Download 2 The ignition system on the MZ202

Radio interference caused by the ignition circuit on
the MZ202 engine.
Author: Oskar Stielau
Email: [email protected]
Last update: 30 July 2007
1 Background
The MZ202 engine is a two cylinder, two stroke built by Compact Radial
(http://www.compactradialengines.com/home.html) in Canada. It produces
60hp and is a popular choice for microlights and single seat gyrocopters. It is
also the standard engine used in the Mosquito ultralight helicopter
(http://www.innovatortech.ca/index.htm).
I built a GyroBee with a MZ202 engine. It’s a really nice engine, always starts
first time and runs very nicely. I prefer it to the 503 I had on my previous gyro.
Since a radio is mandatory where I fly, I bought and installed a handheld
ICOM A6 transceiver on the gyro. Using a good external antenna the radio
works very well, as long as the engine is not running. The first time I tried to
use the radio while the engine was running was a completely different story.
Transmitting was fine, but the radio picked up so much interference from the
engine that reception was hopeless. As soon as the engine stopped running,
radio reception was fine again.
On a MZ202 engine the interference can come from one of 2 systems, namely
the ignition system and the alternator (charging) system. The ignition system
is the obvious source of interference, and was where most of the effort was
focused on.
1.1 Magneto vs CDI
Here’s a brief explanation of the differences between a magneto system and
two types of CDI systems.
Magnetos are electromechanical devices that are driven by the engine. They
are self contained, the input is a mechanical driveshaft and the output is the
spark plug lead. The electrical power for the spark is generated inside the
magneto.
Capacitor discharge ignition (CDI) systems are electronic devices, there is no
mechanical input from the engine. They are, however, not self contained
because they need a source of electrical power.
A DC-CDI system needs a 12VDC power source, which typically comes the
battery. The engine does not need to be turning for the ignition system to be
fully functional.
An AC-CDI system gets its power from a coil in the alternator. This system
does not need a battery, but the engine has to be turning for the coil to
generate electrical power.
For
more
information
on
CDI
systems
http://en.wikipedia.org/wiki/Capacitor_discharge_ignition.
check
out
2 The ignition system on the MZ202
The MZ202 uses a dual CDI (capacitor discharge) ignition system. It consists
of an independent pickup coil, CDI ignition module, and coil for each system.
The system is manufactured by IDM in Italy. (http://www.idmsrl.it/indexEng.asp)
2.1 Pickup coils
The pickup coils are located on the flywheel and have two independent leads.
Each lead has two wires, and these are wired directly to the CDI modules.
2.2 CDI modules
The CDI modules are manufactured by IDM part number 0560538. Physical
dimensions are shown in Figure 1.
Figure 1. Physical dimensions of the CDI module
The modules have two plugs with a total of 6 electrical connections. Figure 2
shows the plugs looking at the CDI module.
Figure 2. Looking at the CDI module plugs.
The 2 pin plug (pins 3 and 6) is used for the pickup input, while the 4 pin plug
(pins 1, 2, 4 and 5) is used for the 12V power supply and the high voltage
output.
The datasheets for the CDI modules show the wiring diagram in Figure 3.
Figure 3. Wiring diagram for the CDI module.
NOTE! The wiring harness delivered with the MZ202 is a bit different to the
above wiring diagram. The actual coil used also has a different configuration
and is explained below.
2.3 Coils
The coils are also made by IDM, part number 025 4047. The coils are not
wired as shown in Figure 3, but have two galvanically isolated windings. The
low voltage winding is driven by the CDI module, while the high voltage coil
drives the two spark plugs which are electrically connected in series via the
cylinder heads and engine block.
Figure 4. Coil and sparkplug wiring.
2.4 Output voltage and current waveforms of the CDI
modules
Voltage and current waveforms of the CDI output are shown in Photos 1 and
2. The peak output voltage of the CDI module is about 300V with a rise time
of about 500ns. The peak output current is 22A, while the RMS current at
6000rpm is about 1.2A.
Photo1: VI waveforms, 10s/div
Photo 2: VI waveforms, 500ns/div
3 The wiring harness on the MZ202
The MZ202 comes with a very nicely made wiring harness which makes
wiring the ignition system a breeze. Simply plug in the pickup coils, CDI
modules and coils, wire in the ON/OFF switches, connect to a 12V battery,
and you’re up and running.
The wiring harness has an interface plug for wiring of the master switch and
ignition ON/OFF switches. The master switch is required to prevent the
battery from going flat when the engine is not running, and needs to be closed
when the engine is running. The ignition ON/OFF switches supply power to
their respective CDI module, and need to be closed for the ignition circuit to
work. A “mag check” is accomplished by opening one of the two ignition
switches at a time, thus removing 12V power to that module. Figure 4 shows
the interface plug detail.
Fig 4. The interface plug.
NOTE! The numbers molded on the plug do not correspond to these
numbers, but are a mirror image.
Figure 5 shows how the wiring harness is made up. In the harness there are
two solder connections, a ground connection joining 9 wires, and a +12V
connection joining 3 wires.
To voltage
regulator
ground
To battery
negative
terminal
To engine
ground
PU1
CDI1
PU2
Ground solder
connection
3
6
2
4
(+)
Coil 1
3
6
2
4
(+)
+12V solder
connection
(-)
CDI2
(-)
Coil 2
Alternator
32516
G G C +B
Voltage
regulator
Interface
plug
Master
CDI1
To battery
positive
terminal
CDI2
Figure 5. The wiring harness as supplied with a MZ202 engine.
A discrepancy with the wiring diagram shown in Figure 3 is clear. Figure 3
shows pin 5 used as ground, while in the wiring harness pin 6 is being used
as ground. A multimeter test indicates that the two pins are internally
connected inside the CDI module.
4 Radio interference measurement
Before even starting it should be made clear that it is impossible to completely
eliminate electrical interference from a running spark ignition engine. Radio
interference in microlight aircraft is not a new issue, and if the topic is raised
there are usually many people who can tell horror stories and attempts to cure
it. It is not limited to a specific engine or ignition brand. It is, however, less of a
problem in metal skinned aircraft for two reasons, enclosing the engine in a
metal skin provides a certain amount of shielding, and metal sheet provides a
very good antenna ground plane which an associated radiation pattern less
susceptible to picking up the RF radiation caused by the engine.
Although the source of RF interference cannot be eliminated, it is, however,
possible to reduce it to such low levels that it is not noticeable, and in some
cases not even measurable. If the interference is not noticeable, it is generally
not considered to be a problem.
The level of radio interference is subjective, and different people will have
different views of what good, bad or terrible radio interference means. When
trying to reduce the level of interference some form of quantification
(measurement) of the level of interference is useful to determine how effective
each measure is in reducing radio interference.
Measuring the level of radio interference can be done in various ways, some
crude and others highly scientific. In this investigation two methods were
used, the one giving an easily and accurately measurable result, and the
other more subjective. The two methods are described below:
4.1 Squelch level method.
The ICOM A6 handheld transceiver used has an adjustable squelch level that
can be adjusted from 0 (most sensitive reception) to 24 (least sensitive
reception).
For the squelch method an unused radio frequency is selected. With the
engine not running the squelch level can be turned all the way down without a
signal being received, typically on the A6 the squelch level could be turned
down to a value of 1.
With the engine running, the squelch level needs to be turned up to prevent
the radio from picking up engine interference. The level at which the squelch
needs to be set at so that the radio no longer picks up the interference gives
an accurate measure of the level of radio interference (a value of 0 results in
continuous reception).
The squelch level depends on various parameters, and these always need to
be the same for all measurements. Some parameters are:
 Antenna gain: The squelch level is strongly dependant on antenna
gain, the worse the antenna the lower the squelch level can be set.
For the tests no attempt was made to decrease the antenna gain,
the antenna used was designed for good performance throughout
the airband.
 Engine speed: All measurements were taken at idle. It was found
that the squelch level needed to be turned up by 1 or 2 at full
throttle.
4.2 Ignition kill method.
This method is more subjective and consisted of selecting the ATIS frequency
of a nearby airport (about 30 miles away). With the engine running the
transmission is monitored and one of the two ignition systems switched off.
The change in radio interference gives a good indication of how much radio
interference is generated by that particular ignition system. If a difference
cannot be detected anymore when one ignition is switched off the level of
interference from that ignition system can be considered to be very low.
Finally if the level of radio noise does not change when the engine is switched
off, the interference can no longer be considered a problem.
To give an indication of how bad the radio interference was it should be noted
that with the engine not running the squelch could be set at 1 without the radio
picking up a signal. With the engine running and the squelch set at the
maximum value of 24 the radio would still occasionally pick up interference
from the engine.
5 Measures taken to reduce interference
A systematic approach was taken to try to reduce the radio interference. The
ultimate aim was to be able to reduce the squelch level with the engine
running to 5 or less.
After each step the level of interference was measured using the squelch level
method.
The steps taken are described below:
Step 1: Shielding of coils and HV spark plug cables.
The coils were shielded using adhesive copper tape from RST Engineering
(http://www.rst-engr.com/rst/catalog/airplane_antenna.html). Photo 3 shows
the shielded coils.
Photo 3. Coils shielded using adhesive copper tape.
The HV leads were also screened using ignition shielding braid from Aircraft
Spruce (http://www.aircraftspruce.com/catalog/elpages/shieldbraid.php). The
shielded leads are shown in Photo 4.
Photo 4. Shielded HV coil leads.
Result of step 1: Squelch level dropped from 24 to 21.
Step 2: Shielding the wires between the wiring harness and the ignition
ON/OFF switches.
Photo 5 shows the braid used (the same as that used for the spark plug
leads). Only the wires between the interface plug and the ignition switches
were shielded, the harness was left alone.
Photo 5. Ignition switch wire shielding.
Result of step 2: Squelch level remained unchanged at 21.
Step 3: Shielding the wires between the CDI modules and the coils.
Again the same braid as used for the spark plugs was used. The wires were
shielded from the coil to where they entered the big join in the wiring harness,
shielding all the way to the CDI module can only be done if the wiring harness
is cut apart.
Result of step 3: Squelch level dropped from 21 to 19. Grounding the shield at
one or both ends did not make a difference.
Step 4: Shielding the CDI modules with copper tape.
The same copper tape used to shield the coils was used to shield the CDI
modules.
Result of step 4: Squelch level dropped from 19 to 17.
Step 5: Removing the original wiring harness and rewiring completely with
short wire runs and twisted wires where possible.
By dismantling the original harness completely, it was possible to tidy up the
wiring substantially. At the same time the ground pins on the CDI modules
were changed as follows:
In the original wiring harness pin 6 of the CDI module is used as the only
ground connection. The large (22A) current pulse to the coil flows through this
pin and also through the soldered ground connection in the harness.
When rewiring pin 5 was used to exclusively conduct the coil current, while
pin 6 was used as the ground for the 12V supply. This simplifies the wiring
between the CDI module and the coil, as two dedicated pins of the CDI
module are now used to drive the coil, and the full length of the wires between
the CDI modules and the coils can be shielded as shown in Photo 6.
Photo 6. Shielded wires from CDI module to coil.
Figure 5 shows how the system was wired.
Figure 5. Final wiring configuration.
Result of step 5: Squelch level dropped from 17 to 15.
Step 6: Placing CDI modules in an aluminium box.
An aluminium box was made and the two CDI mounted inside. It was hoped
that this would improve the shielding provided by the copper tape in step 4.
Photo 7 shows the box with the CDI modules inside, some of the copper
shielding from step 4 is visible.
Photo 7. Aluminium box with CDI modules inside.
Result of step 6: Squelch level remained unchanged at 15.
Step 7: Changing the wiring between the CDI modules and the coils from
shielded wires to co-axial cable.
The shielded wires between the CDI modules and coils were replaced using
shielded
MIL-C-27500
wire
from
Aircraft
Spruce
(http://www.aircraftspruce.com/catalog/elpages/shieldwire.php). The outer
shield was used as the ground wire (pin 5 of the CDI module), while the inside
conductor was used as the high voltage wire (pin 4 of the CDI module). Photo
8 shows the two shielded cables running up to the coils.
Photo 8. CDI module to coil wiring.
Result of step 7: Squelch level dropped from 15 to 12.
During the first 6 steps there was always a noticeable difference when one of
the two ignition systems was switched off. The difference between the two
ignition system was, however, seldom the same (one ignition system would
always generate more noise than the other).
After step 7 there was no longer a noticeable difference between the two
ignition systems. With one ignition system switched off the squelch level with
the engine running could be decreased to 8, but there was no noticeable
increase in interference when the ignition was switched back on again.
When switching off the engine, however, there was still a noticeable change
in radio noise. Radio performance is not “perfect” yet, but can now be
classified as somewhere between “good” and “very good”.
There was no single measure that eliminated or even caused a huge
reduction in radio interference. Most steps resulted in small but measurable
reductions in interference, and the end result was the cumulative effect of all
the small improvements.
6 The Ducati CDI system used by Rotax
Newer Rotax engines also use CDI ignition systems, their systems
manufactured by Ducati. Figure 6 shows a drawing of a typical 2 stroke, 2
cylinder, dual ignition Rotax system. The four stroke Rotax (912 family) uses a
different system which will be briefly touched on a bit later.
Figure 8. The Ducati CDI ignition system used on 2 stroke Rotax engines.
There are two main differences between the IDM system and the Ducati
system as described below.
6.1 Power supply
The Ducati system derives its power directly from the alternator, while the IDM
system derives its power from the 12V battery. The Ducati system can
operate without a 12V battery, but not the IDM system.
On a side note, this is the reason why a MZ202 starts quicker than a Rotax
503, I think most MZ202 owners are impressed at how quickly their engine
starts.
With the IDM system a lot more care needs to be taken when designing and
wiring up the 12V system, as a loss of 12V power to the ignition modules will
cause the engine to stop.
A loss of 12V power could be caused by any one of the following scenarios:
6.1.1 Broken wire
A broken wire anywhere in the ignition 12V supply or ground return loop will
cause the engine to stop. The broken wire could be at the battery, master
switch, kill switches, ignition modules, or anywhere in-between. This loop is
the most critical 12V supply loop, and special attention should be paid to it.
6.1.2 Short to ground
A short to ground in the ignition circuit will cause a voltage drop dependant on
the severity of the short circuit. If the short is severe the engine will stop.
If the ignition circuit is fused and the fuse blows, the engine will stop and it will
be impossible to restart the engine before the fuse is reset/replaced. The
ignition circuit is not fused it must be wired with extreme care to prevent any
chance of the 12V wires shorting to ground.
All circuits not related to the ignition circuit must, however, be fused. This is to
prevent a short in a non critical circuit from pulling down the 12V supply and
stopping the engine.
6.1.3 Master switch accidentally opened
The Master switch should be mounted in such a manner that it is virtually
impossible to accidentally switch it off in flight.
6.1.4 Master switch failing open circuit
If the master switch fails mechanically (due to vibration) or electrically (due to
overloading). If the standard MZ202 wiring harness is used the switch must be
rated to carry the maximum charging current of about 12A.
6.1.5 Battery going flat
If the charging system stops working, it is possible that the battery could be
charged enough to start the engine, but then loose charge and cause the
engine to stop in flight. A low voltage warning system would detect this fault
and give enough time to make a safe landing.
The charging system on the MZ202 engine can deliver about 13A. The
ignition system requires about 1A. The total constant electrical load of all
other loads on the aircraft should thus never exceed about 10A (around
120W).
6.2 Kill switches
With the Ducati system the ignition is killed by shorting a lead to ground, while
for the IDM system the ignition is killed by opening the 12V supply switch.
6.3 Integrated coil
The Ducati system combines the CDI module and coil into one unit. There are
thus no external wires that run between the CDI module and the coil. It has
been shown that this link is a dominant source of RF interference, and the
Ducati system in this respect can be expected to generate less RFI. It is, of
course, possible to mount the IDM CDI modules and the coils next to each
other and have very short leads between them. This is in effect is what Ducati
have done.
6.4 Rotax 912 system
The Rotax 912 ignition system is similar to the IDM system, with separate CDI
modules and coils. The modules are, however, mounted very close to the
coils with the wires between them not more than about 7cm (~3 inches) long.
7 My opinion…
These are my thoughts on the issue, and I’m open to discussion.
7.1 Source of RF interference
It should be realized that in this whole investigation no changes were made to
the engine. Also no changes were made to the CDI modules or coils. The
physical location of the coils and CDI modules was not changed. All that was
changed was the wiring between the various components, and the addition of
external shielding in the form of either copper braid, copper tape or aluminium
sheet.
One can thus not say that a specific engine, or specific ignition system
generates more or less radio interference than another. It is how the different
components are connected to each other that affects the level of interference.
The wires between the CDI modules and the coils seem to be the main
culprits when it comes to RF interference. The measured voltage and current
waveforms confirm this, with a very high rate of rise in the voltage, and a large
associated current peak. Using a shielded wiring structure with the shield
used as the ground conductor greatly reduces both electric and magnetic field
strengths around these wires.
7.2 My recommendations
The MZ202 engine runs beautifully with the supplied ignition system and
wiring harness. If I could fly without a radio I would use the wiring harness.
If I was to build another flying machine using an MZ202 and knew that a radio
was mandatory to fly (like my Mosquito helicopter that’s on its way) I will do
the following:
1. Throw away the wiring harness supplied with the MZ202 engine.
2. Make a aluminium box for the CDI modules, and mount them inside as
close as practically possible to the coils.
3. Wire the coils to the CDI modules using co-axial cable.
4. Complete the wiring as per Fig 5.
5. Do not fuse the ignition system 12V supply.
6. Fuse all other electrical loads.
Icom A6/A24 headset transmit issue
The ICOM A6/A24 handheld transceiver works really well as a handheld, both
transmitting and receiving.
It is possible to plug in a headset using the ICOM OPC-499 headset adaptor,
or alternatively 3.5mm (for speaker) and 2.5mm (for microphone and PTT)
plugs. When doing this there are no known problems when receiving, but
there is a known feedback issue when transmitting. This feedback issue is not
well documented; here are a few links that refer to it.
http://www.aerialpursuits.com/comms/commsfaq.htm#a6squeals
http://www.pprune.org/forums/showthread.php?t=200612
http://forums.flyer.co.uk/viewtopic.php?t=18108&postdays=0&postorder=asc
I did some experimentation using a headset on an ICOM A6 and found the
following:
Using the supplied rubber antenna (FA-B02AR) there was always feedback
when transmitting. It was generally so bad that transmission was not possible.
Using an external antenna, the feedback was dependant on the SWR of the
antenna. With a good antenna (SWR < 1.5) there is no feedback. As the
SWR gets worse, feedback goes from barely audible to so bad that the radio
cannot be used for transmitting.