Knock Sensor
Jeremy Ellis
Fall 2009
Overview
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Background
Sensor
Piezoelectric Effect
System Circuit
Benefits
Common Applications
Challenges
What is knocking?
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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?
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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
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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
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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
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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
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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
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Piston driven engines,
both gasoline and diesel
Challenges (knocks)
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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
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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
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Notice
peak
voltage at
about 7
kHZ.
Ordering Data
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Prices range from approximately $50 to $300
depending on model
Models vary by automobile manufacturer
Aftermarket and OEM models
References
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wikipedia.com
partstrain.com
autoshop101.com
piezocryst.com
misterfixit.com