II.Major
Objectives of the GDI engine
1.The difference between new GDI and current MPI
2.Outline
3.Technical features
III.
Major characteristics of the GDI engine
1.Lower fuel consumption and higher output
2.Realization
of lower fuel consumption
3.Realization
of Superior Output
For
many years, innovative engine technology has been a development priority
of Mitsubishi Motors. In particular, Mitsubishi has sought to improve engine
efficiency in an endeavor to meet growing environmental demands, such as
those for energy conservation and the reduction of CO2 emission to limit
the negative impact of the green-house effect.
In Mitsubishis endeavor
to design and build ever more efficient engines, it has devoted significant
resources to developing a gasoline direct injection engine. For years,
automotive engineers have believed this type of engine has the greatest
potential to optimize fuel supply and combustion, which in turn can deliver
better performance and lower fuel consumption. Until now, however, no one
has successfully designed an in-cylinder direct injection engine for use
on production vehicles. A result of Mitsubishis engine development capabilities,
Mitsubishis advanced Gasoline Direct Injection GDI engine is the realization
of engineering dream.
Mitsubishi Gasoline Direct Injection GDI Engine
II. Major Objectives of the GDI engine
For Mitsubishi, the technology realized for this GDI engine will form the cornerstone of the next generation of high efficiency engines and, in its view, the technology will continue to develop in this direction.
Transition of Fuel Supply System
2. Outline
(1) Major Specifications
(2) Engine Diagram
III. Major characteristics of the GDI engine
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.
(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
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.
2 . Realization of lower fuel consumption
(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.
(2) Combustion of Ultra-lean Mixture
In conventional MPI engines, there were limits to the mixtures leanness
due to large changes in combustion characteristics. However, the stratified
mixture of the GDI enabled greatly decreasing the air-fuel ratio without
leading to poorer combustion. For example, during idling when combustion
is most inactive and unstable, the GDI engine maintains a stable and fast
combustion even with an extremely lean mixture of 40 to 1 air-fuel ratio
(55 to 1, EGR included)
(3) Vehicle Fuel Consumption
Fuel Consumption During Idling
The GDI engine maintains stable combustion even
at low idle speeds. Moreover, it offers greater flexibility in setting
the idle speed.
Compared to conventional engines, its fuel consumption during idling is
40% less.
Fuel Consumption during Cruising Drive
At 40km/h, for example, the GDI engine uses 35% less fuel than a comparably
sized conventional engine.
Fuel Consumption in City Driving
In Japanese 10E15 mode tests ( representative of typical japanese
urban driving ), the GDI engine used 35% less fuel than comparably sized
conventional gasoline engines. Moreover, these results indicate that the
GDI engine uses less fuel than even diesel engines.
Emission Control
Previous efforts to burn a lean air-fuel mixture have resulted in difficulty
to control NOx emission. However, in the case of GDI engine, 97% NOx reduction
is achieved by utilizing high-rate EGR (Exhaust Gas Ratio) such as 30%
that is allowed by the stable combustion unique to the GDI as well as a
use of a newly developed lean-NOx catalyst.
Newly Developed Lean NOx Catalyst (HC selective deoxidization
type)
3 . Realization of Superior Output
(1) Basic concept
To achieve power superior to conventional MPI engines, the GDI engine has
a high compression ratio and a highly efficient air intake system, which
result in improved volumetric efficiency.
Improved Volumetric Efficiency
Compared to conventional engines, the Mitsubishi GDI engine provides
better volumetric efficiency. The upright straight intake ports enable
smoother air intake. And the vaporization of fuel, which occurs in the
cylinder at a late stage of the compression stroke, cools the air for better
volumetric efficiency.
Increased Compression Ratio
The cooling of air inside the cylinder by the vaporization of fuel
has another benefit, to minimize engine knocking. This allows a high compression
ratio of 12, and thus improved combustion efficiency.
(2) Achievement
Engine performance
Compared to conventional MPI engines of a comparable size, the GDI
engine provides approximately 10% greater output and torque at all speeds.
Vehicle Acceleration
In high-output mode, the GDI engine provides outstanding acceleration.
The following chart compares the performance of the GDI engine with a conventional
MPI engine.
