Machine Design

Combustion Engines Burn Super Clean

Gasoline direct-injection technology may foster super-efficient powerplants for future autos.

Sherri Singer
Associate Editor

As automakers race to build more efficient engines, engineers at Mitsubishi Motors Corp. continue to develop and improve the gasoline direct-injection (GDI) engine. This powerplant optimizes fuel delivery and combustion, reducing both emissions and fuel consumption. Unfortunately, Mitsubishi currently uses the GDI engine in Japan and Europe only. The sulfur levels in U.S. gasoline degenerates the GDI engine catalytic converter.

Conventional engines typically use a fuel-injection system which replaces the carburetor. Multipoint injection, widely used today, injects fuel into each intake port. But, fuel delivery and combustion control have limits in MPI engines because the fuel mixes with air outside the intake port, just before entering the cylinder.

Now meet the gasoline direct-injection engine. The GDI is similar to a diesel in that fuel directly injects into the cylinder. But injection timing is controlled to match load conditions. The fuel control provides combustion of an ultra lean mixture of gasoline and air for higher fuel efficiency than diesel engines. Also, a compression ratio of 12.0 compared to that of 10.5 for MPI engines delivers higher volumetric efficiency and response, surpassing conventional engine performance.

Combustion modes
One of two combustion modes is selected, depending on engine load. Stratified charge (ultralean combustion) is delivered on the compression stroke, and homogeneous charge (superior output combustion) is delivered on the intake stroke. The GDI engine adjusts the timing of the gasoline-fuel spray injection for the engine load while maintaining the proper air-fuel mixture for combustion.

For example, at speeds up to about 72 mph, the GDI engine operates in the ultralean mode, consuming less fuel. In this mode, fuel injects towards the curved top of the piston crown (instead of towards the spark plug) during the late stage of the compression stroke. The spray, combined with the airflow in the cylinder, causes it to vaporize and disperse. The result is the appropriate air/fuel gaseous mixture is under the spark plug for ignition.

In the superior-output combustion mode fuel injects during the intake, when the piston descends towards the cylinder bottom, vaporizing into the airflow and following the piston down. By selecting the optimum timing for the injection, the fuel spray follows the piston movement, but does not catch up to it. The piston moves downward and the combustion chamber volume becomes larger, dispersing the fuel spray widely. This ensures the correct homogeneous mixture for ignition of the spark plugs.

Technical aspects
A major obstacle in GDI engine development was spark plug fouling. A narrow spacing configuration, where the fuel injector sat close to the spark plug, allowed easy fuel ignition as the fuel directly hit the plug. This caused soot to accumulate on the plug, eventually leading to fouling. Mitsubishi engineers overcame the problem by injecting fuel toward the piston surface instead of directly at the spark plug. The fuel vaporizes before touching the plug, a result of the fuel spray striking the piston surface and mixing with airflow in the cylinder.

Four features make the difference in GDI engines. The upright straight intake port supplies airflow into the cylinder. Curved top pistons control combustion by helping shape the air-fuel mixture. A high pressure fuel pump (about 15 times normal) supplies the pressure needed for direct in-cylinder injection. High-pressure electromagnetic swirl injectors control the vaporization and dispersion of fuel spray.

Most MPI engines today use horizontal intake ports and mix air and fuel together outside the cylinder before injection. These ports create a vertical vortex called a tumble. In GDI engines, the direction of tumbling air is opposite than that of MPI engines because of upright straight intake ports. These ports direct the airflow down at the curved-top piston, which redirects the airflow into a strong reverse tumble. The reverse tumble carries the fuel vapor toward the spark plug at the center of the cylinder for peak fuel injection.

The curved-top piston has a spherical cavity which controls the shape of the air-fuel mixture and the airflow in the combustion chamber. It suppresses dispersion of the vapor and confines it in a small volume as it moves toward the spark plug for ignition. Also, the piston controls the flame propagation, and enhances and preserves the reserve tumble within the spherical cavity at the end of the compression stroke.

High-pressure swirl injectors prepare the optimum mixture of fuel by controlling the dispersion and atomization of injected fuel as it sprays directly into the cylinder airflow. Before the atomized fuel spray disperses, it vaporizes above the curved piston heads and is carried toward the spark plug in a stratified form. Injected fuel vaporizes in 1.6 msec, producing a stratified gaseous charge around the plug before ignition and an ultralean fuel-to-air ratio of 40 to 1.

During late injection, fuel spray injects into the compressed air in the cylinder. The drag force from high-density air compacts the spray, containing it inside the spherical piston cavity. The atomized fuel vaporizes during the limited interval between the end of injection and the start of ignition.

When early injection occurs, a homogeneous mixture or mist develops and widely disperses over the cylinder. At the same time, the swirling action caused by the injector keeps fuel away from the cylinder liner which could otherwise reduce lubrication.

Environmental issues
Typically, a new technology that reduces one emission sometimes increases another. The first GDI engine, developed in 1996, reduced carbon dioxide levels, but did not affect the nitrous oxide and hydrocarbons. Mitsubishi engineers developed the next GDI engine to cut nitrous oxide 97% (using 30% EGR and a lean Nox catalyst) and hydrocarbon emissions while also reducing carbon dioxide. Also, use of a low-sulfur fuel further reduces emissions while improving fuel efficiency beyond normal GDI levels. An added environmental benefit of low-sulfur fuel is that it reduces the occurrence of acid rain.

Two-stage mixing helps reduce hydrocarbon emissions. The time it takes for a three-way catalyst used in gasoline engines to remove hydrocarbons from exhaust emissions during the start up period drops from approximately one minute to 20 sec. The shorter warm-up time cuts hydrocarbon emissions more quickly, resulting in cleaner air.

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