Propane (LPG) Injection
Propane (LPG) Injection 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 Injection and using Nitrous Oxide on gasoline vehicles to achieve a power increase. Basically, this analogy is correct, 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 Nitrous Oxide has on gas motors.

Exhaust emissions are reduced as a result, with lower quantities of unburned hydrocarbons and fewer particulates (smoke). LPG Injection 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 Propane Injection Work
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 will not self-ignite inside a diesel-fuel compression-ignition engine. 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. 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 Propane Injection Get's You
Normally aspirated engines require different systems to introduce the gas than do turbocharged engines. The results are different as well.

Normally aspirated engines will realize only a modest gain in power by the use of LPG gas. Displacing 1% 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 Normally Aspirated 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 Normally Aspirated 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.

It's worth noting that if you experience a big increase in power on a naturally-aspriated diesel engine after installing an LPG Injection system, then you should go back and check to see that your injection pump was adjusted to provide a nearly stoicheometric 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 Injection. 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.

Types of Propane Injection Systems
Dump Systems
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 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.

Controlled Systems
Theses system 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.

In order to determine the load on the engine in a Normally Aspirated system, a venturi must be placed in the air intake, as diesel engines have no natural intake manifold vacuum. 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.

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 Injection. Since boost is such a reliable indicator of engine load, higher values of Injection can be realized, with tighter control over the results.

Calculating The Amount of Gas To Apply
For the simple dump system on the Pusher, I calculated gas flow in BTU's, since this was about the only way that I could understand how to correlate diesel-vs.-LPG. Here's what I started with:

The Pusher, using a 1.6 liter diesel engine gets around 30 MPG of diesel fuel. A gallon of diesel contains 128,000 BTU's of thermal power. Assuming 60 MPH, the consumption would be 2 gallons per hour, or 256,000 BTU/hr. I was aiming at 10% LPG for a start, so I simply divided 256,000 by 10 = 25,000 BTU of LPG.

Consulting the chart at in the appendix of this page (down towards the bottom), we see that in order to get 25,000 BTU/hr from the regulator that I planned on using (11" WC), I'd need an orifice of 1.5mm diameter, or a # 53 number drill bit.

Calculating for an ordinary VW Rabbit, which gets 50 MPG, we see that fuel consumption is 1.2 gallons per hour at 60 MPH = 153,600 BTU. 10% would be 15,400 BTU, or about 1.2mm/#56 bit. Using the larger orifice from the Pusher would yield about 16% LPG.

Determining gas rates for turbocharged engines is a bit more troublesome, as there is a lot more air available to add fuel and gas to. What I have been told is that gas pressures can run in the 6 PSI range, with orifices of 1/2 to 3/8 of one inch. This is a *lot* of gas, and will require a regulator capable of the increased flow, as well as a much larger storage tank for the gas. See the gas flow calculator in the appendix of this page.

Tuning it up
Adjusting the orifice and/or gas flow for optimum performance could be tricky. Basically, increase the LPG flow incrementally until you see/hear detrimental effects. The first, and most noticeable is hard knocking, or pinging from the engine. If it sounds like someone threw a handful of ball bearings inside your motor when your turn on the gas, by all means TURN IT DOWN!!! The rattling you are hearing is the excessive peak pressure, and is also the sound of your pistons being turned into molten aluminum. If anything, adding LPG to a diesel engine's intake air should make the engine quieter.

Another thing to watch for if your engine has a pyrometer (exhaust gas temperature gauge), is EGT's dropping. LPG promotes more complete combustion, so some of the heat that used to escape through the exhaust pipe is now being converted into mechanical power and transferred to the wheels.

Diesel engines do not react well to LPG Injection when they are cold. Turning on the gas before the engine has warmed properly will result in rough idle and bogging, or lack of pulling power. Let the engine come up to something near operating temperature before turning on the gas. Another reason to have a dash-mounted control switch.

Air Flow
Probably the single most important thing you can do for a diesel is to improve the air flow into and out of the engine. This is particularly important in a Normally Aspirated engine.

Intake Air
First and foremost, remove any and all possible flow restrictions. Several low-restriction air filters are available, such as the K&N brand filters. Increasing the diameter of the intake ducting is also important. On the VW diesel engines, the "snorkel" which leads into the air filter box is designed to minimize intake noise ~not~ maximize flow. Converting to a larger pipe will mean that the engine has less pulling loss, resulting in more air per piston intake stroke, which means you can stuff more fuel (and/or LPG) into the engine, resulting in more power. Check out the 3" ducting on my engine in the photo above in the "Supplying gas to the Intake" paragraph above. This ducting is fed by a cowl scoop at the back of the hood. I like to think that I get at least a little boost pressure out of it.

Temperature of the air is also important. Cooler is better. Cool air is more dense, more oxygen to aid in combustion. The intake end of the pipe leading into the air filter should be supplied with the coolest air possible. Usually, this means from either the grille, a hood scoop or under the front bumper of the car. What you don't want is air that has been warmed after coming through the radiator, in other words, engine compartment temperature air.

Turbo engines benefit from installation of an intercooler, which is essentially an air-to-air heat exchanger that removes the heat produced when the intake air is compressed by the turbine. Cooler boost air will allow greater gains in power through increased fueling, either through adjustment of the injection pump, by Injection, or both.

Getting rid of exhaust gases quickly is also very important. Diesel engines do not benefit from controlled back pressure like gasoline engine do. In all cases, larger diameter exhaust pipes are better. Low restriction exhaust manifolds, down pipes and mufflers all add to power and performance, in dramatic ways. Less exhaust restriction is also beneficial to keeping exhaust gas temperatures lower.

If you are trying for ultimate power, a good old fashioned cylinder head port-n-polish of the intake and exhaust ports works wonders, as do exhaust headers.

Other Mods
Most anything you do to enhance performance will result in the engine running outside of it's design parameters. It may be necessary to compensate for this by adding high performance parts such as racing head bolts/studs, ceramic-coated pistons, additional cooling system enhancements, larger clutch disc and pressure plate, and even a locking differential trans-axle to minimize wheel spin.

How Propane (LPG) Injection Work
  • Gas for Diesels - Why your diesel needs propane injection, and how you can get it (PDF).

Propane Resources
Bully Dog Technologies
Propane Injection systems (Ford, GM, Dodge, John Deere, Volkswagen).
Jeg's High Performance
Turbocharger, supercharger, nitrous, propane kits parts, tools, general parts and accessories.
MSD Ignition
Your performance electronic source for ignitions, coils, distributors, alternators, timing controls, lazer gauges, spark plug wires, starters, fuel management, data acquisition and tuners.
US Diesel Parts
Offers a full-line of high quality, diesel performance products for Ford, Dodge, and Chevy.

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