Question about a Leece Neville 110-555j

[Henry W]

Before you purchase any pulleys did you figure out what the engine operating speed will be? This is most important. I know the recommended engine speed for the KAW FE350 engine is from 3000 to 3600 rpm. I will post more on this later this evening.

Henry

[cohippy]

i was planning on running it around 2000 - 2250rpm

[Mad_Labs]

my trigen unit, the engine drives an st head, an AC compressor for refrigeration, twin 555 alternators, so there will be times that the engine is running providing power for other uses, and if the batteries are fully charged, then yes the controller will go into float mode.  no reason not to under those conditions, and an argument to be made to do so under those other conditions.

bob g

Makes sense! I was sure you hadda reason...

Jonathan

[Henry W]

Hi cohippy,

2200-2250 rpm is way to slow for that engine. Kawasaki, Briggs&Stratton, Kohler, Honda and other major manufactures all designed their comercial air cooled engines to run at the speed range from 3000 to 3600 rpm. There are reasons for this.

One is lower vibration which also reduces stress on the engines internal parts and the equipments parts. Modern single cylinder engines can create frequency that will tear apart an engine and equipment if they are ran continously below recommended engine speed. There is a thing that is called crittical engine speed. It is best to keep engine harmonics / frequency within check.

All manufactures have an engineering application data manual which goes through very throughly what I am trying to explane. They are only available to the Manufactures engineers and OEM's. I have the privilage to have access to read this data.

Two 3000 to 3500 rpm is the speed that the engine is effectively cooled by the fan.

Three The engine will run more efficently at recommended speeds.

Setting the engine up to run at 3000 RPM unloaded, and than running it loaded will see engine speed (rpm) droop. It can be up to 5-10% so this means the engine speed can slow down to 2850 - 2700 rpm's. Look at the attachment I posted. It has the performance curves and it shows at aprox. 2700 rpm is just past peak torque. It is best to have the loaded engine speed set past peak torque curve because of the speed droop that will happen.

Henry

[mobile_bob]

just get two 555's and drive them both, run the engine at rated speed or 3000rpm
if you like, and control the alternators with a 3 step controller, so you can dial in
the right amount of load to fit the engine.

or drive one 555 and something else that is useful such as a refrigeration compressor?

seems useful to me?

bob g

[LincTex]

There are reasons for this.... There is a thing that is called crittical engine speed. It is best to keep engine harmonics / frequency within check.

OK, I understand everything you typed, and from an engineering standpoint all the info you presented should be, and indeed appears to be,  correct.

HOWEVER, in the last 40 years I have played with more air-cooled engines than I could ever count. I have not experienced "failures" that correlate to the possible issues that you stated could occur.

I WILL agree that small diesel engines, by all means, are very sensitive to the load placed on them at the specified RPM they are running at/should be running at/are rated at (critical speed). But gasoline engines, in practice, are actually pretty darn forgiving. I have never experienced unusual overheating problems due to reduced airflow, even at moderate loads (I NEVER mow my lawn at full throttle - EVER!)

And as far as efficiency goes - I say you are splitting hairs on that one.
If I run an air-cooled gasoline engine at 1800 RPM at half the load it is rated for at 3600 RPM, it's going to use about half as much gas. maybe a little more (guessing 5-10%?), but you and I won't be able to tell in the practical sense.

I do thank you for providing such useful information, but I believe cautioning someone against running their 3600 RPM "rated" engine at anything less than that speed is rather silly.

[glort]

I built a " generator" with a car alternator on a 5Hp petrol engine many years ago.
I used it for running some computers and Photo Flashes off an inverter and Battery. There were a few reasons for going this way rather than a normal generator.
The way I set the thing up was with an over sized pulley on the engine so I could run it at about half speed for fuel economy and less racket.

I used that thing 8 hours a day for probably more than 300 Hours.
Under load the engine did work but it was set up so it never bogged down or over loaded.  I did have an oil temp gauge tapped into one of the oil drains and the thing definitely ran cooler than at full speed with a smaller pulley to make the same power at the alternator.

PRoviding an engine is not over loaded at less RPM, it will be burning less fuel than at full revs/ load and therefore producing less heat. I would suggest that the cooling system on most small engines would be more efficient at lower revs than higher ones due to to less bottlenecks and induced turbulence of the air path and also for the fact that slower moving air is well known to pick up more heat than fast moving air.  I would also suggest that the frictional heat buildup is likely to be proportionally  less in a slower turning engine.

When I set my gen set up I was warned about carbon buildup in the engine. I never saw any and I did pull the head several times to see how the thing was going.
Perhaps this was a concern years ago the way fuels and oils were but it certainly didn't cause any problems on my engine.

I have seen engineering papers that state that engine life is magnitudes longer with engines run at 80% less revs and power than those run flat out. Given that a number of small engines are only splash lubricated and have plain bearings ( Hello B&S) under speeding these engines has to be a good thing.
t's no secret that the old slow speed engines like Listers lasted forever and that was in no small part to the low revs they did and the resulting reduced wear.

I also found the fuel consumption to be markedly improves with the engine turning slower. This is also mirrored in Car engine design where some models of vehicle have actually got better economy with a larger engine in a new model  than the older one by using the increased torque and reducing engine speed.

As for Vibration harmonics, I don't see lower engine revs being a concern. These are small, mass manufactured engines not Racing engines.  I have opened up plenty of brand new small engines ( I wouldn't run one without stripping and setting it up properly) and they are far from a precision bit of engineering.  The manufacturing tolerances would be a lot more likely to upset any balance and harmonics that was built into the things and the faster they run, they more the imbalance is likely to cause stress within the components.

In my limited specific experience  in under driving small engines, I can only say that I have seen every benefit and no problems what so ever.

[millman56]

Going on what I have read and seeing some engine performance graphs , it seems that a diesel engines best fuel efficiency as opposed to its best fuel consumption, is around peak torque and best mechanical efficiency rpms,  I think that petrol engines would be the same.

Not sure about slow moving air picking up more heat than fast moving air, do you mean proportionally more?  Laminar and turbulent flow issues apart, more airflow moves  more heat.   

The Lister engines longevity can be due as you say to low speed but also to a very low bearing loading coupled with a modest power output for a given capacity.

Car engines last a long time because its impossible to drive them at their maximum quoted output continuously, I suspect if we could they would not last very long at all.    The continuous rating of automotive engines used as industrial engines is substantially less than their auto rating.

Mark.