Diesel Dual Fuel Conversions
Franz Hofmann has written excellent articles for the Eng-Tips forum FAQ. We highly recommend that you read them if you are considering a dual-fuel conversion for your diesel engine. He has participated in a great deal of research with dual fuel fumigation, both natural gas and propane (see Expert advise on diesel propane fumigation and Propane enrichment for diesels) and is very much against driver-controlled LPG metering. For this reason, we also recommend that you stay away from any driver-adjustable system!
There is great deal of misinformation on the internet and, if your engine is damaged by a dual fuel system, you will be on your own. In reading through the FAQ of one dual fuel system, we came across the following statement:
Should your vehicle require a trip to the dealer for routine maintenance or repairs, the XXXX™ is easily removable and labeled for ease in reinstalling.
This statement clearly acknowledges that their system will result in warranty issues with the manufacturer . Now that these systems are in existence, your dealer will be looking for signs of a dual fuel system installation when you bring it in for service on an engine failure.
Another bit of misinformation is the idea that LPG is a catalyst to improve the combustion diesel fuel. By definition, a catalyst is a substance that causes or speeds up a chemical reaction by the presence of the catalytic substance that in itself remains unchanged. In a diesel engine, for practical purposes, all fuel is already consumed. With a dual-fuel system, both the LPG (or other gaseous fuel) and the diesel fuel will be consumed so there is no way that LPG could possibly be considered a catalyst.
Another dual fuel system web site states that:
Propane when injected into a Diesel Engine enhances combustion by evening out the normally violent explosion that commonly occurs in Diesel Combustion. This evening effect provides a more complete burn, a cleaner burn, a cooler burn and a less violent burn resulting in lower EGT's, more power, more torque, fewer emissions and the system pays for it self with huge increases in fuel mileage!
It is interesting that they would describe Diesel combustion as a "normally violent explosion". Diesel fuel is commonly injected directly into the combustion chamber and relies on the heat of compression to ignite the fuel. The fuel droplets must first vaporize before they can ignite. Long before the fuel mixture approaches stoichiometric, the engine will start producing smoke. Diesel combustion is quite complex and it is extremely simplistic to say that diesels are inefficient and have incomplete combustion because of the smoke produced as engine load approaches maximum. That smoke, although sometimes quite visible, actually represents a very small amount of unburned fuel. Very little smoke, if any, is produced at low loads. However, diesel particulate emissions are definitely hazardous to your health and are present even at idle. See CARB School Bus Study. There is also evidence that soot has a definite detrimental effect on climate (see National Geographic - Melt Zone).