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GASOLINE DIRECT INJECTION
1.0 INTRODUCTION
Gasoline direct injection (GDI) engine technology has received considerable attention over the last few years as a way to significantly improve fuel efficiency without making a major shift away from conventional internal combustion technology. In many respects, GDI technology represents a further step in the natural evolution of gasoline engine fueling systems. Each step of this evolution, from mechanically based carburetion, to throttle body fuel injection, through multi-point and finally sequential multi-point fuel injection, has taken advantage of improvements in fuel injector and electronic control technology to achieve incremental gains in the control of internal combustion engines. Further advancements in these technologies, as well as continuing evolutionary advancements in combustion chamber and intake valve design and combustion chamber flow dynamics, have permitted the production of GDI engines for automotive applications. Mitsubishi, Toyota and Nissan all market four- stroke GDI engines in Japan.
Major Objectives of the GDI engine
• Ultra-low fuel consumption that betters that of even diesel engines
• Superior power to conventional MPI engines
Sophisticated high-pressure injectors capable of producing very fine, well-defined fuel sprays, coupled with advanced charge air control techniques, now make stable GDI combustion feasible. There are impediments to widespread GDI introduction, however, especially in compliance with stringent emission standards. This report addresses both the efficiencies inherent in GDI technology and the emissions constraints that must be addressed before GDI can displace current spark-ignition engine technology.
In this seminar I am intending to familiarize the working of this technology, which has the capability to become the turning point in the development of diesel engine technology.
WHY NOT CARBURETTOR?
All Internal combustion engines burn fuel in air and every fuel has ideal air ratio at which it will ignite or burn as completely as possible. The challenge that faces engineers is to introduce the perfect or precise proportions of fuel and air required for complete combustion. This is commonly referred to as the stoichiometric ratio. Petrol has a stoichiometric ratio of 14.7:1(14.7 parts of air with 1 part of fuel by weight). This ratio has to be maintained under the varying engine loads and conditions. The carb earlier did this metering with its ancillaries. But the carb has its limits and though performance and economy with modern carbs were acceptable, a seamless power delivery and emissions often suffered.
Carburetor has following disadvantages
• Vapour lock
• Perfect air/fuel mixture cannot be obtained
• Lack of throttle response
• Low volumetric efficiency
• Icing – problem in aircraft engines
• Mechanical device
• Compromises on emission
2.0 Transition of fuel supply
3.0 Major characteristics of the GDI engine
3.1. 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.
(2) The GDI engines foundation technologies
There are four technical features that make up the foundation technology. The Upright Straight Intake Port supplies optimal airflow into the cylinder. The Curved-top Piston controls combustion by helping shape the air-fuel mixture. The High Pressure Fuel Pump supplies the high pressure needed for direct in-cylinder injection. And the High Pressure Swirl Injector controls the vaporization and dispersion of the fuel spray.
These fundamental technologies, combined with other unique fuel control technologies, enabled Mitsubishi to achieve both of the development objectives, which were fuel consumption lower than those of diesel engines and output higher than those of conventional MPI engines. The methods are shown below.
In-cylinder Airflow
The GDI engine has upright straight intake ports rather than horizontal intake ports used in conventional engines. The upright straight intake ports efficiently direct the airflow down at the curved-top piston, which redirects the airflow into a strong reverse tumble for optimal fuel injection
Fuel Spray
Newly developed high-pressure swirl injectors provide the ideal spray pattern to match each engine operational modes. And at the same time by applying highly swirling motion to the entire fuel spray, they enable sufficient fuel atomization that is mandatory for the GDI even with a relatively low fuel pressure of 50kg/cm2
Optimized Configuration of the Combustion Chamber
The curved-top piston controls the shape of the air-fuel mixture as well as the airflow inside the combustion chamber, and has an important role in maintaining a compact air fuel mixture. The mixture, which is injected late in the compression stroke, is carried toward the spark plug before it can disperse.
Mitsubishis advanced in-cylinder observation techniques including laser-methods are utilized to determine the optimum piston shape.
3.2. Realization of lower fuel consumption
3.2.1 Basic Concept
In conventional gasoline engines, dispersion of an air-fuel mixture with the ideal density around the spark plug was very difficult. However, this is possible in the GDI engine. Furthermore, extremely low fuel consumption is achieved because ideal stratification enables fuel injected late in the compression stroke to maintain an ultra-lean air-fuel mixture.
An engine for analysis purpose has proved that the air-fuel mixture with the optimum density gathers around the spark plug in a stratified charge. This is also borne out by analyzing the behavior of the fuel spray immediately before ignition and the air-fuel mixture itself.
As a result, extremely stable combustion of ultra-lean mixture with an air-fuel ratio of 40 (55, EGR included) is achieved as shown below.