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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

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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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67
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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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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

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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.
 

Warwagon

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During a tour in 1994 or 1995 at an Intel plant in Rio Rancho I was told about how they got caught by turbo charger lag on a lot of big V16 CAT diesel generators. Big very expensive lag problem both in cost when they tripped on it and to fix it. Pun intended. Long story on how and why. It's been 25 years so best I recall.

The event I recall was prior to or in 1995 and then there was only one 345kV line at the time. A big rig wreck took out the said single power line and not just power to Intel...

So power is out and the Intel plant is switched to UPS battery room power.
Generators start. I lost count of how many V16 CAT diesel sets they had. It was impressive and a lot.
Power comes up on generators.
Load is switched from battery room to gensets.
Black smoke as the gensets go full rack and loose RPM.
Load switches back to battery room due to generators not taking the sudden load.
Generators come back up to RPM.
Load switches to generators that again bog and can't take the load.

This loop went on until the battery room caught fire. :oops:
Some bad things happen when air hoods stop running and chemical vapors used in the process start mixing. :hitfan: This is when they hit the BIG RED BUTTON to abandon the plant. Oh yeah, and a rather big battery room is on fire.

Smoke particles are bolder sized debris on chips. That and other details of the sudden production stoppage ruined nearly everything on the production line. Quite an expensive power outage, battery room fire, production equipment damaged from power loss and/or loss of chemical flow, days of lost production, and ultimately generator performance failure during an emergency.

During the production power outage the battery room was switched to generator(s) all at once. The turbo's could not spool up fast enough before the engine RPM bogged down out of frequency. Then because the load was switched "off" back to battery the sudden no load allowed the turbos slow back down.

The generators needed the turbo's at boost to be able to generate their rated and expected available power. Can you see the calculation that got screwed up or missed here? One of them is: Rio Rancho is at 5282' in altitude and asking full load with no turbo boost to erase the altitude...

The fix that was implemented was spooling the the V16 Cat diesel turbos on compressed air. This way the gensets started, came up to RPM, then the turbo's were brought up to boost on compressed air, and then and only then was the load switched to the gensets.

I'll leave you with the thought and raw sound of the compressed air needed to spool a V16 diesel turbo. Now think of a lot of them for a large production plant where everything stays powered. Yeah think about stalling a V16 at rated RPM by hitting it with a big enough load and the only way around it is to spool the turbo up before it gets the load.

Later in March 2000: Three 345kV transmission lines failed due to a brush fire taking out power to most of the State of NM. The March 2000 outage illustrates "the fix worked", the scale of the problem, and cost of an outage. The attached PDF contains info on the 2000 outage: Online here. (I recall a 2nd transmission line being built around 1995.)
 

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Birken Vogt

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You would think an organization like Intel would have had the generator vendor demonstrate a cold start-full load acceptance test on a facility like that. Sounds like a failure in engineering and a failure to test performance. I would think this could have been discovered without ever having to move the transfer switches.
 

Warwagon

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Again this is merely an example that you can't expect zero to full rated load out of a turbo generator engine without ways to bring the turbo up to boost first. I belive this is what you were wondering with the NA question: what is it's surge ability before the "turbo boost button" gets pressed.

I am sure a lot of things could have been done different and the PDF mentions "lack of understanding" to some cutting edge new equipment when power is lost. I assume, because I don't recall exactly, the plant expansion and loads being higher during production than in warm standby had something to do with it. By the later report we can assume the extreme cost from the failed backup power implementation was given better funding and priority.
 

Birken Vogt

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The nice thing is that these home standby and small-medium commercial units usually don't have to accept full load in one step. It will be a bunch of air conditioners and refrigerators not all of which are on at the same time.

Now if it has been shut down due to lack of fuel or anything of that nature, some stepping of the start might be required on restart.
 
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