Knock Sensor Jeremy Ellis Fall 2009

Overview Background Background Sensor Sensor Piezoelectric Effect Piezoelectric Effect System Circuit System Circuit Benefits Benefits Common Applications Common Applications Challenges Challenges

What is knocking? What are Knocks?? - high frequency vibrations caused by detonation, or the premature burning of fuel inside an engine's cylinders 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. 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! 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 Knock Sensors generate a voltage when vibration is applied to them utilizing the piezoelectric effect Generated voltage is proportional to the acceleration 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. 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) Tuned to engine knock frequency (typically 6-8kHz)

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

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. 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. 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. 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 Piston driven engines, both gasoline and diesel

Challenges (knocks) Piezoelectric elements can be sensitive to more than one physical dimension Piezoelectric elements can be sensitive to more than one physical dimension Operate in harsh environment (dirt, grease, moisture, road salt, etc.) for automotive applications Operate in harsh environment (dirt, grease, moisture, road salt, etc.) for automotive applications Ceramic materials lack long term stability Ceramic materials lack long term stability Sensor can be fooled by things like bad water pump or alternator bearing, or a loose rod bearing 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 Understand engine knocking and its effects Knock Sensors based on Piezoelectric effect Knock Sensors based on Piezoelectric effect Generated voltage is proportional to the acceleration Generated voltage is proportional to the acceleration Knock sensors allow optimum engine performance and protect the engine from potential damage caused by detonation 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. Notice peak voltage at about 7 kHZ.

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

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