Frequent road-users are happy about the distinctly lower fuel price (2008: approx. 0,68 Euro/kg against 1,50 Euro/Liter) and, as opposed to the Diesel engine, the stable taxation. The dense network of filling stations, particularly in most of our neighbouring countries, make it even more attractive. Even the conversion may be worthwhile. Despite the variable expenses of natural gas operation being lower, partly because of the lower consumption, the change-over to LPG, in spite of the natural gas promotion, is more favourable. There are, e.g., conversion kits which fit into the spare-wheel housing, indeed, these are systems which have only a limited range. The same as with natural gas operation, the engine-oil stays cleaner and the spark plugs have to be replaced less frequently. Because LPG is heavier than air, there are sometimes legal restrictions concerning underground parking lots. Otherwise, although a larger additional tank does take up space, because of the very low pressure load of 5 - 15 bar, it is also relative light-weight. Only for cold-starting must there be, at least some remaining petrol on board. When searching for a suitable filling station, keep your eyes open for one that offers Liquified Petroleum Gas. One only needs little time to get used to the 80%- full stop-valve. One also need not be afraid of gas leaking out, because the gas has an odorous additive which gives it a smell, apart from that, the gas-tank is checked every ten years by the Technical Inspection Authorities (in Germany (TÜV).
There are (monovalent operation) vehicles which can only be run on gas, eg., fork-lifters. On the roads however, except for busses or taxis, most of the vehicles can switch from gas to petrol (bivalent operation). Because of the too low compression ratio when running on petrol, this type of engine has a consumption which is approx. 10% higher. Modern engines with knocking sensors can reduce this to about 2%. A common control device, or (as seen in the picture) one specially conceived for LPG operation, provides, through magnetic valves, for the vapourization and pressure reduction. The LPG, depending on intake manifold pressure, Lamda-sensor current, throttle valve position, and RPM among other things, then finds it's way to the invidual cylinder intakes.
The first refitted systems had, apart from the tank with filling connection, also the pressure reducer and the heat exchanger, which could actually only be installed in the engine compartment at that time. Then the carburettor was dismantled and a piece of intake piping with flanges and nozzles was inserted. Through an additional low-pressure connection a volume control was possible. A magnetic valve allowed the possiblity of switching it off. This system even survived the introduction of the Lambda regulation. Thereby however, the gas supply had to be regulated electronically, by means of stepper-motors. It is even possible to provide a diagnosis connection. Then finally, the time of the gas injection arrived. This was competion for the 'blow-in' technology. They are now all regulated by Lambda, but differ according to the arrangement, either central or for each specific cylinder. The latter can either work all together or, partially/fully sequential. In this case, the system then uses the signals to each individual injection valve of the petrol injection and can even cooperate with ESP, ESD and OBD. Of course there are already gas-systems for petrol direct-injection as well. 06/08