LPG conversion

[arcturus]

What would be the "pay back" time for a LPG conversion on a 960 2.9? I pull a van and do about 5k / year in this vehicle. What are the other benefits?

[Mike_Brace]

Payback time is fairly easy to calculate. Assume a 50% saving in fuel costs - it can be better than that - so your payback time is when you have covered the number of miles on lpg at a cost equivalent to conversion costs. The 960 benefits from a sequential injection system which is likely to cost in the region of £1600 so payback would be when you have used about 3500 litres of lpg - depending on consumption I'd guess about 15,000 miles.
Other advantages of lpg are cleaner emissions, cleaner engine and oil. Many people also consider it runs quieter on lpg. Obviously biggest plus for most people is less tha half the price of petrol.

Mike

[arcturus]

In that case, as I only use the vehicle for towing, it would probably take 4 or five years to recover the cost and by that time I would probably have disposed of the vehicle, so not cost effective. Thanks.

[stephen-in-hull]

You could always go to the Continong, and have a back street specialist fit a second hand system. This shouldn't take longer than a day.

I lived in The Netherlands during the 1980s, and around one quarter to one third of the cars on the road there used LPG.

There are good honest LPG retrofit specialists in every major town, and although the Dutch aren't such enthusiastic car DIYers as we are, they are very environmentally conscious, and good modern donor cars are being broken all the while. (I seem to remember that at ten years old the tank has to have a rather elaborate pressure proofing test, but if you buy a system that has seen less than ten years use, and take it abroad...)

The pressure tank, the changeover electricals and the plumbing to conduct engine hot coolant to prevent the pressure reduction valve icing up due to adiabatic cooling are all bog standard bits of kit with practically nothing to go wrong. With new pressure tubing and small parts, and fresh PTFE tape on all the threads, I'm sure that you'd get a great system at a good price.

One problem is the size of the gas tank, and in those days they were mostly largish, but in a cavernous boot like a Volvo 240, not really that much of a nuisance. I think that in the meantime smaller ones, to mimic spare wheels in size exist, enabling you to accommodate a gas tank in place of your spare, and then decide if you wanted to take your spare with you, or your mother-in-law, on a journey by journey basis.

LPG is said to not contaminate engine oil so much as petrol combustion, and I think exhausts last longer, too. I think sparkplugs also ran longer.

One problem (no, not really a problem, but an annual cost that has to be factored in) with SU carburetted engines with LPG is that the LPG vapour provides no lubrication whatsoever to the carb, which petrol spray does. LP gas is introduced through a very crude, but highly effective banjo fitting between air filter and carb. The butterfly valve of the carb provides the usual throttle control, but the poor needle is being drawn up and down, dry, and doesn't like it. At full throttle the tip of the needle often chatters in the jet, wearing it (the needle, mostly), and therefore LPG-fuelled SU carbs need much more frequent needle changes, and if the needle is very worn, might give problems reverting to petrol operation were you to run short of LPG.

Some users recommend always starting on petrol, then switching to LPG a mile or two down the road when the engine is hot (the changeover switch on your dash is typically like a rotary side / headlight switch that electrically operates valves and cuts out the petrol pump). I never bothered.

Another slight problem, which perhaps doesn't apply in our climate, is that the fuel companies change the % proportions of the gas mix at some stage with the onset of deepest winter; this sometimes irritates an otherwise well performing system.

A perennial ptential cause of difficulty is the aforementioned heat exchanger that warms incoming gas from the high pressure, where it drops to low pressure, but only if you do nothing to look after it. A rubber diaphragm separates hot engine coolant water from very cold gas. This in turn means that every bit of grease and oil in the water will condense on the water side of the freezing cold diaphragm, and then will stick there to intercept all the particulate gunge scudding about through the engine's cooling system, and can therefore block the water circulation to the gas system completely. Annually, or more often, if your engine is shedding scale and swarf, this needs to be disassembled and picked out, and ideally a new diaphragm and mini gaskets fitted, maybe a 45 minute job. If you buy the kit of bits, you'll find that three times out of four you won't use them. But the cacky doo that you recover makes you realise what is constantly circulating in your engine.

On a really cold morning it sometimes helps to pour a kettle of boiling water over the gas pressure regulator valve, but our weather rarely gets anywhere nearly as cold as the Cloggies' habitually does.

Another tip is that in course of time air gets entrained into the tank, and it should be vented off manually, until the vapour coming out of the tank vent valve looks white, like steam. Needless to say this has to be done out in the woods, and without a cigarette in your hand, or the engine running. This has to be done, maybe, annually. This enables the tank to be filled completely with gas, and not have to try and compress air whilst being filled.

The other important safety point is that, as every caravaner and yachtsman knows, LPG leakages are colourless and heavier than air. Running an LPG vehicle means that you can never park in an underground car park, for obvious reasons.