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From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Investigation into engine pre-ignition and the effect of warming up the fuel-air charge.

Identifier  ExFiles\Box 34\2\  Scan105
Date  15th May 1918
  
Contd. -4- EH1/LG15.5.18.

engine which demonstrates the effect of warming up the charge
it frequently happens when the ignition is switched on to start
the engine, the coil "buzzes" for some time and then after a
period of time the charge fires. The only effect we can see
has happened is that the spark warms up the portion of the
charge round the plug so that it ignites.

Another point we have investigated lately, is that
if an engine has been running full power and is stopped down
quickly by closing the throttle and switching off, or switching
off without closing the throttle, it will drop in speed without
firing until it is turning very slowly then it will start
firing, pre-igniting and so keep running for a considerable time.
We frequently have had it happen that an engine has been
stopped down and we have gone to turn it after it has stopped for
a minute or two and it has pre-ignited. One would assume that
the ignition takes place due to over-heating ignition plug or
exhaust valve because they form the hottest portions of the
combustion chamber. On our test engine however, we are able
to observe that the ignition plugs and exhaust valves are quite
cool and black when this ignition takes place. We have
removed the ignition wires altogether so as to be sure there is
no leakage in the switch. One would imagine that if the charge
is ignited by any over-heated portion of the combustion chamber,
it would go on firing immediately it is switched off and when
the combustion chamber must be hottest, but when it has cooled
down. The only reason we can see that it fires at the low
speed, is that then the charge is warmed up. There are two

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