Single point mixer-ring open loop

Simple mixer-ring open loop system (1st generation)

Suitable for carburetor or mechanical injection vehicles without a catalytic convertor or lambda sensor.

There are a lot more parts to this system than in the picture!

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Single point mixer-ring open loop systems are the simplest and oldest form of LPG system, known as 1st generation.


Suitable for carburetor or older mechanical injection vehicles only, without a catalytic convertor or lambda probe in the exhaust.


The main system parts comprise of the LPG tank, fill point, vaporiser (reducer), the mixer-ring (as shown in pic) and various valves / solenoids, piping and wiring. A 3 position switch with incorporated LPG fuel gauge is mounted on the vehicle dashboard. The LPG fuel gauge is a row of LED lights, the more LEDs that are lit the fuller the tank.

Why 3 positions on the switch in the dashboard? The switch positions are: run on petrol, no fuel, run on LPG. The no-fuel position is used when switching from petrol to LPG. With carburetor vehicles it is necessary to allow the petrol remaining in the carburetor to be used by the engine before switching to LPG or the engine will get both petrol and lpg at the same time, this can make the engine run too rich and stall. The no fuel position cuts off the petrol feed to the carburetor but does not turn on the LPG system. To change from petrol to LPG you switch to no-fuel mode and wait until the engine starts to cut out before switching to LPG. To switch from LPG to petrol, you can simply move the switch straight back to petrol. This is a basic system and the only system that requires a 3 position switch or having to wait until the engine starts to cut out before switching to LPG, however this is necessary where the vehicle’s original fuel system used carburetors. All other systems have either a 2 position (closed loop mixer-ring) switch where you can switch between petrol and LPG immediately and seamlessly, or a single touch button (sequential systems) to change fuels instantaneously and seamlessly, or fully automatically.


Tech-stuff… How does it work?

The mixer-ring is placed on top of the carburetor or onto the air intake just before the throttle valve. It is precision engineered to allow a linear rate of LPG to be sucked into the engine along with air as with a conventional carburetor. The mixer-ring is connected via a hose to the vaporiser. The hose has an inline manually adjustable screw valve. On these 1st generation systems there are no very complicated electronics, the system is tuned in a similar way to a carburetor by adjusting the inline screw valve and screws on the vaporiser. On these 1st (and second) generation systems, the vaporiser supplies fuel at near atmospheric pressure, so the LPG is literally sucked out of it by the  vacuum supplied to it by the mixer-ring. Mixer-rings must be matched to each individual model of vehicle — they come in a large range of sizes and also have a range of venturi sizes that can be fitted inside. Getting any aspect of a mixer-ring system wrong can be the equivalent of putting the wrong carburetor on a vehicle and it would not run properly.   


As with all aftermarket LPG systems, it is usual practice that drivers start on petrol and drive a minute or so before switching to LPG. This is because the vaporiser requires heat from the engine to convert the high pressure liquid LPG in the tank to a lower pressure gas so it can be used as a fuel for the engine. If the vaporiser were to get too cold, it may not be able to supply enough LPG for the engine if the driver were to suddenly apply heavy use of the accelerator pedal. If you have ever burst a full aerosol or butane lighter you will know what we mean by the above:- a lot of gas escapes at first until the evaporation cools the contents, then the gas escapes much more slowly. If the temperature were kept at the same level (as it is in the vaporiser connected to a warm engine) then the gas would continue to escape at the same rate.




Pictures are of a mixer-ring  and mixer type system vaporisers

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 Site Updated January 2012