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Ignition Lag

This document discusses ignition lag in spark-ignition (SI) engines and ignition delay in compression-ignition (CI) engines. It explains that ignition lag/delay is the time between fuel injection/spark and the start of noticeable pressure rise. In SI engines, a longer ignition lag avoids knocking by allowing more time for flame propagation. In CI engines, ignition delay affects combustion rate, knocking, starting ability, and smoke; it consists of physical and chemical delay periods. The delay is influenced by many engine/fuel factors and allows for gradual burning and pressure rise in CI engines. However, too long a delay can cause higher peak pressures and knocking.

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546 views2 pages

Ignition Lag

This document discusses ignition lag in spark-ignition (SI) engines and ignition delay in compression-ignition (CI) engines. It explains that ignition lag/delay is the time between fuel injection/spark and the start of noticeable pressure rise. In SI engines, a longer ignition lag avoids knocking by allowing more time for flame propagation. In CI engines, ignition delay affects combustion rate, knocking, starting ability, and smoke; it consists of physical and chemical delay periods. The delay is influenced by many engine/fuel factors and allows for gradual burning and pressure rise in CI engines. However, too long a delay can cause higher peak pressures and knocking.

Uploaded by

eafz111
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© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Ignition Lag in SI Engines : This is the first stage of combustion (of three).

It is the time between


the application of spark in the engine and the first point of noticeable pressure rise in the engine. In
the diagram given below, it is indicated by the process AB. A self propagating nucleus of flame is
formed in this period. It’s duration depends on the temperature, pressure, nature of the fuel the
proportion of the exhaust of residual gas (EGR) and also upon the relationship between the
temperature and the rate of the reaction (the combustion of fuel).

In SI engines, the duration of the Ignition lag affects the knocking tendency of the engine. If the
ignition lag is longer than the time required for the flame front to burn through the charge, there will
be no knocking. But, if it is shorter than the time required for the flame to propagate through the
charge (and the temperature of the charge is greater than the auto-ignition temperature of the fuel),
detonation can take place. So, to avoid detonation, a longer ignition lag is preferred accompanied by
a high auto-ignition temperature as far as the fuel properties are concerned.
Ignition Delay in CI Engines : This too just like in the SI engines is a preparatory phase. It affects
combustion rate, knocking, engine starting ability and the presence of smoke in the exhaust. It is
given by the time in between the point of injection of fuel, and the first point of noticeable pressure
rise in the engine. It consists of two distinct periods ie, the physical delay and the chemical delay.
Physical delay is the delay caused because of the time taken by the fuel to atomise, vaporise and
mix with the air in the engine. The chemical delay is caused by the time taken for the reaction to
start and then for it’s rate to increase. This chemical reaction ie the burning of the fuel starts slow
and then accelerates. This is equivalent to the ignition lag in SI engines.
The delay period in CI engines is affected by a number of factors :-
• Compression Ratio
• Engine Speed
• Output of the Engine
• Atomization of the fuel and the duration of injection
• Injection Timing
• Quality of the fuel
• Intake Temperature
• Intake Pressure

The injection in CI engines takes a definite amount of time. It is not instantaneous. Therefore, as the
injection continues, the first few droplets are undergoing the ignition delay while others are just
being injected. They will soon also commence the burning phase. The same can be said for the fuel
which is injected a little later. The fuel injected earlier stars burning while that injected later
undergoes the ignition delay. This leads to a continuous and gradual burning of the fuel with little
accumulation of the fuel. This also causes a gradual pressure rise in the engine thus allowing
smooth operation.
But, if the delay period is longer, the fuel accumulated is more. So, more of the fuel molecules will
reach the burning phase at the same time. This will lead to a higher rate of heat release causing a
higher rate of pressure rise. This is disadvantageous as this will cause high vibrations and jamming
forces in the engine. As the ignition delay period is increased, more fuel burns almost
instantaneously leading to a higher rate of pressure rise and a higher peak pressure as well.
This is the situation of knock in a CI engine and will be accompanied by an audible knocking
sound. So, to keep the tendency of knock to a minimum, the ignition delay period is reduced as
much as possible.
This is the reason why the ignition delay is lower for diesel engines than it is for petrol engines.
For more information regarding the topic, I would recommend the book named below.
Source : Internal Combustion Engines by V Ganesan (4th Edition)

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