Sherpa LPG conversion

The LPG conversion

The running cost of a V8 in a van is potentially rather high, it will do about 20mpg on a good day, so it's logical to do an LPG conversion. So although fuel usage will rise slightly, the cost will drop considerably. The payback time I calculate as being around 7000 miles.

The basic idea was to get as much LPG on board as possible, and also make provision for an internal gas cooker as LPG is basically propane.

So the configuration is two internal large tanks for the engine, which automatically top up a small external tank which is used for the cooker. I ended up with two external fillers, one for each internal tank for two reasons, firstly as it appeared that I couldn't get a complete fill on both internal tanks, and also so I can use two LPG pumps simultaneously!

I've not got many pics of the conversion, but here's one of the internal tanks

And the frame to hold the 20l 'domestic' supply tank beneath the vehicle

Under the bonnet

The vapouriser is an OMVL R90/E. You can see the shutoff valve on the bulkhead with the feedpipe going into the vapouriser. The output from the vapouriser is the top pipe which goes into a plenum mounted mixer. You can also see the timer relay to the right of the vapouriser which shuts off the relay if the spark signal stops (i.e. engine stalls)

There is also a flap opener on the air-flow meter, which is fed via a vacuum reservoir (from a Vauxhall Vectra) which you can see sat on top of the heater duct. The flap opener is hidden below the horns.

Issues with the conversion

I have fitted an Innovate LM1 Lambda display. It's quite clear from the readings that the mixture control of the evaporator is pretty poor, the mixture varies from about 12:1 to 18:1. A more sophisticated version is required.

The other issue is that the ignition timing map as defined by the distributor is very wrong for the type of fuel. A mapped ignition is required to rectify this.

Originally I had a mixer which fitted to the flap type air flow meter. This was quite a restriction on airflow, so I fitted a larger one to the inlet of the plenum chamber. Unfortunately the airflow restriction caused by the air-flow meter caused the engine to suck excessive gas out of the new mixer, so I purchased a vacuum operated 'flap opener'. Nearly there, but the configuration was such that the engine always started on petrol, and changed to LPG once the engine revs reached a certain amount (about 2500rpm). Needless to say, it changed over when I had my foot down accellerating. Thus there is limited vacuum, so the flap opener didn't work, and the restriction caused the mixture to be so rich that the engine choked to death and stopped. Usually when pulling out of a junction!!! The fix was to fit a vacuum accumulator in the vac feed to the flap opener so there was always enough vacuum to activate it.

All images (C) 2005 - 2015

Last modified on: 23rd June 2015 by email the webmaster

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