Working of Start-Stop Systems

Working of Start-Stop Systems

Working of a start stop system:

In automobiles, a start-stop system automatically shuts down and restarts the internal combustion engine to reduce the amount of time the engine spends idling, thereby improving fuel economy and reducing emissions.

In a typical situation the driver releases the accelerator pedal, activates the brake paddle and the vehicle comes to a halt. The driver takes the car out of the gear. The Engine ECU checks the following:

1. Engine is idling and no gear is engaged
2. The wheel speed sensor shows a zero speed
3. Electronic battery sensor shows adequate battery charge for next start operation

When all these conditions are satisfied the engine will wait for some manufacturer specific debounce time and then switches off automatically. The starter pinion engages in the ring gear¹ preparing for the next start. This enables the engine to be started quickly

As soon as the clutch is actuated the starter receives the signal to restart the engine. The engine is started quickly and quietly and is immediately ready for operation again.

The conditions in which the system will go to stop mode may differ with various customers. The above is mentioned to give a general idea

Components of a Start-Stop System

1. Start/Stop sensor motor
2. ECU with Start/Stop functionality
3. Electronic Battery Sensor EBS
4. Cycle resistant battery
5. Pedal Equipment with Sensors
6. Generator

Generally in latest Micro-hybrid systems the separate alternator and starter motor are replaced with a combined motor/generator as it reduce the start time and the noise emission²

In the start/stop system a reinforced starter³ replaces the conventional starter. The start/stop system requires an adapted engine management system, which has additional interfaces to the starter and sensors. Since the start/stop system is an emission-relevant system, it must satisfy the requirements of OBD (onboard diagnosis), i.e., it must be monitored in driving mode and exhaust-gas-relevant faults must be stored in the ECU's fault memory.

Because of the many starting processes it has to manage, the battery must be cycle-proof. It is monitored by a battery sensor, which checks the battery state of charge and signals this to the engine ECU before the internal-combustion engine is automatically switched off. Ancillaries such as the A/C compressor, which are normally driven via the internal-combustion engine and are also required during the standstill phases, must be electrically driven or replaced by other solutions. This also applies to the mild hybrid and the full hybrid, in which the start/stop function can be realized by means of the electric motor.

Notes:

¹Ring gear is a medium carbon steel ring with teeth, it transfer torque from the starter motor pinion to the flywheel to rotate the engine to begin the cycle.

The process of engaging starter pinion during the stop is called as pre-engaging.

²Noise emission is amount of noise emitted from the vehicle. Nosie emission is generally high during starting. Noise emission is one of the major criteria except start time while designing start stop systems.

³Reinforced starter is different from conventional starters to work it with start stop system. In general it has certain differences from conventional starter for better starting time, lesser noise and higher lifetime. Some of the differences in a reinforced starter are

- Electric Motor with Increased size and Performance

- Reduced Gear Ratio

- increased number of teeth in starter pinion

Questions & Answers

What will happen if the battery gets discharged due to battery loads during a long stop?

The behavior of the system is different for various customers. In some cases the engine may start automatically without drivers input. In other cases the system may give indication to driver to start engine by a battery indication lamp.

Both systems have their own advantages and disadvantages. In one hand, if the vehicle starts automatically then the driver may panic while on the other hand if the driver doesn't start after the indication the system may have problems during next start due to low battery.

I want to implement start stop in my car. Is it possible?

Yes, it is possible. But it maybe quite cumbersome, complex and expensive for customers to retrofit the start stop system

For implementing a simple start-stop system you need to make few changes in your car.

E.g. the starter needs to be replaced by an improved starter which can handle more cycles.

You also need few additional sensors like Electronic battery sensor, neutral gear sensor and clutch paddle sensor. Also when you are implementing start stop system you will need electronic circuitry to control Start & Stop of engine. The handling is performed by the engine ECU if the start stop is fitted by OEM. But if your ECU doesn't support it then it can be implemented via an Electronic Control Unit with a small 8-bit microcontroller which communicates with your Engine ECU.

Will start stop system affect my engine performance?

Maybe Yes*, (see disclaimer)

The lubrication systems of internal combustion engines are inherently least effective immediately after the engine starts; Majority of the engine wear occurs at startup, hence the frequent starting and stopping of such systems reduce the lifespan of the engine considerably. Also, the frequent start and stop cycles may reduce the engine's ability to operate at its optimum temperature, thus reducing the engine's efficiency. 

Disclaimer:

I have received information from various reliable websites and system experts. But the questions and answers are based on hypothesis.

*Though I found this information under heading hybrid electric drive, I believe the same applies to normal start stop system. None of the manufacturer mentions it. I consulted with few start stop system experts and they too agree on that there will be some affect on performance. The affect may not be high for a well designed and properly lubricated system.