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This way performance is kept at its optimum while mostly eliminating the risk of engine damage caused by knock (e.g. when running on low octane fuel). [5] An early example of this is in turbocharged Saab H engines, where a system called Automatic Performance Control was used to reduce boost pressure if it caused the engine to knock. [6]
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.
Diagram of uniflow scavenging. Scavenging is the process of replacing the exhaust gas in a cylinder of an internal combustion engine with the fresh air–fuel mixture (or fresh air, in the case of direct-injection engines) for the next cycle. If scavenging is incomplete, the remaining exhaust gases can cause improper combustion for the next ...
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.
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.
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.
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 ...