Post by MCS on Oct 11, 2008 17:57:47 GMT -5
There are a lot of chainsaw owners who read this board and they each have some level of knowledge on how these high speed two cycle engine operate. As we all try to keep these Homelites running, I thought some technical write ups on the different areas might be helpful. No real deep theory just some high level discussion.
Magneto ignitions with points:
Our chainsaw ignition consists of these parts: 1) the flywheel with the embedded magnets, 2) a coil and core assembly, 3) a spark plug, spark plug wire and cap, 4) a set of points, 5) a condenser, which is technically a capacitor in electronic terms, and 6) the kill switch.
Wire passing through a magnetic field creates voltage. The field strength of the magnets, the size of the wire, and the speed the wire moves through the field, all play a part. In our chainsaw, the rotating magnets in the flywheel provide a moving magnetic field and the stationary coil is the wire. The coil, really a transformer, has two sides – a primary and a secondary. One side of the primary is connected to chassis ground and the other side is connected to the points, the condenser, and the kill switch. The secondary side is connected to chassis ground (same wire as primary) and the other side is connected to the spark plug wire. Every time the magnets pass the coil, voltage is generated. Voltage by itself doesn't do us any good, we need current flow and for current to flow in the primary side, we need a complete path. Current leaving a source has to have a path back to the source. In a flashlight, current leaves the battery, goes through the light bulb and switch and returns to the other end of the battery. So, in our primary circuit, when the points are closed or the the kill switch is in “Off” we have a complete current path. Both sides of the primary are “grounded” so to speak. When the magnets pass the coils and core, current will flow in the primary side of the coil – it is a short pulse but that is all we need and the pulse is coupled to the secondary side of the coil. If the kill switch is in Off or the points never open, we get this pulse each time the magnets pass the coil and core but it isn't strong enough to do anything.
When current flows in a coil of wire a magnetic field builds up around the coil of wire. If the current path is abruptly interrupted, the magnetic field rapidly collapses and creates a large voltage spike. In our ignition system, the opening of the points causes the interruption of current flow in the primary circuit thus creating the voltage spike. Our coil is a step up transformer so this voltage spike of 150 volts is stepped up in the secondary to 15,000 volts which is enough to jump the gap of the spark plug. The condenser, which is connected across the points, or in parallel, provides a low resistance path for current when the points open. When the points open, the condenser is in series with the coil. If the condenser was not in the circuit the high voltage spike would cause current to jump across the point gap and burn the points. The size of the condenser has an electrical relationship to the size of the coil but that is outside the scope of our discussion. We only need to know that if the condenser changes value, the primary voltage spike is reduced which will reduce the secondary “spark.” This is why it is common to change the condenser when the points are changed because we don't know if the condenser has changed value through use and age.
When the saw is running and the kill switch is turned to OFF we loose the abrupt interruption of current flow that the points provided. Voltage is still created in the coil circuit as mentioned above but not high enough to jump the spark plug gap.
The position of the magnets in relationship to the coil core and points is fixed by the flywheel key. The opening of the points in relationship to TDC of the piston is fixed by the position of the point box. The setting of the point gap can have a small effect on engine timing. The magnets have to be passing the coil when the points open.
Points have to be changed because they get dirty or pitted and this creates resistance in the primary circuit which reduces current flow in the primary which reduces the output of the coil on the secondary side – the spark plug side.
Magneto ignitions with points:
Our chainsaw ignition consists of these parts: 1) the flywheel with the embedded magnets, 2) a coil and core assembly, 3) a spark plug, spark plug wire and cap, 4) a set of points, 5) a condenser, which is technically a capacitor in electronic terms, and 6) the kill switch.
Wire passing through a magnetic field creates voltage. The field strength of the magnets, the size of the wire, and the speed the wire moves through the field, all play a part. In our chainsaw, the rotating magnets in the flywheel provide a moving magnetic field and the stationary coil is the wire. The coil, really a transformer, has two sides – a primary and a secondary. One side of the primary is connected to chassis ground and the other side is connected to the points, the condenser, and the kill switch. The secondary side is connected to chassis ground (same wire as primary) and the other side is connected to the spark plug wire. Every time the magnets pass the coil, voltage is generated. Voltage by itself doesn't do us any good, we need current flow and for current to flow in the primary side, we need a complete path. Current leaving a source has to have a path back to the source. In a flashlight, current leaves the battery, goes through the light bulb and switch and returns to the other end of the battery. So, in our primary circuit, when the points are closed or the the kill switch is in “Off” we have a complete current path. Both sides of the primary are “grounded” so to speak. When the magnets pass the coils and core, current will flow in the primary side of the coil – it is a short pulse but that is all we need and the pulse is coupled to the secondary side of the coil. If the kill switch is in Off or the points never open, we get this pulse each time the magnets pass the coil and core but it isn't strong enough to do anything.
When current flows in a coil of wire a magnetic field builds up around the coil of wire. If the current path is abruptly interrupted, the magnetic field rapidly collapses and creates a large voltage spike. In our ignition system, the opening of the points causes the interruption of current flow in the primary circuit thus creating the voltage spike. Our coil is a step up transformer so this voltage spike of 150 volts is stepped up in the secondary to 15,000 volts which is enough to jump the gap of the spark plug. The condenser, which is connected across the points, or in parallel, provides a low resistance path for current when the points open. When the points open, the condenser is in series with the coil. If the condenser was not in the circuit the high voltage spike would cause current to jump across the point gap and burn the points. The size of the condenser has an electrical relationship to the size of the coil but that is outside the scope of our discussion. We only need to know that if the condenser changes value, the primary voltage spike is reduced which will reduce the secondary “spark.” This is why it is common to change the condenser when the points are changed because we don't know if the condenser has changed value through use and age.
When the saw is running and the kill switch is turned to OFF we loose the abrupt interruption of current flow that the points provided. Voltage is still created in the coil circuit as mentioned above but not high enough to jump the spark plug gap.
The position of the magnets in relationship to the coil core and points is fixed by the flywheel key. The opening of the points in relationship to TDC of the piston is fixed by the position of the point box. The setting of the point gap can have a small effect on engine timing. The magnets have to be passing the coil when the points open.
Points have to be changed because they get dirty or pitted and this creates resistance in the primary circuit which reduces current flow in the primary which reduces the output of the coil on the secondary side – the spark plug side.
