I take quite a bit of interest in LPG systems. I think it's mainly because LPG conversions get a lot of bad rap which is completely unnecessary. Nearly all problems with LPG conversions are down to a bad install and/or lack of maintenance.
LPG does burn hotter and is also much dryer and doesn't lubricate as well which usually means most cars require a valve protection system.
That said, many cars actually convert to LPG extremely well and don't need any form of 'valve protection fluid'.
Probably the most common valve protection systems come from a company called Flashlube. But these systems aren't just there to protect the valve seats but also to cool the combustion chamber and help lubricate the whole upper cylinder. Flashlube is usually injected around 1000:1 ratio LPG to Flashlube straight into the intake.
There are two different types of system. Electronic systems which have a control module and then use an injector pulse and a pump to control the flow of flashlube. There is also a vacuum fed system which just work off the vacuum from the inlet.
Turbo engines require an electronic system as there is obviously no vacuum in the inlet manifold on boost. Otherwise it's down to user choice.
These systems simply have a dial or a knob to increase or reduce the amount of fluid used. These are worth keeping an eye on. For every litre of LPG used you should use 1ml of fluid. Every few fill ups it takes 5 minutes to see how much fuel you've used, see how much flashlube you've used and adjust the system accordingly. Eventually they will get accurate and will require next to no adjusting.
You can also get 'manifolds' so you can have a separate Flashlube injector for each cylinder.
LPG systems actually use the stock ECU signals to calculate the LPG fueling. The LPG ECU actually takes control of the Petrol injectors and the LPG injectors and has the ability to slowly switch over to LPG, either gradually or one cylinder at a time. Direct injection engines require petrol to flow through the injectors to keep them cool. Therefore LPG converted direct injection petrol cars usually run on 95% LPG and 5% Petrol to which stops the injectors overheating. All this means if you wanted to get your car remapped after an LPG conversion it wouldn't make any difference as the LPG fueling would adjust itself accordingly!
There is also a myth that performance cars can't run on LPG which is utter nonsense as LPG actually has an octane rating of 108ron!
LPG systems do need maintaining but it's not very expensive and doesn't take very long! The fuel filter is changed and it is usually plugged in to ensure it's all set up correctly.
As far as oil is concerned LPG is burns cleaner and therefore oil will actually last much longer when running on LPG.
Diesel Systems...
As far as diesel conversions are concerned then there seems to be a few types of system available.
First is a very cheap and cheerful method where a very basic ECU is used. The signals it uses are manifold pressure and crank speed.
The fueling is adjusted on these very basic signals. Therefore it's likely that you will get LPG being injected on overrun which is no good for fuel economy. Usually these systems have a separate head/coolant temperature sensor to ensure the engine does not overheat. I have been quoted £1400+vat for this system.
Some systems have a 'piggy in the middle' made ECU which monitors the diesel ecu and uses the CAN interface and fuel pulse to mimic a petrol ECU. Then you can hook up any standard sequential LPG system to this ECU to control the fueling. This is a very good system but is very expensive and only one company seems to offer it. This system uses head temperature and a knock sensor as a safety measure to ensure no overheating and no detonation before TDC. This system would cost around £2000+vat.
The other system is by a company called Europegas. (See
HERE) It's very much like the system above but it's all in one unit.
It takes information over the CAN interface to work out it's fueling. It also sits in between the fuel pressure sensor and the ECU to 'dial down' the diesel fueling and replace with LPG. This is a great feature because it means if you get the car mapped it will adjust the fueling to suit.
It also has the safety features of the system above but also has an exhaust gas temperature sensor to ensure no damage is done due to overfueling.
I have been quoted £1500+vat for this system and is probably the only system I would consider getting fitted.
An LPG fumigation system fitted to a diesel will reduce your fuel bills by 15-25%. This is not just because the LPG is half the price as usually only 25% of the fuel that ends up in the cylinder is LPG but because a standard diesel engine only burns around 80% of it's diesel. The hotter burn of the LPG means nearly all of the diesel gets burned. Because of small returns compared to a petrol conversion makes no economic sense for most people, this is why not many people have them fitted. You would need to do around 30,000-50,000miles to see your money back.
Saying that not only will it reduce your fuel bills but it will improve warm up times, slash your emissions, keep your engine cleaner, improve performance, improve throttle response and should overall improve reliability.
I love talking about this sort of thing as I find it really interesting. A well installed LPG conversion that is fitted to a well maintained car will reduce your running costs and reliability dramatically!
In Turkey 37% of their cars run on Autogas. The only reason we get so scared of it is because of piss poor installs and poor maintenance.
