SYSTEMS.
Types of systems - in order of complexity which is really a process of elimination of the newer (costlier) more sophisticated systems on the basis that they are less likely to be of interest here (and if they are - then seek specialist advice).
INJECTION SYSTEMS
To get it out of the way, there are systems that handle LPG as a liquid as they administer it to the engine. But, due to the inconsistent pressure (with temperature) of an LPG holding tank they require a pump inside the tank. These have a poor reputation for reliability - not helped by LPG's lack of lubricating quality. They do have the benefit though of eliminating the need for a reducer/vapouriser which has to be plumbed into the engines cooling system and are quite bulky (and fussy re orientation).
Gas injection though is widely respected and increasingly specified. The usual set up is not dissimilar to a Vortec's spider in that the solenoid injectors which handle the LPG as a gas (as it has previously passed through a reducer/vapouriser) are grouped together then feed the inlet manifold branches via individual supply pipes. This is the preferred method as the injectors are quite bulky and difficult to place advantageously on a manifold in close proximity to the inlet valves.
Both injection systems use the signals sent to the gasoline injectors and via an algorithm make adjustments appropriate to the differing characteristics of the two fuels. The injectors can be batch or sequentially fired depending on the system. This is the preferred system for modern-day engines but, they can throw the vehicle's ECU into fuel trims that can be problematic as they can illuminate the check engine light. They are probably the best system though and anyone seriously considering this for a modern-day vehicle would obviously seek the advice of a seller/installer.
MIXER SYSTEMS
This is where it becomes interesting for the likes of us. I'll explain the simplest open loop system first.
The system comprises a tank, a variety of solenoid valves between it and the reducer/vapouriser, a mixer and a power valve between the reducer/vapouriser and mixer.
Working backwards, the mixer is a venturi fitted upstream of the throttle butterfly (the carb's one is fine as the carb is likely being retained but only a butterfly is actually needed). Air destined for the engine passes through it and creates a slight vacuum which pulls LPG in gas form from the reducer/vapouriser into the mixer's venturi where it is then entrained with the air heading toward the engine. The power valve is akin to a tap but when set retains its setting. It is adjusted during the calibration process and controls maximum LPG flow - in much the same way as a main jet in a carb has ultimate control over fuel supply. The reducer/vapouriser is plumbed into the coolant circuit (via the heater hoses to supply heat as the vapourisation of LPG draws heat and the system could otherwise freeze up). The reducer has a 'sensitivity' setting which is used in conjunction with the mixer's ability to create vacuum and the power valve to enable complete calibration. Note that the reducer/vapouriser does not emit LPG gas under pressure but very slightly below atmospheric (the sensitivity screw adjusts this pressure) hence the need for a venturi. The only other setting is for idling (via an adjustable screw) for conditions when mixer vacuum is low. It can provide LPG gas at a very slight pressure.
That's it. An engine equipped with the above can start on the LPG (just wait a few moments after start-up for the reducer/vapouriser to get some heat before driving off), will happily sit with a carb below it (generally the switching between fuels allows both fuels to be shut off until the carbs float chamber are empty then the LPG is enabled) but cannot be as finally tuned as an closed loop system which requires an O2 sensor which would be vulnerable to dirtying during carb operation.
Closed loop is all the above and is intended for EFI equipped engines with O2 sensors that when running on gasoline do so cleanly enough not to soil their sensors.
The system is calibrated as per the closed loop system but has additionally a stepper motor driven actuator connected electrically to a small ECU. This ECU controls fuel switchover and disabling of the gasoline injectors) but it's main function is to read from the O2 sensors (it also reads from the TPS and CPS but those readings aren't used for mixture control). Once the system is calibrated mechanically (as the closed loop system was) the power valve is then removed and replaced with the actuator which, as it was connected to the ECU during the calibration process and was reading from the O2 sensors, arrived at a 'default setting' akin to the final setting on the power valve. When installed in place of the power valve it goes to this setting. It continues to read from the O2 sensors thus can open and close slightly to keep the mixture strength where it needs to be. How far it can deviate from its default position (in steps) can be programmed into the ECU (very simple procedure) as can it's near closing on over-run (setting and rpm both adjustable).
The main advantages this system has over the open loop one is its ability to (via the actuator) fine tune the mixture to stoichiometric during running and, self tune (the default setting) for temperature and atmospheric changes.
It is simpler than a carb but is a carb with an adjustable main jet under constant control. It integrates well with an EFI engine, doesn't need fuel enrichment from cold (true of all LPG systems) but can cause fuel trim drift under prolonged running on LPG (reverting to gasoline will readjust the fuel trims but takes time). All ignition related controls function as they would under gasoline operation. Optionally, the gasoline pump can be disabled. Both closed and open loop systems can be installed as stand-alone systems - ie no gasoline system with the provision of O2, TPS and CPS sensors for the closed loop system.
Of all the systems, closed loop single mixer systems are the simplest and easiest to work with and give good results. Maybe a little trickier if chasing big power outputs as the mixer size can become critical re lack of vacuum at lower speeds (like throats on a carb) but otherwise simple, reliable and pretty cheap to knock up. They can also (if the actuator is replaced by the pre-set (during initial calibration) power valve) and the solenoid valves hot wired - get you home even if the ECU fails (providing you can disable the gasoline system (pulled pump fuse).
