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Stratified charge engine

The stratified charge engine is a type of internal-combustion engine, similar in some ways to the Diesel cycle, but running on normal gasoline. The name refers to the layering of fuel/air mixture, the charge inside the cylinder.

In a traditional Otto cycle engine, the fuel and air are mixed outside the cylinder and are drawn into it during the intake stroke. The air/fuel ratio is kept very close to the stoiciometric[?], exactly the amount of air necessary for a complete combustion of the fuel. This mixture is easily ignited and burns smoothly.

The problem with this design is that after the combusion process is complete, the resulting mixture contains considerable amounts of free oxygen and nitrogen atoms, which were split apart from the air by heat. These will readily react with each other, creating NOx, a polutant. This is currently addressed with the use of a catalytic converter[?] in the exhaust system, which re-combines the NOx back into O2 and N2

A Diesel engine, on the other hand, injects the fuel into the cylinder directly. This has the advantage of avoiding a problem known as ping that plagues Otto cycle engines, which allows them to run at much higher compression ratios. This leads to a more fuel-efficient engine, which is why they are commonly found in applications where they are being run for long periods of time, like in trucks.

However the Diesel engine has problems as well. The fuel is sprayed right into the highly compressed air, and never has time to mix properly. This leads to portions of the charge consisting almost entirely of air, and others almost entirely of fuel. The ineffecient combustion that results from this poor mixture leads to the presense of other polutants, notably soot.

The Stratified charge design attempts to fix the problems with both designs. It uses a direct-injection system like the Diesel, with its inherant ability to be run at efficient high compressions. However, like the Otto, it relies on gasolines ability to mix quickly and cleanly in order to avoid the poor combustion found in the Diesel.

To do this, the fuel injectors are aimed in order to inject the fuel into only one area of the cylinder. This leads to a very rich charge in that area that ignites easily and burns smoothly. As the combustion proceedes, it meets a very lean area (often only air) where it cools rapidly and the harmful NOx never has a chance to form. The additional oxygen in the lean charge also combines with any CO to form CO2, which is less harmful. The much cleaner combustion allows for the elimination of the catalytic converter, as well as allowing the engine to be run at leaner mixtures, using less fuel.

This is not terribly easy to arrange. The system has been used for many years in slow-running industrial applications, but have proven to be very difficult to develop into an automobile engine[?]. Many attempts have been made over the years, notably in Wankel engine applications, but only the Japanese car manufacturers have pressed ahead with its development. It is estimated that they have spent several hundreds of millions of dollars in R&D since the 1970s.

Today, however, several of these engines are appearing on the market. Mazda and Mitsubishi both have cars using these designs, and Volvo recently teamed with Mitsubishi to produce their designs in Europe. The primary advantage to these engines is fuel economy, running at a leaner setting they use some 15 to 20% less fuel than traditional designs.



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