2006 Lexus IS 250
2006 Lexus IS 250. Click image to enlarge

By Jim Kerr
Photos by Grant Yoxon

Every Fall, a panel of experts from the Automobile Journalists Association of Canada (AJAC) is presented with the newest automotive technologies and the members vote for the best technology. One of the contenders this year for the Best New Technology award is the Lexus D4-S Direct Fuel Injection system. Currently, this fuel injection system is available on 2006 Lexus IS 250 and IS 350 models, and it offers significant improvements in both performance and fuel economy. The D4-S Fuel Injection system uses a combination of port injection and direct injection. Both the port and direct injectors use solenoid type electric injectors controlled by the engine computer but their placement and function is different.

Port fuel injection has been in use for the past 15 to 20 years on most gasoline injected engines. Port injection designs have one fuel injector for each engine cylinder, with these injectors positioned in the air intake ports of the engine. Both multi-port and sequential port injection systems have been used on vehicles. Multi-port systems fire a group of injectors at the same time and sequential injection fires the injectors one at a time in the cylinder firing order. Both types spray the fuel into the intake port behind the intake valve, and the fuel is drawn into the cylinder when the intake valve opens.

2006 Lexus IS 350
2006 Lexus IS 350. Click image to enlarge

There are some emissions improvements available with sequential injection, so it has become the most popular system, and that is the type of port injection used in the Lexus design. The DS-4 port injection supplies some of the fuel and it is also used to cool the air mixture in the intake port for better engine volumetric efficiency.

Direct injection also has one injector per cylinder and this injector, as its name would imply, sprays the fuel directly into the cylinder. Each direct injector can add additional fuel in one of two ways: homogeneous or stratified.

During homogeneous charge operation, the direct injector sprays fuel during the first half of the intake stroke. This is mixed with the intake air.

2006 Lexus IS 250
2006 Lexus IS 250. Click image to enlarge

Then the mixture is compressed, including the fuel provided by the port injectors on the cylinder’s compression stroke. This method of fuel delivery provides a rich mixture that delivers more power from the engine for good performance.

During stratified charge operation, the direct injector sprays fuel into the cylinder near the end of the compression stroke. This fuel is directed by the piston contour to an area near the spark plug so that a rich mixture is present near the spark plug to start combustion. This allows the rest of the cylinder to have a far leaner mixture provided by the port injection, so overall, the air/fuel mixture operates on the lean side. This provides better fuel economy and reduced emissions.

This combination of two injection system designs integrated into one would seem like a complicated application of direct fuel injection. However, this design overcomes the difficulties of complete combustion when direct fuel injection is used by itself in gasoline engines. By using two types of fuel injection and two patterns of injection, the Lexus engine reduces emissions, provides high efficiency and produces horsepower. While I am not able to test emissions output myself, I can tell you that after driving both the IS 250 and IS 350 on the track, it definitely produces horsepower!

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