Lets figure this out! Standby generator, slow down engine but maintain genny

We have a 60kw generator that sometimes needs to run light loads. It consumes more fuel than needed because the engine is buzzing at 1800rpm. I would like to explore slowing the engine but I realize this means breaking the coupling between engine and genny and doing "something" in there. Not even sure what yet. I would be fine adding a pulley and belt so I can run the genny head with a smaller engine but that is not that ideal.


Why no one made a little transmission for these is beyond me. I will have to examine the power curve of the engine a bit to see how it will do but if a Semi can idle using less than 1gal/hr, and I am burning two, I think slowing down should help.

Gen HAS to turn a certain speed to make a certain Hz

Slowing motor/engine down while turning the gen same "fast" speed for the right Hz just uses more power - because motor/engine is working harder

More power needs more fuel

Just chasing your tail looks like to me

You need to maintain frequency, so RPM matters. Unless it is an inverter based generator.

A lean burn carburettor might be a more simple exercise. That is, a progressive fuel delivery based on throttle position.

CalG said:

You need to maintain frequency, so RPM matters. Unless it is an inverter based generator.

Click to expand...

I saw a Honda generator recently that idled down at light load but maintained 60hz. I imagine it had an inverter built in. I have no idea how you'd go about attaching an inverter to an existing generator, would something like a VFD work?

Mud, you were likely looking at an EU3000 inverter.

YES, guys! I fully understand about the genny head rpm to maintain frequency, thus my poor explaination about uncoupling the motor and genny. NO, it would not be loading the engine higher because there is next to no load here!

Basically trying to trying to mod a 60kw into a 20kw.

The real issue is not RPM, since the engine can maintain RPM at a lower throottle setting with light load than with heavy load. The issue is more the efficiency of the carburetor and engine at lower engine RPM, vs at a higher RPM. Always possible that it will burn more at a lower RPM, because the maker KNEW what the RPM had to be, and optimized the thing to operate there.

That means that half power uses more than half the fuel.

It would be set up to use the least possible fuel at near full load, maybe 80%, because buyers will look at fuel usage. What that does for lower powers is not necessarily known, but in general will not follow the load.... after all it would idle at essentially zero power out, and still burns fuel.... so the graph of fuel usage does not go to zero AT zero.

Likely the system is less efficient at lower RPM, because it was not designed to operate there.

What is the fuel?

If diesel one may be able to place ball valves inline with injectors to simply remove a couple cylinders from the engine.

If gas it is more difficult.

If you have 60 kw and only need 20 best is to get correct size and call it a day.

Sent from my SAMSUNG-SM-G930A using Tapatalk

Smart shopping can get you a smaller gas or diesel genny (probably inverter-based) that will run optimally at the smaller loads, and give you added capacity if needed at shop or elsewhere. All for less than you'll spend (time and materials) on your proposed mod. Plus, you're not down in the interim.

Engines have an "efficiency band" just like a "power band". Dropping rpm a lot will get you into a range with lower efficiency which will counter some or possibly all of the savings you would gain by reducing engine speed. Some of the losses are coming from spinning that large gen head and only using a fraction of the capacity.

Other consideration to slowing the engine down would be adjusting the governor to the lower speed. Will it even be stable at that speed or will the governor pulse/surge?

Lastly, have you looked up the power curve of your engine? If you really are only trying to make 20kw you may be surprised how little room drop you can make and still produce 1/3 of the hp in a manner safe to the engine. I'm predicting the 20kw output level of your engine is around 1400 rpm. How much will you really save dropping rpm from 1800 to 1400?

A belt reduction would be the cheapest but more maintenance and does have some catches. The bearings in your direct-couple gen head can't take belt loading so you have to build your belt reduction mechanism then direct-couple the gen head to that. A few shafts and bearings and pulleys and some belts and you're in business. Plus adjusting the engine governor.

You would be much better served to get a properly sized genny... 20kw units are easy to find...

20kw would be a stretch and I probably should not have posted that. More like 5kw. I certainly realize the efficiency curve of an engine, which correlates directly with the torque curve. However, The efficiency is also tied to load. The lower the load, the lower the efficiency.

I know we can go around about it but to be simplistic, start your car and rev it at 1800 while sitting in your driveway, then let it idle and let me know which uses more fuel. Yes, one could argue about spinning the generator rotor but in a car, you are still cranking all the accessories and the input shaft to the trans so there is some load.

I might be chasing my tail, indeed. Was just trying to think out of the box. Finding small 3P generators is not easy and most small ones are the "screamers" which are 3600rpm fuel guzzlers. Since we seem to have a lot of time in which we don't have much more than 2-5kw load, I guess I am looking for the most efficient or easy way to cover that. I know a BIG diesel can idle using less than 1gal/hr so I figured I might be able to consider a bigger/slower engine config rather than the typical "run engine and genny at 1800.

I will look for other options here I suppose.

huleo said:

I know a BIG diesel can idle using less than 1gal/hr so I figured I might be able to consider a bigger/slower engine config rather than the typical "run engine and genny at 1800.

Click to expand...

Where did you hear that ? Diesels are an entirely different animal than gasoline engines, since the air is not throttled. Tonytn's idea was interesting, but other than that, I'd say you're going to spend a lot of time trying to save ten bucks a day.

(Not sure about ball valves at current injection pressures, but the idea of running a 6-71 as a 3-71 to save fuel at low loads is interesting ... although running three cylinders as a compressor might not work so great, you'd want to jake brake those cylinders as well ?)

Nah. Get a smaller genny and just fire up the big one when you need it. If you can save the purchase price on fuel, go for it

Get a smaller genny and just fire up the big one when you need it.

Click to expand...

Totally

My little 3ph 10Kw gas unit is a noisy bugger, so made up a decent length extension to get it away from the shed a bit. Neighbors are far enough away and are, in their own way noisy buggers, so all good.

Got a bit of cool room panel over in the other shed. Thinkin about making a drop over cover to help reduce the noise a bit. Only a thought so far, gotta think about ventilation, exhaust etc...

The cost of your mod would exceed the cost of a correct generator, At the moment the US ARMY is scrapping a bunch of generators The 5kw ones are inverter based. All the ones I have seen are very low hours and in lightweight sound boxes.

You are in Germany and you know this?

5kw 3P? With a normal voltage? I do own an inverter and like it. They idle down when not under hard load, which is pretty much what I would like to do..... Finding quality, small 3P gennies is not easy. They are usually cheap screamer types.

huleo said:

You are in Germany and you know this?

5kw 3P? With a normal voltage? I do own an inverter and like it. They idle down when not under hard load, which is pretty much what I would like to do..... Finding quality, small 3P gennies is not easy. They are usually cheap screamer types.

Click to expand...

I follow the military surplus scene closely. DRMO scraps surplus all over the world. I have a close friend that holds the current European DRMO contract. He has around 70 of these generators currently, but if you search, they are also available from the current US DRMO contract holder (Government Liquidation). You may also note that there are many sizes, but only the 3KW units are the inverter type.

After further thought, you should consider NOT using a generator at all. Inverters are really the way to go for small loads.

steve-l said:

I follow the military surplus scene closely. DRMO scraps surplus all over the world. I have a close friend that holds the current European DRMO contract. He has around 70 of these generators currently, but if you search, they are also available from the current US DRMO contract holder (Government Liquidation). You may also note that there are many sizes, but only the 3KW units are the inverter type.

After further thought, you should consider NOT using a generator at all. Inverters are really the way to go for small loads.

Click to expand...

FWIW Gov liquidation hasn't had the US DRMO contract for nearly a year now... I can't recall the company but it's a large multi-state auctioneer that holds it now.

It's a sensible and nobile desire to not run 'too much' generator. In a real-life generator application, one needs to be in-touch with the amount of fuel they consume, and not do it senselessly.

Unfortunately, there's a whole lot more to a generator's efficiency, than just the engine's speed. Matter of fact, the engine's speed has almost NOTHING to do with it... and yes, a generating plant that uses a synchronous generating scheme MUST have the armature turning at an appropriate speed in order to produce proper frequency... although you could put a 'transmission' between the prime mover (engine) and generator unit, the end result will yield such a small reduction of fuel economy, that you won't be able to reliably measure it.

The OVERHEAD of a running generating plant... is the amount of energy it consumes JUST TO RUN... not to actually drive a load, just to be IN OPERATION.

First, is the overhead that the engine needs just to spin:
-Overcome internal component friction
-circulate lubricating oil
-circulate coolant
-circulate cooling air

Now couple a generator:
-Overcome friction
-Circulate cooling air
-produce DC for field excitation

Even before producting a MILLIWATT, you're burning a fair chunk of fuel energy, and what isn't being turned in to heat, is being turned into noise.

Now, consider the ramifications of slowing down the engine:

The fuel system and tuning of the engine is optimized for the generator's synchronous RPM. If you were to design a generating plant that was intended to run at 1800rpm, how would you tune the engine?

If it were me, I'd set up the intake, exhaust, camshaft, and timing such that the torque curve's PEAK was at 1799rpm, and at 1806rpm, it dropped totally off the planet. I'd also make it so that once it got below 1500, it'd be basically worthless.

Why? Because your load engages with a RUNNING, but NO LOAD machine. When you engage the load, the engine is at basically NO throttle, but at around 61hz. as soon as the engine takes a load, the engine slows JUST FAR ENOUGH so that the governor SEES the load, and opens the throttle. This is called DROOP... in order for the governor to react with proper proportion and gain, the engine's speed must actually VARY from absolutely NO load, to lightest load, to heaviest. The governor MUST see the RPM fall, in order to properly open the throttle to compensate. From that basic point (droop) the governor linkage geometry, governor flyweight design, and governor spring rates determine how much throttle appears as a result of a given amount of droop.

Now consider- you've gone through the trouble of putting an overdrive transmission on the engine, and made a drive-end bearing support plate (the generator head usually only has one bearing... the other is by virtue of the engine's flywheel), and you're running the engine at 600rpm, geared up 3x to put the generator at 1800rpm. Your peak torque is WAY above your running speed... and at this speed, the distributor timing weights are sucked-in to provide reliable idle and safe starting, so the engine has no power... and of course, the governor is totally insensitive to any speed change, because the flyweights are effectively bottomed by spring force. The only thing keeping the engine running is... the carbeurator throttle stop.

Does this enlighten you any?

The BEST solution to having a really big generator, is to have a second, smaller unit. I have THREE... a mid '50's 7.5kw single-phase Kohler, an early '40's 12.5kw three-phase Kohler, and a late '50's military 35kw (very conservatively rated) 240v single-phase. They're all liquid cooled, with the two small ones being four cyl, the big'n is an inline six.

They're all fairly slow runners... the 7.5 is 1800rpm... the other two run 1200rpm.

They're all liquid cooled, because they last longer than air cooled, and don't have shrouds that become plugged up with rodent furniture.

Oh... and all that waste heat you get from a generator? I have hydronic floor heating in the shop and house... and the engines' cooling loops will be used to recapture waste generator heat to warm the shop and house. It will eventually be thorough enough so that I will likely install clutches on the fanbelt drives. Recapturing the generator's waste heat in winter, saves me the need to burn propane in my furnaces too.

Be very careful you don't over cool the engine by capturing the generator's waste heat. It will be very bad for the engine.

DaveKamp said:

The BEST solution to having a really big generator, is to have a second, smaller unit.

Click to expand...

+1. I operate a working boat in Alaska each summer. We have a 60KW main generator but when the boat is not underway we only need from 5-20 KW. Our solution was to get a 20 KW generator and switch as needed. Significantly reduces fuel use and noise. Our source is used trailer-mounted generators used in construction and similar settings. Multi-Quip is a common make and are frequently available from rental companies. Most seem to have sound insulated enclosures and have single and three phase up to 480V.

steve-l said:

Be very careful you don't over cool the engine by capturing the generator's waste heat. It will be very bad for the engine.

Click to expand...

Excellent point, and in a diesel, it can be a little more of an issue, but in practice, It's pretty hard to pull that much heat from a spark-ignition engine. Like an automotive engine, liquid cooled generators have thermostats, and bypasses. If it's a genuine industrial generator, it has under and overtemp switches that're easily used to control cutoff solenoids and circulatory pumps during warmup.

Easy waste-heat recovery is to use a marine exhaust manifold, and circulate coolant from the exhaust manifold, into a heat exchanger... one will not certainly not 'over cool' the exhaust manifold. Many common standby generator engines are also used in marine applications, with raw water coming in at a much lower temperature than return from a recover system will.

A generating system with heat recovery can be run without a directly-driven cooling fan and radiator... that reduces a fair chunk of engine shaft horsepower required, hence, the base fuel consumption drops. In this case, the heat recovery available, is basically ALL the fuel energy that is NOT converted to electricity.

Let's say your base fuel consumption on a spark-ignition propane machine is 2gal/hr, at 92,000btu, and you're driving a 14kw load... that's 182,000btu/hr... if that was a perfect conversion, you SHOULD be getting 54kw or so, but you're only getting 14kw, which means the remaining 54-14=40kw is waste heat and noise. Rough estimate is that about 1kw is noise, that means 39kw of your fuel needs to be recaptured somehow.

39kw = 133,000btu.

If you cool the exhaust, and pull the excess engine heat, you'll get around 100,000btu, because at least 33,000 of that heat will be radiating directly off the engine and generator. If it's in MY generator shed, though, it's warming the room, so that's not really a loss.

100,000btu/hr will take 100,000lbs of water up one degree in one hour... that's 1666 pounds (208 gallons) per minute. If you slow that flow down to say... 3 gallons per minute, that's 24lbs/minute... with a rise of 70 degrees... or double that flow to 6gpm to yield a rise of 35 degrees.

Basically, if you scale your recovery system on BASE fuel consumption cooling, you'll never overcool it... you just need to give it some bypass time to warm up, before starting the recovery program.