Basics of Automotive Electronics: Starter Motor Control

What is starter motor? Why is it required? How it works?

Starter Motor is a DC motor used to start the vehicle.

Starter motor is replacement to hand-crank in old 4 wheelers or alternate to kick start in 2 wheelers.

Generally referred as electric start, since electricity (battery) is used to start the engine.

Starter motor is connected to vehicle’s engine through the flywheel.

When connected to battery, starter motor will rotate the flywheel, and thereby initiating fuel injection and ignition to engine, and starts the vehicle.

Conventional Starter Circuit without ECU (motor connected to battery via key)

Starter Operation control by ECU (motor is connected only when key is turned and ECU allows crank)

Why controlling the starter motor operation?

Component Safety -Starter motor is a high current DC motor. Current through motor coil wounds goes up to 150Amps during starting (to rotate the heavy flywheel). So starter motor should not be ON for longer duration, to avoid heating and damage to motor.
While engaging starter motor, we should make sure that drivetrain is disengaged from engine. This is to avoid whole mass of the vehicle coming onto starter motor and thereby further increasing the motor load.

Passenger / pedestrian safety – Cranking when vehicle is in gear will cause sudden movement of vehicle. This may result in front collision or may cause damage to road crossing pedestrian.

Vehicle safety - If the key is not authenticated, then start of the vehicle should not be possible.

What are the conditions to make sure while engaging the starter motor to engine?

Driver is cranking : Before engaging the starter, make sure that driver is cranking.
Because, in some conditions like key circuit is shorted to battery, then start request will be present at ECU even without driver actually trying to start. This may cause un-intended vehicle movement, which may cause damage to vehicle in front or person crossing by.

Cranking Duration : Starter motor should not be ON for longer duration (mfr specified max.time)However, during cold temperatures starting the engine will take longer than normal time. So, duration of cranking should be temperature based.

Disengaged Drivetrain : Starter motor should not be ON when drivetrain is engaged to engine.So, either Gearbox should be in Neutral or Clutch should be pressed while starting the vehicle. In AT, gearbox should be in Neutral and Brake should be pressed while starting.

Engine rpm : While engaging the starter motor to engine, make sure that engine is not running already. This will damage the teeth of the starter motor. Also, once the engine speed crosses idle rpm, then starter motor should be disengaged.

Battery Voltage : Engaging the starter motor to battery at battery voltage lower than startable threshold, then it will further discharge the battery without any useful work. So starter motor should not be engaged at low battery (ex. less than 8V-9V in cars)

Other features possible by controlled start :

Apart from above mentioned controls, some of the comfort features are also possible like,

Push Button Start : Keep the key in your pocket only. Just press a button on dashboard to start the vehicle. Button will hold the start request from driver until successful start is done.

Engine Start-Stop : Engine start-stop functionality is possible by having control over starter relay by ECU. When the vehicle is in stop condition in traffics, just by pressing clutch or brake or accelerator pedal ECU will send start request and vehicle will start automatically without driver actually cranking.

Request by other ECUs : Start can be done after recognizing start request from other ECUs like Body Control Module.Features like push button start are normally processed by BCM and will be sent to ECU over CAN.

Immobilizer : If key authentication is failed, then ECU will not allow the start.

Emergency Start : Even though starter is not allowed to engage if the drivetrain in engaged to engine in normal driving conditions, if driver is panicked and trying to start the vehicle, then using specific patterns of crank, brake, clutch and accelerator pedal emergency situation is detected and start is allowed overriding powertrain conditions or starter time durations or battery conditions.

Start request by ECB (Engine Compartment Button) : In commercial vehicles engines are placed under head of the vehicle. While doing engine inspection or repair head is tilted to access engine. A start button is provided in the engine compartment for easier service.

Motor warranty calculation : Starter motors come with warranty for certain number of starts (150,000 to 300,000 starts). Counting the number of successful starts happened is possible connecting through ECU.

Some other possibilities :

Disable via vehicle loan/insurance provider : If the driver has not paid the loan amount or timely insurance premium, then providing companies can disable the start of the vehicle remotely, after few initial warnings/remainders. Of course, vehicle should be fitted with communication system.