[WingsofWar]

I was talking to a friend today that I haven't heard from in a long time. He eventually got a job with Lexus and has worked on a regular basis the 2GR/4GR-FSE and we got to talking about the subaru engine with Yamaha heads and D-4S. Actually he was not to keen on the idea of working on it or being one of very few people who have experience with tuning DI, he doesn't want to help people out of their failures.

He did though taught me some stuff about D-4S that was really interesting.

Gasoline used in a conventional fuel injected engine has a desired burn rate of stoich 14.7:1, but Gasoline used in a direct injected engine has a burn rate of stoich 40:1. So when adjusting fuel pressure on the common rail we are looking at 4x times the pressure seen in a fuel rail. This also changes AFR outputs and tail pipe emissions characteristics and tuning using conventional methods without applying new theory will damage the engine.

During Intake Charge on a conventional engine we see AFR mixtures similar to tailpipe AFRs. On Direct injection engines, we actually desire 15:1-20:1 Lean AFRs during intake stroke. Unlike conventional engines which will damage at those ratios. And turbo applications we don't want to reach lower than 13AFRs as its actually too rich and is wasting optimal power.

For the D-4S, direct injection is not used during idle, and low load. Port injection is the primary fuel system. At mid load and cruise port injection and direct injection are used in tandem. At High load and WOT port injection is not used and direct injection controls primary function.

“The D-4 incorporates a highly optimized combustion chamber: The piston crowns are fitted with a lipped, cup-like formation that focuses the injected air/fuel mixture tightly around the spark plug for maximum burn.

Intake air is drawn through helical ports that create a high degree of horizontal swirl, which Toyota says combines with the lipped combustion chamber to not only maintain highly stable combustion -- a quintessential problem for lean-burners -- but also to stratify the air/fuel mixture.

Additional swirl is promoted with special swirl-inducing high-pressure injectors. The end result is a fuel-rich mixture in the direct vicinity of the spark plug, with extremely lean air/fuel ratios near the cylinder walls.”

When looking at tuning options, we come across the problem of the different environment we see in the tailpipe. With Ultra-Lean conditions, higher NoX levels, and a higher overall exhaust temperature. A number of Wideband Oxygen Sensors (WBO2) are seeing more errors and inaccurate AFR readings. So a Wide Range Oxygen Sensor (WR02) was developed but is still not widely available.