Effects of Spark Advancement

• As the spark advancement increases, combustion is initiated more early which leads to cylinder pressure rising rapidly while the piston is still completing the compression stroke. This creates excessive cylinder pressures and even produces shock waves in cylinder. This condition is shown by the ragged top of curve 2.
• The engine will run rough and tend to overheat as the spark is advanced more, resulting in loss of power.
• Over advanced spark may cause the cylinder peak pressure to occur at TDC, thereby causing higher than normal maximum pressures to occur, hence promoting detonation.

Effects of Spark Retardation

• Retarded spark helps to decrease the engine knock because comparatively lower maximum engine pressures are produced.
• The maximum pressure occurs late in the cycle during the power stroke and is not as high as that of the normal case. So the power output is lesser due to spark retardation.
• The engine runs hot due to late burning and causes damage to exhaust valves and ports.
• Spark retardation causes exhaust gas temperatures to be higher thereby promoting additional burning of hydrocarbons in the exhaust manifold.
• Fuel economy is adversely affected.
• Spark retardation causes lesser emissions at idling operation of engine.
• The normal curve indicates smooth engine running when the maximum pressure is built up when piston reaches TDC.