The stratified charged engine is an internal combustion engine in which air-fuel ratio is not equal throughout the cylinder. It uses the fuel charge consisting of two layers of air-fuel mixture. A rich mixture is provided close to the spark plug by a small auxiliary inlet valve and a lean mixture in the cylinder through the main inlet valve. This combination of a rich mixture near the spark plug and a lean mixture in the cylinder allowed stable running, yet complete combustion of fuel and low exhaust gas emission.
Traditional Otto cycle engine :
In 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 ratio. The mixture is easily ignited and burns smoothly.
The problem with this design is that after the combustion process is complete, the resulting mixture contains considerable amount of free oxygen and nitrogen atoms. The creates Nox , pollutant.
Traditional diesel engine :
In diesel engine the fuel is injected into the cylinder directly. In diesel engine 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 result from poor mixture leads to the presence of other pollutants.
The stratified charge design attempts to fix the problems with both engines . It uses a direct injection system like the diesel, with its inherit 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 diesel.
Features of stratified charged engine :
- A stratified charge engine concentrate a rich mixture near the sparkplug(air-fuel ratio is less than 14.7:1) and lean mixture (at air fuel ratios 50:1 or greater) throughout into the cylinder.
- This technique enables the use of ultra-lean mixture (50:1) or greater that would be impossible with carburettor fuel injection hence reduce the fuel consumption.
- Stratified charge engine could operate unthrottled as does the diesel engine.
- Stratified charge engine is surrounded mostly by air, which keeps the fuel flame away from cylinder walls and finally reduce emission and heat losses.
- They have significantly high HC and NOx emissions .they can be minimized by using Exhaust Gas Recirculation (EGR). In this a catalytic converter in exhaust system can further oxidise CO and HC emission from engine.
Direct injection stratified charge gasoline have significantly higher fuel economy than conventional throttled engines . by stratified the air fuel mixture in the centre of the combustion chamber and keeps the hot products away from the walls, hence the heat loss can be reduced
By the Exhaust Gas Recirculation system, their is sufficient gain in thermal efficiency or elimination of throttle losses, increased compression ratio.
Stratified charge make it possible to lower the temperature at which the fuel is sprayed.
The advantage on this system on fuel economy is (15 to 20)% less fuel than the traditional engine.