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 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 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 NO_x, a polutant. This is currently addressed with the use of a catalytic converter in the exhaust system, which re-combines the NO_x 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, and allows the Diesel 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 inefficient combustion that results from this poor mixture leads to the presence of other pollutants, 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 inherent 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, often a small "subcylinder" at the top of the main 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 NO_x 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.

After years of trying, this layout has proven to be not 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 its 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 1980s models of Civic, Accord and City, is a form of stratified charge engine which had wide market acceptance for some time. The CVCC system had conventional inlet and exhaust valves, and a third, supplementary inlet valve, which charged a volume around the spark plug. The spark plug and CVCC inlet was isolated from the main cylinder by a perforated metal plate; upon ignition flame fronts shot into the very lean main charge through these perforations, ensuring complete ignition. In the Honda City Turbo, such engines produced a high power to weight ratio at up to 7,000 rpm and above.

Today, however, several of these 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 this design, and PSA Peugeot-Citroën has developed such an engine. The primary "sales advantage" to these engines is fuel economy, running at a leaner setting they use some 15 to 20% less fuel than traditional designs.