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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.
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.
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 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.
Cool flames may contribute to engine knock – the undesirable, erratic, and noisy combustion of low-octane fuels in internal combustion engines. [2] In a normal spark-ignition engine, the hot premixed flame front travels smoothly in the combustion chamber from the spark plug, compressing the fuel/air mixture ahead.
In diesel engines in particular, EGR systems come with serious drawbacks, one of which is a reduction in engine longevity. For example, because the EGR system routes exhaust gas directly back into the cylinder intake without any form of filtration, this exhaust gas contains carbon particulates. And, because these tiny particles are abrasive ...
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 term "hot air engine" specifically excludes any engine performing a thermodynamic cycle in which the working fluid undergoes a phase transition, such as the Rankine cycle. Also excluded are conventional internal combustion engines , in which heat is added to the working fluid by combustion of fuel within the working cylinder.