On the market there are three main types of diesel/LPG systems.

The solenoid/jet method is the oldest and most crude system available on the market. Basically the system consists of a solenoid to turn the system on and off and a jet to control the flow of gas.
 
The problem with this method is that while the system is on, the flow of gas is constant when the engine is not. This mean that while the vehicle is idling, there is too much gas going in, damaging the engine. While pulling a caravan or horse float, the engine is working hard, there is not enough gas, giving poor performance and fuel savings.
 
Unfortunately the majority of diesel/LPG systems on the market use this method because it's cheap to make and fast to install, allowing the manufacturer and installer to make a quick buck before it all turns pear shaped. Tuning is done by unscrewing the jet and replacing with a bigger or smaller jet. These systems damage engines and give poor fuel savings.

Being the cheapest to make also means these are the cheapest to buy and give truth to the phrase “Buyer Beware”.
 
The stepper and venturi method is an improvement over the solenoid/jet method but still falls way short of a true injection system. This system uses a stepper to open and close a valve gradually to meter the gas flow and a venturi to allow the gas to be sucked into the engine.
 
The killer with the stepper/ventrui system is the response time. These systems make take up to 20 seconds to catch up with the engine. Just think how long it takes you to turn your garden hose on from fully open to fully closed and back again. These systems work just as fast.
 
With a response time like that, you have already overtaken the car ahead before the gas has caught up with the engine. If you hit the brakes hard, the system is putting gas into the engine long after the diesel has stopped, damaging the engine.
 
A true injection system uses a needle injector, which is the same injector used in all modern petrol/LPG systems. The flow of gas is metered by pulsing the injectors rapidly. This rapid pulsing allows the gas to be exactly measured while giving an almost instantaneous response time.
 
The faster, more accurate delivery means that better performance and fuel savings can be achieved using less gas and without damaging the engine.

 

All of these systems claim to “Inject” in their marketing but only a true injection systems use a needle injector. When selecting a system ask your installer if they use a jet, ventrui or a needle injector cause "Oils ain't Oils".