مهندسی مکانیک

Gasoline Direct Injection Engine

1.The difference between new GDI and current MPI
2.Outline
3.Technical features

1.Lower fuel consumption and higher output
2.Realization of lower fuel consumption
3.Realization of Superior Output

I. Introduction

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

• Ultra-low fuel consumption that betters that of even diesel engines
• Superior power to conventional MPI engines

1. The difference between new GDI and current MPI
For fuel supply, conventional engines use a fuel injection system, which replaced the carburation system. MPI or Multi-Point Injection, where the fuel is injected to each intake port, is currently the one of the most widely used systems. However, even in MPI engines there are limits to fuel supply response and the combustion control because the fuel mixes with air before entering the cylinder. Mitsubishi set out to push those limits by developing an engine where gasoline is directly injected into the cylinder as in a diesel engine, and moreover, where injection timings are precisely controlled to match load conditions. The GDI engine achieved the following outstanding characteristics.

• Extremely precise control of fuel supply to achieve fuel efficiency that exceeds that of diesel engines by enabling combustion of an ultra-lean mixture supply.
• Very efficient intake and relatively high compression ratio unique to the GDI engine deliver both high performance and response that surpasses those of conventional MPI engines.

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

3. Technical features

• Upright straight intake ports for optimal airflow control in the cylinder
• Curved-top pistons for better combustion
• High pressure fuel pump to feed pressurized fuel into the injectors
• High-pressure swirl injectors for optimum air-fuel mixture

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.

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.

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 10E15 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.