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Autogas Advantages: One Customer's LPG Conversion Experience.

Neil Birkitt, Editor of Volkswagen Driver magazine, had always been curious about what advantages a LP Gas fueled car could provide for him. He explains his experience when he finally converted to LPG.



IF YOU RECALL, from the introductory article in the January issue, the intention with this Mk 4 Golf V6 4Motion was always to convert it to run on cheaper LPG (Liquid Petroleum Gas).

It was an idea that had been planted in my mind early last year, when I put together a feature on an R32 that had been converted to LPG, for our March issue. The idea of a high-performance four-wheel-drive hatchback, with effectively ‘TDI’ fuel economy, was quite an attractive one, and has become more so ever since…


Well, I couldn’t afford an R32, but – back in late October 2012 – when I spotted an advert for an X-reg Mk 4 Golf V6 4Motion in the Club GTI classifieds, for sale in Birmingham, I immediately recognised its potential for my project. Priced at just £750, it was about as cheap as they come, and although it needed some paintwork and a few odds and ends sorting out, it was just what I wanted for the forthcoming Winter. My friend Andrew Chapple, at Volkswizard in Birmingham, was very helpful, taking delivery of the car for me and arranging the paintwork and a few odd jobs for the MoT. I also had the offer of a set of UltraGrip 8 Winter tyres, supplied by Goodyear, also in Birmingham, and these were fitted for me before I took delivery in November, just in time for the cold, wet and snowy Winter. I’ve already raved about the performance of these tyres, and just can’t say enough good things about them. The combination of the smooth power and tractability of the 2.8 V6, the Haldex four-wheel drive system, and the incredible grip of the 205/55 R16 Goodyears made the Mk 4 Golf the supreme driving machine for daily journeys across the snow-bound Chilterns. And even now, in early April, it is still cold enough to justify using Winter tyres!


But, the Mk 4 Golf V6 4Motion was never renowned for its fuel economy, and no matter how much I’d saved on buying this car in the first place, it would soon have become an expensive indulgence unless I made progress with the LPG conversion. Running on unleaded petrol, it was averaging 27-28 mpg, with around 30 mpg on a steady motorway cruise. Even driving extremely carefully, it couldn’t better 35 mpg on a long run. It was soon costing me the best part of £20 a day just to drive to work and back, 64 miles each way, let alone any other excursions. But I’d already discussed my plans with LPG conversion specialist Prins Autogas, based in Southampton, and they’d agreed to convert the car for me. It took a while before our logistics became mutually practicable, but in mid-February I drove the Golf down to Southampton, before using my old Audi 80 TDI again (45 mpg, but on expensive diesel) for several weeks while the Golf was in the workshop. Normally, the conversion would be completed and fully tested well within the week, but there were a couple of issues and delays to overcome. The original inlet manifold gasket was found to need replacement, after the inlet manifold was removed but – not available from stock here in the UK – this had to be specially back-ordered from Germany.

It was also considered sensible to fit new spark plugs and leads, and these were ordered through ECP in Milton Keynes. The plug leads are ProSpark double silicone, and the spark plugs are NGK LPG LaserLine, Iridium-tipped plugs specially designed for use with LPG. I think I can forgive the fact that the original consignment only included four spark plugs; after all, the majority of Mk 4 Golfs have only four plugs, or none at all…


From research for the previous article, I already knew that the Prins VSI (Vapour Sequential Injection) conversion is a cut above the average LPG installation and a world away from the rather agricultural conversions of old. Designed and manufactured by Prins Autogassystemen, in Eindhoven, Holland, it has its own electronic control unit, which interfaces with the standard VW ECU to precisely control the LPG delivery to the six individual OEM-quality Keihin injectors which are fitted into the inlet manifold. The injectors operate sequentially, achieving a high degree of efficiency for both optimum performance and economy. LPG is stored under pressure, in a liquid state, in the 50-litre tank which takes the place of the spare wheel in the boot floor, with the filling point mounted nearby on the external bodywork, in this case on the offside rear bumper. A tank sender measures the fluid level in the tank and communicates this, via the ECU, to the switch on the dash. From the tank, the LPG is pumped to the reducer, mounted in the engine bay, which is where the pressure is reduced to transform the LPG from liquid to gaseous state prior to injection. A temperature sensor on the reducer controls the switchover from petrol to LPG. The engine always starts up on petrol and switches over when the temperature is adequate, usually within less than a mile.


Between the reducer and the injectors is a filter, which sifts out any pollution from the dry gas, to protect the injectors, and this also incorporates a pressure/temperature sensor. Solenoid valves, on the tank and on the reducer, cut off the flow of LPG when switching back to petrol, when switching off the ignition, or in the event of an error determined by the ECU. The injector rail is fitted close to the inlet manifold; in the case of this V6, the six individual injectors can be seen in a row along the front of the engine, tapped into the inlet manifold, close to the front slam panel.

Controlling the petrol injectors during the LPG mode is an Injector module, while the LPG ECU (VSI Computer) – mounted in the scuttle panel close to the original ECU – processes all the incoming signals and drives the different actuators. The engine always starts up on petrol, using the standard engine management and injection system, and – if so selected – switches over to LPG automatically as soon as the engine temperature is adequate. Even on a very cold morning, with thick frost on the windscreen, I find that the switchover occurs automatically within the first mile or so.

The system can also be switched manually between LPG and petrol, using the small press switch on the dash, in this case mounted to the dash panel immediately adjacent the steering wheel.


This switch also incorporates four green LEDs which indicate the fluid level in the LPG tank, while a single red LED indicates when the system is running on petrol, either on start-up or when it has switched automatically back to petrol injection when the LPG is exhausted, in which case it also beeps until the switch is pressed to select purely petrol mode. It’s still early days with the findings. With several press cars to road-test, so far I’ve only completed a couple of long journeys with the Golf V6 on LPG, on each occasion covering around 220-230 miles per tankful. In each case, the LPG refuel required around 45 litres (9.9 gallons) of gas, costing £34.65. That’s at 77p per litre, although I’ve since discovered a Shell station on one of my regular routes where Autogas is just 72p per litre, which will save a couple of quid per refill. Converted to mpg, the LPG consumption works out at only 22-23 mpg, but when you calculate the comparison with the cost of petrol to cover the same distance, at an average of 28 mpg, you find that it would require 33 litres of unleaded at £1.39 per litre, costing £46.


There are also a couple of provisos with this calculation, in that – for the first 1,000 miles or so – the LPG installation is set to run rich, something which will be readjusted at its first check-up and service. It’s also realistic to comment that I’ve probably been driving slightly faster on LPG than I would have been on petrol, aided and abetted by the fact that the standard fuel gauge no longer registers any movement while driving! Of course, you also need to budget for £20-worth of unleaded every now and then, to keep sufficient petrol in the fuel tank to cater for the start-up phase and to allow some reserve for occasions when the LPG runs out and a gas refuel might not be close to hand. On a recent journey up to Cadwell Park and back, for the Time Attack meeting on Easter Monday, I covered almost the whole 250-mile round-trip on one tankful of LPG, although I’d also put in £20 of unleaded to give me some leeway by way of a reserve situation.


Clearly, I’ll be monitoring the logistics and overall economics of the conversion in the long-term, but – on current form – the LPG conversion appears to be saving me around £10 per 200 miles, equivalent to a fuel economy of 38 mpg when comparing with the cost of petrol. In my case, clocking up over 650 miles a week just on basic daily commuting alone, and often amounting to as much as 1000 miles a week, the switch to LPG is saving between £30 and £50 a week – more than enough to pay for another full tankful. I’ve not yet tested the full performance, either by timed acceleration tests or a rolling-road dyno session, but subjectively there is no detectable loss of performance or engine response when running on LPG; if anything, the engine feels smoother and quieter than when running on petrol.



LPG (Liquid Petroleum Gas) is a mixture of propane gas and butane gas, produced during the refining of oil and natural gas. The exact ratio can vary, in winter more propane, in summer more butane. It is normally odourless but – for safety – a distinctive odour is added to it. It has a high octane rating (typically 102-108 RON) and burns more smoothly and cleanly than petrol, with a considerable reduction in harmful CO2 and NOx emissions and particulates. With virtually no acid or carbon combustion byproducts, it also greatly reduces contamination of the engine oil.

Although its lower energy density means that the actual rate of fuel consumption is somewhat higher with LPG than with petrol or diesel, this is more than offset by the considerably lower cost per litre, primarily due to lower taxation.



The Prins Autogas LPG VSI conversion typically costs £1500 + VAT for a 4-cylinder engine, £1750 + VAT for a 6-cylinder and £2,000 + VAT for an 8-cylinder, with a 2-year warranty on parts and injectors. A service costing £120 is recommended every year, or every 20,000 miles.

Assuming an average fuel cost saving of £40 per week, the installation on my V6 4Motion could well have paid for itself in about a year.



Of course, there’s not much point in saving on the cost of fuel if you have to drive miles to find it, and the logistics of running on LPG do need a bit more thought than usual. Not all fuel stations stock it, although there are plenty of maps, websites and apps which list the ones that do. In my case, a BP station within a mile of my home has Autogas – in fact, I drive right past it every morning, and every night – and the motorway service stations at Toddington, near our office, also have it. There are only a couple of outlets on my alternative cross-country journey, but the Shell station at Beaconsfield Services (M40), only a few hundred yards off route, has it at just 72p per litre. The bottom line is that you’ll never really be caught out, because if the LPG runs out during a long journey you can always run on petrol in the meantime. But, with careful planning, you can add an extra dimension to your journeys, not to mention enjoying the satisfaction of paying what is virtually half price for your fuel.



Something I discovered, quite early on, is that there are several different fittings for connecting the gas hose to the tank filler. Essentially, a nozzle inserts into the filler aperture, engages on the keyway and twists to lock in place, before clamping securely in place with either a lever, collar or by locking a trigger, before holding the button to fill with gas. You also need to keep your bare hands clear of the small gas discharge when releasing the nozzle, as it can chill your flesh uncomfortably, although it’s no more irksome than splashing petrol or diesel on yourself. It can be a bit daunting at first, certainly not as simple as just poking in a normal filler nozzle and pulling a trigger, but clear instructions, with diagrams, are usually posted at the refill point and it soon becomes second nature to hook up. There are also different fittings for LPG outlets abroad, and an appropriate adapter will be required if travelling on the continent.