---
title: "Sensors, actuators and ECU"
subject: "Automotive Technology"
chapter: 11
key: "semester_5/automotive_technology:sensors-actuators-ecu"
source: https://www.bctengineer.com/q/semester_5/automotive_technology/sensors-actuators-ecu
site: BCT Engineering
---

# Sensors, actuators and ECU

Automotive Technology · Chapter 11: Automotive Electronics

- Asked in: year not given, 6 marks
- Video: [How the Engine Management System (EMS) Works – ECU, Sensors & Actuators Explained](https://www.youtube.com/watch?v=aHzflESaPTA) — TechBehindAutomotive
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/automotive_technology/sensors-actuators-ecu
- Whole chapter: https://www.bctengineer.com/md/semester_5/automotive_technology/chapter-11

## Question

What is an electronic control unit (ECU)? With a block diagram, explain how sensors, ECU and actuators work together. Give examples of four sensors and three actuators used in an engine.

## Short answer

The ECU reads sensors like MAF, throttle position, coolant temperature, and oxygen sensor, processes signals with microprocessor and maps, and drives actuators such as fuel injectors, ignition coil, and idle air control. Closed-loop lambda feedback maintains stoichiometric mixture.

## Answer

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.

```text
 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

| Type | Device | Function |
|---|---|---|
| Sensor | Mass air flow (MAF) | Measures intake air mass |
| Sensor | Throttle position (TPS) | Driver's demand |
| Sensor | Coolant temperature (CTS/NTC) | Warm-up enrichment |
| Sensor | Crankshaft position (Hall/inductive) | Speed and timing |
| Sensor | Oxygen (lambda) | Mixture feedback |
| Actuator | Fuel injector (solenoid) | Delivers fuel |
| Actuator | Ignition coil | Spark |
| Actuator | Idle air control / electronic throttle, EGR valve | Idle speed, NOx control |

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What does the ECU do?
   A. Only stores fault codes
   B. Replaces the battery
   C. Generates the spark directly
   D. Reads sensor inputs, uses stored maps to calculate outputs and controls actuators
   Answer: D. The ECU is the on-board computer that processes inputs and drives actuators. (Key point: ECU role)

2. Why does the ECU use an A/D converter?
   A. To reduce battery voltage
   B. To amplify actuator current
   C. To produce AC
   D. To change analogue sensor signals to digital for processing
   Answer: D. Sensors give analogue signals, which must be digital for the microprocessor. (Key point: ADC)

3. Which sensor gives closed-loop mixture correction?
   A. Knock sensor only
   B. Throttle position
   C. Crankshaft position
   D. Oxygen (lambda) sensor
   Answer: D. Oxygen sensor feedback lets the ECU correct injection to keep the mixture near 14.7:1. (Key point: Closed loop)

4. Which are actuators?
   A. MAF, TPS, CTS
   B. Fuel injector, ignition coil, idle valve/EGR valve
   C. Oxygen and crank sensors
   D. Dashboard lamps only
   Answer: B. Injectors, ignition coils, idle air control and EGR valves carry out the ECU's commands. (Key point: Actuators)

## Related in Automotive Technology

- [Engine management system and ADAS](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Aengine-management-and-adas) — Automotive Technology, chapter 11
- [Phases of combustion in a CI engine](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Acombustion-ci-engine-phases) — Automotive Technology, chapter 3
- [Engine displacement, compression ratio and power](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Aengine-displacement-bmep-numerical) — Automotive Technology, chapter 1
- [Air flow and fuel flow of an SI engine](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Afuel-consumption-air-flow-numerical) — Automotive Technology, chapter 2
- [Two-stroke versus four-stroke engines](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Atwo-stroke-vs-four-stroke) — Automotive Technology, chapter 1

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- [How the Internet and Web work together](https://www.bctengineer.com/question.md?key=semester_7%2Fweb_technology%3Ainternet-and-web-together) — Web Technology, chapter 1
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- [Closed loop control system block diagram explained](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Awhat-is-cs-bd-closed-loop) — Control Systems, chapter 1, asked 4×