Effects of Fuel-Air Ratio on the Flame Propagation for S . I . Engines د٘ق٘ىا خجغّ شٞصبر –

As a result of the rapid development in the various transport means, a great concern was shown in the design and development of spark-ignition engines to achieve the best efficiency and performance and to guarantee a full combustion of the fuel inside the combustion chamber to avail all the energy of the fuel entering the cylinder. This work involves the study of the reaction equations of the combustion for stochiometric and rich cases, and the study of combustion products and their effects on the environment.


Introduction
Flames can be divided into stationary and propagating flames. The propagation of the flame front happen rapidly through the mixture while stationary flame front is more or less stationary in space. Stationary flames are also categorized into three classes according to the nature of premixing of the reactants. In the first, the fuel oxidant are introduced separately into combustion chamber and the rate of burning is mainly depends on the rates of diffusion of the fuel and oxidant are kept separate, many industrial practices use the diffusion flames. The second general type of stationary flames is that in which the fuel and oxidant are premixed before introduced into combustion and is known as premixed flame. In the third, the fuel and oxidant be partially premixedwith additional air supplied through a separated section to complete combustion. This type known as double flame (inner premixed and outer diffusion flame). In spark ignition engines the type is premixed flame with spherical coordinate.
When a spark plug is fired, the high voltage, high energy discharge ignites the air-fuel mixture between and near the electrodes. This creates a spherical flame front that propagates outward into the combustion chamber. In the initial stage, the flame front moves very slowly because of its small original size. It does generate enough energy to quickly heat transfered the surrounding mixture and thus propagates very slowly. If the temperature of fuel -air mixture is not high (less than SIT,spark ignition tempreture), the flame will be distinguished. It is desired to have a rich airfuel mixture around the electrodes of the spark plug during ignition. A rich mixture ignites more rapidly, has a faster flame fronts speed, and gives a better start to the overall combustion process.
Understanding of the mechanism of flame initiation and flame front propagation is considered to be important. More important is the effect of the operating condition on flame in ICE, towards which the research is directed.

Combustion reactions:
Spark ignition engines obtain their energy from the combustion of hydrocarbon fuel with air, which converts chemical energy of the fuel to interval energy in the gases within the engine. There are many types of different hydrocarbon fuel components, which consists mainly of hydrogen and carbon. The maximum amount of chemical energy that can be released (heat) from the fuel is when it reacts (combusts) with a stochiometric amount of oxygen. Stochiometric oxygen is just, enough to convert all carbon in the fuel to CO 2 and all hydrogen to H 2 O.
At the end of compression stroke the cylinder contains the mixture of air and fuel mixed with each other through carburetion process in carburetor. Air is used as the source of oxygen to react with fuel.
For actual combustion in a S.I. engines, the equivalence ratio is a measure of fuel-air mixture relative to stochiometric conditions, which is defined as [1,2]:-Φ = (FA) act / (FA) stoich = (AF) stoich / (AF) act (1) where, FA = m f / m a : fuel-air ratio AF = m a / m f : air-fuel ratio m a : mass of air m f : mass of fuel when, Φ < 1 running lean, oxygen in exhaust Φ > 1 running rich, CO and fuel in exhaust Φ = 1 stochiometric, maximum energy released from fuel spark ignition engines operate normally with an equivalence ratio in the range 0.9 to 1.2 depending on the type of operation.

Mixture preparation and fuelair ratio
The composition of the working fluid (fuel-air) mixture for spark ignition engines after compression stroke consists of air, fuel and residual gasses. In the analysis of working fluid at the trapped conditions, three cases are possible:- for the hydrocarbon fuel employed in S.I. engines, the number of hydrogen atoms can be obtained from the following equations:m = 2n + 2 (3) to obtain the number of moles (a 1a 4 ) a mass balance for carbon, hydrogen, oxygen and nitrogen is performed: For Carbon a 1 = n (4) For Hydrogen a 2 = m/2 (5) For Nitrogen a 3 = 3.76 x (6) For Oxygen a 4 + a 2 / 2 + a1 = x a 4 = xnm / 4 (7)

B-stochiometric mixture (Φ = 1):
C n H m + xO 2 + 3.76 xN 2 → a 1 CO 2 + a 2 H 2 O +a 3 N 2 (8) Mole balance of carbon, hydrogen, nitrogen gives: For Carbon a 1 = n (9) For Hydrogen a 2 = m / 2 (10) For Nitrogen a 3 = 3.76 x (11) C-Rich mixture (Φ > 1): C n H m + xO 2 + 3.76 xN 2 → a 1 CO 2 + a 2 H 2 O +a 3 N 2 + a 5 CO (12) Where a 5 is the number of moles of CO. The mass balance of carbon, hydrogen, nitrogen and oxygen gives: For Carbon a 1 + a 5 = n (13) a 1 = na 5 For Hydrogen a 2 = m / 2 (14) For Nitrogen a 3 = 3.76 x (15) For Oxygen a 5 + a 2 + 2 a 1 = 2x (16) a 5 = 2x + m / 2-2a 1 (17) a 5 = 2x + m / 2 -2n + 2 a 5 a 5 = 2n +2x + m / 2 (18) and a 1 = n -2n + 2 xm / 2 the total number of moles of fuel vapour, residual gas and air in the cylinder at trapped condition can be calculated from the The composition at trapped conditions is calculated as follows: NC n H m = K 1 (21) NCO 2 = a 1 K (22) NH 2 O = a 2 K 2 (23) NH 2 = 3.76 x + a 3 K 2 (24) NO 2 =x + a 4 K 2 (25) NCO = a 5 K 2 (26) Where K 1 and K 2 are scale factors reducing the mole numbers to the proper size to fit in the engines. Where, N a : number of moles of air plus fuel (vapor / gas) in a mixture containing 1 mole of fuel N pr : number of moles of products formed from the combustion of N a N T : number of moles of fuel (vapor / gas) and air in to engine at trapped condition N r : number of moles of residual exhaust from previous cycle in the engine Heywood, showed that the typical residual fraction in spark ignition engines range from (5% -20%).

Environmental effects of exhaust gases
Carbon dioxide is an odourless, colourless gas, which is faintly acidic and nonflammable. Carbon dioxide is a molecule with the molecular formula CO 2 . The linear molecule consists of a carbon atom that is doubly bonded to two oxygen atoms, O=C=O. Although carbon dioxide mainly consists in the gaseous form Due to human activities, the amount of CO 2 released into the atmosphere has been rising ex-tensively during the last 150 years. As a result, it has exceeded the amount sequestered in biomass, the oceans, and other sinks CO 2 is one of the most important greenhouse gases which convert sunlight reaches the earth into heat and trap it near the earth's surface, so that the earth is warmed up, yet, the increase in CO2 concentration in the air will lead to serious environmental problems that the earth's climate is changing because the temperatures are rising. This unnatural addition to the greenhouse effect is known as global warming. It is suspected that global warming may cause increases in storm activity, Melting of ice caps on the poles, which will cause flooding of the inhabited continents, and other environmental problems During the last decade, Humans have been increasing the amount of carbon dioxide in air by burning of fossil fuels. About 22% of the current atmospheric CO 2 concentrations exist due to these human activities. Carbon dioxide emissions have risen from 280 ppm in 1850 to 364 ppm in the 1990s, Increasing carbon dioxide emissions cause about 50-60% of the global warming.
Increasing CO 2 concentrations have also various bad impacts on health, the primary health dangers of carbon dioxide are: -Asphyxiation. Caused by the release of carbon dioxide in a confined or unventilated area. This can lower the concentration of oxygen to a level that is immediately dangerous for human health. -Central Nervous System Effects: High levels of CO can affect even healthy people. People who breathe high levels of CO can develop vision problems, reduced ability to work or learn, reduced manual dexterity, and difficulty performing complex tasks. At extremely high levels, CO is poisonous and can cause death [4].-Smog: CO contributes to the formation of smog, ground-level ozone, which can trigger serious respiratory problems.

1.
The best combustion process occurs when the mixture is stochiometric fuelair ratio which means a complete combustion for all the fuel entering the combustion chamber.

2.
When the mixture is lean, excess air will be emitted. 3. When the mixture is rich, CO will be emitted to the atmosphere as a reaction product which has a serious bad impact on the environment.