Stratified Charged Engine
Published on Jan 01, 2017
The stratified charged engine is an internal combustion engine in which air-fuel ratio isn't equal throughout the cylinder. It uses a fuel charge consisting of two layers i.e. layering of fuel/air mixture. A rich mixture is provided close to the spark plug by a small auxiliary inlet valve and combustion promotes ignition of a lean mixture in the remainder of 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 emissions.
Decreasing the CO2 emissions is possible only with the decrease of fuel consumption and this can be achieved most effectively by operating the engine with the stratified charge principle. It is seen that stratified charge engines have the potential to attain a reduction in CO2 emissions up to 19%.Also stratified charge is surrounded mostly by air, which keeps the fuel and the flame away from the cylinder walls for lowest emissions and heat losses.
It similar in some ways to the Diesel cycle, but running on normal gasoline. This method of operation delivers a reduction in fuel consumption that can reach 40% when the engine is running at very low charge. It causes significant gain in thermal efficiency also.
The approach of a stratified charge engine with exhaust gas recirculation (EGR) has been widely introduced in automobile. It 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. This 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 amounts of free oxygen and nitrogen atoms. These will readily react with each other, creating NOx, a pollutant. This is currently addressed with the use of a catalytic converter in the exhaust system, which break the NOx back into N2 and O2 .
A Diesel engine, on the other hand, injects the fuel into the cylinder directly. This has the advantage of 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 engines. 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.
The prototype of stratified charged engine is developed by HONDA of JAPAN. This engine uses a conventional engine block, piston, spark plug.
In this system the combustion chamberisdividing so as to create a pre-combustion chamber where the sparkplug is located so that rich mixture is produced close to the spark plug. The head of the piston is also modified. It contains a toroidal (donut-shaped) cavity that imparts a swirling movement to the air contained by the cylinder during compression. As a result, during injection, the fuel is only sprayed in the vicinity of the spark plug and lean mixture is in the cylinder throughout.
In production gasoline DI (Direct Injection) engines, charge stratification is achieved by using the wall-guided spray with enhanced gas motion. Rich mixture is designed to come to the spark gap at appropriate timing.
A stratified charge system, which have a thin fan-shaped fuel spray and a shell-shaped piston cavity.
A stratified charge engine only pulls air through the transfer system. The fuel required for combustion is forced into the cylinder through an injector placed in the top of the cylinder (head). The injector sprays a fuel/air mixture in the form of a fuel cloud into the cylinder. Surrounding this cloud is air supplied by the transfer system. As the cloud is ignited and burns, the surrounding air provides almost complete combustion before the exhaust port opens. For stratified charge engine, it is well know that lean, stratified combustion can reduce fuel consumption and gain some merits in gasoline spark-ignited, direct injection engines for several reasons. Fuel spreads in a thin film over the wall and is evaporated by the air swirling in the chamber to form the stratified charge.
In order to realize the stratified combustion, the cylinder mixture formation in time,
spatial control is essential. Stratified charge engine could operate unthrottled as does the diesel engine. First, unthrottled operation allows for a significant reduction in pumping loss, especially at low loads. Second, the lean mixture being compressed has a higher ratio of specific heats. This allows for a more efficient compression and expansion process. Third, there are lower wall heat losses in the cylinder because of the centralization of the mixture away from the walls.
A stratified charge engine concentrates a rich mixture near the spark plug (air-fuel ratio is less than 14.7:1) and lean mixture (at air-fuel ratios of 50:1 or greater) throughout into the cylinder.
To do stratification, 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 proceeds, 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.
In a stratified charge engine, the fuel is injected into the cylinder just before ignition. This allows for higher compression ratios without "knock," and leaner air/fuel mixtures than in conventional internal combustion engines.
All the subtlety of engine operation in stratified mode occurs at level of injection. In this air-fuel ratio is free to range from rich limit of homogeneous to lean limit of stratified combustion and the combustion mode is varies between homogeneous and stratified as per need.
This comprises two principal modes:
1. LEAN MODE:
It corresponds to operation at very low engine load.
2. NORMAL MODE:
When it runs at full charge and delivers maximum power.In the first mode, injection takes place at the end of the compression stroke. Because of the swirl effect that the piston cavity creates, the fuel sprayed by the injector is confined near the spark plug. As there is very high pressure in the cylinder at this moment, the injector spray is also quite concentrated. The “directivity” of the spray encourages even greater concentration of the mixture. A very small quantity of fuel is thus enough to obtain optimum mixture richness in the zone close to the spark plug, whereas the remainder of the cylinder contains only very lean mixture. The stratification of air in the cylinder means that even with partial charge it is also possible to obtain a core of mixture surrounded by layers of air and residual gases which limit the transfer of heat to the cylinder walls.
This drop in temperature causes the quantity of air in the cylinder to increase by reducing its dilation, delivering the engine additional power. When idling, this process makes it possible to reduce consumption by almost 40% compared to a traditional engine. And this is not the only gain. Functioning with stratified charge also makes it possible to lower the temperature at which the fuel is sprayed. All this leads to a reduction in fuel consumption which is of course reflected by a reduction of engine exhaust emissions. When engine power is required, injection takes place in normal mode, during the admission phase.
This makes it possible to achieve a homogeneous mix, as it is the case with traditional injection. Here, contrary to the previous example, when the injection takes place, the pressure in the cylinder is still low. The spray of fuel from the injector is therefore highly divergent, which encourages a homogeneous mixture.
Features Of Stratified Charged Engine
• A stratified charge engine concentrates a rich mixture near the spark plug (air-fuel ratio is less than 14.7:1) and lean mixture (at air-fuel ratios of 50:1 or greater) throughout into the cylinder.
• This technique enables the use of ultra-lean mixtures (air-fuel ratios of 50:1 or greater) that would be impossible with carburetors or conventional fuel injection hence reduces the fuel consumption.
• Stratified charge is surrounded mostly by air, which keeps the fuel and the flame away from the cylinder walls hence lowest emissions and heat losses.
• Stratified charge engine could operate unthrottled as does the diesel engine.
• They also have significantly higher HC and NOx emissions. However it can minimize by using Exhaust Gas Recirculation (EGR). In this study, EGR was simulated by using dilution gases, such as CO2 and N2.A catalytic converter in exhaust system can further oxidise CO and HC emission from engine.
Direct-Injection Stratified-Charge Gasoline Engines have significantly higher fuel economy than conventional throttled engines. By stratifying the fuel-air mixture in the center of the combustion chamber and keeping the hot burnt products away from the walls, heat losses can be decreased.
The stratified charge engine is designed to reduce the emissions from the engine cylinder without the use of exhaust gas recirculation systems, which is also known as the EGR or catalytic converters.It causes significant gain in thermal efficiency reduction or elimination of throttling losses, increased compression ratio and lean combustion.Functioning with stratified charge also makes it possible to lower the temperature at which the fuel is sprayed. All this leads to a reduction in fuel consumption which is of course reflected by a reduction of engine exhaust emissions.The primary advantage to these engines is fuel economy i.e. 15 to 20% less fuel than traditional engine.
1. ASME technical paper on stratified charged engine
2. Stone, R., Introduction to Internal Combustion Engines. SAE International Inc., 1992.
3. Blair, G.P., The Design of Two-Stroke Engines, Society of Automotive
Engineers, Warrendale, Pennsylvania, February 1990, pp 672, SAE ref no.: R-
104, ISBN 1-56091-008-9.
4. The Influence of Swirl on the fresh Charge Processes in
AUniflow-Scavenged Two-Stroke Engine, SAE Paper No.: 866642,1999
5. Wikipedia “stratified charged engine”
6. C.D.Wood,“Unthrottled Open-Chamber Stratified-Charge Engines”, SAE Paper 780341, 1978.
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