Super knock is an enhanced version of the characteristic sound associated with conventional engine knocking. It is distinguished from regular knocking as it is triggered by the pre-ignition. In traditional engine, on the other hand, one of the main causes of knocking is the auto-ignition of petrol before it is entirely consumed in the cylinder of an internal combustion engine.
Although it’s all about probability, modern engines have more chances to get a super knock. It is because newer engines are geared towards increased performance and efficiency. Smaller engines with turbochargers are preferred to reduce vehicle weight, however, this also results in increased load and pressure on the engine component.
More pressure means more heat, which can lead to various problems. One such problem is the low speed pre-ignition (LSPI). The problem normally occurs in engines with low rpm or low engine speed at higher throttle or load conditions. For instance, taking off from a dead stop from a red signal or cruising on the highway in high gear at full throttle. Your car will be at low rpm and high load condition, a recipe for LSPI.
Consequently, it is LSPI that leads to the destructive super knock in engines.
Hot Spots in Engine
Light knocks are not as destructive as super knocks. To understand how each occurs, let us consider a cylinder in an internal combustion engine. During the 4-stroke cycle in the engine, the combustion occurs through ignition from a spark plug. However, sometimes the event of combustion can occur due to hot spots in the cylinder.
Hot spots are basically localised overheated regions that are sufficient enough to cause a combustion on their own.
There are various reasons that can cause hotspots including lack of heat transfer from coolant or excessive combustion temperature over the stock limit. Adjusting air fuel ratio and valve timing is one way to prevent hot spots.
Light Knock vs Super Knock in Engines
The light knock occurs when the piston is already moving downward after the spark plug ignition. The impact of resulting detonation from hotspots is reduced and is not much destructive. In fact you may not even notice light engine knocks while driving.
The super knock in car engines occur when the piston is still moving upward and hotspot combustion is triggered before the spark plug ignition. During the compression stroke, there are two countering forces.
The expansion from hotspot combustion tries to push down the piston while the piston is forced upward by the crankshaft. If the compression stroke is unable to deal with the high pressure from pre-ignition, it will result in damage to the piston or connecting rod.
Avoiding Super Knock in Engines
To know how to avoid it, you need to know what causes engine knock. Super knock in engines usually occurs due to low-speed pre-ignition (LSPI), which is still not fully understood. Finding the exact cause of pre-ignition that leads to super knock might be unknown, but the researchers have found problems that are linked to it.
For instance, car engine oil is one factor in increasing the probability of LSPI. Motor oil can enter the cylinder in various ways including leakage from valves, sprayed oil from squirters or through a crankcase ventilation system.
Motor oil composition also plays an essential role in decreasing the chances of LSPI. Calcium content, which is present in the form of detergents to prevent contaminants from sticking to the engine itself, can increase the occurrence of LPSI. Researchers suggest replacing calcium with magnesium can reduce the probability of LSPI.
The timing of pre-ignition decides the intensity of the engine knock. Earlier pre-ignition has increased intensity in addition to other factors like the presence of unburnt fuel or oil leakage into the cylinder.
LSPI has multiple causes, out of which some are not really understood. One way is to use the right composition of motor oil in modern cars. It seems downsizing engines to meet fuel economy standards in addition to emission requirements is reaching its peak. High-performance small engines tend to run hot, which gives another challenge to the lubrication industry.
Some manufacturers are using car ECU to avoid conditions leading to LSPI. To effectively counter the LSPI, there is a need for next gen motor oils. Regular car maintenance is the key to avoid most problems including engine knock.
However, if your car frequently requires workshop visits, explore your options with used cars for sale in the UAE.
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