Understanding LPG fuel systems

Above left is a mixer ring, on the right a pressure reducer (or vaporiser). There are many more parts that are fitted but these are the main parts. The mixer goes just before the throttle housing on the engine, so all air entering the engine passes through it (note there are different types of mixer which can look completely different but they all work in the same way). An LPG feed from the reducer is connected to the mixer and the LPG is literally sucked out of the reducer and mixed with the air entering the engine. Here’s how it works - The little holes on the inside of the mixer are where the LPG comes out. Because the mixer is designed to create a slight resistance to the airflow above the holes, the holes are partially exposed to engine vacuum which sucks out the LPG into the flow of air that is going into the engine. Mixers come in many different sizes and have interchangeable venturi’s (the middle part which causes the air restriction and has the holes). In addition to a manual control valve fitted in the LPG feed pipe, the size of the venturi controls how much LPG is sucked into the engine. These systems work in much the same way as a petrol carburettor. The advantages are that they are the cheapest form of LPG system, very simple, not much to go wrong, and can be the best or only option for a carburettor or older fuel injected engine. The disadvantages are that the mixer creates a slight restriction to the air flow into the engine so can lower the peak power of the engine slightly. Backfiring is a possibility, but no more so than with petrol carburettors with a properly set up system. Bare in mind that a backfire could destroy a plastic engine inlet manifold, air filter, or if fitted to a fuel injected engine a backfire could damage an air flow sensor mounted in the air inlet stream, but if your vehicle has a carburettor it will have a metal inlet manifold for the same reason. As there is no closed loop control of the air/fuel mixture there may be certain driving conditions where the air/fuel mixture isn’t optimised, so economy and drivability might not be quite as good as with a closed loop system. We wouldn’t fit this type of system on a vehicle with a catalytic convertor, because an incorrect mixture can burn catalytic convertors out (carb’ vehicles won’t have a catalytic convertor). But again, this is just the same as a carburettor. Carburettors wear out quite quickly, LPG mixers don’t wear. You may have more power and get more mpg from one of these systems than you do with an old carburettor. Of course, it will be cheaper to run on LPG in any case as LPG is much cheaper to buy. When converting any carb vehicle to LPG, the system cannot be made to change between fuels instantly because there is no way of instantly stopping the flow of fuel from the carburettor. The changeover switch for carb vehicles has three modes, to switch from petrol to LPG you press the button once, which cuts off the fuel supply to the carb but doesn’t yet turn on the LPG. When the fuel that is remaining in the carb starts to run out the engine will hesitate, then you press the button again and the system switches to LPG mode and carries on running. When changing from LPG to petrol you press the button once and the system allows petrol to the carburettor but keeps the engine running on LPG. When the carb fills with enough petrol the engine will start to hesitate as it is now getting both petrol and LPG so will have a very rich mixture. When it hesitates you press the button again and the LPG is turned off leaving the system running only on petrol. Usually carb vehicles will start happily on LPG so there is no need to start on petrol and LPG is used all the time.