How Motorcycle Start/Stop system works

Not too long ago manufacturer started releasing scooters with start/stop system. It saves a lot of fuel by automatically turning off the engine when stopping for 3 seconds. But when you want to go again just twist the throttle and the engine comes back to life again. High Tech huh? Honda calls this system ACG or Alternating Current Generator and Yamaha calls this SMG or smart motor generator. Both of them might have different names but they work the same way.

alternating current generator


Start/stop System vs Conventional System

On Honda scooters, when you press the engine start button the sliding gear engages the gear of the engine, which then rotates and starts the engine. It disengages when you release the engine start button. On Yamaha scooters, when you push the start button the starter rotates and drives the starting gear by the use of a starting fowl or a one way clutch. The conventional starters rely on a starter.

Start/stop system vs conventional starters

The new start/stop system don’t have a starter instead uses stator and flywheel to turn the wheel. On conventional starter system the flywheel and the stator only acts as a generator for the motorcycle lights and charging the battery.

In the motorcycle start/stop system when engine is off, then you press the start button the battery supplies a current to the stator. The stator transforms the current into a magnetic field which thens rotates the flywheel. The flywheel is attached to the crankshaft. This will then get the engine started.

Start/stop system magnetic fields

When engine is running, The flywheel and the stator acts like a generator for charging the battery and supplying power to other bike component. Just like the conventional system.

Please know that a generator has the same component as a motor. The only difference is the direction which the current flows and the magnetic fields.


 How does a start/stop system works when driving

1.  When you turn on the Idling stop switch the ECU goes into the start/stop systemSwing back acg

2. As you are driving around and you stop at an intersection, The speed sensor will detect that your speed is at 0km/hr. The sensor will then relay the information to the ECU and after 3 seconds turns of the engine.

3. As the engine stops the, The piston is rotated backwards to near bottom dead center. This is called the “swing back“. This helps the engine have a running start which will make it start easier and saves a lot of power.

4. When you want to go, you twist the throttle. There is a sensor at the throttle that alerts the ECU to start the engine. The ECU then tells the battery to supply power to the Stator so the flywheel will rotate and start the engine.





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