Detonation results due
to instantaneous burning of large amount of charge. This is observed at the
starting of combustion.
Mostly Detonation is occurred because of following reasons:
Detailed Explanation:
- For petrol engines, the octane number determines how likely your fuel is to detonate.
- The higher the octane number, the harder it is to ignite
- If you use low octane fuel, it will combust easier.
- The problem is, when combustion occurs before the spark plug is lit, 2 flame fronts will collide inside your cylinder.
- When they 'hit' each other, you will hear a ping sound, this causes irregular pressure gradients in your cylinders. Over time, this can lead to damage engine due to elevated temperatures, fatiguing of metal due to pressure spikes greater than design intent.
- The engine will suffer from decreased performance as the unplanned combustion event will push against the planned combustion event, absorbing energy that would otherwise be used to build head and pressure at the right crank angle, which would push down on the piston as intended, leading to design intent power and torque.
- If detonation occurs to far before top dead center (TDC), then this may cause force against the piston to turn backwards slightly, hence results in force against the rotation of your crankshaft.
- Forced Induction (Turbocharged/Supercharged) are more likely to detonate. Air is pressurized as it enters the combustion chamber, more pressure, more temperature, easier to ignite air/fuel mixture**You must use high octane fuel for forced induction engines**A charge air cooler will help prevent detonation, increase air density allowing more fuel to be used creating more power.
Effect
of Detonation:
1. Noise and Roughness.
2. Vibration.
3. Mechanical Damage.
4. Decrease in power output
and efficiency.
5. Pre-ignition.
Mostly Detonation is occurred because of following reasons:
- Low Octane Fuel
- High Compression engine
- High Mileage engine
Factors
affecting Detonation or Knocking in SI engine:
1. Compression ratio:
The pressure and
temperature at the end of compression increases with increase in compression
ratio. This in turn increase the maximum pressure during the combustion and
creates a tendency to knock.
2.
Supercharging:
It is the process of
allowing the charge at a pressure higher than atmospheric pressure. Because of
supercharging the density and temperature increases there by knocking tendency
increases. Super charging is generally done on CI engines
3.
Increasing inlet temperature:
It results in increase in
knocking.
4.
Increase in load:
Increase in load result in
increase in temperature of the cylinder, there by rising the temperature and
hence knocking tendency increasing at high loads.
5.
Advancing the spark:
when the spark is advance
the burning gas is compressed by rising piston and therefore the increase in
temperature will be much more and hence the knocking tendency increases. To
avoid detonation spark must be retarded.
6.
Flame travel distance:
Flame travel distance
should be small to avoid detonation.
7.
Spark plug location:
spark plug which is located
centrally has minimum tendency for knocking because the flame travel distance
decreases.
8. Engine
size:
For larger engine size the
flame travel distance will be longer and hence knocking chances are high,
because of this reason SI engine sizes are generally limited.
9.Location
of exhaust valve:
There are two hot spot zone
in SI engines. ie., spark plug & exhaust valve.
If the exhaust valve is
very far from the spark plug the temperature around exhaust valve region will
be high.
If the spark plug is very
far it will take much time and during this time the charge near exhaust valve
would autoignitied due to high temperature and hence exhaust valve must be
located as close to spark plug as possible. So the flame travel distance is
minimized.
10.
Turbulence:
With
increase in turbulence the flame speed increases and hence the chances of
detonation would decreases.
11.Engine
speed:
Increase in engine speed increases turbulence & hence knocking tendency
decreases.
12.
Octane rating of fuel:
For better antiknock properties octane number must be high.