Tech Stuff

Maximizing GDI system efficiency

Dieter Lorentz and Ryan Barr are Senior Training Instructors, Bosch Automotive Training

Complete GDI systems include high-pressure valve/injectors, high pressure pump, fuel pressure sensor, fuel rail and electronic control unit. Illustration courtesy Bosch
<p>Complete GDI systems include high-pressure valve/injectors, high  pressure pump, fuel pressure sensor, fuel rail and electronic control  unit. Illustration courtesy Bosch</p>

Engine management systems can be divided into two main categories: manifold or port injection, and gasoline direct injection (GDI) systems. These systems have many similarities, and major differences between the systems are primarily fuel delivery and control and operating pressures.

The need for higher fuel economy and lower emissions has been the driving force behind the changes to these systems since their inception. In the last six years a shift has been underway — many manufacturers are downsizing engine displacement and utilizing direct injection coupled with turbocharging to increase fuel economy without sacrificing engine performance.

Both of these systems use electronic throttle control, returnless fuel systems, and a majority of vehicles incorporate variable valve timing and lift and variable geometry intake manifolds. Manifold or port injection and direct injection systems utilize many of the same sensors to operate.

In the manifold injection system, the air/fuel mixture is generated outside the combustion chamber, in the intake manifold. The fuel injector sprays the fuel directly onto the intake valves where together with the intake air it is drawn into the cylinder.

GDI engines generate the air/fuel mixture in the combustion chamber. During the intake stroke, only combustion air flows past the open intake valve and into the cylinder. Using a high-pressure pump that is driven off of the camshaft, fuel is then injected directly into the cylinders under a variable high pressure, which can be over 2,000 psi.

In a typical manifold injection system, as the engine speed and load change, fuel pressure remains constant. The powertrain control module (PCM) will control the injectors for a longer period of time to allow for the additional fuel required. At full load, the on time of the injector is as much as 20 milliseconds long.

In a direct injection system, the length of injector on time as well as the pressure is controlled, and a GDI system does not utilize as much time to inject the same required amount of fuel as in a manifold system, so the open time of the injector is increased, and also the fuel pressure is increased using the high pressure pump.

Because of the higher pressures involved, the fuel rail and high pressure components are different. GDI systems use a low-pressure in-tank pump similar to a manifold type injection system, to supply the high-pressure pump with fuel.

Diagnosing and repair

Any problems associated with these systems are similar, and all of the basics apply.

Fuel delivery and volume, air leaks, etc., affect both of these systems. A good starting point, if there is a problem, is to check the scan tool data.

False air will have the same effect on either of these systems, causing drivability issues like hesitations or poor idle quality. Possible problems can show up as fuel trim issues like low fuel delivery due to a restricted fuel filter or line.

Tags: Bosch 
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