Note: Descriptions are shown in the official language in which they were submitted.
The present invention relates to improvements in ignition coils.
It is well known tha~ energy is delivered by the secondary winding
o~ the ignition coil at a high voltage, but comparatively low current. It
is a characteristic of spark plugs that they rcquire a much ~igher voltage
to achieve the initial ionization of the electrode gap, than they need to
maintain the current flow across the gap once ionization has taken place.
This di~ference in voltage is roughly 6,000 volts to 1,000 volts for a warm
engine, but may rise to as much as 17,000 volts to 1,000 volts in a cold
engine.
Since modern internal combustion engines are required to burn
leaner mixtures at large spark advance angles, in order to produce low
emission levels, the standard ignition coil is no longer able to supply the
high spark energy required for these conditions of operation.
It has already been proposed to overcome the above problem by
providing an ignition system having two coils, the first coil providing a
high voltage in order to ionize the gap across the electrodes of the spark
plug whilst the second coil is initiated a short time later to provide a low
voltage high energy discharge across the already ionized gap.
One such system for achieving this double discharge across the gap
is disclosed in British Patent No. 695,4~2 ~Rolls Royce Ltd.). The disclosure
in this patent shows the use of two separa~e coils, the secondary winding
of the high voltage coil being connected across an auxiliary and main gap
provided in series. Whilst the secondary winding of the low voltage coil is
connected across the main gap only through a diode.
The disadvantage of the ignition system disclosed in British Patent
No. 695,442 is that the high voltage coil has to ionize two gaps. Also whilst
the diode prevents currenk from the high voltage secondary winding from feed-
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ing back to the low voltage secondary winding, there is nothing to prevent
current flow in the other direction and this could happen once the voltage
across the secondary winding of the high voltage coil has decayed below
the level of the voltage across the secondary winding of the low voltage
coil since the auxiliary gap would still be ionized.
It is an object of the present invention to overcome partially or
wholly disadvantages e~hibited by prior art systems.
According to the present invention, there is provided an ignition
systcm for an internal combustion engine said system including: a first
ignition coil ha~ing a high tuxns ratio between its secondary and primary
windings; a second ignition coil having a relatively low turns ratio between
its secondary and primary windings in comparison with the first coil; means
for supplying the voltage generated in the two secondary windings to tne same
distributor system or spark plug of the internal combustion engine; means
for electrically isolating the two secondary windings; first means for switch-
ing off the current in the primary winding of said first ignition coil in
order to produce a high voltage to ionize the gap across the spark plug;
second means for switching off the current in the primary winding of said
s0cond ignition coil at a constant predetermined crank-shaft position after
the current in the primary winding of the first coil has been switched off,
in order to maintain the voltage across the gap so as to provide a long
spark duration.
In this arrangement, the energy provided by the first secondary
winding ionizes the gap across the spar~ plug, and then both the secondary
windings supply spark energy simultaneously for 2-3 milliseconds once the
spark ~ap has been ionized~ the second secondary winding supplying the
majority of the energy necessary to sustain the spark once the gap has been
ionized.
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In a preferred embodiment the electrical isolating means comprises
a pair of diodes connected between respective one ends of the secondary
windings and the distributor system or one electrode of the spark plug.
The first and second primary current switching means may comprise
first and second opto-electronic triggers operated through separate chopper
discs driven in synchronism with the engine.
The present invention will now be described in greater detail by
way of examples with reference to the accvmpanying drawing, which is a circuit
diagram of one preferred form of double ignition coil system according to
tlle invention using two opto-electronic triggers for providing high spark
energy to an internal combùstion engine operating on a lean fuel mixture.
Referring to the drawing, the ignition system includes ignition
coils 1 and 2, having respective primary windings Pl and P2 and respective
secondary windings Sl and S2. The primary windings Pl and P2 are colmected
in series with respective power transistors Tl and T2 across the 12 volts
battery supply o the internal combustion engine. The ends of the secondary
windings Sl and S2 not connected to their respective primaries, are connected
to a distributor Z through respective diodes Dl and D2. The distributor Z
distributes the energy to the spark plugs 3 in the correct sequence of
cylinder firing, in accordance with conventional practice.
; The power transistors Tl and T2 are switched by respective
"Lumenition" ~Registered Trade Mark) bistable opto-electronic triggers com-
prising respective trigger modules Xl and X2. The trigger modules Xl and
X2 are energized from respective photo-transistors Bl and B2, as a result of
infra-red radiation from respective gallium arsenide lamps Al and A2 after
passillg through the apertures or slots of respective discs Cl and C2, driven
in synchronism with the engine.
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The primary windings Pl and P2 have about 200 turns, whereas, the
secondary windings Sl and S2 respectively have 20,000 turns and 2,000 turns.
The secondary winding S2 is of substantially thicker gauge wire, in order
that it has a lower resistance. The open circuit voltages across the second-
ary windings Sl and S2 are of the order of 35,000 volts and 3,500 volts
respectively.
In the case illustrated in the drawing, the slotted or apertured
discs Cl and C2 are arranged so that the disc Cl is set to switch of its
associated power transistor Tl a few crankshaft degrees in advance of the
switching of the second power transistor T2 by the disc C2. Thus, the power
transistor Tl, on switching off, causes a high secondary voltage of 35,000
volts to be developed in the secondary winding Sl, to cause the initlal
ionization of the compressed fuel mixture across the spark gap. A few
crankshaft degrees later, -the second power transistor T2 switches off, to
cause a voltage of 3,500 volts to be developed across the secondary winding
S2. Since this secondary winding S2 has larger diameter wire than tlle
secondary winding Sl, it can carry a larger current, this current supplement-
ing the current flowing in the winding Sl in order to maintain the necessary
energy to the spark plug to ensure complete combustion of the lean fuel mix-
ture in the cyllnder.
The advantages of the above described system lie in the fact thathigher spark energy will be available in the combustion chamber, thus
permitting complete combustion of lean fuel mixtures, whilst ensuring that
the spark duration is such that the spark is not present during the combus-
tion period.
The present invention is no~ limited to petrol or, or that matter,
diesel spark ignition internal combustion engines, and could be equally
applicable to iring gas, or even oil, burners in industry.
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