their claim to fame was their short-stroke engines. That short-stroke reducing piston speed and travel - and according to Ford, making a better engine.

Personally, I'll take a long-stroke, high low-end torque engine anytime - unless I'm running a chain saw. That's why Case tractors had such good lugging power, along with the 5.9 Cummins in Dodge trucks. Super-long stroke means the piston has lots of leverage to turn the crank. Kind of like the difference from turning a 6" long wrench and a 12" long wrench.

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135 Fan

10-19-2010 20:25:56
68.149.56.30



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 Re: HP-RPM-Diesel Fuel economy question in reply to jdemaris, 10-19-2010 18:57:16  
Perkins always advertised the advantages of their engines having a 5"(long) stroke. Dave



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buickanddeere

10-20-2010 04:03:18
209.240.125.206



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 Re: HP-RPM-Diesel Fuel economy question in reply to 135 Fan, 10-19-2010 20:25:56  
That would make the perkins a faster running engine at 2250 rpm than the Kubota at 3600rpm. There is a market for small light weight diesels to operate 3000/3600 rpm loads such as generators.



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Aaron Ford

10-19-2010 17:43:51
98.157.103.136



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 Re: HP-RPM-Diesel Fuel economy question in reply to Blue_Tractor_Man, 10-19-2010 14:25:19  
Looking at your dyno chart, this layman reads it as 5hp is achieved at any point on the curve from 1600 to 3200. The torque curve appears to peak around 2100-2300. This would indicate a good energy capture of the fuel between these speeds. The fuel curve appears approach the lowest point of its curve at 2300. I would run as close to 2300 as possible. Just my read.

Aaron



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jdemaris

10-19-2010 14:29:03
67.142.130.35



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 Re: HP-RPM-Diesel Fuel economy question in reply to Blue_Tractor_Man, 10-19-2010 14:25:19  
The peak of the torque curve is just about always where the engine will run the most efficiently. That's the range of 2000 - 2200 RPM on your engine. That seems to most efficient range on most diesels.



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buickanddeere

10-19-2010 18:23:47
209.240.125.206



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 Re: HP-RPM-Diesel Fuel economy question in reply to jdemaris, 10-19-2010 14:29:03  
I agree. 2000-2200rpm is near peak torque rating and the engine is making enough rpms to develop the required HP. Enough rpms to avoid lugging and not excessive to cause extra wear.



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Blue_Tractor_Man

10-19-2010 17:34:39
75.216.207.203



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 Re: HP-RPM-Diesel Fuel economy question in reply to jdemaris, 10-19-2010 14:29:03  
I wonder if this particular engine was set up to run at 3000 RPM?

Do these little engines have the capability of timing variations based on RPM?



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pkurilecz

10-19-2010 19:47:56
68.88.70.197



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 Re: HP-RPM-Diesel Fuel economy question in reply to Blue_Tractor_Man, 10-19-2010 17:34:39  
To answer this question properly requires the flowrate (Q) -- head (H) chart for the pump.

I suppose that this is for an irrigation pump.

From the Q - H chart, you can get the RPM that the pump has to operate at to deliver the Q and H. From this you will obtain the HP that is needed to power the pump. You can also ensure that the pump is operating where it is most efficient.

From the engine HP - Speed graph you can then determine which RPM will supply this HP. Be sure to use the continuous ratings.

However; since the RPM of the engine will probably not match the pump RPM, you will then need to determine the driver and driven pulley sizes and belt requirements. (5 Hp is well within the capability of a single belt, but I would use two for reliability. Nothing worse than a belt breaking just after starting and you don't find out until afternoon.)

You will then need to figure about a 10 to 15% power loss by the belt drive. Refigure the engine speed and recalculate the pulley sizes.

Also, check your pump application. If the pump is generating too much head for the application, then you are wasting power and as a result fuel.

If less head is needed, you can put a smaller diameter impeller in (or machine down the existing one). Most pumps have different Q -- H curves for different diameter impellers.

hth

pkurilecz

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Ken Macfarlane

10-20-2010 08:01:34
156.34.142.102



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 Re: HP-RPM-Diesel Fuel economy question in reply to pkurilecz, 10-19-2010 19:47:56  
You also don't have the full fuel map for the engine, the efficiency at full output is shown but not the part load efficiencies. That type of chart looks like a topo map.



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