Mechanical Engineering - IC Engines and Nuclear Power Plants - Discussion

Please anyone explains it.

Advancing the spark timing will decrease knocking tendency as well as brake power of SI engine.

Yes, advancing the spark timing reduce knocking tendency.

No, it will decrease knocking.

Answer is correct only, Advancing Spark TIming increases temp n press. In cylinder block which inturn increases knocking.

Advancing spark increases the knocking tendency because compressing a burnt gases increases the temperature and pressure this causes auto ignition of the end charge.

Timing advance is required because it takes time to burn the air-fuel mixture. If the ignition spark occurs at a position that is too advanced relative to piston position, the rapidly expanding air-fuel mixture can actually push against the piston still moving up, causing knocking (pinging) and possible engine damage.

To extract maximum power from the engine the spark is ignited, slightly before TDC so that the peak pressure occurs just after piston reaches TDC and expansion stroke starts. This enables us to extract maximum power from it.

However advancing the spark timing too early, will result into ignition of the mixture before the piston reaches TDC. This causes the mixture to exert opposing force on the piston. In addition the ignition causes a much higher pressure and temperature due to the ignition occurring in the compressive stroke resulting into pre-ignition/detonation.

Ignition timing advancing means that the spark plugs are firing earlier in the compression stroke, farther from TDC. The air and fuel mixture in the combustion chamber doesn't burn right away.