Some things that I haven't seen mention of yet pertaining to twin plugs and
multiple sparks.
As we all know, max power comes from running slightly lean and getting
complete combustion. It is a fine line to walk, however, and many cars have
to run richer than optimum to prevent knock and other mixture related
problems.
If you can control mixture evenly, and control the flame front (as in some
modern engines), you can run quite lean. Under these conditions there are a
couple of factors to consider. The environment in the combustion chamber is
not a room filled with gas waiting for an ignition source. It is a room
whose floor may be moving at 3000 fps with gas coming and going at 50 cfm.
Not exactly a simple environment for anything to happen, let alone to have
it happen in a couple of milliseconds.
In order to get the most out of the fuel you are using, you need to burn it
all. Multiple sparks are employed as a fairly simple way to try to reignite
the mixture should it happen to go out or to ignite patches that didn't get
consumed initially, that may have swirled in front of the plug. At higher
RPM there simply isn't time for this, so a single, long duration, spark
works even better.
Twin plugs is a slightly different monster. A deliberately designed twin
plug system, such as BMW motorcycles. Will have one plug on each side of
the cylinder, they are designed to light the mixture from both sides and to
have the flames meet in the center of the chamber. This halves the distance
necessary for the flame front to travel, and thus halves the time for
combustion. With all of this saved time you could afford to spin the engine
to twice the RPM or you can move the firing event with respect to piston
position to get more complete burn at more optimum times and thus more power
per event. Depending on what your priorities are. Remember that in some
engines, depending on geometry and RPM, the pistons can outrun the flame
front.
In BMW's case, they turn on the extra plug under low to moderate load to
allow especially lean running conditions. So lean that a single plug would
not be able to support combustion because the flame front is too weak to
cross the piston center in the time available. If memory serves, they may
also vary timing between them to alter burn characteristics under these
conditions. At full throttle or high load conditions, they can turn off one
of the plugs an operate in a conventional manner if they so desire, similar
to the conditions where the emission system would operate in open loop mode.
This is an factor in facilitating automobile emissions getting lowered while
efficiency is going up. If less fuel can be used to make the same power,
consumption is reduced. If complete combustion can be assured, there will
be little or no hydrocarbons in the exhaust. The Nox and CO in the exhaust
can then be handled by the Cat. A cat cannot clean hydrocarbons, and a rich
condition will clog a cat very quickly.
James Nazarian
71 MGBGT-V8
71 MGB Tourer
87 325es
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