Gasoline direct injection

Gasoline Direct Injection

Gasoline Direct Injection [GDI]

ABSTRACT

To meet the emissions requirements, and to obtain further improvement in fuel economy, the automobile research centers are booming towards a newer engine technology with the use of electronics.

This paper deals with a solution by use of “Electronic controlled Direct Injection of gasoline” in the SI engines. This system is used to reduce pollution and to improve fuel economy. In the GDI engine, petrol is injected directly into the cylinder with precise timing, eliminating waste and inefficiency. By operating in two modes, Ultra-Lean Combustion Mode and Superior Output Mode, the GDI engine delivers both unsurpassed fuel efficiency and superior power and torque.

This paper deals with the various components used in this GDI system, various characteristics of GDI system and comparative study with the conventional, MPFI and GDI system. It is estimated that within the shortest time all the S.I.Engines will be developed to suit the GDI system. The GDI system will meet the emission norms EURO-III and beyond.

Introduction:

Day to day increasing population of vehicles threatens a lot due to the polluting effect of harmful gases from the tail pipe of an automobile. The conventional carburetor system is not enough to suit with the pollution norms. The further advancement of the system finds a way to utilize the electronic systems to be used in the fuel injection system (MPFI). The GDI system is developed to get the more benefits than the MPFI engines.

Drawbacks of Carburetor System:

• The precise control of the fuel over the wide range is not possible.

• Due to that the fuel economy decreases.

• The pollution level increases.

What is GDI system?

The gasoline direct injection system is a method of introducing the petrol directly into the cylinder using electronic systems. This is an advanced method of MPFI engine. The solid lubricated electronic injectors are used to inject the fuel in to the engine cylinders directly.

The difference between new GDI and current MPI

For fuel supply, conventional engines use a fuel injection system, which replaced the carburetion 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. In the GDI system, 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.

A GDI Engine:

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

Major characteristics of the GDI engine
1. Lower fuel consumption and higher output
(1) Optimal fuel spray for two combustion mode

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 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, enabledto 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 Air flow

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.

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.

(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.

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.

Lean NOx Catalyst (HC selective deoxidization type)

3 . Realization of Superior Output 
(1) Basic conceptTo 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 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.

Conclusion:

Thus the GDI engine is an engine which combines the fuel economy of diesel with the performance of petrol. This engine conserves resources and generates fewer emissions due to complete combustion. The main advantage of the GDI system is its better performance in the lean modes also. Due to that the two main benefits fuel economy and less emission can be achieved. This GDI system draws a way to attain the EURO-III and beyond.