Gasoline Direct Injection has been around for longer than people think, by about 25 odd years, the basic reason why gasoline direct injection has become more prevelant is because of the lower financial costs involved and more importantly better control systems, which are a world away from the mechanical based control systems of old.
The advantages over a conventional MPFi system are as follows:
1 . Lower fuel consumption and higher output
(1) Optimal fuel spray for two combustion mode
Using methods and technologies unique to Mitsubishi, the GDI engine provides both lower fuel consumption and higher output. This seemingly contradictory and difficult feat is achieved with the use of two combustion modes. Put another way, injection timings change to match engine load.
For load conditions required of average urban driving, fuel is injected late in the compression stroke as in a diesel engine. By doing so, an ultra-lean combustion is achieved due to an ideal formation of a stratified air-fuel mixture. During high performance driving conditions, fuel is injected during the intake stroke. This enables a homogeneous air-fuel mixture like that of in conventional MPI engines to deliver higher output.
Ultra-lean Combustion Mode
Under most normal driving conditions, up to speeds of 120km/h, the Mitsubishi GDI engine operates in ultra-lean combustion mode for less fuel consumption. In this mode, fuel injection occurs at the latter stage of the compression stroke and ignition occurs at an ultra-lean air-fuel ratio of 30 to 40 (35 to 55, included EGR). Superior Output Mode
When the GDI engine is operating with higher loads or at higher speeds, fuel injection takes place during the intake stroke. This optimizes combustion by ensuring a homogeneous, cooler air-fuel mixture that minimized the possibility of engine knocking.
To realise the former part of the objective, modifications included a curved cylinder head to facilitate differential mixing of the fuel with the air to facilitate lean burn.
A reverse tumble effect(to promote mixing of fuel) was also created with the help of a straight up intake port
The tumble effect is further enhanced by an injector which creates a swirl pattern of fuel when it is injected.
The basic problem we encounter while trying to enhance fuel efficiency is that after a certain limit, the fuel mixture stop igniting within the cylinder, and ignites in the exhaust manifold(what we refer to as backfiring)... ..this is mainly due to the fact the air does not have enough fuel to ignite. In this type of an engine, a much leaner charge can be ignited as the fuel is sprayed around the sparkplug and its injected at the end of the compression stroke, and so it doesn't disperse fully into the air, and a differential air fuel ratio is achieved within the cylinder, the enhanced NOx volumes associated with ultra lean burn are handled by increasing the exhaust gas recirculation ratio.
The application of this engine is quite favorable for turbocharging and supercharging as this engine has a wider operating range of overall air fuel ratios, which is something that limits the efficiency of forced induction engines.
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