Automotive Electronics

The ECU is the on-board computer of the engine. It reads inputs from sensors, calculates the best output from stored maps and controls the actuators.

 SENSORS ---> [ Input   ]
 MAF, TPS,    [ circuits] -> [Microprocessor + memory] -> [Output drivers]
 CTS, O2,     (ADC,        (ROM maps, RAM)             -> ACTUATORS
 crank, knock  filters)                                   injectors, ignition
                                                          coil, idle valve, EGR
              <---- CAN bus / diagnostic ---->

Working

1. Sensors convert physical quantities (temperature, air flow, speed, position) into electrical signals.
2. The ECU changes analogue signals into digital using an A/D converter and compares with stored 3D maps.
3. It calculates injection pulse width, ignition timing and idle speed, and sends signals to power transistors (drivers).
4. Actuators carry out the action. Oxygen sensor feedback gives closed-loop correction.
5. The ECU stores fault codes (DTC) for diagnosis through the OBD port.

Examples

Need for in-vehicle networks

A modern car has 50 to 100 ECUs (engine, ABS, airbag, body, infotainment). Connecting each pair by separate wires is heavy, costly and unreliable. A serial data bus lets all ECUs share data over only two or a few wires, reduces weight, and makes diagnosis easy.

CAN (Controller Area Network)

Developed by Bosch (ISO 11898).

 ECU1    ECU2    ECU3    ECU4
  |       |       |       |
 ==== CAN_H =====================
 ==== CAN_L =====================
 120 ohm terminator at each end

• Two twisted wires, CAN-H and CAN-L, carry a differential signal (dominant bit: H = 3.5 V, L = 1.5 V; recessive: both 2.5 V), so noise is rejected.
• Speed 125 kbit/s to 1 Mbit/s (high-speed CAN for engine, ABS).
• Message frame: start bit, identifier (11 bit, shows content and priority), control, data (0 to 8 bytes), CRC, acknowledge, end.
• Multi-master: any node can send. Arbitration by the identifier is bitwise: the lower number wins, so there is no collision. Error checking is strong.

LIN (Local Interconnect Network)

A cheap, single-wire, master-slave bus for simple, slow devices such as mirrors, window lifts, seat motors and rain sensors.

LIN nodes connect through a gateway to the CAN network.

(a) Engine management system (EMS)

The EMS is the combined electronic control of fuel, ignition and auxiliary systems by one ECU.

 Sensors (air, throttle, temp, crank, O2, knock)
          |
        [ECU]
   |      |        |         |
 Fuel   Ignition  Idle     EGR/EVAP/
 injection timing speed    VVT

Functions

1. Fuel control: calculates the basic injection time from air mass and speed, and corrects for temperature, battery voltage and lambda feedback (closed loop at 14.7:1).
2. Ignition control: sets spark timing from speed, load and knock sensor (retards the spark upon knock).
3. Idle speed control with electronic throttle.
4. Emission control: EGR, purge, secondary air, catalyst monitoring (OBD).
5. Others: fuel cut-off in overrun, cooling fan control, variable valve timing, self-diagnosis and limp-home mode.

(b) ADAS - Advanced Driver Assistance Systems

ADAS uses sensors and software to help the driver and increase safety. Sensors are radar (77 GHz, distance and speed), camera (lane, signs), ultrasonic (parking) and lidar.

Other: blind-spot detection, parking assist, traffic sign recognition, driver drowsiness alert.