LPG: supplement or replacement?

One of the most controversial fuels, yet highly popular among taxi drivers, is LPG. Once in a while there are plans to convert jeepneys and other public service vehicles from Diesel to LPG, but those plans still don't catch up. The increased weight and the space requirements for the fitment of a gaseous fuel system are still the main concerns, but there are other aspects to keep in mind. While an ordinary Diesel engine with mechanical injection is likely to last nearly forever, also requiring fewer replacement parts on the long run, those petrol engines converted to LPG are going to require spark plugs and ignition coils to be replaced once in a while, not to get into the matter of the higher sensibility of a spark-ignition engine to adverse environmental conditions such as the floods and tropical storms. Okay, more up-to-date electronically-controlled Diesel engines have their flaws too, but they can still retain some of the adaptability to alternative fuels ranging from ethanol to biodiesel and going through waste frying oil that can be blended with animal fats too.

LPG is not exactly a safe fuel, not just because it can displace oxygen while switching from the liquid phase to vapour but due to the lower safety standards of LPG reservoirs compared to the ones used in vehicles fueled by CNG (Compressed Natural Gas) which can be released in a safer way in case of crash or during a fire with a lower risk of explosion. On another hand, finding a suitable replacement for LPG to be used with those precarious systems is actually harder than switching from fossil Natural Gas to biomethane (landfill gas, sewage gas, swamp gas, among other popular names). Why should we ever look at LPG as the last word in gaseous fuels disconsidering the complexity and impracticality of Hydrogen?

At its favour, gaseous fuels actually can be used as a supplement to enhance the combustion process in a Diesel engine. Be it either Natural Gas or LPG, these fuels absorb heat when a small amount is fumigated into the inlet stream of the engine as a supplemental injection, therefore expanding and displacing oxygen as much as an EGR valve does when it dumps exhaust gases back into the inlet stream. On the other hand, the gaseous fuels are not going to dump soot and other impurities back inside the engine as the recirculating exhaust gases do, and since both LPG and methane have shorter carbon chains they increase the flame spread inside the combustion chambers, providing an overall cleaner burn and it goes further by requiring a lower volume of Diesel fuel. This way, it improves the fuel saving of Diesel while the environmental impact is decreased. A petrol engine converted to LPG will require it to start on petrol until it heats up to the operating temperature and then the cooling stream is used to vaporize LPG in order to avoid a freezing of the inlet manifold, unless it uses a forklift carburettor specially designed to operate only with LPG or a liquid-phase direct injection layout similar to the latest Australian Ford Falcon Eco-LPI.

At least for a while, LPG may fare reasonably as a supplement, never as a replacement, to Diesel fuel.