Hi!
I am running my dailydriver on LPG since about 10years. Here in Germany the LPG thing got “mainstream” quite late, about the time I got mine converted.
At our neighbours, the Netherlands, it was common decades earlier to run LPG on cars, since the 70’s think. Maybe more Dutch members can confirm on that more precise.
In Japan Taxis run on propane since the 80’s, as per NGK-homepage. Did not knew that, learned that when I searched their homepage and found special spark plugs for LPG-use and some comments on how much history/experience this (Japanese!) company has with LPG through the Jap. Taxi-business.
As
@thethee explained correctly, the older/original systems worked with an evaporator, so the lpg was not liquid anymore but a gas, when sucked in. Could be used with carburated, mechanically injected and electronically injected cars.
A major producer of the old days is US company IMPCO, still in business.
The most modern LPG systems are fully electronically controlled and do have injectors, so your engine breaths liquid LPG. Thus less power-losses (example is the JTG system from ICOM, that I am running in my 92 BMW 535i).
As mentioned: the LPG technique is nothing new, works good if the system (and the installation!) is a quality one and saves you money. My personal breakeven was after about 20,000mls. The more fuel your engine consumes, the earlier comes the break-even.
The system can be installed if desired without robbing you trunk-space: there are smaller tanks available that get installed instead of the spare-tire.
Also I have a 2003 Chrysler Grand Voyager 3.3 V6 (think is named Town&Country or Pacifica or Caravan or Minivan in the US) that also runs on LPG. Manufacturer in this case is very reputable company PRINS.
Longterm reliabilty depends on what make/engine you have: mandatory are hardened valve-seats, which any catalytic converter equ. car should have.
But still no guarantee to resist, if engine-materials-quality is low: e.g. modern OPEL or many French cars.
Usually: the older the car the better ;-)
Also there are systems injecting a valve-saving fluid, most popular is FLASHLUBE.
MOST IMPORTANT LAST: Chrysler did build Propane-cars from factory!!! As early as 1981! Look here:
Chrysler Propane Cars: Dodge Diplomat, Plymouth Gran...