knocking The metallic sound produced by a spark-ignition petrol engine under certain conditions. It is
caused by rapid combustion of the
unburnt explosive mixture in the
combustion chambers ahead of the
Ûame front. As the Ûame travels from
the sparking plug towards the piston
it compresses and heats the unburnt
gases ahead of it. If the Ûame front
moves fast enough, normal combustion occurs and the explosive mixture is ignited progressively by the
Ûame. If it moves too slowly, ignition
of the last part of the unburnt gas
can occur very rapidly before the
Ûame reaches it, producing a shock
wave that travels back and forth
across the combustion chamber. The
result is overheating, possible damage to the plugs, an undesirable
noise, and loss of power (probably
due to preignition caused by overheated plugs). Knocking can be
avoided by an engine design that increases turbulence in the combustion
chamber and thereby increases Ûame
speed. It also can be avoided by reducing the compression ratio, but
this involves loss of efÜciency. The
most effective method is to use highoctane fuel (see octane number),
which has a longer self-ignition
delay than low-octane fuels. This can
be achieved by the addition of an
antiknock agent, such as lead(IV)
tetraethyl, to the fuel, which retards the combustion chain reactions. However, lead-free petrol is
now preferred to petrol containing
lead tetraethyl owing to environmental dangers arising from lead in
the atmosphere. In the USA the addition of lead compounds is now forbidden. New formulae for petrol are
designed to raise the octane number
without polluting the atmosphere.
These new formulae include increasing the content of aromatics and oxygenates (oxygen-containing
compounds, such as alcohols). However, it is claimed that the presence
in the atmosphere of incompletely
burnt aromatics constitutes a cancer
risk.
Knoevenagel reaction A reaction
in which an aldehyde RCHO reacts
with malonic acid (CH 2 (COOH) 2 )
with subsequent loss of CO 2 to give
an unsaturated carboxylic acid
RCH=CHCOOH. Thus, the chain
length is increased by 2. The reaction
is base-catalysed; usually pyridine is
used. The reaction is named after
Emil Knoevenagel (1865–1921).
knotane See molecular knot.
knot theory A branch of mathematics used to classify knots and entanglements. Knot theory has
applications to the study of the properties of polymers and the statistical
mechanics of certain models of phase
transitions.
Knudsen Ûow See molecular
flow.
Kohlrausch equation An equation that describes the molar conductivities of strong electrolytes at low
concentration, i.e. Λ m = Λ
0
m – Κc
½,
where Λ m is the molar conductivity,
Λ
0
m is the limiting molar conductivity
(the molar conductivity in the limit
of zero concentration when the ions
do not interact with each other), Κ is
a coefÜcient related to the stoichiometry of the electrolyte, and c is the
concentration of the electrolyte. It is
possible to express Λ
0
m as a sum of
the contribution of each of its ions.
The Kohlrausch equation was Ürst
stated in the 19th century by the German chemist Friedrich Kohlrausch
(1840–1910) as the result of a considerable amount of experimental work.
With its characteristic c
½ dependence, the equation is explained quantitatively by the existence of an ionic
atmosphere round the ion as
307
Kohlrausch equation
k
www.AzShimi.ir www.AzShimi.com
caused by rapid combustion of the
unburnt explosive mixture in the
combustion chambers ahead of the
Ûame front. As the Ûame travels from
the sparking plug towards the piston
it compresses and heats the unburnt
gases ahead of it. If the Ûame front
moves fast enough, normal combustion occurs and the explosive mixture is ignited progressively by the
Ûame. If it moves too slowly, ignition
of the last part of the unburnt gas
can occur very rapidly before the
Ûame reaches it, producing a shock
wave that travels back and forth
across the combustion chamber. The
result is overheating, possible damage to the plugs, an undesirable
noise, and loss of power (probably
due to preignition caused by overheated plugs). Knocking can be
avoided by an engine design that increases turbulence in the combustion
chamber and thereby increases Ûame
speed. It also can be avoided by reducing the compression ratio, but
this involves loss of efÜciency. The
most effective method is to use highoctane fuel (see octane number),
which has a longer self-ignition
delay than low-octane fuels. This can
be achieved by the addition of an
antiknock agent, such as lead(IV)
tetraethyl, to the fuel, which retards the combustion chain reactions. However, lead-free petrol is
now preferred to petrol containing
lead tetraethyl owing to environmental dangers arising from lead in
the atmosphere. In the USA the addition of lead compounds is now forbidden. New formulae for petrol are
designed to raise the octane number
without polluting the atmosphere.
These new formulae include increasing the content of aromatics and oxygenates (oxygen-containing
compounds, such as alcohols). However, it is claimed that the presence
in the atmosphere of incompletely
burnt aromatics constitutes a cancer
risk.
Knoevenagel reaction A reaction
in which an aldehyde RCHO reacts
with malonic acid (CH 2 (COOH) 2 )
with subsequent loss of CO 2 to give
an unsaturated carboxylic acid
RCH=CHCOOH. Thus, the chain
length is increased by 2. The reaction
is base-catalysed; usually pyridine is
used. The reaction is named after
Emil Knoevenagel (1865–1921).
knotane See molecular knot.
knot theory A branch of mathematics used to classify knots and entanglements. Knot theory has
applications to the study of the properties of polymers and the statistical
mechanics of certain models of phase
transitions.
Knudsen Ûow See molecular
flow.
Kohlrausch equation An equation that describes the molar conductivities of strong electrolytes at low
concentration, i.e. Λ m = Λ
0
m – Κc
½,
where Λ m is the molar conductivity,
Λ
0
m is the limiting molar conductivity
(the molar conductivity in the limit
of zero concentration when the ions
do not interact with each other), Κ is
a coefÜcient related to the stoichiometry of the electrolyte, and c is the
concentration of the electrolyte. It is
possible to express Λ
0
m as a sum of
the contribution of each of its ions.
The Kohlrausch equation was Ürst
stated in the 19th century by the German chemist Friedrich Kohlrausch
(1840–1910) as the result of a considerable amount of experimental work.
With its characteristic c
½ dependence, the equation is explained quantitatively by the existence of an ionic
atmosphere round the ion as
307
Kohlrausch equation
k
www.AzShimi.ir www.AzShimi.com