So, what about propane?
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It is Roush that advertises that their propane pickup truck fuel economy is down 10%. From what you've described with intake manifolds blowing up, it sounds like the trucks you've been driving are EFI engines with old-style fumigation (mixer) conversions.
Hanging an Impco mixer on the inlet of a modern EFI engine is the wrong way to convert it because these systems are usually very prone to intake manifold backfires. Injection conversions do not backfire and their fuel economy is typically much better than 80% (and often 90%) of the vehicle's gasoline economy.
Hanging an Impco mixer on the inlet of a modern EFI engine is the wrong way to convert it because these systems are usually very prone to intake manifold backfires. Injection conversions do not backfire and their fuel economy is typically much better than 80% (and often 90%) of the vehicle's gasoline economy.
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What system did you use? What problems are you having with it?
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Trucks running on propane
We ran trucks on propane for many years. Had as many as 11 running on propane at a time. All these trucks were equipped with an under dash switch to switch them from propane fuel back to gasoline. The switch had a neutral on it, when switching fuels you would switch to neutral until all of that fuel had burned and your engine started to die, then you would switch to the fuel you wanted to use, propane or gas. If you failed to switch to neutral the engine would knock or quit completely sometimes, the engines simply did not like gasoline and propane mixed.
If you were expecting cold weather (32 degrees or below) you would always switch them to gasoline the night before. The engines did not start as well on propane in below freezing weather. If the engine was warm there was no problem, it was when the engine was completely cold plus cold weather.
We rarely did this, but it's best to start and run your engine on gasoline until it is warmed up. A propane system runs the propane thru a heat exchanger, thus warming the propane before going into the engine. If the engine is cold the propane will not be heated and thus the cold propane hitting the valves is extremely hard on the valves, while they are being heated on one side and hit with cold propane on the other. With propane running thru a heat exchanger using water heated from your radiator, it has a tendency to keep your engine from overheating also.
Propane has 110+ octane (which is an anti-knock agent) an engine that would knock on gasoline would never knock on propane, no matter how hard you treated it. Propane has a lower BTU rating than gasoline, thus getting on average 1 to 2 miles lower mileage per gallon of propane than gasoline. Propane having a lower BTU rating also means a slight loss of power.
When we first converted to propane we were changing oil at 3000 miles. The oil at 3000 miles looked like new oil. We then started changing oil at 6000 miles, then finally 9,000 miles before it looked like it even needed to be changed. Also spark plugs seemed to last forever, they looked like new plugs when we would removed them. Even though these were company trucks, and were sometimes abused by company personnel we had trucks lasting 300,000 to over 400,000 miles before trading them in. And even then the motors were mostly OK, it was other parts of the truck that were wearing out.
The only problem we ever had running our trucks on propane was with valves, they could become warped sometimes and the heads had to be taken off and valves replaced. This was only in a few instances and was never a major problem. I do remember looking at the heads when they were removed, there was very little sign that they had been used or they looked brand new. The top of the pistons appeared the same as the heads.
I am certainly no expert at anything, just relaying the experience we had running propane for many years...
If you were expecting cold weather (32 degrees or below) you would always switch them to gasoline the night before. The engines did not start as well on propane in below freezing weather. If the engine was warm there was no problem, it was when the engine was completely cold plus cold weather.
We rarely did this, but it's best to start and run your engine on gasoline until it is warmed up. A propane system runs the propane thru a heat exchanger, thus warming the propane before going into the engine. If the engine is cold the propane will not be heated and thus the cold propane hitting the valves is extremely hard on the valves, while they are being heated on one side and hit with cold propane on the other. With propane running thru a heat exchanger using water heated from your radiator, it has a tendency to keep your engine from overheating also.
Propane has 110+ octane (which is an anti-knock agent) an engine that would knock on gasoline would never knock on propane, no matter how hard you treated it. Propane has a lower BTU rating than gasoline, thus getting on average 1 to 2 miles lower mileage per gallon of propane than gasoline. Propane having a lower BTU rating also means a slight loss of power.
When we first converted to propane we were changing oil at 3000 miles. The oil at 3000 miles looked like new oil. We then started changing oil at 6000 miles, then finally 9,000 miles before it looked like it even needed to be changed. Also spark plugs seemed to last forever, they looked like new plugs when we would removed them. Even though these were company trucks, and were sometimes abused by company personnel we had trucks lasting 300,000 to over 400,000 miles before trading them in. And even then the motors were mostly OK, it was other parts of the truck that were wearing out.
The only problem we ever had running our trucks on propane was with valves, they could become warped sometimes and the heads had to be taken off and valves replaced. This was only in a few instances and was never a major problem. I do remember looking at the heads when they were removed, there was very little sign that they had been used or they looked brand new. The top of the pistons appeared the same as the heads.
I am certainly no expert at anything, just relaying the experience we had running propane for many years...
Last edited by Spanky1; Aug 27, 2011 at 02:17 PM. Reason: more info
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From the description of Spanky1's trucks, it sounds like the trucks were using the basic dual fuel system for carbureted vehicles. Often, the converter would be mounted relatively far from the mixer and this is the main reason that these types of conversions were hard to start in cold weather.
Propane systems deliver a GAS to the engine and not a LIQUID. When the engine is hot, the converter is producing hot propane vapor at whatever temperature the engine is operating (likely around 180°F) and typically at a pressure of -1.5" WC (Water Column). That is, the propane pressure delivered to the engine is a slight vacuum, which makes the system fail-safe because fuel will only flow when there is enough air flow in the mixer to create a vacuum. When the engine is shut down, the propane vapor cools to ambient temperature by morning and contracts to a greater vacuum in the process. If you have a long vapor line from the converter to the mixer, it will take some cranking for the converter to build pressure back up to -1.5" WC so the engine will start.
To overcome this problem, it is best to locate the converter as close as possible to the mixer. It is often useful to use cold start enrichment like the Start Assist Valve (PN SV) or the Vacuum Power Valve (PN VPV).
Burned or warped valves occur because the fuel mixture is still burning when the exhaust valve opens. This is generally due to either running rich fuel mixtures (because they tend to burn slower) or to having the wrong advance curve (because propane burns at a different rate than gasoline) or both.
Modern injection conversions are completely different than the old Impco-style fumigation systems. They all start up on gasoline and automatically switch over to propane once the engine is warm. This ensures that gasoline never becomes stale in the tank and allows smaller converters to be used in tight engine compartments. When set up correctly, the fuel trims on propane are the same as on gasoline so getting the right fuel mixture is not an issue.
As for engine oil, it's true that it stays cleaner on propane. However, you can't go by color when deciding when to change it. Oil life also depends on additive depletion and only used oil analysis can determine how long an oil may be run.
Propane systems deliver a GAS to the engine and not a LIQUID. When the engine is hot, the converter is producing hot propane vapor at whatever temperature the engine is operating (likely around 180°F) and typically at a pressure of -1.5" WC (Water Column). That is, the propane pressure delivered to the engine is a slight vacuum, which makes the system fail-safe because fuel will only flow when there is enough air flow in the mixer to create a vacuum. When the engine is shut down, the propane vapor cools to ambient temperature by morning and contracts to a greater vacuum in the process. If you have a long vapor line from the converter to the mixer, it will take some cranking for the converter to build pressure back up to -1.5" WC so the engine will start.
To overcome this problem, it is best to locate the converter as close as possible to the mixer. It is often useful to use cold start enrichment like the Start Assist Valve (PN SV) or the Vacuum Power Valve (PN VPV).
Burned or warped valves occur because the fuel mixture is still burning when the exhaust valve opens. This is generally due to either running rich fuel mixtures (because they tend to burn slower) or to having the wrong advance curve (because propane burns at a different rate than gasoline) or both.
Modern injection conversions are completely different than the old Impco-style fumigation systems. They all start up on gasoline and automatically switch over to propane once the engine is warm. This ensures that gasoline never becomes stale in the tank and allows smaller converters to be used in tight engine compartments. When set up correctly, the fuel trims on propane are the same as on gasoline so getting the right fuel mixture is not an issue.
As for engine oil, it's true that it stays cleaner on propane. However, you can't go by color when deciding when to change it. Oil life also depends on additive depletion and only used oil analysis can determine how long an oil may be run.
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From: In a house, in a small town
I would not use propane in a general automotive situation, industrial sure because it's readily available because it's normally on site for fork lifts and other vehicles. They other thing is it takes more fuel to get the same horse power and will use more fuel because propane does not burn any where near the same temp as gasoline. Everyone I talk to says they are terrible in cold weather and will not start unless they are plugged in during the winter. The other thing is propane is a very dry fuel and it will dry out valve seals and you will start to consume oil at some point. All of our Yale fork lifts are powered by GM 4.3s and most of them consume oil and our even older units you really have to watch oil levels or you can run them out.
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According to propanecarbs.com, propane has a peak flame temperature of 3614°F compared with gasoline's peak flame temperature of 3591°F, a difference of 23°F. The reason that you use more propane than gasoline is that propane has about 73% of the energy of a gallon of gasoline (84,250 BTU/gallon vs 113,602 BTU/gallon). Air-fuel ratios are calculated on a mass-basis and propane actually has more energy than gasoline (19,904 BTU/lb vs 18,211 BTU/lb) in this respect.
The reason that propane engines are a bit less powerful than gasoline engines is because propane (as a gas) displaces more air from the combustion chamber than gasoline (as an atomized liquid). This results in a slightly lower volumetric efficiency, which tends to improve it's relative fuel economy by reducing pumping losses. Also, the converter tends to be a bit flow-restricted at high power outputs, which causes the fuel mixture to become leaner.
Propane's dry nature doesn't affect the valve seals because these are constantly bathed in oil. Since there are no additives in propane, engines without hardened valve seats (as are common with unleaded fuels) often experience valve recession. If forklifts burn oil through their valve seals, it's because they either have a lot of hours or they weren't properly maintained.
As for cold weather operation, properly set up and maintained propane systems have no problems running in Canadian winters on the prairies. Only as you approach -40°F will low tank pressure potentially cause a problem.
The reason that propane engines are a bit less powerful than gasoline engines is because propane (as a gas) displaces more air from the combustion chamber than gasoline (as an atomized liquid). This results in a slightly lower volumetric efficiency, which tends to improve it's relative fuel economy by reducing pumping losses. Also, the converter tends to be a bit flow-restricted at high power outputs, which causes the fuel mixture to become leaner.
Propane's dry nature doesn't affect the valve seals because these are constantly bathed in oil. Since there are no additives in propane, engines without hardened valve seats (as are common with unleaded fuels) often experience valve recession. If forklifts burn oil through their valve seals, it's because they either have a lot of hours or they weren't properly maintained.
As for cold weather operation, properly set up and maintained propane systems have no problems running in Canadian winters on the prairies. Only as you approach -40°F will low tank pressure potentially cause a problem.