Volvo 819 E - Page 14

Group 28 Ignition systems
Des ig n and funct ion - General
Fig. 1 'Hot' plug Fig. 2 'Cold' plug
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• Spark plugs : Use of the correct type of spark plug, correctly ins ta lled and replaced at th e re commen ded in tervals ,
is
essential to t he satisfactory operation of the ign it ion system. Assuming the engine to be in good co ndition , ad -
herence to these recommendations will ensure maximum plug life, making the component one of the most re-
liable in the ignition sys tem .
In
this context, it may be of interest to consi der some of the demands impose d on the spark plugs and to discuss a
number of concep ts.
- As the means used to igni te the mixture in the combustion chamber, the spar k plug in it iates the co mb us t ion pro-
cess. The plugs are designed to supply up to approx. 25 sparks per second per cylinder at 6000 rlmin in a 4-c ylinder
e ngine , from a high-tension source which, in the case of electronic ig nition systems, often operates at over 30 kV .
-
Spark plugs must be capable of withstanding the extremely rapid temperature and pressure flu ctu ation s which oc-
cur in the combustion chamber. During the combustion process, th e temperature rises at in terval s to a value of
perhaps 2500"C and the pressure to approx. 60bar ( 870 psi). Almost immediately, during the induction stro ke, the plug
co mes in contact with the cold, un co mpressed fuel l air mixture , which is likely to be at the same temperature as the
outside air.
- To e
nsure operation within th e correct operating temperature range, spark plugs are made wit h different heat rat·
i
ng s (or ranges) for different engine types . If the operating temperature is too low (i.e. below about 400
Q
C), the plug
w ill rapidly become coated with combustion residues. This type of fouling weakens the spark and ca uses the en-
gine to miss. On the other hand, if the opera ti ng temperature is t oo hig h (ove r approx . 1000 ~ C), the fuellair mixture
may be ignited by the in candescen t plug surfaces, initiating uncontrolled combus t ion. Every engine manufacturer
specifies the app r opriate heat rat ing on the basis of parameters such as sp ecific engi ne output, probable runn i ng
conditions and clima tic conditions.
- A plug with a high heat rat ing has a greater thermal retention capacity and conducts less heat away from the com-
bustion chamber. This type is normally used on low-speed engines operating at relatively low com bustion tem -
peratures.
Plugs of t his type are also known as 'hot' plugs and are provid ed with a long insula tor nose as shown in
Fig. 1.
- A
spark plug with a low heat rating has a lower thermal retention capacity and conduc ts a grea ter amount of heat
away from the combus t ion chamber. This type is normally used on engines with high specific outpu ts operating at
relat ively high combustion temperatures . Plugs of thi s typ e are also kn own as 'co ld' plugs and feature a short in ·
sula t or nose as shown in Fig. 2.
- Fitting a plug with the recommended heat rating will ensure that the correct working temperature is reached quick -
ly wi
thout t he r isk of overhea t ing. This also assumes that the t igh t ening torque is within the speci fied limit s, since
the thermal conductivity will be dependent on th e degree of contact be tw een the cylinder head and plug . The
spark gap should also be checked; an excessive gap will cause the engi ne to miss.
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