Kevin Warnock

As you know, I am in the process of reconverting my RTS bus conversion. I did a minimum conversion in order to legally title it as a motorhome in California. The California Department of Motor Vehicles inspected and signed off on the conversion and changed the title to ‘automobile housecar.’ This entitled me to register and insure it as a motorhome, which costs less than a passenger bus to insure. I had to do this conversion quickly, and I didn’t have the time to create the type of vehicle I really wanted. I sold my old MCI bus conversion recently, and that gave me the funds to redo aspects of my RTS conversion.

One of the most severe problem with my first conversion of the RTS was that I didn’t insulate it. I left the existing insulation in the walls and ceiling. That insulation was fiberglass encased in plastic wrap. It was thin and not particularly effective. I’ve since discarded this insulation and have received a written quote from Tri County Insulation in Santa Clara, California some time back now. I couldn’t have the work done because I didn’t have the money at the time. Now I do, thanks to selling my MCI conversion. I’m back in touch with Tri County and they still want to do the work. I hope to get the RTS spray foamed in the next week or so, if all goes well.

One of the other problems with my RTS is it did not have a generator. It has installed a powerful Trace SW4024 inverter with a bank of Trojan T105 batteries. This system can power up to 4,000 watts of AC appliances at once. It’s a true powerhouse. It’s discontinued now as I’ve had it since 2001, but you can still get them on EBay, and it’s still considered the gold standard of inverters for bus conversions.

I’ve thought a lot about what kind of generator to install in my RTS, and this article details my plans.

Wrico International 8,000 generator

The normal way bus nuts handle generator power is to put a 5,000 to 20,000 watt diesel generator in a storage bay. This can cost $5,000 or more, and these monsters weigh perhaps 400 pounds and consume half a gallon of diesel per hour, which is way too much for a modern future oriented vehicle.

What I have in mind is a propane engine driving a large frame 24 volt alternator. The 24 volts DC would be routed through the Trace SW4024 inverter I mentioned above, and this would give me up to 4,000 watts of AC power. This inverter can sustain that output continuously if it is ventilated enough to keep it cool. I don’t forsee any situation where I would need to push it to the limit full time however.

I want to use propane because the emissions are cleaner, and the fuel is easier and safer to handle than gasoline. It’s also cheaper, in part because road tax is not added to the price per gallon. But another huge benefit is that propane doesn’t go bad like gasoline does, so the generator stays cleaner and is easier to start after long periods of non use. Gas generators are notorious for getting gummed up with stale fuel and being hard to start if not used frequently enough. I already converted my Honda eu1000 generator to propane using a kit from US Carburation Kit Center, which sells kits for numerous generators and engines. You send them the carburator from your engine, and they modify it and plumb it to accept propane. Then you reinstall it on your engine and hook up a source of regulated propane and your gasoline engine now runs on propane. It’s simple.

The discount tool store Harbor Freight sells gasoline engines for unbelievably low prices. For example, with a 20% off discount coupon one can always find online or in some print magazines, an electric start 11 horsepower gasoline engine can be purchased for under $300 including sales tax! This is a fantastic deal. One could buy a spare to keep on board for on the road replacement if something went wrong with the primary engine.

Harbor Freight 11 horsepower gas engine

My idea is to use a big enough alternator to take full advantage of the 11 horsepower output, but to choose pulleys so that even when the engine is running at idle speed that useful DC power is produced. This may take more than one pulley, like in a drill press, where the operator can change the speed by manually grabbing the stopped V-belt and putting it on a different pair of pulleys.

These Harbor Freight engines run at 3,600 RPM at full output, but their idle RPM is much lower – I would guess around 1,800 RPM. I predict that the noise output drops dramatically at idle, like the Honda EU series of Inverter based portable AC generators. I’m sure the fuel use drops dramatically as well. I suspect that even at idle an 11 horsepower engine can produce at least 3 horsepower. This is around 1,000 watts of power when converted through an alternator and inverter to AC. This is enough power to run a mini-split 9000 BTU air conditioner. If I need more power, I can just adjust the engine throttle to run the engine faster, and more power will come out. I should be able to rig up a remote throttle control so I could change the engine speed from inside.

One of the main reasons bus nuts go with huge diesel generators is the big ones usually run at 1,800 RPM and are water cooled. By running an 11 horsepower generator at idle, I hope to get to the 1,800 RPM speed. Now I need to make the generator water cooled… but how?

When I was thinking up my efficient motorhome air conditioning idea, I discovered how inexpensive car radiators can be. They can be had for under $100 each for smaller ones. Even the smallest should be fine for my application, as cars have engines much larger than 11 horsepower.

My idea is to build a soundproof box for the engine, and put a car radiator inside the box with the radiator fins directly in the path of the airflow from the engine fan. Then, connect this radiator via rubber radiator hose to a second radiator outside the box, which could be close to or far from the box, as needed. An electric fan mounted on this outside radiator would send air over the fins to cool them. A water pump would circulate water between the two radiators, and this pump would be powered directly by the starting battery for the engine, which is constantly charged by the built in alternator on the engine which keeps the starting battery fully charged. So, the pump would only run when the engine is running, which is what the plan requires.

The dual radiator system will make the generator system quiet because the generator box no longer needs a large fresh air source, so the box can be sealed and insulated, except for a small pipe to provide combustion air and a small pipe for the combustion exhaust. The area of these two pipes should be under 2 square inches. This is almost nothing compared to how much area would be needed to directly air cool an 11 horsepower engine.

Another huge benefit to my proposed system is that the box can be custom fit to just contain the engine and radiator. This won’t take much space compared to building box for an air cooled generator, which would need multiple sound baffles and sound foam in the baffles to quiet the noise. With my design, I predict the volume required is 1/2 to 1/3 what would be required otherwise.

Since the air in and exhaust out portals are so small, it will be difficult for any engine fire to cause catastrophe, because an engine fire couldn’t get unlimited oxygen to burn. It will be a simple matter to build a trap door for these inputs and outputs that will snap shut if there is a fire, and with no oxygen, the fire will go out almost immediately. Many RVs and bus conversions burn to the ground when a generator catches fire, so my proposed system will save money and lives if widely adopted by others.

The trap doors I envision would be spring loaded, and held open with meltable links that would melt and come apart if they get too hot, as they would in an engine fire. This kind of trap door is used in HVAC systems to slow the spread of fire in buildings, so the technology is proven and cheap.

For extra safety, a propane detector can be installed that is linked to a gas shutoff valve. When the detector senses propane, the shutoff valve loses electricity, and since it’s spring loaded as well, it closes and shuts off the flow of propane. An engine without propane or oxygen won’t burn for long.

Now that I’m a TechShop member, I have access to a metal shear and bending brake. A shear will cut sheet metal like scissors cut paper. A bending brake will bend sheet metal. Thus, I can make the engine box out of steel or aluminum, for added fire safety. I am taking a class in TIG welding at Techshop on January 24, 2010, so I hope to be able to weld the box seems to be certain the box can be tight enough to stop a fire.

I’m thinking about mounting the generator I describe here on the roof deck I envision for my RTS, over the driver’s compartment. The inverter is now installed in the back of the RTS, but I have built a special compartment in the front stairwell, under the passenger seat. I did this because large inverters hum when on, and I want that noise as far from the bedrooms as possible. My RTS has two bed areas, one mid ship and one in the rear. So if the inverter is in the front stairwell area, with two residential doors separating it from the closest bed area, I don’t think anyone will be able to hear the inverter, even if parked in a quiet spot. I need to check the RV code book to see if there is a restriction to roof mounting generators. I don’t suspect that there is one, but I want to do things correctly. The big benefit to roof mounting is the engine will be outside the living area, so I can just direct the exhaust out of the box into the atmostphere, without having to worry about the safety concerns of running exhaust pipes through or underneath living areas.

I’ve thought about the above generator concept quite a bit, and I like the idea. It’s more efficient to use an alternator to charge batteries than to use a conventional AC generator to power a battery charger to charge batteries. Alternators are designed to charge batteries, and they are cheap and plentiful, though the Ample Power brand alternators I linked to above are not cheap!

I love the idea that I can dial in how much power I need, to balance noise, fuel consumption and charging rate to best suit the circumstances. I hope to build this system this year, and I’ll certainly post pictures and test results here to my blog. Please subscribe if you’re interested in updates to this entry.