SPARK IGNITION ENGINE

In SI engines the air and fuel are usually mixed together in the inake system before entering to the engine cylinder using a carburetor or fuel injection system .

Fuel-management systems,whether of the
carburettor or injection types,have the task
of preparing an optimum air- fuel mixture .

Fuel management by means of manifold
injection permits the optimum adaptation
of the air - fuel mixture to every operating
phase of the engine . It also ensures a
lower level of Pollutants in the exhaust gas.

In spark - Ignition systems , fuel management
is by means of either a carburettor or a fuel
-injection system .

Essentially, the power, the fuel consumption
and the exhaust-gas composition of a spark
-ignition engine depend upon the air-fuel ratio

Perfect ignition and perfect combustion only take place within particular air-fuel ratios . In the case of gasoline ( petrol) , the ideal air-fuel ratio is about 15:1 ( Stoichiometric ratio ) .

Deviations from this ratio affect engine operation .The amount of fuel to be injected depends upon load, engine speed and the particular exhaust-gas regulations in force at the time.Depending upon the mode
of operation , ( idle, part load or full load ) , a different air-fuel ratio is optimal in each case.

In automobile application ,the temperture of the air entering the intake system is controlled by mixing around air with air heated by contact with the exhaust manifold . the ratio of mass flow of air to mass
flow of fuel must be held approximately constant at about 15 to ensure reliable combustion.

In SI Four stroke engine for example , to maintain high mixture flows at high engine speed ( and
hence high power outputs ) the inlet valve , which open before TC , closes substantially after BC.

During intake , the inducted fuel and air mix in the cylinder with residual burned gases remaining from
the previous cycle .

After the intake valve closes , the cylinder contents are compressed to above atmospheric pressure and temperture as the cylinder volume is reduced .

Between 10 to 40 crank angle degree before TC ignition start at spark plug .A turbulent flame develops from the spark discharge , propagates across the mixture of air , fuel , and residual gas in the cylinder.

As fuel-air mixture burns in the flame , the cylinder pressure rises above the level due to compression alone forming a curve called the motored cylinder pressure it is the pressure reace obtained from a motored on nonfiring engine .

There is an optimum spark timing which ,for a given mass of mixture air fuel inside the cylinder ,gives maximum torque .we can defines the maximum brake torque timing ( MBT ) as the minimum spark advance for best torque .

About two-thirds of the way through the expansion stroke , the exhaust valve starts to open. the cylinder pressure is greater than the exhaust manifold pressure and blowdown process occurs.the burned gases flow through the valve into The exhaust port and manifold until the cylinder pressure and exhaust pressure equilibrate.

The exhaust valve remains open until just after TC. the intake opens just before TC .the valves are opened and closed slowly to avoid noise and excessive cam wear.to ensure the valves are fully open when piston velocities are at their highest , the valve open periods often Overlap.

If the intake flow is throttled to below exhaust manifold pressure ,then backflow of burned gases into the intake manifold occurs when the intake vavle is first opened .