Improving Internal Combustion with Direct Injection

Direct injection engines -- found in vehicles ranging from sports cars to snowmobiles -- are becoming more common as manufacturers look to improve the internal combustion engine by squeezing out more horsepower, less emissions and better fuel efficiency.

Several GM engine families presently use Spark Ignited Direct Injection (SIDI) technology, including the 2.0L turbocharged 4-cylinder engine in the Saturn SKY and Pontiac Solstice and the 3.6L V6 engine in the Cadillac CTS. (Fig. 1) Some industry projections indicate that in a few years, one in six engines will be built with direct injection. The numbers illustrate the value of the technology: the 300 HP 3.6L V6 in the new Chevy Camaro is rated at an estimated 29 MPG highway.

Direct injection refers to the fact that the fuel is injected directly into the combustion chamber. Direct injection is not new. In fact, it's a basic principle of the diesel engine. A diesel engine depends on the heat of compression to ignite the diesel fuel the instant it's injected into the cylinder (this is Compression Ignited Direct Injection or CIDI). By comparison, SIDI engines inject gasoline directly into the air in the cylinder and the resulting mixture is then ignited by a spark plug. (Fig. 2)

The injection system used on SIDI engines differs from standard fuel injection in one important way. With SIDI, the fuel is injected at a much higher pressure (up to 2,176 psi or 15,000 kPa) because it's injected directly into the combustion chamber rather than into the intake manifold (port injection).

Benefits of SIDI

When compared with port injection, benefits of SIDI include:

- lower emissions, particularly at startup

- higher compression

- better fuel economy (particularly on turbocharged engines)

- increased horsepower

All gasoline engines depend on atomizing the fuel before combustion. But with carburetors, throttle bodies, and even port injection, the atomization takes place at a distance from the combustion chamber. By the time the atomized fuel and air reach the combustion chamber, some of the fuel has separated out and collects on intake surfaces. SIDI eliminates all of this by atomizing the fuel directly in the combustion chamber. As the fuel atomizes, the air and fuel mixture are cooled, enabling the use of a higher compression ratio.

Better atomization contributes to a reduction in pre-ignition and detonation, which is why the SIDI engine can operate at a higher compression ratio and consume less fuel. SIDI allows the engine to run on a leaner mixture (more air, less fuel) at full power. SIDI also allows a higher compression ratio, which in turn provides better fuel economy at part and full throttle.

Direct injection permits controlling a richer mixture around the spark plug, making it easier to ignite when the engine is cold. The SIDI engine runs smoother and generates lower emissions during cold start and warm-up.

Components

The fuel delivery components of SIDI are divided into two groups, the low pressure system and the high pressure system.

Low side components are essentially the same as those in a port injection system. The fuel tank contains a modular reservoir assembly (fuel pump). Fuel pressure in the low side is approximately 60 psi (410 kPa).

Low pressure lines deliver the fuel from the in-tank fuel pump to the engine compartment. (Fig. 3)

A stainless steel pipe, containing the fuel pulse dampener (Fig. 4) and fuel pressure service valve, connects the fuel line to the high pressure pump. The fuel pulse dampener may have an audible clicking sound when the engine is idling.

The high side starts at the high pressure pump (Fig. 5), which is located at the rear of the cylinder head. It is driven by extra lobes on the camshaft. This pump is capable of delivering fuel at a pressure of 2,176 psi (15,000 kPa).

The high pressure pump incorporates the fuel rail pressure (FRP) regulator. The FRP is operated by the ECM, using pulse width modulation, to provide the fuel pressure commanded by the ECM. And the high pressure pump also incorporates a pressure relief valve.

The fuel rail delivers fuel from the pump to the injectors. A fuel pressure sensor is attached to the fuel rail, and it contains a diaphragm and strain gauges. Both the fuel rail and pressure sensor are made of stainless steel. A silicone-free lubricant should be used before mounting the sensor.

Electro-magnetic fuel injectors are used. (Fig. 6) They mount to the cylinder head and spray fuel directly into the combustion chamber. Precision-machined holes generate a cone shaped spray pattern. The injector hold-down clamp must be replaced if removed.

The ECM contains a converter that steps up voltage from 12V to 65V and charges a capacitor. The capacitor provides 65V to open the injector. Then the ECM provides pulse width modulated 12V to hold the injector open for the prescribed time.

SIDI Service Highlights

A scan tool can be used to perform the following (see the appropriate Service Information for details):

- Cylinder Power Balance

- Fuel Injector Balance

- Fuel Pressure Control

- Fuel Pump on/off

- Fuel System High Pressure Reduction

Low side system pressure relief is the same as with standard fuel injection systems.

High side system pressure relief can be done two ways:

- Turn off the engine and wait at least two hours, then cover the high pressure fitting with a shop towel and loosen

- Use a scan tool to command the fuel pump relay off, run the engine until it stops (approximately 30 seconds). Turn off the ignition and confirm fuel pressure with the scan tool.

To remove and service injectors, it may be necessary to use special tools. Check the Service Information for more details about the right tools.

When removing injectors, the following must be discarded and replaced:

- fuel injector hold-down clamps

- O-rings

- plastic spacers

- fuel injector seal

When removing the high pressure fuel pump, the following must be discarded and replaced:

- fuel pump bolts

- fuel pump gasket and O-ring

- high pressure fuel pipe

When installing the high pressure fuel pump, be sure the roller lifter is oriented properly, the camshaft is at base circle, and the number 1 piston is at top dead center (TDC) on the exhaust stroke.

- Thanks to Mike Militello and GM Powertrain