How autogas works in a car
Autogas, or LPG for short, is a mixture of propane and butane that becomes liquid under pressure. It is stored in its own steel tank, usually in the spare wheel well or as a cylinder in the boot. A valve on the tank limits filling to about 80 per cent so that the liquid gas can expand when it warms up.
From the tank the gas flows to the vaporiser, which is connected to the engine’s cooling circuit and turns the liquid back into gas. Gas injectors feed it into the intake manifold, just ahead of the intake valves. A separate control unit reads the commands of the petrol injection and converts them for the gas. The engine starts on petrol; once it is warm, the system switches to gas by itself.
Signs that your LPG system should go to the workshop
- A smell of gas: switch to petrol, stop safely, switch off the engine, ventilate, do not smoke; if the smell persists, do not drive on and call us or a breakdown service
- The engine judders only in gas mode
- The system does not switch to gas or keeps switching back to petrol
- The engine management light comes on in gas mode
- Gas consumption rises for no obvious reason
Which engines are suitable
The conversion is simplest with port injection: the gas injectors sit right next to the petrol injectors. With direct injection, the petrol goes straight into the combustion chamber; such engines can often be converted too, but with more effort and not in every case. Often a little petrol is then injected in gas mode as well to cool the petrol injectors; that costs part of the saving.
LPG does not lubricate the valve seats the way petrol does. Engines that are considered sensitive to this therefore get a dosing unit for a valve-protection additive. We clarify before the conversion whether your engine is one of them.
LPG in an American V8
In a large V8, LPG makes a particularly noticeable difference to the fuel bill. The 5.7- and 6.4-litre HEMI from RAM, Dodge, Chrysler and Jeep have port injection, as does the 3.6-litre Pentastar V6, and are therefore considered well suited. The current V8s from Chevrolet and GMC and the Hurricane six-cylinder in the newer RAM 1500, on the other hand, use direct injection; there we check case by case whether a conversion makes sense.
With eight cylinders and a lot of power, the vaporiser and gas injectors must be large enough and the control unit designed for eight cylinders, otherwise there is not enough gas at full load. Pickups and large SUVs often have more room for the tank than a smaller car. More on this under workshop for US vehicles.
Refuelling and range with two tanks
Not every filling station sells LPG; common fuel apps show the nearest pump. Abroad, you will need an adapter for the filler connection in some places. Because the car has two tanks, the ranges add up. When the gas runs out, the engine carries on with petrol without stopping. Still, always leave some petrol in the tank, because the engine starts on it and the fuel cools the petrol pump.
In gas mode the engine uses more litres, because LPG contains less energy per litre. So compare the cost per 100 kilometres, not the price per litre. A simple calculation shows when the conversion pays for itself: the cost of the system divided by the saving per kilometre gives the distance it takes. Include the maintenance of the gas system in the calculation.
Installation and approval
Before the conversion we look at the engine: mileage, condition, fault memory and ignition system. An engine that runs roughly on petrol will not run any better on gas. After installation we tune the system on the road. An inspector from a recognised testing organisation then approves it, and it is entered in the vehicle documents. Also tell your motor insurer about the conversion.
Maintenance and gas system inspection
At the intervals set by the system manufacturer, the gas filters are changed, the lines checked for leaks and the tuning checked. This fits in well with servicing. The spark plugs matter too: gas needs a stronger spark, so worn plugs often show up first as misfires in gas mode.
In Germany, every car with LPG has a gas system inspection (Gasanlagenprüfung) as part of the HU, the general roadworthiness test. It is also required after major repairs to the system and if an accident or fire has affected it. After a repair on the low-pressure side, a check for leaks and function is enough. The inspection covers the tank, valves, lines and leak-tightness, among other things. We prepare your vehicle for it; more under HU and AU.
What customers often ask
Does the engine have less power on LPG?
Depending on the engine, there is a small measurable difference. With a well-tuned system most drivers notice little of it in everyday driving, especially as a large engine has plenty in reserve anyway.
Can a diesel be converted to LPG?
A conventional LPG system is built for petrol engines, in which a spark plug ignites the mixture. A diesel ignites without a plug, purely through compression. Such a system is therefore not intended for diesel cars.
How safe is an LPG tank?
The tank is made of thick-walled steel. Valves on the tank shut off the gas flow if a line is damaged and release pressure in a controlled way in extreme heat.
Do you also maintain LPG systems that were fitted elsewhere?
Talk to us and tell us the manufacturer of the system; it is given in the installation documents and usually on the control unit. We will then tell you in advance what we can do.