LPG injection into diesel engines (long)

ScottFromNZ · Post by **ScottFromNZ** » 25 Apr 2004, 15:37

Interesting article.....www.mrsharkey.com/lpg.htm
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. 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 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. 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 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 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 "colliding flame fronts" 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, which then causes back pressure, preventing the proper expansion of the ignited fuel leaving the swirl chamber.
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.
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.
See the link above for construction details.

oilyspanner · Post by **oilyspanner** » 25 Apr 2004, 15:52

Interesting stuff
Stewart

rabenson · Post by **rabenson** » 25 Apr 2004, 23:42

I must admit that I was under the impression that the only problem with Nitrous Oxide injection, on a gasoline or diesel was down to the extra strain on components due to the increased power output. Won't the same apply to this mod or am I wrong... again...

Ron

ScottFromNZ · Post by **ScottFromNZ** » 25 Apr 2004, 23:47

You would be absolutely correct, Ron.

AndersDK · Post by **AndersDK** » 25 Apr 2004, 23:49

I think you're right Ron -
But instead of just going for more power, there is the obvious option to choose for better economy, & less pollutions [;)]

rabenson · Post by **rabenson** » 26 Apr 2004, 00:36

Good point, Anders - I suppose any efficiency improvement has to be good!
Ron

Post by **philhoward** » 26 Apr 2004, 00:58

If it gives a 10% hike in power, vrtually all engines have that built in "safety factor" anyway. I believe that it's the smoothness factor which applies to diesels (unless you do 50,000 miles a year) as the cost advantage versus the amount used doesn't make it a totally monetary advantage. LPG as a replacement fuel on a petrol, on the other hand...
I believe its down to the different burning properties of LPG which gives you less of the fabled "diesel knock", as you are effectively replacing some of the diesel with LPG - the 10% mark sounds like figures i've heard before.

mbunting · Post by **mbunting** » 26 Apr 2004, 17:24

Nobody has picked up on the fact that you could REDUCE fuelling considerably at the pump, keeping the HP around 90 by using the LPG, but using less diesel !

Post by **philhoward** » 26 Apr 2004, 17:51

That's effectively it...as you are adding fuel (LPG), then there's less needed to be "added" by the diesel pump. As diesels run at a constant air/revolution (i.e. always lean) you need less throttle input for a given speed/power. You could reduce the max fuelling and fuel rate to compensate, or go the other way and get another 8-10% power instead at max throttle...the choice is yours!