Multi-spark ignition is very useful especially in the case
of startings at low temperature and at low rpm range. Basic
idea, is to apply to spark plugs instead of only one spark, a
spark-burst having big energy. In this case, combustion of
air/fuel mixture is much better and the emissions are more
reduced. In addition, through burning improvement, the
consumption of fuel can be reduced.
Why synchronized multi-spark, or what means this?
Special literature abounds in multi-spark EID schematics.
These have in common the fact, as the breaker-points dont control
directly EID, but an oscillator, which will generate a
succession of impulses, and these impulses shall command EID.
This aproach has tw1o major deficiencies:
First spark doesnt match exactly with the moment of breaking
points; so, it has an aleatory delay toward this. This is
equivalent to an aleatory modification of ignition advance,
which will leads to non-uniform run of engine.
At high rpm range, the time betw1een tw1o impulses of
multi-spark device can become comparable with the time betw1een
breaker-points impulses; this shall lead to an unstable
operation of engine, with trepidations and knockings. To avoid
this trouble, is necessary to switch-off the multi-spark
device when rpm of engine exceeds a certain value.
With these in mind, I imagined the device described forwards.
Few calculations elements
The crankshaft velocity of an internal combustion engine is given by following formula:
n = revolution speed of engine crankshaft (rpm)
M = strokes number (2 or 4)
N = number of sparks per second (sparks frequency, in Hz)
B = number of ignition coils
C = cylinder number
For usual four stroke engines, with 4 cylinders and a single ignition coil, the formula becomes :
From where :
In fig.1 is shown an EID equipped with synchronized multi-spark module.
Shaping block has the role to provide fixed length impulses (2
mS) at each breaker-points opening. In this way are eliminated
the false impulses which appear due contacts vibrations.
As shown in drawing, shaped impulse triggers directly the EID
and act as START impulse for multi-spark module. If rpm of engine
is under speed limit, the module will generate a series of
supplementary impulses that, through an OR gate, will generate
supplementary sparks by EID. When speed limit is reached (for
example, 2000 rpm), supplementary impulses stops at module
output, thus no supplementary sparks will be generated.
The module uses for control the shaped impulses from breaker
points. The time betw1een tw1o consecutively impulses depends on
rpm engine and has the values shown in upper table.
From whole T interval, only in the first half of this will be
generated supplementary sparks, after the main spark produced by
the breaker points. This is very important, because generating
sparks outside of half of the interval, the spinning distributor
could apply these sparks to next cylinder, and this could be very
harmful for mechanical parts of engine.
In fig. 3 is shown the block-diagram of the multi-spark module.
At breaker-points opening, the shaping circuit (not shown in
drawing) produces a square impulse having 2 mS. This, named BP,
is applied to EID by an OR gate and generate the main spark.
In multi-spark module, during 2 mS interval, a sequence timer
(a counter with decoded outputs) accomplishes the initialization
of circuits (full operations will be detailed later). When
impulse BP disappears, the gate P2 is opened and the counter N1
receives impulses with 1 mS period, from clock generator. This 8
bits counter measures, in fact, the duration betw1een tw1o
breaker-points impulses. It can count maximum 255 impulses, each
having 1 mS (see the table, this correspond to 120 rpm, far
below the free running speed !). At next BP impulse, P2 close and
the counting stop. The number stored inside N1 is in fact the
time length betw1een tw1o BP impulses.
The sequence timer copy the number stored in N1 to N2, after
this resets counter N1. When BP becomes low level, N1 restarts
the counting. In the same time, the up/down counter N2, starts
counting the impulses having 0.5 mS period, which comes via gate
P1. It counts down, but with double speed. In this way the
counter N2 reach to 0 after T/2 time. The counter N4 and gate P5
makes the impulses for supplementary sparks (2 mS length).
This counter works only if INH signal is at low level. The
fip-flop FF1 marks the interval T/2 in which will be generated
supplementary sparks. It is reseted when N2 reach 0. The gates P3
and P4 unlock the flio-flop and start supplementary sparks.
Also, these gates switch-off the multi-spark function when engine
speed limit is reached (in this case, ~ 2000 rpm). How works
this ? In the upper table we can see at about 2000 rpm, the time
length betw1een tw1o BP impulses is 15 mS.
This means as after a counting cycle, the first 4 bits of
counter N1 will be 111 and next 4, 0000. In this case, P3 gate
output will be at low level, and the same value for P4 output.
The flip-flop FF1 will be not set, and as result, no
supplementary sparks. If the speed engine decrease (time length T
increase), the last 4 bits of N1 will have at least one 1 and
the flip-flop will be set. This allow to appear supplementary
sparks until flip-flop will be reseted by borrow impulse of N2.