By Jim Marotta

Trying to choose between performance and economy is often a very difficult decision, and usually there has to be a compromise between the two. However, one technology is helping to power gasoline engines to new heights of performance, while pushing fuel mileage forward by leaps and bounds: gasoline direct injection. The photos below from Bosch illustrate the key elements of a gasoline direct injection system.

Back in the Day

Direct fuel injection is not a new idea. Diesel engines have always used the technology, which squirts fuel under high pressure into an engine’s cylinders rather than into the intake manifold. In a diesel engine, the process of injecting fuel directly into the combustion chamber at the top of the compression stroke initiates and controls combustion. The Robert Bosch Company perfected the modern mechanical injection pump for small diesels in Germany in 1927.

Bosch developed the first automotive direct injection system using gasoline, and Mercedes introduced it on the 1955 Mercedes Benz 300SL. Engine power was double that of its carbureted counterparts and allowed a top speed of up to 161 mph, making it the fastest production car of its time.

How Direct Injection Works

It might be easiest to see how direct injection works by comparing it to traditional fuel injection methods. With conventional fuel injection, injectors supply all cylinders with a mist mixture of air and fuel, averaging a constant 14.7:1 ratio, known as a stoichiometric mixture. Vacuum draws the mixture into the cylinder where the spark plug ignites it. The throttle valve determines how much of the air/fuel mixture enters each cylinder, keeping the mixture at an optimum 14.7:1 ratio. Lean air/fuel mixtures won’t ignite well, creating excessive NOx and hydrocarbon emissions which the catalytic converter must then capture and process.

In a direct injection engine, the injection nozzle is located inside the combustion chamber and injects a finer spray, like that from an atomizer bottle. Each solenoid-controlled injector has minute outlet holes which exude a fine mist. Injectors positioned to the side of each cylinder, aim the fuel into the cylinder, adjacent to the spark plug, and alongside the intake and exhaust valves.

Like an atomizer bottle spray, the fine mist generated by each solenoid-controlled injector's tiny outlet holes creates a well-atomized air/fuel mixture. Injectors spray fuel into the cylinders at pressures of up to 2,150 psi, about 35 times more intense than port fuel injection. (Courtesy of Bosch.)

A high-pressure fuel rail feeds each bank of cylinders using individual injectors and a fuel rail pressure sensor to help the powertrain control module precisely control fuel pressure. Some systems fire the injectors multiple times during one injection event at pressures of up to 2,150 psi, which is about 35 times more intense than port fuel injection.

Each bank of cylinders has a high-pressure fuel rail that feeds the individual injectors, and a high-pressure pump with a rail pressure sensor that helps the vehicle powertrain control module precisely control fuel pressure. (Courtesy of Bosch.)

When the injectors spray fuel into the cylinder, a relatively small, precisely-shaped volume of ignitable air/fuel mixture surrounds each spark plug. Areas inside the combustion chamber, but away from the spark plug, merely contain air or recirculated exhaust gas. This stratification of the charge allows the engine to burn mixtures with a much higher rate of air than conventional engines. The cushion of non-combustible gas around the combustion chamber also means less combustion heat to evacuate. This improves the thermal efficiency of the engine, increasing fuel economy.

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

Since direct injection charge stratification works best at low and medium loads in the lower half of the engine speed range where traditional gasoline engines are least efficient, it allows direct injection engines to obtain up to 21 percent better fuel economy.

How Working Together Improves Performance and Economy

The future of direct injection involves coupling the system with other technologies, such as turbocharging and automatic engine stopping and restarting. By integrating multiple technologies, automakers can develop smaller, more fuel-efficient engines, while improving performance.

Turbocharging direct injection engines is a promising fuel economy technology. A turbocharged, direct-injection engine combines existing and proven technologies, allowing manufacturers to meet future emission standards using existing catalytic converters. Automakers can apply this technology across a manufacturer’s entire engine portfolio, including flexible fuel applications.

Other technologies maximize direct injection engine thermodynamics. In economy mode, an insulating blanket of air and recirculated exhaust gas helps keep heat away from the cylinder walls and head. In high-powered mode, the engine creates more heat. By controlling the operating speed of the water pump, especially during economy mode operation, a reduction in drag on the engine provides improved fuel economy. Variable intake and exhaust timing is especially efficient with gasoline direct injection and turbocharging systems. The variable engine timing enabled by camshaft phasing can optimize the combustion process. Also, valve overlap at low rpm can be adjusted by the engine controller to increase the turbocharger response, reducing turbo lag.


With almost every manufacturer offering at least one direct injection engine, technicians should be seeing these vehicles in their bays for service.

“The biggest item to consider when servicing (direct injection) systems is the high voltage and fuel pressures the systems generate,” says Al Krenz, director of service for Bosch North America. A direct injection system typically will operate between 725 psi up to 2050 psi, so bleeding down the fuel system properly is important.”

“Always follow the manufacturer’s procedure to relieve the fuel system before performing any repairs to the system,” Krenz recommends.

Carefully diagnose the fuel injector voltage signals. High-pressure injectors typically actuate at approximately 70 volts and 10 amps, with the capability to rise over 120 volts.

As with diesel direct injectors, carbon can build up on the injector tip and interfere with fuel distribution and atomization. While typical port injectors produce a fuel droplet of approximately 165 microns, direct injectors atomize a much smaller fuel droplet size of only 65 microns, so even the slightest loss of fuel delivery will adversely affect engine drivability, power output, fuel economy and exhaust emission.

Direct injection and other complementary systems provide performance and economy by using very complex technologies. If you want to learn more about how specific direct injection and other technologies work, become a subscriber and receive all system service instructions.

A muscle car enthusiast and drag racer, Jim Marotta is a freelance automotive writer with more than 20 years experience in the automotive industry.