Tips From An Automotive Engineer #3 - 8 injectors, 4 cylinders? (D-4S Basics)

[BRZ]

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.

[pszuvela]

Thanks for the informative posting BRZ.

[onbooSTi]

Nice write up. I was wondering about the D4-S and now I know. I read what you said about Cobb, but what needs to be done in order for tuning? Will this effect mapping of the engine? Make it more complicated for tuners? In the end, do you think it was a wise move to use this system or just a ploy to get Toyota's name on the engine?

[BRZ]

The world is going to direct injection. Expect every car on the road to use it 10 years from now. Its just plain better than standard fuel injection, and as the technology improves, its negative effects will be deminished significantly. Engine startup emissions will probably be reduced significantly as auto makers come up with better in cylinder designs and injector construction.

Cobb i'm sure will attempt to come up with a system to tune this engine, as will most other companies, as it is likely to be used in the wrx/sti when it is revamped over the next few years. The twin injection systems is going to make tuning much more complicated that it would be with one system, and I see most tuners trying to bypass the port injectors completely and just run the engine on direct injection when in closed loop. I don't know if this will be possible or not at the moment, Toyota/Subaru has not obviously released any information on the inner workings of the system for me to be able to tell you that. But that is a possiblity. If that cannot be done, tuners likely will have to make adjustments to two maps to make up one tune rather than the standard one set of adjustments.

However, a bright spot is that Subaru did anounce a while back that its ecu would be completely open. Meaning that it would allow for modification of all parameters within the ecu, essentially making the car 100% tuneable in every aspect. So tuning will definately be possible, but maybe not that easy.

[piotrush]

Another plus of having a port injection system is that it might reduce the risk of carbon build-up on the valves over the long term. I used to work at a MINI dealer and one of the issues they had was that at some point the valves would cake up with carbon deposits and would cause CELs, rough running, loss of power, and engine damage in extreme cases. Our foreman said that was because the air fuel mixture does not 'clean' the valves since it no longer moves past them. I'm sure poor choice of fuel quality had a lot to do with it with many owners, but I'm glad to hear that both systems are in use on the BRZ.

[BRZ]

Yes, the gasoline definately does clean off the valves as it enters the cylinder. But usually any build-up of carbon deposits is caused by an improper fuel mixture or very poor valve timing and bad gasoline. Direct injection and cam timing is getting to the point where air and deposits are not having the chance to move back into the intake, and thus can't add deposits to the intake valves. So i don't see this being a problem down the line.

[raventare]

Is there a chip for the Lexus tuning yet...that will answer any questions as to whether it is easy to do or not. Cobb will be able to do it for sure as well as any chip/tuning manufacturer. We ALL need to write to all the aftermarket companies to get them interested in supporting our cars or it will be years till they get around to it. The more requests they get for parts, the more likely they will build something. Sure, there are some on the "bleeding edge" of this development but there are many more under the proverbial rock. The squeaky wheel will get the grease...or so they say, whoever THEY are.

Almost forgot, chips are fine but the MOST important thing is a "map" to go with it and those you will want from a "go to" place so you can take care of any problems that may come up...Just checked and Lexus IS350 chips start at $70...a chip with proper mapping should cost a few hundred dollars and as much as several thousand for special cars. As long as we are sans-turbo it should be on the low end of the scale as we will have fewer interfaces...and definitely YMMV

[BRAZEN]

Thanks for posting. I like.