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September 15, 2010
Direct fuel injection
Fuel economy with less emissions
Mark Lemay
Due to tighter emission regulations and the demand for higher fuel economy, manufacturers are rapidly switching to direct fuel injection technology. GM/Saturn, Mazda, Volkswagen/Audi, BMW and Mercedes Benz are using it at this time. It addresses the problems related to poor engine efficiency due to incomplete combustion and heat dissipation, wasted heat energy and poor fuel vaporisation.
Direct fuel injection is similar to regular fuel injection, except that the fuel is injected directly into the combustion chamber (figure 01), as opposed to conventional multi-point fuel injection where fuel is injected into the intake manifold behind the intake valve. It enables the use of a stratified mode (ultra lean burn) for improved fuel efficiency and low emission levels at low engine loads and idle speeds.
Stratified Mode
This mode is leaner than Lambda 1.0 (14:7 to 1 or leaner A/F). This is used for light load and idle. Note that fuel is injected very late in the compression stroke just before the ignition takes place. Since the fuel quantity is very low, there is little need for compression to further atomize the fuel.
Homogenous Mode
 This mode is equivalent or richer than Lambda 1.0 (14.7:1 and richer A/F) and is used when power is called for in higher load situations. The fuel is injected on the intake stroke similar to current port systems (Figure 02). Note that engines using DI also have higher compression ratio.
Controlling fuel mixture
In order to control the fuel mixture under these two conditions, a wide band O2 sensor is used to measure the leaner mixtures when the engine is in stratified mode. Since the lean mixture requires more air the throttle plate is opened much further than the operator demand, which also results in less pumping loss. In the homogenous mode, where more torque is required, the throttle returns to obey the driver’s input. Let’s compare a wide band sensor with a standard O2 sensor :
Conventional O2 sensor
 In this illustration (Figure 03), there is plenty of oxygen on the fresh air side and not enough on the exhaust side. As O2 move across the sensor element a voltage is produced. Since more O2 movement means more voltage the amount of O2 in the exhaust controls the amount of movement and voltage produced.
A Wide band O2 sensor combines a conventional oxygen sensor (reffered to as a NERNST cell) and an oxygen pump cell
Oxygen Pump cell is basically a conventional O2 sensor that has both electrodes exposed to the exhaust stream (Figure 04). Because both electrodes are exposed to the same oxygen content there is no voltage produced.
 By applying current to the electrodes the movement of O2 is forced from one side of the sensor to the other, creating an O2 « pump ». Changing the direction of the current (Figure 05) will then change the direction of O2 movement. The PCM controls the pump cell which in turn controls the voltage of the NERNST cell (Figure 06). Practically, this means that the direction of current flow and the amount of voltage applied to the circuit are directly related to the A/F ratio, thus enabling the PCM to measure the air fuel mixture.
 Fuel delivery
Here is a short description of the main components involved in a DI system :
• Low pressure fuel pump (conventional), which must be verified before testing the high pressure system.
• High pressure fuel pump is a single piston design, PCM controlled
and driven off one of the camshafts using a 2 or 3 lobe cam. The pressure varies between 500 (idle) to 2200 psi (high load).
• Injectors (Figure 07) used in DI must be able to respond very quickly. Hence the armature is relatively heavy and the return spring is much stronger than conventional injectors. In order to accomplish this injectors are fired using much higher voltages. Typical injector voltages range from 65 to 90 volts. In stratified charge mode the injector ON times can be as low as 0.5 ms.
 • Fuel pressure control valve can be controlled via a bypass style regulator or a Flow control valve that limits the amount of fuel entering the high pressure pump.
• Fuel pressure sensor (Figure 08) is a 3 wire sensor with an output between 0 to 5.0 volts. It acts like a potentiometer and is mounted onto the high pressure fuel rail. Low voltage equals low pressure, and voltage will increase as fuel pressure increases.
 Caution
• Since the PCM uses a boost capacitor to increase the voltage to 65 volts, be careful when probing the flow control valve wires.
• Also watch for the fuel system being under extreme pressure that could cause serious injuries.
• Before opening the fuel system bleed off the system pressure by removing fuel lines or opening the fuel system
High fuel pressure relief procedure
1- Connect a scan tool and command the fuel pump relay OFF, shutting off the low pressure fuel pump.
2- Start the engine and let it run until it stops (20-30 sec.)
3- Turn the ignition OFF
4- Using the scan tool confirm that there is no pressure left. If not repeat procedure.
IMPORTANT
If a scan tool is not available disable the fuel pump and start the car. Wait for the engine to stall out before carefully removing any fuel system components
Replacing injectors
You will need special tools to install the Teflon seal (Figure 09) on the combustion chamber end of the fuel injector.
Use the installation cone
Install the resizing tool over the seal to install the seal and be careful not to damage it
Diagnosis and testing
For any abnormal conditions, like misfiring, malfunctions, lack of power or else, retrieving faults code and testing circuits patterns and voltages are part of the usual procedure. Familiar components are tested as such while new ones must be tackled with caution. Referring to the manufacturer’s manual is a must. Reading TSB will also be very useful in some cases.
Mark Lemay is the owner and primary trainer for Auto Aide. Mark conducts technical training classes for many different organizations including AARO, Uni–Select and Auto-Sense. He also runs a diagnostic service trouble shooting problem vehicles and uses many of these cars as case studies in his classes.
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