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Knock Sensor

Knock Sensor. Jeremy Ellis Fall 2009. Overview. Background Sensor Piezoelectric Effect System Circuit Benefits Common Applications Challenges. What is knocking?.

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Knock Sensor

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  1. Knock Sensor Jeremy Ellis Fall 2009

  2. Overview • Background • Sensor • Piezoelectric Effect • System Circuit • Benefits • Common Applications • Challenges

  3. What is knocking? • What are Knocks?? - high frequency vibrations caused by detonation, or the premature burning of fuel inside an engine's cylinders

  4. How can it be prevented? • Why do we care?? - Knocks can rob power from the engine and worst, destroy the engine itself. • How can we prevent knocking?? – lower compression ratio, higher octane fuel, change driving habits or……MECHATRONICS! Need a tow??

  5. The Sensor • Knock Sensors generate a voltage when vibration is applied to them utilizing the piezoelectric effect • Generated voltage is proportional to the acceleration • Due to the vibration, a counter weight inside the sensor is applying pressure on the piezo element, this pressure creates an electric charge in the piezo element which is the output signal of the sensor. • Tuned to engine knock frequency (typically 6-8kHz)

  6. How it works - Piezoelectric effect • Discovered by Pierre and Jacque Curie in late 19th century • Principle began use in industrial sensing applications in 1950’s • Piezoelectric elements generate voltage when pressure or vibration is applied to them • Ceramics and single crystal materials

  7. Knock Sensor Circuit • Once signs of detonation are detected (i.e. knocking), the knock sensor sends a voltage signal to the engine management computer which retards the spark timing slightly to avoid detonation.

  8. Benefits • Vehicle engines work more efficiently and produce more power when operating near the detonation limit. • Although simple, knock sensors allow optimum engine performance and protect the engine from potential damage caused by detonation.

  9. Common Applications • Piston driven engines, both gasoline and diesel

  10. Challenges (knocks) • Piezoelectric elements can be sensitive to more than one physical dimension • Operate in harsh environment (dirt, grease, moisture, road salt, etc.) for automotive applications • Ceramic materials lack long term stability • Sensor can be fooled by things like bad water pump or alternator bearing, or a loose rod bearing

  11. Summary • Understand engine knocking and its effects • Knock Sensors based on Piezoelectric effect • Generated voltage is proportional to the acceleration • Knock sensors allow optimum engine performance and protect the engine from potential damage caused by detonation

  12. Back-Up Charts

  13. Sample Voltage versus Frequency • Notice peak voltage at about 7 kHZ.

  14. Ordering Data • Prices range from approximately $50 to $300 depending on model • Models vary by automobile manufacturer • Aftermarket and OEM models

  15. References • wikipedia.com • partstrain.com • autoshop101.com • piezocryst.com • misterfixit.com

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