Natural Gas and Propane as Motor Fuels?


I see this sort of thing all the time. Using natural gas and propane as a motor fuel. During my tenure as a mechanic I worked on various heavy trucks and passenger cars that ran on either propane or natural gas. First of all natural gas has less energy output than either gasoline or propane. It requires large, heavy tanks than can contain high pressure liquified natural gas. It takes anywhere from 8-12 hours to refuel a natural gas-powered vehicle, not 15 minutes like the gas stations. Propane systems for vehicles in existence today will not work with modern vehicles because the propane industry has dragged its feet and have not kept up with current computer control fuel management system using digital fuel injection. Propane makes less horsepower than the same engine running on gasoline, and less torque too because it doesn’t have the energy content gasoline does. To make matters even worse, it has  high combustion temperature, and it goes into the cylinders as a dry gas. There is absolutely no lubrication to valve guides, upper cylinder rings. In fact because of it’s higher combustion temperatures, compression rings lose their ductility and become less elastic taking a set to where they finally decided to expire. This causes massive amounts of blow by past the compression rings that gets lost going into the crankcase. I’ve seen propane engines that leaked on the average of 45-55% into the crankcase or past burnt valves. These engines were in such poor shape that they would not run on gasoline without a complete overhaul.

    Natural gas engines have similar maladies as well. For all intents and purposes you can lump the natural gas engines in with the propane engines when it comes to areas problematic to these types of fuels.

    I’ve also heard that propane and natural gas engines don’t dilute crankcase oils. Well they don’t. But they do something just as bad. They increase the viscosity of the oil. This happens because the lack of liquid combustion residues leaking past cylinder rings into the oil doesn’t occur. Instead the higher heat from propane fuels boils away the light ends of the engine oil increasing its viscosity. I’ve seen 10-30W oil that had the viscosity of 40W single weight oil.  Multigrade oils won’t remain multiweight in propane engines. Low viscosity oils are important in providing faster lubrication into critical engine areas during cold start-ups etc.  Higher viscosity oils can’t do this as well. They also increase the drag on the oil pump, cause more windage problems for the crankshaft assembly. Crankcase oils in propane engines still experience mechanical degradation from shearing just as gasoline engines do so you can’t claim extended oil changes with a propane or natural gas fueled engines if you want to keep your bearings in one piece.

    Because of higher under-hood temperatures I’ve seen wiring harnesses become cooked and brittle, requiring their entire replacement. Certain engines cannot be satisfactorily run on propane. Case in point is the 392 International Harvester engine. True this engine for reasons unknown to me have been run on propane extensively. The school buses I worked on had these engines in them that were running on propane. The problem with these engines are manifold. First of all propane takes a hotter ignition because it’s a dry gas and has higher spark plug inter-electrode resistance than gasoline engines do. The IH engines I worked on had marginal ignitions on them for gasoline let alone propane or natural gas. So they needed a hotter system like MSD aftermarket distributors and boxes to run they way they should. Second problem is in the combustion chamber in the heads. The intake and exhaust valves are very close to each other. There’s very little material around the valve seats and because of the higher combustion temperatures and lack of a decent amount of valve seat area to sink the heat away from the exhaust valves would instead crack between the intake and the exhaust valve seats. Due to propane’s lack of lubrication, valve seat recession occurs as well. Couple this with the cracking problem and you have a cylinder head that’s ready for the scrap yard.

Valve seat recession isn’t just a problem for IH engines. I”ve seen big block and small block Chevrolet’s and large and small block Fords with valve seat recession problems. In fact I saw a recession problem so bad that the edges of the exhaust valve gradually eroded away from the outer radius and stuck itself into the area of the exhaust port below the seat. This caused a rocker arm failure and a bent pushrod, and a collapsed lifter on the same cylinder.

If all this isn’t bad enough, then you come to the problem of trying to make this stuff work on new cars. An overview of a propane induction system is in order first so you understand how it works and the problems with trying to adapt this antique technology on today’s cars. First of all propane is stored inside a low pressure tank. Most motor vehicle tanks are required to safely contain 312 psi. Most propane tank pressures even on the hottest days only run around 110-120 psi. The propane is pumped into the tank as a liquid. It stays that way as long as it’s under pressure. If you release the pressure it converts itself to a gas at minus 44 degrees below zero. Think of it as watching a glass of 7-up bubbling away. Propane looks similar.  To get the propane into a usable form for the engine it must be carried from the tank as a liquid up to a device called a converter. Before the converter it goes thru a safety device called a  fuel lock off valve. This filters the liquid, and lets it pass to the converter if this valve  has manifold vacuum applied to it. If it does not, it will not allow propane to flow into the converter. Okay so the converter is just what it sounds like. It converts liquid propane into a gas and steps down the pressure to about 1/2 psi for use in the mixer. The converter has engine coolant circulating thru it to keep it from freezing up as it flashes the liquid propane off to a gas.  So why not just convert the liquid into a gas in the tank? Well it’s a little thing called BTU loading and wetted surface area. An engine requires a huge BTU load to run compared to the wetted surface area of the fuel tank. What is wetted surface area?  It’s the amount of internal surface in a propane cylinder that can vaporize a given amount of propane for the load it’s connected to. If you increase the load and don’t increase the surface area, the cylinder will freeze down. First frost forms on the outside of the cylinder due to the moisture in the air around it. As this process continues the frost freezes into ice. The ice effectively insulates the tank walls from the warmer air and the process degenerates as time goes on. As this process continues the pressure in the tank continues to drop because less and less liquid is being converted into a gas. The way to get around it is to extract the propane from the cylinder in a liquid form and flash it off to vapor using the converter.

As the propane exits the converter it gets sucked into the mixer. The mixer is nothing like a gasoline carburetor. It has very few parts by comparison. It has a primitive diaphragm controlled idle circuit and an even more primitive power control. A large bolt with a flat plate attached to it that works similar to a faucet. There is a diaphragm controlled slide that allows the propane vapor to mix with air sucked in thru the top of the mixer so that it creates a stoichiometric air to fuel ratio the engine can run on. This system is typical of almost all propane vehicle induction systems.

   One company I know of experimented with liquid propane injection and it worked very well. It even allowed for all the vehicles OEM on board systems to remain.  This company has since gone out of business and I don’t know where their research went.  The propane company I worked for converted several new small school buses over to propane and some larger ones. This was done by installing an old-fashioned propane mixers in front of the throttle bodies. Then the port injectors were disconnected. Some were removed. The buses had to have the ECM (electronic control module) removed and sent back to a company to reflash the firmware in them for a propane fuel curve. Any such modifications to a new vehicle would most surely void the manufactures warranty.

   I could build a motor that would run on propane. Several modifications should be made to the engine before. A ring set that can handle elevated combustion temperatures should be used, I’d probably use a gapless top ring. I would also increase piston skirt to cylinder wall clearance a little. I’d use liquid injection instead of the antiquated vapor systems. I’ve seen the vapor systems explode and blow air cleaners off motors, bend throttle shafts on throttle bodies they were in front of, blow up plastic intake manifolds on newer GM small blocks. All this takes is a back fire thru the intake track which happens more often than not on propane systems. Hardend valve seats in the heads would be in order. Sodium filled or valves made from some exotic materials such as ICONEL, Stellite, or maybe even Titanium to help with the heat.

   I’m not so quick to jump on the propane/natural gas band wagon as some are. Propane is just as expensive as gasoline maybe more so. Until someone in the propane industry decides that the old way of getting propane into an internal combustion engine needs to be looked at again, I’ll pass on its use as a viable motor fuel. I know because I worked in the propane industry for over 12 years. I did everything from unload rail cars, I  worked in a huge gas plant, I  worked on fleet vehicles powered with it. I ain’t making this stuff up.

Dave

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