Description-
The Tendency of the engine to knock is greatly affected by the shape of the combustion chamber. the combustion chamber of an I-head engine is bounded at the top by the cylinder head, inlet valve, exhaust valve, and spark plug; and at the bottom by the piston head and top compression ring. There are two general shapes- wedge and hemispherical. The shape determines turbulence, squish, and quench. These three factors affect the engine to knock.
Mechanical Factors-
1. Turbulence-
The proper shape of the combustion chamber imparts turbulence to the air-fuel mixture entering it. Turbulence assures more uniform mixing of air and fuel so that the combustion is more uniform. Turbulence also reduces the time required for the flame front to travel through the compressed mixture.
2. Squish-
Squish means to push through small areas. As the piston nears TDC, it squishes or squeezes or pushes the air-fuel mixture at the end of the compression stroke, out of the squish area. As it squishes out, it promotes turbulence and thus further mixing of the air-fuel mixture.
3. Quench-
It has already been stated that knocking results when the temperature of the last part, the mixture goes too high and explodes before the flame front reaches it. If some heat is extracted from this part of the mixture, its temperature will not reach the detonation point. The closeness of the cylinder head to the piston and the relative coolness of these surfaces causes the heat to be extracted from the mixture so that tendency for detonation to occur is quenched.
4. Smog-
The automobile engine and the fuel system release to the atmosphere a variety of gaseous compounds, including unburned hydrocarbons, and carbon monoxide. The compounds contribute to the formation of smog. The shape of the combustion chamber also influences the number of contaminants in the smog. The wedge combustion chamber has a large surface area and produces a great percentage of contaminants per power stroke than the hemispherical combustion chamber. Incomplete combustion deposits, which ultimately cause detonation.
Combustion chamber shape-
The shape of the combustion chamber matters in the detonation. Combustion chamber means the chamber where the power stroke is held. There are two types of combustion chamber shapes-
1. Wedge Combustion Chamber-
In the wedge combustion chamber, the spark plug is located at the center of the dome and hence the flame front has to travel a greater distance to reach the end of the wedge. However, the end of the wedge has a quench area that cools the last part of the air-fuel mixture to prevent detonation. It also imparts turbulence to the mixture.
2. Hemispherical combustion chamber-
In a hemispherical combustion chamber, the spark plug is located at the center of the dome, and hence the flame front has a relatively short distance to travel. Also, there are no distance pockets of the last part of the mixture to detonate. The chamber has no squish or quench area, however, there is relatively little turbulence.