market dynamics
Motorcycle without high speed, run fast! Why does the length of the magnet boss affect the maximum speed?
Release date:2022.05.12

Recently maintenance of a pedal motorcycle, it seems to be the imitation of Honda CH125, driving more than 48000 kilometers, according to the owner of the description, his car appeared power decline phenomenon, to a maintenance shop to replace the cylinder, piston and ring and other parts, after the assembly, power without any improvement.

The repairman checked item by item and found that the magneto flywheel had lost magnetism. After replacing the new part, the fault was still found.

Then the electronic igniter, high voltage package and stator assembly produced by the same manufacturer were replaced, but the fault was still not eliminated, and the top speed was only about 40km/h.

There are many reasons for engine power decline, in addition to the checked items, the fuel system is not smooth (including carburetor) is one of the factors that can not be ignored.

Maintenance personnel carefully checked the fuel tank cover vent hole, air filter element, negative pressure switch in the fuel system of the vehicle, cleaned the carburetor, and replaced a new carburetor for comparative test, no fault point was found.

Then, the cylinder compression pressure and the installation of gas timing were checked successively, and the spark plug sleeve and the high pressure damping cap were removed to test the fire. The spark was strong and powerful, and the root of the fault was not found.

Reluctantly, the ignition timing lamp was held to check the ignition timing of the magneto, and it was found that when the engine speed was between 1500 and 7000r/min, the ignition advance Angle should gradually rise from 13±2° to 31±2°. However, when the crankshaft speed had risen to 9000r/min, the ignition advance Angle barely reached about 22°, and the engine speed rose slowly. That's weird.

According to the analysis of magneto ignition principle, the fault should be caused by the excessively long arc of trigger boss.


Replace the magneto flywheel with the same length as the original boss, carefully remount the engine, start the vehicle, warm up the vehicle a little and then drive on the road, slowly increase the throttle, the vehicle acceleration sensitive, the maximum speed reached 85Km/h.

People can not help but ask why the length of the magneto flywheel boss has such a big impact on the ignition advance Angle, and even affect the maximum speed of the motorcycle?

According to the principle of internal combustion engine, the design of motorcycle magneto flywheel trigger boss is the basis of the whole engine ignition advance Angle design, which directly determines the generation of pulse trigger signal and the correct ignition advance quantity at high speed.

This is because the trigger boss on the magneto flywheel has two key features.

The first is the starting position of the flywheel boss relative to the spline center hole (i.e. relative to the Angle of the flywheel spline axis line); The second is the length of the flywheel boss.

The position of the trigger boss is determined by the installation position of the trigger coil mounted on the body. It is calculated repeatedly (10 ~ 15° in advance according to the magnetic pole center of the trigger coil and the Angle relative to the TDC), and the specific position has been determined in the factory manufacturing, which is the mechanical advance of the engine.

This mechanical advance is the mechanical advance Angle of the engine and the origin of the coordinates of the CDI electronic igniter.

After the whole ignition system design is completed, it is necessary to fine-tune the mechanical advance according to the actual operation of the engine.

The next thing to decide is the length of the trigger boss. This length is actually the CDI ignition advance Angle, which is the optimal ignition advance Angle for the engine at high speed.

The length of trigger boss should be determined according to the electronic advance Angle. The process is very complicated. It is necessary to design and modify repeatedly.

Second, is the ignition system pulse trigger signal generation, common external rotor magneto trigger, mainly used to trigger the signal.

Trigger signal is actually only speed signal, it reflects not the pulse voltage, but the frequency of fast and slow.

The relationship between the flywheel boss and the magnetic pole of the trigger coil is to generate voltage signal by changing the size of the air gap in the magnetic circuit of the coil winding.

When the flywheel rotates, it drives the boss to rotate together. When it is just close to the magnetic pole sensor of the trigger coil, the gap decreases from large to small. When turned away from the middle position, the gap from small to large, so that the trigger coil winding induced pulse voltage.

The peak potential of trigger signal increases and the frequency of trigger signal increases with the acceleration of speed.

The clearance between the trigger contact and the flywheel boss is called the trigger clearance. The trigger gap is actually the air gap of the magnetic circuit, the smaller the air gap, the less flux loss, the greater the induced electromotive force in the trigger coil.

However, due to the impact of processing costs and material costs, the gap cannot be too small. It is generally controlled at about 0.7mm (maximum 1.30mm).

Again, the trigger signal output from the trigger is required on the waveform, generally positive after negative, the following introduces the characteristics of the motorcycle electronic igniter and the generation of the trigger pulse waveform.

The ignition characteristics of the electronic igniter of the four-stroke motorcycle engine are generally designed as a second-order polyline, that is, the low-speed section and the high-speed section each correspond to a nearly fixed ignition advance Angle, and the intermediate transition section is connected with a diagonal line (the ignition curve of the four-stroke engine is shown in the figure above and the figure below).

The difference of ignition advance Angle between high and low speed segments is usually determined by the length of trigger bump on the magneto flywheel

Trigger boss technical parameters, reflect the crankshaft Angle position information, igniter is to judge this moment to adjust the ignition advance Angle, that is to say, its position and length directly determine the generation of pulse trigger signal and high speed under the correct ignition advance amount.

For the convenience of expression, the starting point of the flywheel trigger boss (that is, the side closest to the center of the flywheel keyway) is temporarily taken as point A, and the end of the trigger boss length (that is, the side farthest from the center of the flywheel keyway) as point B.

When point A of the flywheel rotor trigger boss just enters the trigger contact (i.e., magnetic pole) area, the magnetic flux through the trigger coil increases gradually, forming the positive half wave rising edge of the trigger pulse.

When point A of the boss coincides with the center of the trigger head (the magnetic pole center, which we call point C), the trigger pulse reaches the positive half-wave peak.

After that, due to the magnetic flux through the trigger coil is gradually reduced, so the formation of positive half wave falling edge, until the trigger contacts all into the boss, positive half wave end.

At this point, the flywheel continues to rotate, because the trigger contact is still within the boss range, the magnetic flux of the trigger coil does not change, so its induced electromotive force is zero.

When point B at the end of the trigger boss length starts to pass through the trigger contact, the magnetic flux through the trigger coil begins to decrease. When point B completely leaves the trigger contact, the magnetic flux in the trigger coil will decrease to zero, which forms the negative half wave of the trigger pulse.

It is not difficult to see that the length of radian occupied by the trigger bump on the flywheel affects the amount of magneto trigger signal input to the integrated CDI to varying degrees, that is, it affects the process of starting Angle and reaching the maximum Angle at what speed, that is, it affects the optimal ignition advance Angle.

After the event, maintenance personnel carefully ask users turned out to repair the car, the malfunction magneto flywheel is borrowed from other engine, think that as long as the flywheel taper on the tip of the cone hole and crankshaft installation, magnetic steel internal and external dimensions are the same, each other can general, not considering the flywheel convex platform length difference influence on the engine ignition advance Angle.

Due to the fault flywheel boss is too long, so that the engine in the range of often used speed of ignition advance Angle is too small, directly affect the normal work of the ignition system, reflected to the motorcycle, although increase the throttle, the highest speed of the engine is not up, the speed of only about 40km/h strange fault.

Find the cause of the fault, replace the magneto flywheel accessories with the same length of the boss, carefully reassemble the disassembled parts, start the engine at a touch, and the motorcycle starts on the road with sensitive acceleration. After the warm-up, step on the accelerator, and the maximum speed reaches 85km/h, confirm that the original car fault is completely eliminated.

Each car design factory, magnetic steel boss and igniter are matched, neither can be replaced at will magnetic steel, also can not be replaced at will igniter.

These theories explain why the magnetic steel flywheel and the ignition of the original car of many models can not be replaced with each other. Some models with large differences in the length of the boss can not be replaced even if the ignition socket is exactly the same.

But with the development and progress of technology, intelligent igniter programmed by single chip microcomputer has now solved the problem that the igniter is not universal. Regardless of the length of the boss, it can be calculated in real time through the internal program to get the best ignition advance Angle.