The LPG Conversion

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PG, or liquid petroleum gas, is one of the more topical ways of reducing your fuel expenses. This is because the Government, after years of stating that LPG would not be subject to the same duty as petrol and diesel cars, would become subject to a tax levy proportionate with the damage they cause to the environment (or something similar).

However, LPG remains cheap. Although the cost varies, it is typically half that of unleaded petrol. Most cars running on LPG are usually set up as “dual fuel” vehicles, i.e. they can run on both unleaded and LPG. Most are started on unleaded and then switch over to LPG after a given period of time. This helps keep the petrol injectors in good condition.

One disadvantage of this “dual fuel” set up is that the vehicle needs to be optimised for either unleaded or LPG. Typically, vehicles are optimised for unleaded and so fuel consumption is somewhat poorer when running on LPG. The figure varies, but some people quote that the engine uses approximately 20% more LPG than it does with unleaded - and it produces slightly less power. Modern sequential LPG injection systems are more expensive but more efficient.

LPG burns significantly cleaner than unleaded and this is important for oil longevity - essentially oil and sparkplugs remain healthy for longer.

LPG conversions do have one disadvantage, especially for small cars, in they require a relatively large LPG tank, and this intrudes on luggage space. It is possible to use the spare wheel area if this is inside the vehicle, but in the case of the Ka the wheel is slung under the vehicle. Thus, this requires further investigation. The Endura-E is a suitable engine for conversion (there are relatively few engines that are not suitable), so this aspect is not a problem.

One other problem of LPG is measuring the fuel consumption of the car. If the system uses unleaded for starting and then switches across to LPG, when one goes to fill the gas tank the calculated fuel consumption figure will not be accurate. The solution to this would be to use LPG for the most part, and occasionally topping up the unleaded tank (or running on unleaded) and calculating the average fuel consumption using both tanks. This way, one may use a significantly higher amount of LPG relative to unleaded, but we maintain the same basis for fuel consumption figures.

Subject to the installation cost, sequential (multipoint) LPG systems cost between £1,500 and £2,000. This should make it significantly less than a TDCi Ka conversion.

Example Fuel Costs

The above spreadsheet shows that with an assumed fuel consumption figure of 40 mpg (at the time of writing, Kermit’s fuel consumption average is just 38·3 mpg, however most of our mileage will be on a run so I can reasonably extrapolate his fuel consumption to reduce), and that when running on LPG the engine will only be 80% as efficient compared to when running on unleaded (therefore the assumed LPG mpg is 32).

The above shows that we will recoup the estimated £1,600 outlay in just over 52,000 miles.