8.2 Is knock the only abnormal combustion problem?
Description
This article is from the Gasoline FAQ, by Bruce Hamilton with numerous contributions by others.
8.2 Is knock the only abnormal combustion problem?
No. Many of the abnormal combustion problems are induced by the same
conditions, and so one can lead to another.
Preignition occurs when the air-fuel mixture is ignited prematurely by
glowing deposits or hot surfaces - such as exhaust valves and spark plugs.
If it continues, it can increase in severity and become Run-away Surface
Ignition (RSI) which prevents the combustion heat being converted into
mechanical energy, thus rapidly melting pistons. The Ricardo method uses an
electrically-heated wire in the engine to measure preignition tendency. The
scale uses iso-octane as 100 and cyclohexane as 0.
Some common fuel components:-
paraffins 50-100
benzene 26
toluene 93
xylene >100
cyclopentane 70
di-isobutylene 64
hexene-2 -26
There is no direct correlation between antiknock ability and preignition
tendency, however high combustion chamber temperatures favour both, and so
one may lead to the other. An engine knocking during high-speed operation
will increase in temperature and that can induce preignition, and conversely
any preignition will result in higher temperatures than may induce knock.
Misfire is commonly caused by either a failure in the ignition system, or
fouling of the spark plug by deposits. The most common cause of deposits
was the alkyl lead additives in gasoline, and the yellow glaze of various
lead salts was used by mechanics to assess engine tune. From the upper
recess to the tip, the composition changed, but typical compounds ( going
from cold to hot ) were PbClBr; 2PbO.PbClBr; PbO.PbSO4; 3Pb3(PO4)2.PbClBr.
Run-on is the tendency of an engine to continue running after the ignition
has been switched off. It is usually caused by the spontaneous ignition of
the fuel-air mixture, rather than by surface ignition from hotspots or
deposits, as commonly believed. The narrow range of conditions for
spontaneous ignition of the fuel-air mixture ( engine speed, charge
temperature, cylinder pressure ) may be created when the engine is switched
off. The engine may refire, thus taking the conditions out of the critical
range for a couple of cycles, and then refire again, until overall cooling
of the engine drops it out of the critical region. The octane rating of the
fuel is the appropriate parameter, and it is not rare for an engine to
require a higher Octane fuel to prevent run-on than to avoid knock [27,28].
Obviously, engines with fuel injection systems do not have the problem, and
idle speed is an important factor. Later model carburettors have an idle
stop solenoid which partially closes the throttle blades when the ignition
key was off, and thus ( if set correctly ) prevents run-on.
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