Under normal operating conditions, combustion is initiated towards the end of the compression stroke at the spark plug by an electric discharge. … Abnormal combustion in spark ignition engine majorly occurs as
knock and surface ignition
.
What is meant by abnormal combustion?
The Abnormal Combustion:- ➢
When the combustion gets deviated from the normal behavior resulting loss of performance or damage to the engine
. ➢It is happened when the charge inside the cylinder may be ignite before the flame front reach it.
What is normal combustion?
In normal combustion, the piston is on its way up the cylinder toward Top Dead Center. It’s
compressing a mixture of fuel and air above it
. At a point, before TDC, a spark passes across the gap of the spark plug electrodes and that sets the mixture to burning, creating a tiny flame kernel.
What is an example of abnormal combustion?
There are three forms of abnormal combustion:
Detonation, Pre-Ignition and Detonation of fuel before Ignition
or (Pre-Ignition due to Detonation). The process of burning the fuel in the combustion chamber has always been the most misunderstood concept by people who modify engines and tune engines.
What is the effect of abnormal combustion?
Prevention of auto-ignition, and its resulting abnormal combustion, is a longstanding problem for spark-ignited internal combustion engines. This type of abnormal combustion
triggers engine knock
, which causes not only uncomfortable noises but also engine damage.
What is the process of combustion?
Combustion is a
chemical process in which a substance reacts rapidly with oxygen and gives off heat
. The original substance is called the fuel, and the source of oxygen is called the oxidizer. … During combustion, new chemical substances are created from the fuel and the oxidizer. These substances are called exhaust.
What are the effects of knocking?
Knocking can
cause overheating of the spark-plug points, erosion of the combustion chamber surface, and rough, inefficient operation
.
What causes abnormal combustion?
Abnormal combustion. 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
(beyond the delay period of the fuel used), detonation may occur.
What is the process of normal combustion in petrol engine?
In a conventional SI engine, the fuel is
completely vaporized and homogeneously premixed with air and residual gas
, when combustion occurs. Under normal operating conditions, the mixture is ignited by the spark plug in a optimized moment of the cycle (usually 20-40° before TDC).
How abnormal combustion processes can be avoided in CI engines?
Methods to avoid abnormal combustion in SI engines :
Using fuel having high octane number
.
Retarding the MBT timing of spark
.
Reducing intake temperature and pressure
.
What causes high combustion temperatures?
Possible causes for the damage
Mixture too lean
, resulting in higher combustion temperatures. Damaged or leaking valves, or insufficient valve clearance, causing the valves to not close correctly. The combustion gases streaming past significantly increase the temperature of the valves, and the valves start to glow.
What must be mixed with fuel in order for it to burn?
chemical mixture of semisolids, liquids and gases
. … When the spark plug ignites the fuel and burning progresses smoothly through the fuel mixture. Stages of normal combustion. Spark occurs, flame spreads smoothly, flame form heat with piston, and pressure is mix piston goes down.
What is the meaning of pre-ignition?
Pre-ignition (or preignition) in a spark-ignition engine is a technically different phenomenon from engine knocking, and
describes the event wherein the air/fuel mixture in the cylinder ignites before the spark plug fires.
What are the stages of combustion in CI engine?
- Ignition Delay Period. …
- Period of Uncontrolled Combustion. …
- Period of Controlled Combustion. …
- After Burning.
What is delay period and what are the factors that affect it?
I C ENGINES – II Air Motion & Combustion •Factors Affecting the Delay Period : –
Quality of fuel Self ignition temperature
is the most important property of fuel which affects the delay period. A lower self-ignition temperature results in lower delay period. – Fuels with higher cetane number give lower delay.
How does time factors affect the phenomenon of knocking?
Time factors consist of
engine speed and spark timing
. An increase in engine speed increases the turbulence of the mixture considerably resulting in increased flame speed, and reduces the time available for preflame reactions. Hence knocking tendency is reduced at higher speed.
