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Turbocharging Vapor Fuel Generator Engines

Birken Vogt

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The world of generator engines is always changing. It seems that now, more than ever, small engines are getting turbocharged to meet certain power levels.

What level of boost is required to drive say a 2.5 liter engine to say 40 kw? Where the NA power level of said engine would be 25 kw at 1800 RPM? Any comment on the effect of turbocharging on engines of this nature? I know it has been done off and on a lot in the past but seems more prevalent lately.
 

Vanman

Subscriber
Last Subscription Date
07/10/2019
One thing working in your favor when using gaseous fuels is their high octane equivalent. I want to say that propane is around 100 octane, and natural gas is maybe 115. Can get away with a lot of boost, and not have to take out much timing.

When you start packing in the BTU continuously my concern would be the temperature of critical parts like pistons and exhaust valves.

Keith
 

dkamp

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25kw to 50kw is doubling, which in terms of fuel/air inlet, means you'll be looking somewhere around 12psi of boost. The question is... what is your engine's stock compression ratio, and will it withstand the equivalent thermal and physical load of doubling it's torque?

The reason why I say 'torque', is because in a synchronous application, additional output can't come by virtue of increased RPM.

15kw requires 20.2shp, which at 1800rpm is 58.94 ft-lbs. Going to 40kw is 52shp, which at 1800rpm is 151.73ft-lb.

To make that electrical output, you'll need to more-than-double the available torque.
 

Jim McIntyre

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07/10/2019
Well, 14.7 psi of (intercooled) boost pushes twice the fuel/air mixture into the engine. So, something less than that - maybe 10 psi?
 

Vanman

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07/10/2019
Interesting. I’d rather have one that uses a turbo to achieve rated power rather than 3600 rpm. Since most generators are operated well below rated load most of the time, it seems that you would have the longevity of 1800 rpm, the economy of a small engine, yet the extra power available when you actually need it. Done right I can see it being pretty slick!

Keith
 

Birken Vogt

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That is my thought as well.

Most of the time generators these days are sized to appease the governing authority's idea of what it should be rather than any realistic load profile. Might as well do it with a turbocharger instead of having to use a V8 where a 6 or 4 would do in real life.
 

dkamp

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He wants to go from 25 kW to 40. Tourque at 40 kW would be 160% of the 25 kW value.
You're correct... I have no idea where I came up with 15kw... a little dain bramage go salong way... you are correct.

Keith- using a turbocharger has it's advantages in SOME ways, but not so much others. The downside to running a turbocharger on a generator is maintaining stability. In order for the synchronous generator to stay stable AND responsive, it must have lots of torque available RIGHT AT the point of governance, and even MORE torque slightly (like... 1hz'worth) below.

For a turbocharger to work well, it NEEDS to have exhaust pressure always available, which means there has to be enough fuel burning to create that pressure.

A generator has basically four load states- first one is full rated load... the 100% duty cycle point. Next one is it's rated overload state (which includes a maximum duty time for 'safe' recovery). {My big red generator has an overload rating of something like 150% for 24hrs at 100 degree F ambient temp]. The next, is a partial load (somewhere between zero and 100%, and finally, ZERO load.

When designing a robust, stable, yet responsive generating plant, the prime mover's torque curve predictability is EVERYTHING.

The big huge Gotcha... is Minimum Fuel Consumption. When that machine is running at zero load, the governor is keeping the throttle almost closed... just what it takes to keep the engine spinning at synchronous speed, against the bare minimum static load. The cooling fan, oil pump, coolant pump, battery recharging, excitiation, control electric requirements and gen-head cooling airflow are it's only loads.

When a turbocharger is employed in this application, it MUST be set up to create boost at basically NO load. Why? Because if it doesn't, the throttle-response to load is non-linear, and unstable. A quick chop of electrical load can yield overboost, and a quick load applied results in sag before boost builds.

That being said, they work much better in a diesel than in a spark ignition system, because the diesel is fuel-only in it's control... excessive boost pressure does not cause a fueling problem, and as such, they can run a turbo sized to develop substantial boost, and then blow off all kinds of excess pressure at high load, with no ill effect.

Also keep in mind that the generator's greatest foe, is heat... getting rid of it... both the engine, and in the generator head... while a larger generator engine will create waste heat too... the turbocharger induces smaller areas of higher temperature, rather than being evenly distributed and carried to a radiator. Having that, inside an enclosure, is not a good thing.

IMO, the best solution to having good power in a generator, is to have substantially more available than the load requires... and this is a classic solution in 'real' generators, for more than one reason... one being that having more than enough torque means it CAN withstand high loading for extended timeframe, and can respond to large loads with no apparent side effect, but a very important one, is that in a multifuel application, the same machine can carry the same output rating, regardless of the fuel quality or type. My big 35kw will churn out it's rated (and then some...) regardless of wether it's running on 100LL avgas, or 84 RON tiger-piss, or Propane, or Natural Gas.

The best generators in terms of durability, reliability, serviceability, and dependability, are the simplest ones. I think these three hot-rodder's adages certainly apply here:

1) There is NO replacement for Displacement
2) The only substitute for cubic inches is cubic money
3) A turbocharger means you ran out of space for more, or bigger pistons.
 

Motorhead

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Age
67
Last Subscription Date
07/09/2019
I have seen several Onans with what looked like 460 ci Ford V-8's and they were turbocharged. When you are running an engine at only 1800 rpm, you are not really in the sweet spot to where the engine is producing good horsepower. The turbo is there to help with that and to also increase cylinder pressures which with natural gas helps in making better power. I have a boost gauge on my 5.9 Cummins and I can run down the highway at 4-5 psi boost. I hit the throttle and it can get as high as 36 psi. I can be running at 1300 rpm and the turbo helps pick up the engine speed quick. The turbos on generators seem to be really matched for set speeds and they don't seem to run a waste gate. I believe you could put a turbo on a Ford 300 running a 35-45kw set and get more KW.
 

Vanman

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Last Subscription Date
07/10/2019
Another thing to consider is that the turbo would (hopefully!) be properly sized for the engine that never exceeds 1800 rpm. So even at no load, the engine is at maximum speed as far as the turbo is concerned. As such it will be zinging right along, even at no load. So the turbo lag would be considerably less than what you would experience in an automotive scenario.

Keith
 

dkamp

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Yes, if they were to turbocharge the stationary application, they would size the turbo wheels small, so that even at a very lightly loaded circumstance, there would be boost, but the total boost would be maintained low... like... half an atmosphere at the max... probably a quarter to a third ATM. They would also increase the amount of static load (like, more cooling fan belt load) so that the engine's minimum throttle load was always high enough to keep the boost around the wastegate limit.

Realize that in really BIG generators, particularly diesel -based spark-ignition gaseous, that a turbosupercharger is frequently used... a turbo that has a gear drive and overrunning clutch, that below a certain exhaust pressure level, an overdriven gear system feeding the overrunning clutch spins the turbo compressor, so that it is always generating a base level of boost.
 

Birken Vogt

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Thank you for the thoughts and info.

Be all this as it may, turbocharged generators were somewhat common in the past and now seem to be getting more common again.

All these from the major manufacturers are of course rated to accept full load in one step so they must have figured it out somehow. The length and size of the intake tracts I have seen is quite long but they are putting the throttle plate right before it enters the head so it's after the turbo and all that. The turbos I have seen these days are wastegated, and also very small physically. They are compressing a fuel/air charge mixed right at the compressor inlet.
 

Wayne 440

Registered
When I think of a small turbocharged gas set, the 20 and 30kW Kohlers from about 30 years ago come to mind. Both 4 cylinder Fords, the 30kw with a turbocharger. The 30kw really liked fuel, and (to me anyway) was hard to get set up for good governor response, while the 20kW was easier on fuel and generally required little more than hook it up, flip the switch, watch it work and call it done.

To me, adding a turbo= adding something else to create problems. The ideal engine for 40kW on gas was probably the 300 Ford, now essentially dead thanks to the EPA and their alleged wisdom.
 

dkamp

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I'm certain the 300ci Ford would be good for 40kw on gas... just hafta pick the RPM range where the fuel energy would suit it best in terms of torque thermal stability, and responsiveness.

Birk- the one thing that turbos on gaseous and gasoline DO that challenges governance in a way that diesels do not... is pressure reactions on the throttle plate and linkage. A diesel's speed control being totally on fuel-racking, there's no side-effect to boost, but on a throttle-plate, the turbo increases pressure against the throttle when close to closed... and this makes the governance 'ginky'... and that's one of the reasons why you'll find generators in that performance class never running in less than about a 50% load... so the throttle is always at least half-open... it's 'real' working range is between half open, and 3/4 open.
 

Birken Vogt

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Good info there. I love sorting out governor issues, not. One thing is for sure. The manufacturers are selling these things by the boatload here in California now. We will definitely hear about it if governor issues start to crop up.
 
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