LPG Fumigation

Page One

Lately, I've been getting quite a lot of e-mail from folks who are interested in the LPG Fumigation system that I have installed on the Pusher, so it makes some sense to have a page dedicated to that subject, with at least as much as I know about it, and some links to other resources.

What it is...and why:

Simply stated, Propane (LPG) Fumigation is the introduction of gaseous propane into the air intake of a diesel engine for the purposes of attaining more power, economy, or both. The parallel is often made between fumigation and using Nitrous Oxide on gasoline vehicles to achieve a power increase. This analogy is similar, although the properly implemented use of LPG on a diesel engine will actually result in a better-running engine without the possible damaging effects that N2O has on gas motors.

Exhaust emissions are reduced as a result, with lower quantities of unburned hydrocarbons and fewer particulates (smoke). LPG fumigation will even clean up the odor of diesel fuel in the exhaust, making the smell from the tailpipe of an engine utilizing it much less objectionable.

How it works:

Introducing LPG gas into the combustion air intake of a diesel engine acts as an accelerant, promoting the even burning of the diesel fuel, and more complete combustion, resulting in more power being produced. Many web pages and forum posts will call LPG a "catalyst" but this is not correct, as LPG creates no change in the molecular makeup of either the air or the diesel fuel.

Propane by itself resists self-ignition inside a diesel-fuel compression-ignition engine due to it's high flash point and narrow fuel-to-air ratio. During the compression stroke, the air/LPG mixture is compressed and the temperature is raised to about 400°C, not enough to ignite the LPG, which has an ignition temperature of about 500°C. In the small concentrations that LPG fumigation uses, the LPG mixture is not rich enough to be overly flammable and is more difficult to ignite. When the diesel fuel is atomized into the cylinder under high pressure, it immediately self-ignites (diesel ignites at about 385°C.), and causes the LPG to burn as well. Since the LPG is in mixture with the air, the flame front from the diesel spreads more quickly, and more completely, including igniting the air/fuel mixture which is in contact with the cylinder walls, which are cool in comparison to the super-heated air inside the combustion chamber. Much of the cleaner burning of the fuel is attributed to this ignition against the "cooler" components of the engine, and accounts for raising the percentage of combustion from a typical 75% for a well-tuned diesel engine running on pure diesel fuel alone, to 85-90% with the addition of LPG. Obviously, this more complete combustion also gives a nice boost in power, with an accompanying increase in fuel economy and reduction of pollutants.

What to Expect:

OK, here's where we have to draw a distinction between engine types. Normally-aspirated engines require different systems to introduce the gas than do turbocharged engines. The results are different as well.

Normally-aspirated (N/A) engines will realize only a modest gain in power by the use of LPG gas. Displacing 0.5% of the intake air with LPG will result in a small power increase, perhaps 5-8%. Nearly no increase in power will be noted at full throttle, assuming that your injection pump is correctly adjusted already. Attempting to provide more gas to the engine will not increase performance, and will in fact lead to a condition not unlike pre-ignition in a gasoline engine. This has been attributed to excessive peak pressure inside the combustion chamber, and may have a lot to do with the fact that most N/A engines are also IDI (Indirect Injection), which means that the diesel fuel is not injected directly into the combustion cylinder, but instead enters a "swirl chamber" where ignition takes place. The flame front then shoots out of the swirl chamber into the combustion chamber, where it combines with the air (and LPG) to force the piston down in a power stroke. Apparently, these engines have a problem in that the flame front exiting the swirl chamber ignites the LPG/air mixture, all of which burns instantly instead of in a metered, controlled manner as it would during the normal diesel injection window.

I have had satisfactory results on my VW 1.6 N/A engine when adding LPG at a rate of 8-10% of the BTU rating of the diesel the engine is using. It may be possible to turn the fuel up, but I do know for sure that too much fuel does not increase power, and causes the engine to make very unhappy noises.\r\n It's worth noting that if you experience a big increase in power on a naturally-aspirated diesel engine after installing an LPG fumigation system, then you should go back and check to see that your injection pump was adjusted to provide a nearly stoichiometric air-to-fuel ratio at maximum throttle without the LPG turned on. It's possible that the significant power boost you are seeing is due to the engine now being adequately fueled for the first time. The additional BTU content of the LPG is simply being substituted for the diesel fuel that you haven't been injecting all along.

Turbocharged diesel engines are able to realize a significant increase in power by using LPG fumigation. While the usual suggested increase is considered to be approximately 20%, by careful management of the gas introduction, power gains of up to 40% are possible. My understanding is that it is a very fine line between lots of extra power and a dose of LPG that will render an engine scrap metal in a hurry, so consider carefully before you decide to "turn it up".

Turbo engines are by design blessed with a lean air-to-fuel ratio, and can be fed concentrations of LPG up to about 6-8% of the intake air volume. TDI (Turbo Direct Injection) engines have shown dramatic power increases when properly fumigated with LPG, combined with an "Upsolute" chip, or computer engine management upgrade. (Of course, these modifications will void any manufacturers warranties...)

Types of systems:

I am aware of two basic fumigation systems.

The first, I call the "dump" system, which means that you pick a value of propane to feed to the engine, either by calculation, or by trial-and-error, and you simply "dump" it into the air intake. Little provision is possible for correcting gas flow depending on engine load, so the system is probably only optimized for one type of load demand. The advantage is that this type of system can be cobbled to together by backyard experimenters like myself at a low cost. The disadvantages are that you will probably need to error on the side of caution to make sure that you aren't overloading your engine with too much gas, and that the system doesn't compensate for variations in engine speed, load, etc.

The second system is much more sophisticated, and uses a variety of sensors and controls to monitor engine performance and load, and adjust the gas flow to suit the need at the moment. Most commercially available systems will be of this type.

Levels of fumigation are best utilized in the mid to high power ranges. Determining when the engine is operating in these ranges is problematic. Accelerator (fuel lever) position sensing is not a reliable indicator for load on the engine due to most diesel engines being governed. Additionally, diesel injection pumps have internal fuel adjustment and regulation devices which make fuel lever position inaccurate in relation to engine load. On modern diesel engines using management computers may mean that the accelerator position means very little to the actual engine load, the injection system being "fly-by-wire". The vehicle operator may have the accelerator to the floor, while the computer is waiting for boost to build, or limiting fueling due to engine RPMs, etc.

Intake air volume is not a particularly good indicator of load for naturally-aspirated engines. Since diesel engines are "throttleless", they operate without a butterfly plate restriction in the intake air flow. As a result, the flow of air through the engine is mostly constant for any given engine RPM, independent of load. On turbocharged engines, air flow can be regarded as a reliable indicator of load while running "on boost".

There are systems that have been designed around intake volume to regulate LPG gas flow to the engine. Since diesel engines have no natural intake manifold vacuum, a venturi must be placed in the air intake. A sample of the vacuum produced by the venturi is fed to a metering system, either electronic or mechanical, which adjusts the gas flow to suit the circumstances.

The installation consists of a specially designed, branched duct fitted to the fresh air inlet of the existing
air filter. The branch is connected by hose to a standard automotive demand-type LPG mixer.
(Photo and text courtesy of the Society of Automotive Engineers)

Turbo engines have a great indicator of load built in. It's a fairly simple process to take a sample of the boost pressure developed by the turbocharger and use that to control the metering system. Most commercially made systems are designed for turbo engines, both for this reason, and because of the greater power gain that the turbo realizes from fumigation. Since boost is such a reliable indicator of engine load, higher values of fumigation can be realized, with tighter control over the results. Remember that turbochargers are not simple volume-driven compressors, but actually use some of the heat contained in the exhaust gases, converting it into mechanical energy to produce boost. More load means more heat, which means more boost, which means more fueling, which means more heat, well, you get the idea, it's an upward-spiraling chain of events.

Mechanically supercharged engines aren't going to provide the accuracy of turbo boost, due to the boost pressure being a product of engine RPM rather than load.

The best and most reliable indicator of engine load is fuel flow, but this is difficult to measure accurately. Diesel fuel injection systems pass a greater flow of fuel through the pump than the engine is consuming, the excess being returned to the fuel tank. A differential fuel monitor must be designed and installed which can measure fuel arriving to the injection system and returning to the tank, the difference in the flows being the fuel the engine is consuming. Obviously, this is beyond the the scope of this document, although modern electronic engine management systems may have this fuel flow information available in the engine data stream. Megasquirt, anyone?

Next, we explore the construction of a basic LPG system.