Gasoline direct injection. (Courtesy of Bosch)

A conventional fuel-injected engine supplies all cylinders with a mist-like mixture of air and fuel that is flushed into the cylinders during the intake stroke and ignited by the spark plug. The throttle valve determines how much of the air/fuel mixture enters each cylinder.

In a direct-injection engine, the injection nozzle location is inside the combustion chamber, and each solenoid-controlled injector’s tiny outlet holes generates a finer mist, creating a well-atomized air/fuel mixture.

Each bank of cylinders has a high-pressure fuel rail that feeds the individual injectors and a high-pressure pump with a fuel rail pressure sensor on each rail that helps the vehicle powertrain control module precisely control the fuel pressure.

The direct injection system allows more precise control of the fuel pressure. (Courtesy of Bosch)

Indirect injectors positioned to the side of each cylinder aim the fuel directly into the cylinder, adjacent to the spark plug and alongside the intake and exhaust valves. These injectors spray fuel into the cylinders at pressures of up to 2,150 psi. That's about 35 times more intense than port fuel injection.

To control the injection valves, common-rail injectors use a rapid-action actuator made of piezo crystals. The movement of the piezo package transmits non-mechanically and entirely without friction to the rapidly switching nozzle needle. This doubles the injector's switching speed, allowing more precise measurement of the amount of fuel injected and thus leading to a reduction in harmful combustion products. (Courtesy of Bosch)

Surrounding the spark plug is the relatively small, precisely shaped volume of ignitable air/fuel mixture that results when the injector sprays fuel toward the spark plug just before ignition. When the injector sprays fuel into the cylinder, the shaped piston crown guides the air/fuel mix to the spark plug.

The precise metering of fuel and control of combustion chamber temperatures enables compression ratios of nearly 12:1 without the need for premium fuel, because direct injection reduces the tendency of engine knock. The higher compression ratio alone increases efficiency by about two percent.

Automakers realize major reductions in fuel consumption when direct injection is combined with technologies such as turbocharging. Mixing existing and proven technologies in a synergistic manner can offer double digit fuel economy benefits with a much lower cost than hybrid technologies.

When servicing a direct injection system, keep the following tips in mind:

  • A direct injection system typically will operate between 725 - 2050 psi, so bleeding down the fuel system properly is important. Always follow the manufacturer’s procedure to bleed the high pressure system down before performing any repairs to the system.

  • Use caution when diagnosing the voltage signals of the injectors. The high-pressure injectors typically actuate at approximately 70 volts and 10 amps, with the capability to rise over 120 volts.

  • Because the injectors occupy space in the combustion chamber with the piston, valves, and spark plugs, carbon can build up on the tip of the injector and interfere with the distribution and atomization of the fuel. Even the slightest loss of fuel delivery will have an adverse effect on the engine’s drivability, power output, fuel economy, and exhaust emissions.

  • Injectors can have varying types of spray patterns, depending on the engine requirements. While typical port injectors produce a fuel droplet of approximately 165 micron, direct injectors atomize a much smaller fuel droplet size of only 65 micron, so fuel system cleanliness and fuel quality are important.

Whether your vehicle is fuel-injected or carbureted, be informed with ChiltonDIY’s comprehensive online repair procedures, inspections, specifications, diagnostics, and technical service bulletins for your next repair.

A muscle car enthusiast and drag racer, Jim Marotta is a freelance automotive writer with more than 25 years experience in the automotive industry.