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Knock Sensor Jeremy Ellis Fall 2009 Overview Background Sensor Piezoelectric Effect System Circuit Benefits Common Applications Challenges What is knocking? What are Knocks?? - high frequency vibrations caused by detonation, or the premature burning of fuel inside an engine's cylinders 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?? 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) 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 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. 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. Common Applications Piston driven engines, both gasoline and diesel 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 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 Back-Up Charts Sample Voltage versus Frequency Notice peak voltage at about 7 kHZ. Ordering Data Prices range from approximately $50 to $300 depending on model Models vary by automobile manufacturer Aftermarket and OEM models References wikipedia.com partstrain.com autoshop101.com piezocryst.com misterfixit.com
