20-11-2012, 03:21 PM
ENGINE KNOCKING And Alternate fuels
ENGINEKNOCKING.ppt (Size: 1.78 MB / Downloads: 49)
What is knocking
In an engine , when the fuel remaining after normal combustion ignites explosively by high pressure and temperature produced from normal combustion ,rather than spark from spark plug , it causes knocking or detonation.
This explosive release of energy is what causes the characteristic "knocking”. It creates the characteristic metallic "pinging"
Sound.
Causes of knocking
The main driver for knock is ignition timing. As the ignition is advanced - the spark is fired earlier in the engine operating cycle - the pressure and temperature during the combustion event become higher, thus making knock more likely.
Happens when fuel burn ignites irregularly and prematurely
Usage of fuel with low octane rating
Poor design of engine and due to their own structural problem
ABNORMAL COMBUSTION (DETONATION)
When unburned fuel/air mixture beyond the boundary of the flame front is subjected to a combination of heat and pressure for a certain duration, detonation may occur. Detonation is mainly the instantaneous, explosive ignition of at least one pocket of fuel/air mixture outside of the flame front. A local shockwave is created around each pocket and the cylinder pressure may rise sharply beyond its design limits. If detonation is allowed to persist under extreme conditions or over many engine cycles, engine parts can be damaged or destroyed.
Reduction of knocking
Reducing air and coolant temperatures is always good for suppressing knock (colder air is also more dense, allowing the engine to take in a greater mass of air.)
The strategy for reducing knock - and so allowing more ignition advance (and hence more torque) - is to use a fuel with a higher octane rating. Higher octane fuels contain more components such as benzene that are resistant to self-ignition, and less of components such as n- heptane that self-ignite relatively easily.