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CATEGORIES (articles) > Engines > Technical > Stratified charge engine explained

Stratified charge engine explained

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 stoichiometric, which is defined as the exact 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 combustion process is complete, the resulting exhaust stream contains a considerable amount of free single atoms of oxygen and nitrogen, the result of the heat of combustion splitting the O2 and N2 molecules in the air. These will readily react with each other to create NOx, a pollutant. A catalytic converter in the exhaust system re-combines the NOx back into O2 and N2 in modern vehicles.

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

However the Diesel engine has problems as well. The fuel is sprayed right into the highly compressed air and has little time to mix properly. This leads to portions of the charge remaining almost entirely air and others almost entirely of unburnt fuel lacking for oxygen. This incomplete combustion leads to the presence of other pollutants such as polycyclic aromatic hydrocarbons and the plainly visible exhaust soot.

The stratified charge design attempts to fix the problems with both fuels. It uses a direct-injection system, like the Diesel, with its inherent ability to be run at efficient high compressions. However, like the Otto, it relies on gasoline's 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 to inject the fuel into only one area of the cylinder, often a small "subcylinder" at the top, or periphery, of the main cylinder. This provides a rich charge in that area that ignites easily and burns quickly and smoothly. As the combustion process proceedes it moves to a very lean area (often only air) where the flame-front cools rapidly and the harmful NOx has little opportunity 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 and allows the engine to be run at leaner mixtures, using less fuel.

After years of trying, this layout has proven not to be terribly easy to arrange. The system has been used for many years in slow-running industrial applications, but has generally failed 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 piston-engine development. It is estimated that they have spent several hundreds of millions of dollars in R&D since the 1970s.

Honda's CVCC engine, released in the early 1970s models of Civic, then Accord and City later in the decade, is a form of stratified charge engine that had wide market acceptance for considerable time. The CVCC system had conventional inlet and exhaust valves and a third, supplementary, inlet valve that charged an area around the spark plug. The spark plug and CVCC inlet was isolated from the main cylinder by a perforated metal plate. At ignition a series of flame fronts shot into the very lean main charge, through the perforations, ensuring complete ignition. In the Honda City Turbo such engines produced a high power-to-weight ratio at engine speeds of 7,000 rpm and above.

Today, however, several stratified charge engines are appearing on the market. Mazda and Mitsubishi both have cars using these designs, Volvo Cars recently teamed with Mitsubishi to produce their designs in Europe, Audi has a car using the design and PSA Peugeot Citroƫn has developed such an engine. The primary "sales advantage" of these engines is fuel economy. They run at a leaner setting and use, about, 15 to 20% less fuel than non-stratified designs.

CATEGORIES (articles) > Engines > Technical > Stratified charge engine explained

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