Gasoline Direct Injection
Gasoline Direct Injection (GDI) is a fuel-injection technology which is injects fuel directly into the combustion cylinder instead of the valve port as in conventional fuel-injection. It improves fuel economy, CO2 emissions and power.
GDI was first used in the 1955 Mercedes-Benz 500SL (Gullwing) using mechanical injection. Ford built 100 V8s in the 1970s with direct injection, but cancelled the project due to unreliable electronics; expensive pumps and injectors; and excessively high Nitrogen emissions.
In 1996, Mitsubishi released the first volume production GDI engine, with the 4G94. Mitsubishi overcame the issues Ford faced in the 1970s and is today considered the leader in GDI technology.
GDI is similar to diesel direct injection in that it injects a high-pressure burst of fuel directly into the cylinder. The pistons in a GDI engine use a concave design, allowing for fuel to be concentrated around the spark plug, making it easier to ignite lean mixtures.
Fuel is injected at different times of the combustion cycle, depending upon load. Under light loads, the fuel is injected just prior to the ignition of the spark plug allowing precise control to ignite ultra-lean air-fuel mixtures at up to 40:1. Under light loads, the engine produces increased Nitrogen emissions, which are cleaned by a special catalytic converter. When running in this mode on some models, a GDI-ECO light will come on on the instrument panel to indicate that ultra-lean mode is being used.
Under moderate loads, the GDI system goes into Superior Output Mode and fuel is injected during the intake stroke and mixed with air, running on a standard stoichiometric fuel-air mixture.
Under heavy load, the GDI system goes into Two-Stage Mixing Mode and fuel is injected twice during the combustion cycle making the engine run slightly richer than the stoichiometric mixture.
GDI engines generally have a higher compression ratio than their conventional counterparts generally using a 12.5:1 ratio. This is achieved by using fuel to cool the air as it comes into the combustion chamber, reducing the chance of detonation.
In a GDI engine, air normally enters the engine directly through the top of the cylinder head, directing air down onto the top of pistons.
GDI has been applied to the following engines:
- Mitsubishi GDI engines operate best on low-sulfur fuel, which is used in Japan. Using high-sulfur fuel can cause the catalytic converter to fail and for injectors to clog. European domestic market GDIs use recalibrated electronics to avoid the problem by not allowing the system to fully enter Ultra Lean Mode.
- Carbon is known to build up in the throttle body, inlet tract and valves as the EGR and PCV systems recycle gases into the engine. In a conventional engine, fuel limits the carbon build up and fuel injector cleaners can be used to clean inlet tracts and valves. Decoking is often required on GDI engines.