STAG 500 DIS - more than sixth generation

© ACSTAG 500 DIS offers the best of two worlds - it combines direct injection of liquid state LPG through petrol injectors and port injection of vapour state LPG through injectors fitted in the intake manifold

The STAG 500 DIS is so innovative that AC decided to have it patent-protected. It is the effect of a completely new approach to DI petrol engine conversion, being a development of liquid state direct LPG injection systems, but without their imperfections related to the use of the car's original high-pressure petrol pump for boosting autogas pressure. The STAG 500 DIS makes do without it and the pump is deactivated in LPG mode.

Eliminating the petrol high-pressure pump from the autogas system was inspired by the fact that adapting such pumps to work with LPG instead of petrol is often difficult, sometimes actually even impossible in everyday conversion shop conditions. More often than not pumps have to be removed from the engines and shipped to the autogas system manufacturer for modification, which consumes time and money.

Removing the pump from the equation has the added value of eliminating the problem of vapour locks forming in the pump. Vapourisation of autogas in the pump is highly unwanted and if it occurs, it immediately affects engine performance, causing its stalling.

So how did AC's engineers address the issue?

They used external LPG pumps instead, included in the STAG 500 DIS conversion kit. There are two series-connected units in the system, together producing a pressure of up to 20 bar. Compressed fuel is pumped into a tee, where a part of the fuel is directed into the petrol injector rail (bypassing the high-pressure petrol pump) and the rest goes to a reducer, where it's vapourised and transported further into vapour state LPG injectors mounted in the intake manifold.

And so the STAG 500 DIS system uses liquid state autogas injection through the petrol injectors (like in 6th generation systems) and an additional vapour state LPG injection into the channels of the intake manifold (like in 4th generation systems). The liquid state injection accounts for some 30% of the fuel applied to the engine's combustion chambers, the remaining 70% is indirect vapour state injection.

The system uses as many as possible components from conventional (4^th generation) solutions offered by AC so far, such as the reducer, the vapour state injectors and the solenoid valves. Furthermore, the STAG 500 DIS utilises typical, affordable autogas tanks. There is a difference in the multivalve, however, since it needs an additional fuel return connector.

Importantly, there is no risk of mixed fuels entering the LPG tank. Even though autogas and petrol do get mixed to a limited degree in the switchover phase, the LPG return valve doesn't open until the autogas-petrol mixture is completely burned out in the combustion chambers.

Most importantly, though, the new system from AC is 100% versatile (it can be used on any car with a DI engine, with no individual injection strategy maps required for each engine code) and its calibration is similar to that of Q-generation autogas systems (a quantifier is used). Also, the STAG 500 DIS is suitable for converting DI engines featuring additional indirect petrol injectors, which are used to lower emissions in the engine's warm-up phase and partial load operation. A positive side effect of such injectors is keeping the channels of the intake manifold clean and free of deposits as petrol removes unburnt oil particles. Port-mounted LPG injectors provide the same side effect, as confirmed during tests and everyday use of the STAG 400 DPI system.

The new system is also easy to maintain thanks to the use of typical components and external autogas pumps. However, finding the right location for the pumps may sometimes prove difficult in smaller cars due to limited amount of available room.

As the STAG 500 DIS was being developed, AC simultaneously worked on a 6^th generation solution of their own, i.e. one that applies 100% of LPG in liquid state directly into the combustion chambers throug the car's original petrol injectors. The injection strategy utilised by the STAG 500 DIS eventually won, because it's simpler and more affordable. From an installer's perspective, converting a car with the STAG 500 DIS is practically the same as with a conventional sequential autogas system. The only major additional operation is connecting the LPG bus to the high-pressure petrol pump, which allows to replace petrol with LPG in the injection rail and to separate the pump from the fuel system after switching over to autogas.