Multiple-spark ignition system for internal combustion engines, particularly for motor vehicles

BACKGROUND OF THE INVENTION 
The present invention relates to ignition systems for internal combustion 
engines and, more particularly, is concerned with a multiple-spark 
ignition system (for example, with a double spark). 
These systems, also known as "twin-spark" systems, are used to an 
ever-increasing extent in equipping sports cars. 
DESCRIPTION OF THE PRIOR ART 
The ignition systems used until now in twin-spark engines are of 
conventional electrical and electronic type. 
By way of reference, two twin-spark ignition systems of known type will now 
be described with reference to FIGS. 1 and 2 of the appended drawings. 
In general, and also in the description of the invention which follows, 
reference will be made to systems intended to be applied to four-cylinder 
engines. These systems thus operate with four pairs of spark plugs 1, 2, 
3, 4, each associated with one of the cylinders and including two plugs 
indicated by the suffices a and b respectively. 
The system illustrated in FIG. 1 corresponds in practice to the duplication 
of two ordinary single-spark ignition systems. 
The high-tension ignition signal generated by two ignition transformers 5a, 
5b (of which the part relating to the piloting of the primary, of 
electronic or mechanical type, is not illustrated here) is fed to 
respective distributors 6a, 6b which supply the individual plugs. 
As already stated, two ignition systems are thus present, of which the 
first (5a, 6a) supplies the plugs 1a, 2a, 3a, 4a and the second (5b, 6b) 
supplies the plugs 1b, 2b, 3b and 4b. 
FIG. 4 illustrates two graphs, indicated B), showing the generation of the 
ignition pulses by the two transformers or coils 5a and 5b; this must 
occur at every rotation of the engine shaft through 180.degree., in 
synchronism with the ignition sequence in the various cylinders 1, 2, 3, 
4, as also illustrated in the graph A) in the same FIG. 4. 
In order to store the energy necessary for generating the spark, therefore, 
the primary of each coil 5a, 5b must pass current each 180.degree. of the 
engine. 
The power dissipated by the Joule effect is: 
P.sub.J1 =i.sub.eff.sbsb.1.sup.2 .times.R.sub.P1 where 
i.sub.eff1 =effective primary winding current 
R.sub.p1 =primary winding resistance. 
At the instant when the primary current is cut off, the stored energy is 
transferred to the secondary and conducted to the plugs 1a...4b through: 
the high-tension coil-distributor leads (7a, 7b), 
the distributors (6a, 6b), and 
the distributor-plug high-tension leads, generally indicated 8a and 8b. 
The energy losses between the secondary winding and the plugs can in this 
case be evaluated as about 50.div.60% of which about 40% is through just 
the distributor (6a, 6b). 
The striking of the arc between the electrodes of the plug occurs when 
there is a sufficiently high voltage difference between them. The high 
tension is generated across the ends of the secondary of the coil at the 
instant when the primary current is cut off and, other conditions being 
equal, the higher the impedance seen by the secondary, the higher will be 
the high-tension available. 
The determinations made by the Applicant on a specific coil in relation to 
the system of FIG. 1 indicates that this tension usually reaches a maximum 
value slightly greater than 20 kV (negative or positive relative to earth) 
within about 60-80 microseconds. 
The system shown schematically in FIG. 2 applies the so-called "lost-spark" 
principle. In this case, four transformers or coils 9, 10, 11, 12 are 
present, each of which supplies a respective pair of plugs associated with 
different cylinders. In the embodiment illustrated here, the coil 9 
supplies the plugs 1a and 4a, the coil 10 the plugs 2b and 3a, the coil 11 
the plugs 2a and 3b, and the coil 12 the remaining plugs 1b and 4b. 
The part relating to the piloting of the primaries of the coils, not shown 
in the drawing, will always be of the electronic type. 
Supposing now that ignition is effected through the coils 9 and 12: a spark 
will be generated both in the plugs of the cylinder 1 and in those of the 
cylinder 4. If it is also supposed that the cylinder 1 is in the 
compression stage and the cylinder 4 is in the exhaust stage, only the 
spark in the plugs 1a, 1b of the cylinder 1 will cause ignition of the 
mixture, while the spark in the plugs of the cylinder 4 will have no 
effect as regards combustion and will thus be "lost". 
The same is true for the coils 10 and 11. 
In the superposed graphs, generally indicated (C) in FIG. 4, which 
illustrate the generation of the ignition signals by the coils 9, 10, 11 
and 12 with reference to the ignition sequence of graph A, it is seen that 
each coil passes a current at every 360.degree. rotation of the engine 
shaft. 
The Joule power dissipated relative to the previous example will be 
P.sub.J2 =P.sub.J1 /2 (assuming that the parameters of the coils are all 
the same). 
The energy losses between the secondary winding and the plug of the 
cylinder under compression are almost equal to those of the system of FIG. 
1. Only 40.div.50% of the energy available to the secondary is transferred 
to the active plug. 
In this case, the determinations made by the Applicant, again with 
reference to the same physical and dimensional parameters as considered 
previously, in the case in which cylinder 1 is under compression and 
cylinder 4 is in exhaust, with a negative discharge polarity on the plugs 
for the cylinder 1 and a positive polarity for the cylinder 4 and with the 
arc-sparking tension on the plug of the cylinder 4 (which is lower than 
that of the cylinder 1 in that it is in exhaust) of about 5 kV, show that 
the tensions across the secondary of the coil may reach a value of the 
order of about 25 kV. 
OBJECT AND SUMMARY OF THE INVENTION 
The present invention thus aims to provide a multiple-spark ignition system 
which is improved over the prior art both as regards a better efficiency 
of the transfer of the ignition energy to the spark plugs and as regards 
the increase in the ignition tension available. 
According to the present invention, this object is achieved by an ignition 
system for internal combustion engines, in which at least one cylinder of 
the engine has an associated plurality of spark plugs, and including at 
least one energizing source which supplies a respective combination of 
spark plugs, characterised in that all the plugs of the respective 
combination are associated with a respective cylinder of the engine.

DETAILED DESCRIPTION OF THE INVENTION 
In FIG. 3, a twin-spark ignition system is generally indicated 20 and 
includes four ignition transformers (or coils) 21, 22, 23 and 24 with 
respective associated supply circuits for the primary, not illustrated 
here since it is of known type. 
As in the case of the system of FIG. 2, each coil 21, 22, 23, 24 supplies a 
pair of plugs both associated with the same cylinder of the engine. 
More precisely, in the embodiment illustrated, the coil 21 supplies the 
plugs 1a and 1b, the coil 22 the plugs 2a and 2b, the coil 23 the plugs 
3a, 3b, and finally the coil 24 the plugs 4a and 4b. 
In this case, as illustrated in the graphs generally indicated (D) in FIG. 
4, the generation of the ignition signal by each coil may occur at each 
720.degree. of rotation of the engine shaft, that is, at a frequency equal 
respectively to a quarter and a half of the frequencies used in the 
prior-art systems illustrated in FIGS. 1 and 2. 
For this type of application, moreover, the coils 21-24 could have a 
smaller weight and bulk than the coils 9-12 used in the lost-spark static 
ignition system of FIG. 2 (considered from now on by way of comparison), 
while ensuring the high tension and energy needed for correct ignition. 
By way of reference, it should be noted that the power dissipated by the 
Joule effect P.sub.J2 (FIG. 2) and P.sub.J3 (FIG. 3) will be respectively: 
P.sub.J2 =R.sub.p2 .times.i.sub.eff2.sup.2 in the system of FIG. 2, and 
P.sub.J3 =R.sub.p3 .times.i.sub.eff3.sup.2 in the system of FIG. 3 
where R.sub.p2 and R.sub.p3 represent the resistances of the primary of the 
coils and i.sub.eff2 and i.sub.eff3 the effective values of the currents 
which pass through them. 
FIG. 4, this will be: 
##EQU1## 
For the same length of the winding of the primary R.sub.p2 =R.sub.p3 and 
hence 
EQU P.sub.J3=1/2 P.sub.J2 
When it is wished to dissipate the same power to the two coils, it will 
then be: 
P.sub.J2=P.sub.J3 and this gives: 
##EQU2## 
Consequently, the resistivity of the material (usually copper) of the 
winding being indicated .rho., the sections of the winding in the two 
cases being indicated S.sub.2 and S.sub.3 respectively, and the length of 
the wire being L, this gives: 
##EQU3## 
and the same length of wire, this gives: 
##EQU4## 
with the consequent possibility of reducing the radius of the winding by a 
factor of 1/2 in the case of the system of FIG. 3. 
The section occupied by the primary winding in the windows of the core may 
thus be reduced by about 30%. 
Similar considerations may be made regarding the loss in the secondary 
winding. The Joule losses are reduced by 50% for these as well and, if 
practical criteria did not militate against falling below a certain 
diameter of wire (about 6/100 mm), the same percentage reduction could 
also be achieved in the dimensioning of the latter as can be achieved for 
the primary. In the system according to the invention, however, the 
advantage remains for the secondary in that the Joule losses not recovered 
by the reduction in the copper result in a smaller rise in the working 
temperature of the winding itself. This is useful in that, in a preferred 
embodiment of the invention, it allows the coil to be mounted directly on 
the two plugs supplied, where the temperature is rather high. 
Consequently, in the hypothetical case in which the section of the iron of 
the coils 21-24 is kept constant, the area of the windows which receive 
the windings could be reduced with a consequent reduction both in the 
volume and weight of the iron necessary, as well as in the weight of 
copper already indicated. 
Whenever it is wished to reduce the iron of the core rather than the 
copper, one could follow the concept of keeping the section of the wire 
constant and increasing the number of turns, reducing the section of the 
magnetic core proportionally. 
The quantity of energy stored in the primary is the same in both the 
systems of FIGS. 2 and 3 taken in comparison with the case in which 
neither the primary inductance nor the current I which passes through it 
are variable. In the system of FIG. 3, with each coil 21-24 mounted 
directly on the two plugs supplied by it, there are no transmission losses 
or such losses are very limited. All the energy present in the secondary 
will thus be transferred and divided in approximately equal parts between 
the plugs of the cylinder in compression. 
It may thus be stated that the quantity of energy available for good 
combustion in the system of the invention is at least equal if not 
actually superior to that of the lost-spark system of FIG. 2. 
Since the two plugs supplied by each coil 21-24 are in the same cylinder in 
compression, the tension needed to initiate the discharge should be equal 
for both. 
In practice, it will be understood that, because of lack of homogeneity in 
the medium, gap differences, or differences in the electrodes, the spark 
will always be struck first on one plug and then on the other. The 
determinations carried out by the Applicant on the system of FIG. 3 in 
conditions substantially like those to which reference has been made above 
for the lost-spark system of FIG. 2 show that, in the case of the system 
according to the invention, which uses the same coils as those in FIG. 2, 
the high tension available at the second plug is 30 kV and is thus much 
greater than that of the lost-spark system. 
To summarise, the system according to the invention provides, relative to 
the system of FIG. 2 (and with even more reason with respect to the system 
of FIG. 1) for: 
coils of smaller bulk and loss; 
elimination of all the high-tension leads and the losses associated 
therewith; 
spark energy and high tension at the plugs which are the same or even 
greater for the same energy stored in the primary and for the same 
coil-turn ratio. 
A further advantage of the invention lies in the fact that, at the instant 
when the first gap is penetrated (that is, at the initiation of the spark 
at the first plug of each pair), a tension pulse is delivered to the 
second plug, improving the operation of the plugs when they are dirty. 
It is in fact known that the dirt on the plug corresponds to a resistor 
placed in parallel with the gap and having a value which is smaller the 
greater the amount of dirt. 
Hence, in the hypothetical case in which one plug is more dirty than the 
other (statistically it is very improbable that the two plugs of each 
cylinder will be equally dirty at the same time) and thus one branch 
resistance is less than the other, for reasons explained above, the 
discharge will occur first on the one in the better state (which has a 
bigger R) and then on the one in the worse state. 
The high tension present at the latter has the impulsive component which 
tends to improve the operation of the plug itself.