How does LPG conversion affect my catalytic convertor?

To discuss this, we must first run through the workings of the catalytic convertor (cat.) when running on Petrol.

The main player in the cat. is Platinum, a metal that will react with certain gases at particular temperatures.

Perhaps you ran a glow plug engine in a model as a child. The glow plug was connected to a battery for initial ignition but could be removed after the engine was running because the platinum coil in the glowplug did just as its name suggests - It glowed very hot as it reacted with the alchohol content of the fuel, thus sustaining ignition. In this context the platinum is a "Catalyst", because it has provoked a reaction (ignition).However, it would only do this when hot and is not capable of igniting the fuel during a cold start.

The Platinum in the cat. of a Petrol engine performs a similar function, although this time it is not igniting the fuel in the cylinder but employed to "burn off" some of the harmful components of the exhaust gases, i.e. unburned fuel (Hydrocarbons) that would otherwise be released into the atmosphere.

The cat. will only "light up" when it has reached a temperature of some 350 degrees C. It relies on hot exhaust gases from initial start up to supply the heat. After that temperature is reached, the cat will burn most of the Hydrocarbon content of the exhaust gas(es), reducing harmful emissions.

The cat. will also perform this function when LPG is used as a fuel so its function is similar. In normal operation it cannot be harmed by LPG use, and in fact has to deal with none of the exhaust products that erode it, (i.e. acids) resulting from combustion.

Possible problems

The main threat to the operational life of the cat. is the same as when running on Petrol - If the engine is overfuelled (running too rich) the cat. will have to burn more Hydrocarbons and its temperature will increase. Indeed, in extreme cases it may glow white hot. This may not only burn out the heart of the cat. (the Platinum catalyst) but cause it to loosen and eventually fall apart inside due to the resulting expansion and contraction. Replacement can be very expensive.

OK, so how is this relavent to your planned LPG conversion?

If an "open loop" LPG system is fitted to a catalysed vehicle, it will seem to run fine for many miles, and there may never be a problem. BUT if overfuelling during the overrun (i.e. when coasting downhill) occurs the cat. may be glowing white hot (running overtemp.) and eventual damage will result.

A "closed loop" LPG system constantly "talks" to the engine management system and then adjusts the mixture to the optimum required. If the engine is in the overrun state (for instance) the amount of fuel supplied will be reduced so that the exhaust does not include excessive Hydrocarbons. A further benefit is that the cat. will not be mechanically damaged by running overtemp.

When Go LPG! quote you for an LPG conversion the price will reflect the fitting of only a closed loop system to a vehicle that has a catalytic convertor fitted. The price will be initially more expensive than an open loop system but your catalytic convertor will last longer and the atmosphere will be less polluted.

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