What is Engine Knocking - Its Causes, Easy Diagnose & Fixes

 

Have you ever noticed an unwanted sound from your engine? 

The tapping or pinging sound from your engine may be due to engine knocking. When pre-detonation takes place inside the engine, knocking occurs. Pre-detonation is the spontaneous combustion due to the remaining air-fuel mixture of previous cycles. Knocking creates a sound and vibration in the cylinder creating irreparable damage. If knocking is high, it could break the crankshaft due to its vibration. This could lead to catastrophic failure by creating physical pores in the piston crown and cylinder walls. 

 

What is Engine Knock?

The piston inside the cylinder compresses the air-fuel mixture inside the combustion chamber. A spark plug is placed at the top of the cylinder to initiate combustion. In uncontrollable situations, self-ignition takes place without the initiation of the spark plug. Meanwhile, the spark plug ignites the air-fuel mixture from another point. Thus inside the cycling, two points are ignited, and the flames from these two points collide with each other. This detonation creates a vibration and sound in the engine and is called engine knocking.

Knocking was first observed by Harry Richardo in the 20th century. Harry discovered knocking during his research on the failure of the aircraft engine. 

In ideal conditions, the pressure inside the cylinder increases steadily as the piston moves up and decreases as the piston lowers. When knocking occurs there is a rapid change in the pressure and it affects the engine performance and high heat is generated. This high heat can even melt the piston crown.

 

Ways to Prevent Knocking

The prevention methods are different for petrol and diesel engines. Let us check them out in detail: 

 

Prevention of Knocking in Petrol Engines

 

 

The quality of fuel is the primary reason for knocking. The knocking in the spark-ignition engines can be reduced by increasing the octane number of the fuel. Octane is a saturated hydrocarbon with 8 hydrogen molecules. It has anti-knocking properties therefore, additives are added to increase the octane number in fuel. 

In the 1920s, tetraethyl lead or lead was used as an additive to increase the octane number. But lead gets accumulated in the catalytic converters. It leaves a residue in the convertor and prevents contact between the convertor and exhaust flames.  Moreover, the lead had severe health effects on children and adults. Thus, the EPA act (Environment Protection Act) completely stopped the usage of lead in the late 20th century.

 

Prevention of Knocking in Diesel Engines

In diesel engines, the compression takes place without the help of a spark plug. Instead, it uses compression pressure to ignite the air-fuel mixture. Hence it is also called compression ignition engines or CI engines. The knock in the diesel engine occurs due to the residual fuel left in the combustion chamber. This residual fuel ignites at a different time with respect to the inlet air-fuel mixture, thus creating engine knocking. The chances of knocking in diesel engines is increased by the rise in the compression ratio. The higher the compression ratio higher the temperature inside the combustion chamber, and the greater the chances for knocking. 

Like Octane for petrol engines, the Cetane number is the indicator for the quality of diesel. The higher the cetane number, the fuel ignites readily, and there is a lower chance for detonation. 

 

 

Modern Ways to Prevent Engine Knocking

The modern vehicles come with a knock sensor to control the ignition time. A knock sensor is primarily a piezoelectric sensor capable of converting mechanical vibrations into electrical signals. The knock sensor is attached to the engine block or near the intake manifold. It detects vibration and sends signals to the ECU unit. The ECU determines whether the ignition has to be altered or not. Engines with knock sensors are more efficient by reducing fuel consumption and increasing the engine's horsepower. 

Recent advancements like replacing carburettors with fuel injection systems have a positive impact on reducing premature ignition. The fuel injection system injects the fuel at high pressure, resulting in better vaporisation. This has reduced the chance for fuel to remain as a residue in the cylinder, and the ignition time is not delayed. 

Though rapid advancements in the automobile industry have paved the way for hybrid and electric vehicles the dependency on IC engines is still existing. Researchers are constantly working on finding new ways to improve the efficiency of an IC engine. BMW, Ford, Toyota and Volkswagen are proceeding with research to replace turbochargers with superchargers. It is the role of an IC engine analyst to run tests and determine the causes for its failure. The IC engine analyst participates in DFMEA(Design failure mode and effect analysis) cycles and proposes new designs. Skill-Lync is offering the Internal Combustion Engine Analyst course which covers software like Python and Cantera used to analyse engines.  Learn more about IC engines through Skill-Lync courses.