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In spark-ignition internal combustion engines, knocking (also knock, detonation, spark knock, pinging or pinking) occurs when combustion of some of the air/fuel mixture in the cylinder does not result from propagation of the flame front ignited by the spark plug, but when one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front.
To detect knock, a piezoelectric knock sensor (basically a microphone) bolted to the engine block responds to unique frequencies caused by engine knock. The sensor generates a small voltage that is sent to the electronic control unit , which processes the signal to determine if, in fact, knock is occurring.
The first, called Combined Combustion System or CCS, is based on the VW Group 2.0-litre diesel engine, but uses homogeneous intake charge. It requires synthetic fuel to achieve maximum benefit. The second is called Gasoline Compression Ignition or GCI; it uses HCCI when cruising and spark ignition when accelerating.
The reduced engine speeds allow more time for autoignition chemistry to complete thus promoting the possibility of pre-ignition and so called "mega-knock". Under these circumstances, there is still significant debate as to the sources of the pre-ignition event. [3] Pre-ignition and engine knock both sharply increase combustion chamber temperatures.
Squish is an effect in internal combustion engines which creates sudden turbulence of the air-fuel mixture as the piston approaches top dead centre (TDC). [ 1 ] [ 2 ] In an engine designed to use the squish effect, at top dead centre the piston crown comes very close (typically less than 1 mm [ 2 ] ) to the cylinder head.
The contact breaker is operated by an engine-driven cam.On an engine with a distributor, the contact breaker can be found beneath the distributor cap.The position of the contact breaker is set so that it opens (and hence generates a spark) at exactly the optimum moment to ignite the fuel/air mixture.
Its purpose was to permit high levels of boost pressure in multiple-stage turbochargers, and thus high power at high altitudes, without causing detonation that would destroy the engine. The high pressures brought high temperatures of inlet air, making engines prone to knock. This use and storage stabilization methods were important military ...
However, the advent of fuel injection and electronic ignition has made most of the reverse-flow head's advantages redundant in a modern engine and as a result the design has lost its popularity. The reverse-flow head still enjoys some popularity among enthusiasts including Leyland Mini, Chrysler Slant-6 , Holden and Ford Inline 6 fans.