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Gasoline Direct Injection (GDI)

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Gasoline Direct Injection (GDI) is a type of fuel injection system that sprays gasoline directly into the combustion chamber. Like engines equipped with Multiport Fuel Injection (MFI) systems, there is a separate fuel injector for each of the engine's cylinders. But instead of mounting the injectors in the intake manifold so the injectors spray fuel into the intake ports in the cylinder head, the GDI injectors are mounted in the cylinder head and spray fuel directly into the combustion chamber instead of the intake port.

The fuel bypasses the intake valves entirely and enters the cylinder as a high pressure mist. Fuel may be injected at any point during the intake stroke, or if the engine is running in low load ultra-lean mode, the fuel may not be injected until some point during the compression stroke. The air/fuel mixture is then compressed and ignited by a spark as the piston approaches top dead center. The exploding air/fuel mixture generates heat and pressure that pushes the piston down during the power stroke. The burnt exhaust gases are then pushed out of the cylinder during the exhaust stroke.

High Pressure Fuel Injection

Direct injection requires extremely high operating pressures (up to 2200 PSI) compared to conventional multiport fuel injection systems that typically require only 40 to 60 PSI. Direct injection requires more delivery pressure to overcome compression pressure inside the cylinder and to delivery a higher volume of fuel in a shorter period of time.

gasoline direct injection fuel rail
Gasoline Direct Injection fuel rail for a V6.

With ordinary MFI fuel injection, the fuel is sprayed into the intake port which is under vacuum. The fuel mist is then drawn into the combustion chamber along with the incoming air, mixed together during the compression stroke, and then ignited by the spark plug. With GDI, only air is drawn past the intake valves because the fuel is sprayed directly into the into the combustion chamber during the compression stroke.

Some engines with direct gasoline injection do not have a conventional throttle because the throttle is not used to control engine speed and power. The engine computer does that by varying the time and amount of fuel that is injected into each cylinder. Eliminating the throttle means there is no restriction to incoming air and little or no vacuum in the intake manifold. This reduces the normal pumping loses caused by the throttle plates and intake vacuum for improved engine efficiency.

As the piston comes up during the compression stroke, fuel may be injected into the cylinder at any point prior to ignition. The timing of the injection will depend on engine speed, load and operating conditions. In some situations (such as light cruise), fuel may not be injected until the piston has almost reached Top Dead Center on its compression stroke. Additional injection pulses of fuel may also be delivered once the initial mixture ignites to keep the flame burning during the power stroke.

Advantages of Gasoline Direct Injection

Spraying fuel directly into the combustion chamber as compression is building, and during and after initial combustion allows the engine to make more power using less fuel. Engines with GDI can tolerate extremely lean fuel mixtures (up to 40:1) under light load and cruise conditions. The net result is typically 15 to 20 percent better fuel economy compared to multiport fuel injection.

The ability to closely control the fuel mixture and give the engine just what it needs at just the right moment also means GDI engines can handle higher static compression ratios. The Buick 3.6L V6 has a compression ratio of 11.3 to one, which helps improve combustion efficiency and power. The Mazda Skyactiv-G 2.0L and 2.5L engines have a 14:1 compression ratio for even higher efficiency. GDI engines usually produce more horsepower than those with multiport injection systems.

Gasoline Direct fuel Injection Problems

No new technology is trouble free and gasoline direct injection is no exception. Because fuel is injected directly into the combustion chamber rather than the intake port, the fuel provides little or no "cleaning effect" to keep carbon and soot from building up on the intake valves. As the miles add up, a layer of carbon deposits accumulates on the intake valves. As the deposits build up on the valve face, they may prevent the intake valves from sealing causing a compression leak, engine misfire and loss of power. Heavy carbon accumulations on the intake valves can also restrict airflow, hurt power at higher engine speeds and cause a drop in fuel economy and performance. Carbon deposits on the intake valves may also flake off and pass through the combustion chamber and into the exhaust. If the engine is equipped with a turbocharger, there is a chance the carbon could damage the turbine fins in the turbocharger. For more information, see Intake Valve Deposits in Gasoline Direct Injection Engines.

The soot buildup problem tends to be worse in direct injection engines that are used mostly for short trips. The intake valves never get hot enough to burn off the deposits. And if the valve guide seals allow too much oil to dribble down the valve stems, the carbon buildup goes even faster.

The fix for dirty intake valves is to clean the valves with some type of chemical cleaner sprayed into the throttle body, intake manifold or directly into the intake ports. Another repair option in some cases is to remove the intake manifold and spray solvent directly into the intake ports in the cylinder head, or to blast clean the backside of the intake valves with a soft media such as walnut shells, baking soda or plastic beads. For extremely heavy carbon deposits, it may be necessary to remove the cylinder head to clean the valves.

Another problem with gasoline direct injection is that like diesel injection, the fuel has less time to mix with the incoming air before it ignites. The stratified charge effect that direct injection produces also allows richer mixtures near the spark plug and injector, and leaner mixtures further away from the spark plug and injector. The result is that the combustion process can form larger particles of soot similar to that of untreated diesel exhaust. The size and quantity of the particles varies depending on the volatility of the fuel and other operating conditions.

Current particulate emission regulations in the U.S. allow up to 10 mg/mile of particulates. But if future particulate emission regulations require lower levels, some type of exhaust after-treatment similar to that which is now being used on clean diesel applications may be required. Diesel engines have soot traps and urea injection systems for after-treatment.

Gasoline Direct Injection Applications

Gasoline direct injection is used on a variety of late model engines: Audi, BMW, GM, Ford, Hyundai, Kia, Lexus, Mazda, MINI, Nissan, Porsche, VW and others. Some recent domestic applications include Ford Ecoboost engines (which are also turbocharged) in the 2010 Focus & Edge and 2011 Explorer, and the DI 3.6L V6 engine in the 2010 Buick LaCrosse and Enclave, 2010 Cadillac STS and CTS, 2010 Camaro V6, 2010 Chevy HHR SS, 2010 Chevy Traverse and GMC Acadia. The 2014 Corvette LT1 also has direct injection.

By 2016, almost half of all new vehicles sold in the U.S are prediced to have gasoline direct injection engines.

Buick 3.6L v6 direct injection combustion chamber

Shown above is a cutaway of the combustion chamber inside a Buick 3.6L V6 Direct Injection engine.

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