Variable affecting detonations.
1. Temperature : Increasing the temperature of the unburned mixture increase the possibility of knock in the SI engine following are the effect on SI engine parameters on the temperature of the unburned mixture:
1. Increasing the compression ratio increases both the temperature and pressure.
2. Supercharging. Also increase both temperature and density, which increase the knocking tendency of engine.
3. Delay period decreases with increase of coolant temperature, decreased delay period increase the tendency to knock.
2. Density : Increasing the density of unburnt mixture will increase the possibility of knock in the Engine. The engine parameters which affect the density are as follows: Increased compression ratio increase the density. Increasing the load opens the throttle valve more and thus the density. Supercharging increase the density of the mixture. Increasing the inlet pressure increases the overall pressure during the cycle. The high pressure end gas decreases the delay period which increase the tendency of knocking. Advanced spark timing: quantity of fuel burnt per cycle before and after TDC position depends on spark timing. The temperature of charge increases by increasing the spark advance and it increases with rate of burning and does not allow sufficient time to the end mixture to dissipate the heat and increase the knocking tendency
3. Time: Increasing the time of exposure of the unburned mixture to auto-ignition conditions increase the possibility of knock in SI engines. Flame travel distance: If the distance of flame travel is more, then possibility of knocking is also more. This problem can be solved by combustion chamber design, spark plug location and engine size. Compact combustion chamber will have better anti-knock characteristics, since the flame travel and combustion time will be shorter. Further, if the combustion chamber is highly turbulent, the combustion rate is high and consequently combustion time is further reduced; this further reduces the tendency to knock. Location of sparkplug. A spark plug which is centrally located in the combustion chamber has minimum tendency to knock as the flame travel is minimum. The flame travel can be reduced by using two or more spark plugs. Location of exhaust valve. The exhaust valve should be located close to the spark plug so that it is not in the end gas region; otherwise there will be a tendency to knock.
4. Composition: Molecular structure: Increasing the carbon-chain increases the knocking tendency and centralizing the carbon atoms decreases the knocking tendency. Unsaturated hydrocarbons have less knocking tendency than saturated hydrocarbons. Air-fuel mixture: A too rich mixture is especially effective in decreasing or eliminating the knock due to longer delay and lower temperature of compression. Humidity of air: Increasing atmospheric humidity decreases the tendency to knock by decreasing the reaction time of the fuel.