Knock SensorJeremy EllisFall 2009

OverviewBackgroundSensorPiezoelectric EffectSystem CircuitBenefitsCommon ApplicationsChallenges

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 orMECHATRONICS!

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The SensorKnock Sensors generate a voltage when vibration is applied to them utilizing the piezoelectric effectGenerated 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 effectDiscovered by Pierre and JacqueCurie in late 19th centuryPrinciple began use in industrialsensing applications in 1950sPiezoelectric elements generatevoltage when pressure or vibrationis applied to themCeramics and single crystal materials

Knock Sensor CircuitOnce 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.

BenefitsVehicle 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 ApplicationsPiston driven engines, both gasoline and diesel

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

SummaryUnderstand engine knocking and its effectsKnock Sensors based on Piezoelectric effectGenerated voltage is proportional to the accelerationKnock sensors allow optimum engine performance and protect the engine from potential damage caused by detonation

Back-Up Charts

Sample Voltage versus FrequencyNotice peak voltage at about 7 kHZ.

Ordering DataPrices range from approximately $50 to $300 depending on modelModels vary by automobile manufacturerAftermarket and OEM models

Referenceswikipedia.compartstrain.comautoshop101.compiezocryst.commisterfixit.com

Knocking (also called knock, detonation, spark knock or pinging) in spark-ignition internal combustion engines occurs when combustion of the air/fuel mixture in the cylinder starts off correctly in response to ignition by the spark plug, but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front. The fuel-air charge is meant to be ignited by the spark plug only, and at a precise time in the piston's stroke cycle. The peak of the combustion process no longer occurs at the optimum moment for the four-stroke cycle. The shock wave creates the characteristic metallic "pinging" sound, and cylinder pressure increases dramatically. Effects of engine knocking range from inconsequential to completely destructive. Single crystal materials examples - quartz, tourmaline and gallium phosphate Widespread applicationsPiezoelectric effect is found in many applications but knock sensors seem associated with the automotive industry