INTRODUCTION: -
What is Stratified Charge Engine?
The stratified charge engine is a type of internal-combustion engine which runs on gasoline. It is very much similar to the Diesel cycle. The name refers to the layering of the charge inside the cylinder. 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.
Stratified charge combustion engines utilize a method of distributing fuel that successively builds layers of fuel in the combustion chamber. The initial charge of fuel is directly injected into a small concentrated area of the combustion chamber where it ignites quickly. As the combustion process continues, it travels across the top of the piston to a lean area of the chamber, where cooler temperatures reduce the formation of harmful NOx emissions. Subsequent additional small injections of fuel can be introduced to propagate the flamefront and manage piston knock. This arrangement works well in slow constant speed applications, but has proven difficult to manage across the wide range of speed and load incurred in automotive uses.
Examples:
Honda has used a stratified charge design in many of its "lean burn" Civic models.
PRINCIPLE:-
The principle of the stratified charge engineis to deliver a mixture that is sufficiently rich for combustion in the immediate vicinity of the spark plug and in the remainder of the cylinder, a very lean mixture that is so low in fuel that it could not be used in a traditional engine. On an engine with stratified charge, the delivered power is no longer controlled by the quantity of admitted air, but by the quantity of petrol injected,as with a diesel engine.
Figure -1
Fresh air + EGR
Air + petrol
Stratified charge
HOW DOES IT WORK?
One approach consists in dividing the combustion chamberso as to create a pre-combustion chamber where the sparkplug is located. The head of the piston is also modified. It containsa spheroid cavity that imparts a swirling movement tothe air contained by the cylinder during compression. As aresult, during injection, the fuel is only sprayed in the vicinityof the spark plug.But other strategies are possible. For example, it is also possibleto exploit the shape of the admission circuit and useartifices, like “swirl” or “tumble” stages that create turbulentflows at their level.All the subtlety of engine operation in stratified modeoccurs at level of injection. This comprises two principalmodes: a lean mode, which corresponds to operation at verylow engine load, therefore when there is less call on it, and a“normal” mode, when it runs at full charge and delivers maximumpower.In the first mode, injection takes place at the end of the compressionstroke. Because of the swirl effect that the pistoncavity creates, the fuel sprayed by the injector is confinednear the spark plug. As there is very high pressure in thecylinder at this moment, the injector spray is also quite concentrated.The “directivity” of the spray encourages evengreater concentration of the mixture. A very small quantity offuel is thus enough to obtain optimum mixture richness in thezone close to the spark plug, whereas the remainder of thecylinder contains only very lean mixture. The stratification ofair in the cylinder means that even with partial charge it isalso possible to obtain a core of mixture surrounded by layersof air and residual gases which limit the transfer of heat to thecylinder walls. This drop in temperature causes the quantityof air in the cylinder to increase by reducing its dilation, deliveringthe engine additional power. When idling, this processmakes it possible to reduce consumption by almost 40% comparedto a traditional engine. And this is not the only gain.Functioning with stratified charge also makes it possible tolower the temperature at which the fuel is sprayed. All thisleads to a reduction in fuel consumption which is of coursereflected by a reduction of engine exhaust emissions.When engine power is required, injection takes place in normalmode, during the admission phase. This makes it possible to achieve a homogeneous mix, as it is the case with traditionalinjection. Here, contrary to the previous example, whenthe injection takes place, the pressure in the cylinder is stilllow. The spray of fuel from the injector is therefore highlydivergent, which encourages a homogeneous mix to form.
THEORY:-
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 or gasoline) 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 stoichometric. A stoichometric 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 or NOx.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 (or, with supercharged engines, 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. The engine may also run on a much leaner overall air/fuel ratio, using stratified charge. Combustion can be problematic if a lean mixture is present at the spark-plug. However, fueling a petrol engine directly allows more fuel to be directed towards the spark-plug than elsewhere in the combustion-chamber. This results in a stratified charge: one in which the air/fuel ratio is not homogeneous throughout the combustion-chamber, but varies in a controlled (and potentially quite complex) way across the volume of the cylinder. A relatively rich air/fuel mixture is directed to the spark-plug using multi-hole injectors. This mixture is sparked, giving a strong, even and predictable flame-front. This in turn results in high-quality combustion of the much weaker mixture elsewhere in the cylinder. Direct fuelling of petrol engines is rapidly becoming the norm, as it offers considerable advantages over port-fuelling (in which the fuel injectors are placed in the intake ports, giving homogeneous charge), with no real drawbacks. Powerful electronic management systems mean that there is not even a significant cost penalty.With the further impetus of tightening emissions legislation, the motor industry in Europe and North America has now switched completely to direct fuelling for the new petrol engines it is introducing.
It is worth comparing contemporary directly-fuelled petrol engines with direct-injection diesels. Petrol can burn faster than diesel fuel, allowing higher maximum engine speeds and thus greater maximum power for sporting engines. Diesel fuel, on the other hand, has a higher energy density, and in combination with higher combustion pressures can deliver very strong torque and high thermodynamic efficiency for more 'normal' road vehicles.
HISTORY
The principle of injecting fuel directly into the combustion-chamber at the moment at which combustion is required to start was invented by Rudolf Diesel, but it has been used to good effect in petrol engines for a long time. The Mercedes 300SL 'Gull wing' of 1952 used direct fuelling, though Mercedes-Benz subsequently switched to port fuelling for other models.
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.Jaguar Cars in the 1980s developed the Jaguar V12 engine, H.E. (so called High Efficiency) version, which fit in the Jaguar XJ12 and Jaguar XJS models and used a stratified charge design called the 'May Fireball' in order to reduce the engine's very heavy fuel consumption.
Stratified Charge Engine with Two-Stage Combustion:-
Figure-2
Two-stage combustion mechanism in twin swirl combustion (1, zone containing pure air; 2, spark plug; 3, turbulizer; and 4, zone containing the fuel-rich mixture).
Stratified Charge Engine With Two-Stage Combustion Mechanism Shows 17% Reduction in Fuel Consumption Without Direct Injection. Two-stage combustion mechanism in twin swirl combustion (1, zone containing pure air; 2, spark plug; 3, turbulizer; and 4, zone containing the fuel-rich mixture). A team of researchers from Istanbul Technical University(ITU) in Turkey has presented a 1.6-liter stratified charge gasoline engine featuring a twin swirl combustion chamber operating with a two-stage combustion mechanism and experimentally shown that it can deliver a 17% reduction in fuel consumption with a 7% increase in power compared to a conventional 1.6-liter port-injected engine..
The two-stage combustion mechanism was originally proposed by a team comprising researchers from Azerbaijan Technical University (AzTU), Warsaw Technical University (WTU), ITU, and Middle East Technical University (METU). In conventional gasoline engines, every part of the cylinder contains a mixture having an excess air ratio (λ) of approximately 1. Stratified charge engines have frequently stoichometric mixture (λ = 1) only near the spark plug and lean mixture in the cylinder, globally. For the special case of stratified charge engines operating with a two-stage combustion mechanism, there is a lean mixture in the cylinder globally as well; however, there is a fuel-rich mixture in the vicinity of the spark plug. The non homogenous mixture in stratified charge engines is obtained usually with the modification of the piston geometry. The geometry of the intake manifold can also be modified. Because there is a lean mixture in the combustion chamber globally, stratified charge engines have a lower knock tendency than the conventional gasoline engines. Because of this fact, the compression ratio (ε) of a stratified charge engine can be higher than the compression ratio of a conventional gasoline engine; ε ≥ 12 is possible. A higher compression ratio leads to a higher efficiency. The absence of throttle losses in part load operation in combination with the ability to use higher compression ratios leads to lower fuel consumption.
The proposed combustion chamber looks like a figure “8” and is separated into two zones. The spark plug mounted part of the combustion chamber contains a fuel-rich mixture with an excess air ratio of 0.6-0.8, while the other part contains pure air. The fuel is injected into the intake manifold and fed into the zone containing the fuel-rich mixture. The intake manifold is designed for the two-stage combustion mechanism, such that it increases the swirl effect and volumetric efficiency. The counter-rotating swirling motion—which occurs during the intake and compression cycles of the engine—does not allow the mixing of the two zones until ignition time. This allows stratification of the air-fuel mixture across the load range. Because the swirl motion occurs with the start of the intake cycle, the air-fuel mixture can be prepared in the intake manifold (outside of cylinders). Therefore, current electronic injection systems or carburetor engines can be used with this method. In other words, special and expensive direct-injection systems are not required, such as in gasoline direct injection (GDI) engines, where the injection of fuel into the cylinder reduces the time available for evaporation and mixing.
The two-stage combustion mechanism can also reduce emissions of criteria pollutants. Because the liquid phase of the gasoline does not contact the cold wall of the cylinders, and because the counter-rotating swirling motion reduces the contact of the flame with the piston, the stratified charge engines with the twin swirl combustion chamber produce lower hydrocarbon (HC) emissions. Incomplete combustion products (CO and H2 produced during the combustion of the rich mixture (λ = 0.6-0.8) at the first stage can be burned in the second stage of combustion with the effect of the swirl motion. The lack of oxygen in the rich mixture and low combustion temperature at the first stage of combustion do not allow NOx formation.
Direct Petrol Injection.
The differences between Petrol and Diesel.
It is commonly known that a Diesel engine of the same capacity as its Petrol counterpart is more fuel efficient (approximately 10%). The main reasons why a diesel returns better economy is because of its ability to run very lean Air Fuel Ratios, better thermal efficiency aided by its higher Compression Ratio (CR) and significantly less pumping losses at part load due to the lack of a throttle valve.
Diesel engines are not that fussy about the measures of fuel they receive, as long as they get some, they’ll burn it and produce useable power. Petrol on the other hand is far more choosy. If the Air Fuel Ratio (give or take a few ratios) isn’t around the stoichometric value then it really doesn’t want to burn (Stoichometric is the term that identifies the Air Fuel Ratio that offers the most complete burn resulting in the lowest emissions for the hottest flame. For unleaded petrol, it is 14.67:1, which is commonly rounded to 14.7:1. The stoichometric value for other fuels varies with their energy content.) Trying to run a petrol engine any leaner results in partially burnt fuel, unstable combustion and high Hydro Carbon (HC) and Carbon Monoxide (CO) emissions. Getting better economy from a Petrol Engine.
Engineers for years have tried to combine the economy of a Diesel engine with the power of a Petrol Engine. There are two main ways of achieving better economy with a petrol engine. The first one is to get the engine to burn very lean mixtures (lean burn engine) and the other is to create a localized stoichometric cloud of mixture at the spark plug (stratified charge engine). The goal of the stratified engine is to run at Wide Open Throttle (WOT) and control the power in much the same as a Diesel by introducing varying amounts of fuel. Under light load conditions it is possible to run AFR’s as high as 60:1.The stratified charge is not a new concept, Ricardo were experimenting with the technology back in 1922. Early stratified engines used traditional carburetors along with a separate mixing chamber to mix the chemically correct AFR mixture which was then introduced into the ‘Clean air’ in the combustion chamber before ignition.
Types of GD-i Engines.
There are two types of stratified engine, and these differ in the way the air enters the combustion chamber. The swirl method is similar to a Diesel concept in that air rushes into the combustion chamber in an axial motion. This motion centralizes the chemically correct cloud of mixture towards the centre of the chamber in the vicinity of the spark plug. The other method uses what is called reverse tumble. The air entering the combustion chamber from the intake valve is deflected in a circular motion in the opposite plane to the swirl motion. The air hits the cylinder wall adjacent to the intake valve and then down towards the piston. These engines use special ‘Ski jump’ shaped pistons to guide the air and fuel towards the spark plug.
Reverse tumble is probably the most suitable stratified charge delivery system as this has already been successfully demonstrated on Mitsubishi’s GD-i range of vehicles.
Limitations of GD-i.
Even using modern injection technology, it is still not possible to run in stratified mode throughout the rev and load range of the engine. Thus, it is only possible to run in stratified mode at part load. The engine switches over to homogenous mode (early injection) at high speed conditions because there is insufficient time to inject the fuel late into the compression stroke and get the fuel to adequately mix into a cloud of combustible mixture. Injecting the fuel too early when the piston is near Bottom Dead Centre (BDC) results in the fuel missing the ‘ski jump’ on the piston.High load conditions are not possible in stratified mode either as injecting such a large quantity of fuel will result in an ultra rich cloud of mixture at the spark plug that wont burn. Attempting to continue Injecting fuel very late into the compression stroke results in the cloud of mixture hitting the piston when it is near to Top Dead Centre (TDC) that results in the cloud of mixture overshooting the spark plug.