Idea for least costly diesel generator... will this work?

[Kevin Warnock]

While browsing the Backwoods Solar website I came across a kit they sell called the Backwoods Battery Booster Kit. This sells for about $1,000 and gets you a 6.5 HP gas engine and a 24 volt alternator, plus the frame and pulley and electrical components to hook it all up.

The idea is you use this to charge your battery bank and power your inverter for less money than buying a generator. The kit includes an amp meter and dial so you can dial in whatever amps you like, and the engine slows down or speeds up to match the amps drawn from the engine. This is like the eco throttle on the Honda portable generators that I like, except you have to dial in the power manually.

The Backwoods Solar site sells the kit without the engine, and says you can use any engine 3 to 6.5 HP.

On the Kohler website, I found this air cooled 6.5 HP diesel engine:

http://www.kohlerengines.com/common/resources/kd350d.pdf

The engine that comes with the Backwoods kit is 3,600 RPM and so is the Kohler diesel engine. The spec sheet for the Kohler says you can still get 2,500 watts of power out of it even at 1,000 RPM or so.

I talked with the Backwoods Solar people, and they said the Kohler diesel should work with their kit, as the kit only connects to the alternator, not the engine.

I would use this system to provide 24 volts to my Xantrex SW4024 inverter, from which I would draw 110 volt AC power as needed, including enough to run two mini splits, which take 900 watts each when running.

The Kohler documentation says the diesel engine produces 5,000 watts of energy. They also list the capacity as 6.5 HP. The Backwoods Solar site says I will get 30 amps of power at 24 volts from the 6.5 HP gas engine they are selling.

How do I calculate if my proposed setup is large enough to produce say 4,000 watts of AC power through my inverter? The Kohler spec of 5,000 watts suggests it is. But I suspect I need a larger alternator. How large?

My general hope is to get the smallest engine I can to keep fuel use down, but still have enough power to provide the running watts for the two 900 watt mini-splits, which are 9,000 btu each. Note I have a 225 amp battery bank at 24 volts, so I have reserve for starting surges up to 10,000 watts according to Backwoods Solar.

I would build this Kohler engine into a quiet box with fans to cool this air cooled engine. Kohler is checking to see if a remote fuel tank is supported. Remote start is supported already, as is a 24 volt starter motor for the engine.

Thanks,

Kevin

[Dreamscape]

I can't help with your question, just wanted to say you are sure doing your homework. Very well written topic.

I enjoyed reading every word!

Paul

[HB of CJ]

Best guess is that it would work....but perhaps at a little less than your provider implys.  Also the rule of thumb is that air cooled mills NOT EXPRESSLY designed for it do not do well enclosed and cooled with an auxillary fan.  They tend to overheat.  These air cooled mills usually do much better out in the open.  The Kohler might work fine using a mix of alternate fuels.

About 15 years ago we/I made up a $20 batt charger (ni-cads) using a retired reel type lawnmower using a 5hp Briggs and a used 50 amp alternator off of some car.  Welded a bracket mount which hung the alternator off a single belt off the Briggs.  Worked great at 20 amps or sooss.  The alternator spun fast, the mill much slower.  Perhaps others will crunch your numbers. 

[Sean]

While browsing the Backwoods Solar website I came across a kit they sell called the Backwoods Battery Booster Kit. This sells for about $1,000 and gets you a 6.5 HP gas engine and a 24 volt alternator, plus the frame and pulley and electrical components to hook it all up.

The idea is you use this to charge your battery bank and power your inverter for less money than buying a generator.

Actually, that is a generator.  It just happens to be a DC generator, not an AC one.

...

The Kohler documentation says the diesel engine produces 5,000 watts of energy. They also list the capacity as 6.5 HP. The Backwoods Solar site says I will get 30 amps of power at 24 volts from the 6.5 HP gas engine they are selling.

How do I calculate if my proposed setup is large enough to produce say 4,000 watts of AC power through my inverter? The Kohler spec of 5,000 watts suggests it is. But I suspect I need a larger alternator. How large?

4,000 watts at 120VAC comes out to ~167 amps at 24VDC.  However, your inverter is not 100% efficient, and you also need to account for losses in the cables.  Figure that at ~15%, so you'd need at least 192 amps.  Call it a 200 amp alternator.

Note that 4,600 watts (after the 15%) is a little over 6 horsepower.  Since no alternator is 100% efficient in converting mechanical horsepower to electricity, you need to account for that as well.  50% is a good figure to use, so that means you need 12 HP.

If you have an engine that can apply 12 HP directly to the alternator at the alternator's most efficient RPM, you would have what you need.  Of course, the Kohler you are talking about is only half that.

DC generators, BTW, are a very efficient way to charge the batteries and provide AC power through the inverter.  However, this is at the expense of redundancy.  If your inverter fails for any reason, you will now have no source of AC power available unless you are near a shore outlet.  That might be a problem if you have, for example, a household fridge.  Just something to consider.

-Sean
http://ourodyssey.blogspot.com

[Tim Strommen]

Kevin, Paul, Sean -

We did bring up this topic a while back (albeit at a different voltage).  A lot of work went into explaining alternator load and engine power curves and how to match pullies to both.

If you are building a system that has a lot of AC appliances/loads, it's probably more efficient to use AC directly for those loads.  However, if you have a lot of appliances/loads which are low-voltage/DC, or you want to charge batteries - the DC genny is most likely the better approach.

I'm also of the opinion that if you want to play with solar or wind power generation on a bus with a battery bank - DC make-up power is also the way to go (fewer conversions).

As for redundancy, I agree with that point - to a point.  If you are trying to reduce the AC appliances on your bus, you may be able to do smaller point of load DC-to-AC inverters (for each appliance, on an as-needed basis).  If you follow that approach, you may have mutiples of the same inverter onboard, which you can swap between devices should one of them fail.  I would imagine this is probably cheaper than a single large inveter like a Xantrex or Trace.  I think BoogietheCat even suggested taking out the on/off power switch from these smaller inverters and hard-wiring them to things like thermostat contacts in a refridgerator, so that the thermostat actually controls the inverter instead of having a low-power inverter "detect" when a high-power inverter needs to be switched on for a load (this is a genius idea and is capable of saving a good deal of power typically lost to "ghost" loads).

FYI, Backwoods Solar is not the only company selling the DC generator idea - Victron Energy (manufacturer of marine power products) has a white-paper titled "Electricity on Board", in which they show the math/costs/benefits behind using a DC genny.  It's probably a good read if you are seriously considering this approach.  I recommend reading the whole document - but if you just want the highlights, start at section 8 on page 40.

-Tim

[Kevin Warnock]

Sean,

Just a note that I also have a Honda EU1000i on board that I converted to propane, so I will have backup 110V even if the other system were to malfunction. My fridge is a 24 Volt NovaKool brand, and many things that take electricity on board are 24 Volts - furnace, pumps, lights etc...). I do have some 110 loads like the TV, microwave and AC.

Given your recommendation for 12 HP for 4,600 watts after all the inefficiencies are taken into account, would that mean that with 6HP I can expect 2,300 watts of practical power? If so, that's perfect, and all I need, as it will let me run two 900 watt ACs plus a few small loads all day for minimal diesel. If I need more, I can turn on the Honda EU 1000i for another 900 watts. I want to avoid propane for the big generator so I don't have to get a costly horizontal propane tank. However, maybe someone knows where I can get a deal on such a tank, maybe 100 lbs of propane? With the huge price difference between propane and diesel, and the fact that a propane engine is going to cost less than the diesel, maybe I should just go propane only for the two generators. My water heater and stove are also propane. My furnace is diesel.

Thanks for all the great comments so quickly everybody.

Kevin

[jjrbus]

Instead of a mounted 100 lb propane tank, how about several 20,30,40 lb tanks. much eaiser to haul a tank or two to be refilled than taking the whole bus. I find for me the 30lb tanks are the most I want to handle.  HTH JIm

[RTS/Daytona]

There is a GENERAL "rule of thumb"  2Hp per 1000watts

example -> your Honda 1000i 1.8hp = 900 watts (inverter/dc generator)

here's some helpful links for 24 volt 200 amp alternators
Leece-Neville 4974PA 4964PA - 4740JB 4742JB  ??

http://news.prestolite.com/drupaldocs/PP-1184-US_Quick_Reference_Guide.pdf

http://www.elreg.com/files/MARINE%20CATALOGUE.pdf

http://www.elreg.com/product/1006.aspx

I'd check with NICK@ Minco Bus -> http://www.nimcobus.com/
for a 150-200 amp 24 volt air cooled alternator (School bus? / Flxibles ?)

Pete RTS/Daytona

[Kevin Warnock]

Pete,

Thanks for the great links on alternators. Nimco has 24 volt RTS alternators for $350 on their website. Since I have an RTS, maybe I should get one of those, since I could borrow it in an emergency for the bus if I needed to.

What speed do alternators need to go at? Are there some online calculators where I can figure pulley sizes that you would recommend?

To all:

I am really leaning toward propane now since it's $2 less per gallon. I found a huge 200 lb horizontal tank on EBay I am thinking about for $600 brand new. I like the fact that the smell should be much less with a propane generator too. Is that correct?

I use the bus infrequently, so I don't want to stick with gas. I have a much bigger choice of engines if I stay with gas type engines, which I assume can all be converted to propane cheaply. I am inclined to go with Honda. Any thoughts on the best brand? I was planning to buy from Northern Tool, which seems to stock dozens of gas engines at good prices. Any comments about buying an engine from Northern Tool?

Thanks,

Kevin

[Tim Strommen]

The speed fo the DC alternator depends on the power output required of the alternator by the load AND the manufacturer specified output power/RPM table (this is provided by the manufacturer - although every rebuilt alternator I've ever purchased came with a performance report from the manufacturer).


As a rule of thumb, most alternators don't like to be spun at over 6K RPM (bearing/material limitation).  If your engine peaks out at 3K RPM, you'd need a 2:1 pulley drive to get to that point.  Just remember that if you do that - you need to be sure the engine has enough power to drive that multiplied load.

I'd really recommend reading this thread we did on this topic a while back - many of these questions you are bringing up have been gone over in great detail (the math is included): http://www.busconversions.com/bbs/index.php?topic=3163.0


One of the big benefits of using a DC alternator is the variable output current vs. speed relationship.  With an AC alternator, you must spin the rotor at exactly the right speed or you will get bad power out (not fixed to 60Hz or 50Hz).  With DC alternators, the voltage regulator takes care of the voltage at the output - the only requirement is that the alternator be spun fast enough to output the required ammount of power to the load.  This means you don't have to run your engine at 3600RPM to get 10Amps out of a 100AMP alternator (you may be able to get away with only idling or fast idling at 800RPM).  This can translate into a big fuel savings in the short and long term.

Just remember that propane doesn't contain as much energy as unleaded gasoline or diesel.  This means you will use more "gallons" or propane, than you would diesel or unleaded to do the same ammount of work.

Propane contains about 92,000BTU/gallon, while diesel and unleaded contain 140,000BTU and 125,000BTU respectively. Even at $2/gal, you'd need more like $3 of propane to do the same work as a gallon of $5 diesel - you also have to carry more propane to get an equivalent run-time, so you'll be carrying more tank weight etc.

-Tim

P.S.  I've attached a graph from the previous thread on this topic which should illustrate why you can't just pick an arbitrary RPM to spin the alternator to.  With the initial design, a 2.25:1 ratio was suggested to only match the RPM values of each part (the engine to the alternator).  As you can see from the graph of the power bands of the engine and alternator in this ratio, any time the red and pink traces (the engine power curve) fell below the dark blue trace (the alternator output curve), the system was in risk of stalling the engine or snapping the belts.  For Ray's setup - he eventually settled on a 1:1 ratio belt drive, since the engine would never be in a position over its entire power band, in which the alternator could load down the engine beyond what it could produce.  This meant he wouldn't get the full 160Amp rating of his alternator - but he could get just about 150Amps if he turned up the engine RPMs all the way. -T

[RTS/Daytona]

RTS's and most other 6v and 8V detroit diesel engines use

GEAR DRIVEN - OIL COOLED ALTERNATORS

so they wouldn't work for your application

ask NICK @ Minco or Luke what they have in used air cooled alternators

[Charles in SC]

That looks to be a pretty loud engine if I am reading their db sheet correctly.

[buswarrior]

the new coaches, both transit and highway are big into belt drive air cooled now.

Lots of options at the transit scrapper near you, or the current OEM's.

happy coaching!
buswarrior

[Kevin Warnock]

I am now fully exploring a gas engine converted to propane, despite the lower energy content per gallon. The piping will be easier and propane costs much less and the smell of the exhaust I assume will be less.

That said, can anyone recommend a quiet gas engine in the 6 to 12 horsepower range?

Would you think it makes sense to get maybe an engine toward the l2 HP range, but with the intention of running it close to idle speed to keep the noise low? Seems if I run it at 2,000 RPM I might get 5 or 6 HP out of it, which is all I need lots of the time. If I need more power, I can turn it way up too.

Pete, thanks for the reminder that RTS alternators are oil cooled. I'll get a 200 amp air cooled model, roughly.

Thanks,

Kevin

[Tim Strommen]

You asked:

"...Would you think it makes sense to get maybe an engine toward the 12 HP range, but with the intention of running it close to idle speed to keep the noise low?..."

A few questions:

• Are you going with a belt drive?
• Are you intending to use a fixed or variable drive ratio?
• What is the peak amperage demand of your alternator for this system?
• Are you planning on any control logic for this system?

If you are going with a belt drive - and you are planning on some control logic for this - I personally like the idea of a variable ratio belt drive system.  Much like a transmission in a car, it allows you to "tune" the drive to the load and the power the engine is outputting.  If you are using direct drive our you don't plan on having any control logic for this system - you should expect to have to control the engine speed manually (and change it frequently to maintain efficiency), and you will have to make some concessions on the output power from the genny (it won't be able to reliably produce peak in a direct drive or 1:1 drive setup).

I would recommend looking for a water-cooled, non-turbocharged engine to keep it quiet.  You will probably want something with two or more cylinders to keep the vibrations down (or put on a heavy fly-wheel).  Running and engine slower will almost always result in less noise produced - higher frequencies are harder to "mask" with background noise.  Also take a look at the output curves for different engines (remember that if you are modifying the engine to run on propane the manufacturer numbers are not going to be acurate), you may find some engines which use less fuel and make more power at lower revs than others (typically these will be multi-cylinder engines).

Remember that before you go and buy an engine - you need to know what the loads will be.  If you have the alternator, you can already surmise this value.  If your alternator produces 200Amps at 24volts this is 4.8kWatts electrical output (NOT MECHANICAL INPUT!!!).  Considering the conversion efficiencies of a belt drive and alternator, you are looking at more like 7kW required of the engine.  If you have a propane-converted engine, you'll probably need to be shopping around the 9kW range of gasoline-powered engines, which means you're probably actually looking at a 12HP engine TO START WITH...  If you use a fixed high-ratio drive and no control logic, you will be probably looking at a similar curve as I had attached earlier - which means you may be setting yourself up for a stalled engine.

-T