Quote:
Originally Posted by 27litres
In the case of diesel it's a true "dual fuel" in that the LPG supplements the diesel burn. I dont know the ins and outs of it, so I dont know how they get the gas in to the cylinders without throttling, or how they reduce the diesel injection, or balance stoich ratios (I would assume it's only feasible on common rail cars).
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It is immensely more complicated to convert diesels, as I found out when I spent several years of my life developing
this about 10 years ago (you can see a detailed explanation of our solution and the technical difficulties in the owner's manual
here). I've never been involved in the physical/mechanical conversion side of things (my business partner does that) but I believe the injectors have 'straws' on the end, which enables the gas to be injected closer to the inlet valves (to minimise 'robbing' of the gas by adjacent cylinders).
The diesel reduction is, to some extent, automatic. Most diesel ECUs will reduce the diesel injected in response to the feedback they receive from sensors when a secondary fuel is introduced. However, for greater levels of substitution, it's usually necessary to remap the diesel ECU. Injecting the right amount of gas is the biggest challenge. Pretty much every system in existence uses an independent gas ECU that has to be custom mapped to the engine and requires connectivity to much of the same sensors that the diesel ECU does. Compare that to the older LPG kits installed on conventional petrol fuel injection systems, which are merely 'following systems'. All they had to do was to proportionately copy and reproduce the petrol injection output.
The problem with modern common-rail diesels is that the injection timing is very complex, and varies a lot from system to system. Unlike the conventional indirect-injection petrol systems of yesteryear, which just squirt an amount of fuel (determined by a simple square-wave duty-cycle), modern direct-injection diesel systems inject many small pulses of fuel (some just a few microseconds long), at different times in the cycle and for various reasons (some are tiny 'pilot' pusles, for example, that reduce diesel 'clatter' under certain conditions). Deciphering all that was the challenge in designing the system I developed, to make something which could 'universally' determine how much diesel was being injected at any moment with a minimal amount of information (just fuel rail pressure and a current-flow signal from a single diesel injector). It took a huge amount of complex calculations, 60,000+ lines of code and a many, many late nights.
