As I'm sure many of you are well aware, the new Subaru BRZ and Scion FR-S both use Toyota's fancy D-4S direct injection system. But I bet most of you haven't a clue how this system actually works.
Yup, its pretty complex. 8 injectors for a four cylinder engine, at first glance it doesn't make any sense. But let me assure you, there is a reason for Toyota's madness. D-4S actually stands for D-4 Superior. This is essential a modified version of Toyota's D-4 direct injection system. In this system both conventional port injection and direct injection are combined into a hybrid setup for fueling the engine. This means each cylinder will get one direct injection injector, and one port injector, and the ecu will choose which to use based on the engines needs.
The traditional port injection system operates at 45 Psi and is used primarily during startup and idle on its own. The reason behind this being that it is more effiecient than the direct injection system under these conditions and will produces less emissions. Obviously in this day and age, this is a primary concern. Then at part throttle acceleration and cruise conditions, the system transisitons to spray part of the needed fuel from the port injectors, and part from the direct injectors. The amount sprayed from each is calculated by a complex algorithm in the ecu. Essentally as you increase the throttle opening, the direct injection system is more heavily used to support the engine. Then at wide open throttle, the direct injection system kicks in fully and is used on its own to provide all of the fuel. The higher pressure direct injection system offers more precise control over the fuel injection and maximizes performance of the engine.
While the traditional port injection system is a cleaner and perfectly effective system, in order to get air and fuel mixed properly the engineers must design features such as swirl control valves, high tumble cylinder heads and swirl intake runner setups into the engine. These design features limit airflow into the engine and significantly slow down air velocity entering the cylinder, which in turn lowers the amount of air that can be forced into the cylinder. Using the direct injection system, which evenly distributes fuel into the air as it enters the cylinder, eliminates the need for these features and allows the engineers to maximize the flow of the engine and keep high intake velocity. This allows a 2.0L engine that may have been maxed out at 170 hp with port injection, to reach 200 hp with direct injection technology.
Essentially with Toyota's D-4S system, the car uses the port injectors to lower vehicle emissions and the direct injection system to maximize power and torque output. Personally, (and i'm sure many of you feel the same way) I see no reason to have the port injectors. All they do is clean up the cars emissions during idle, startup and open loop conditions, something I consider unecessary. I'm sure this car will vastly exceed the governments emissions requirements, but it looks good for advertising and gives the company more emission credits. However there is a positive here... Direct injection injectors are VERY expensive to upgrade, because they must withstand high cylinder pressures and temperatures over 1500 degrees. Since the system does not use the port injectors during WOT, you could in theory, use them to add more fuel when you maxed out the in cylinder injectors when adding a turbo, without having to shell out a ton of cash to upgrade your injectors. The mixture wouldn't be as good, and thus your engine wouldn't be as efficient as a purely direct injected system would. Its more of a bandaid type solution, but for those who are borderline on fuel or want to make the switch to E-85 without having to spend much cash, this might be a way to save a little money.
This is not the first car to use this system. The Lexus IS 350 has been using it since its inception. So I don't think anyone will see any major reliablity issues. However tuning may be difficult to master at first. Most likely a company like Cobb will produce a nice system to easily compensate for extensive modifications made to the engine, hopefully one that allows for the addition of a turbocharger.