Knock Sensor: Piezoelectric Engine Detonation

Working Principle

A knock sensor is a piezoelectric accelerometer tuned to detect the characteristic vibration frequency of engine knock (detonation). Engine knock occurs when the air-fuel mixture auto-ignites ahead of the spark-initiated flame front, creating destructive pressure waves in the combustion chamber at 5–15 kHz. The knock sensor contains a piezoelectric ceramic element (usually PZT) that generates a voltage when mechanically stressed by these vibrations.

The ECU monitors the knock sensor output, comparing the signal amplitude during the “knock window” (a specific crank angle range near TDC) against a threshold. If knock is detected, the ECU retards ignition timing to prevent engine damage.

Electrical Characteristics

Parameter	Typical (Bosch 0 261 231 120)
Type	Resonant piezoelectric accelerometer
Resonant Frequency	~7 kHz (tuned for engine knock band)
Bandwidth	5–15 kHz (knock-relevant range)
Sensitivity	~25 mV/g at resonance
Output	AC voltage (0–5 V peak, proportional to vibration)
Impedance	>1 MΩ (high impedance, capacitive source)
Mounting	Bolt-on (M8) to engine block
Operating Temperature	−40 to +150 °C

Interfacing with an MCU

The knock sensor output is a high-impedance AC signal. Use a bandpass filter (centre frequency ~7 kHz, bandwidth ~5 kHz) followed by a peak-detect and rectify circuit (or an envelope detector) to produce a DC level proportional to knock intensity. Feed this to an ADC. Professional ECUs use a dedicated knock IC (e.g., TI TPIC8101) that handles filtering, integration, and windowed gain.

Calibration

• Background noise floor: Measure the sensor output at various RPM levels with no knock; this sets the adaptive threshold baseline
• Knock threshold: Typically 2–3× the background level at a given RPM; too sensitive = false retard, too insensitive = missed knock
• Torque verification: The mounting bolt must be tightened to the specified torque (typically 20 Nm); incorrect torque changes the sensor’s coupling to the block
• Induced knock test: Advance timing aggressively on a dyno to induce controlled knock and verify the system detects and retards correctly

Code Example

/*
 * Knock Sensor — Basic Detection (Arduino)
 * Wiring: Sensor signal through bandpass filter + envelope
 *         detector → A0 (0-5V DC proportional to knock)
 * NOTE: Real ECU systems use dedicated knock ICs (TPIC8101)
 */
#define KNOCK_PIN A0
#define THRESHOLD 200  /* ADC counts — tune per engine */

int background = 0;

void setup() {
    Serial.begin(115200);  /* Fast baud for real-time */
    Serial.println("Knock Sensor Ready — warming up engine...");

    /* Establish background noise level at idle */
    long sum = 0;
    for (int i = 0; i < 100; i++) {
        sum += analogRead(KNOCK_PIN);
        delay(10);
    }
    background = sum / 100;
    Serial.print("Background ADC: ");
    Serial.println(background);
}

void loop() {
    int raw = analogRead(KNOCK_PIN);
    int delta = raw - background;

    if (delta > THRESHOLD) {
        Serial.print("** KNOCK DETECTED ** Level: ");
        Serial.print(delta);
        Serial.println("  Action: retard timing 2-3 deg");
    }

    /* Update adaptive background (slow filter) */
    background = (background * 99 + raw) / 100;
    delay(1);  /* Fast sampling for knock detection */
}

Real-World Applications

Limitations

• Signal processing complexity: Raw output contains valve train, injector, and accessory noise; sophisticated filtering is mandatory.
• RPM-dependent noise: Background level changes with RPM; requires adaptive thresholding.
• Mounting critical: Incorrect bolt torque or wrong location on the block dramatically reduces sensitivity.
• Single frequency tuning: Resonant sensors are sensitive to a narrow band; knock frequency shifts with cylinder bore size.