23-11-2012, 06:12 PM
Stratified Charge Engine
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INTRODUCTION:
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. Conventionally, a four-stroke (petrol) Otto cycle engine is fuelled by drawing a mixture of air and fuel into the combustion chamber during the intake stroke. This produces a homogeneous charge: a homogeneous mixture of air and fuel, which is ignited by a spark plug at a predetermined moment near the top of the compression stroke. In a homogeneous charge system, the air/fuel ratio is kept very close to stoichiometric. A stoichiometric mixture contains the exact amount of air necessary for a complete combustion of the fuel. This gives stable combustion, but places an upper limit on the engine's efficiency: any attempt to improve fuel economy by running a lean mixture with a homogeneous charge results in unstable combustion; this impacts on power and emissions, notably of nitrogen oxides.
If the Otto cycle is abandoned, however, and fuel is injected directly into the combustion-chamber during the compression stroke, the petrol engine is liberated from a number of its limitations.
First, a higher mechanical compression ratio (maximum combustion pressure) may be used for better thermodynamic efficiency. Since fuel is not present in the combustion chamber until virtually the point at which combustion is required to begin, there is no risk of pre-ignition or engine knock.
ADVANTAGE OF BURNING LEANER FUEL-AIR MIXTURES:
Higher Thermodynamic Efficiency:
The spark ignition engine output is controlled by means of a throttle which varies the quantity of the mixture inducted during the suction stroke while keeping the bye mixture strength nearly constant. In contrast to it, in the diesel engine which is an unthrottled engine the output is control by varying the amount of fuel injected into a constant amount of air in every cycle. Thus the petrol engine operates with in a very narrow range of fuel air ratios whereas the diesel engine operates over a much wider range of mixture strength.
Reduced air pollution:
The use of leaner overall mixture strengths results in reduced amount of nitrogen (NOx) and carbon monoxide. The hydrocarbons are also low. The single most important factor resulting in higher hydrocarbons is also low. The single most important factor resulting in higher hydrocarbons in exhaust is the flame quenching at the combustion chamber walls. If the charge stratification is used, this quenching is drastically reduced, if not altogether removed, because almost pure air will be present near the combustion chamber walls at the parts loads.
DISADVANTAGE WITH CARBURETOR:
Some problems associated with comfortable running of the carburetor are discussed here.
Ice formation:
The vaporisation of the fuel injected in the current of the air requires latent heat and the taken mainly from the incoming air. As a result of this, the temperature of the air drops below the dew point of the water vapour in the air and it condenses and many times freeze into ice if the temperature falls below dew point temperature.
Vapour Lock:
The improved volatility of modern fuels and the necessity of providing heat to prevent the ice formation, has created carburetion difficulties due to vaporisation of fuel in pipes and float chamber. The heating may also occur due to petrol pipes being near the engine. If the fuel supply is large and supply is small, a high velocity will result causing high vacuum. This causes considerable drop which may also cause the formation of vapour bubbles. If these bubbles formed accumulate at the tube bend, then they may interrupt the fuel flow from the tank or the fuel pump and engine will stop because of lack of fuel. Vapour lock is formed because of rapid bubbling of fuel and usually happens in hot summer.
Back Firing:
During the starting of an engine under cold working conditions, the usual manipulation of the choke varies the mixture from too lean to too rich. A very lean mixture will burn very slowly and the flame may still exist in cylinder when the exhaust valve is about to open. The fresh charge in the intake manifold is about to open. The fresh charge in the intake manifold is not so diluted as when inducted into the cylinder and mixed with the clearance gases and consequently burn more rapidly than the charge in the cylinder. If lean charge comes in contact with flames existing in the cylinder, there will be flash of flame back through the intake manifold, burning the charge therein and causing the customary back firing in the carburetor.
ADVANTAGES OF FUEL INJECTION OVER CARBURETOR:
Fuel injection is a system of fuel delivery for mixture with air in an internal combustion engine. It has become the primary system used in automotive engines, having replaced carburetors primarily in the 1980s.
The primary functional difference between carburetors and fuel injection is that fuel injection atomizes the fuel by forcibly pumping it through a small nozzle under high pressure, while a carburetor relies on the vacuum created by intake air rushing through it to add the fuel to the airstream.
The fuel injector is only a nozzle and a valve: the power to inject the fuel comes from farther back in the fuel supply, from a pump.
The fuel injection eliminates several intake manifold distribution problems. One of the most difficult problems in a carbureted system is to get the same amount and richness of air-fuel mixture to each cylinder. The problem is that the intake manifold acts as a storing device, sending a richer air fuel mixture to the end cylinders. The air flows readily around the corners and through various shaped passages. However the fuel, because it is heavier is unable to travel as easily around the bends in the intake manifold. As a result, some of fuel particles continue to move to the end of the intake manifold, accumulating there. This enriches the mixture going the end cylinder. The center cylinder closest to the carburetor gets the leanest. The port injection solves this problem because the same amount of fuel is injected at each intake valve port. Each cylinder gets the same amount of air-fuel mixture of the same mixture richness.
STRATIFICATION BY FUEL INJECTION:
In the fuel-injection stratified-charge engine, the fuel is sprayed directly into a piston bowl to form a central cloud of combustion mixture surrounded by air. The spark is fired during the injection period, meaning that fuel is sprayed into an existing flame as in the diesel engine.
The first approach:
The first attempt to obtain charge stratification was made by Ricardo. In it a relatively rich mixture was formed at the spark plug by an auxiliary spray while another spray injecting fuel along the major axis of the combustion chamber formed a leaner mixture.
This combustion could give combustion over a relatively large range of overall mixture strengths and allowed very lean engine operation giving efficiencies as high as 36%. However, the range was limited at higher speed and at loads higher than about 50% of the full load the engine did not work properly, presumably due to too rich mixture near the spark plug.
Prechamber stratified charge engine:
Later Ricardo used a small prechamber(next page) fitted with the injector and the spark plug. In this a rich mixture is formed near the spark plug by fuel supply from the fuel injector and a carburetor supplies lean mixture to the main combustion chamber. The auxiliary charge burns in the prechamber, and issues out, through its throat, into the main chamber, and burns lean mixture present there. Thus a leaner mixture can be burned.
This approach also involves many problems of engine operation. The first problem is that of getting good performance over the full load range. At part loads the lean mixture bare effectively burned out at full load due to improper fuel distribution and incomplete and incomplete scavenging of the prechamber the rich mixer is not properly burned.