Generally, knocking is identified by weird or particular sound from the engine during acceleration and deceleration. There is a saying that, knock happens due to sudden acceleration. But, knocking is not related with sudden acceleration and deceleration. In low speeds, when you accelerate suddenly without downshifting, you'll feel a jerk all of the sudden where the engine stumbles to power up. This is known as engine snatching not knocking. Knocking can be identified by metallic sound from the engine block while twisting the throttle. There are different terms such as pre ignition, detonation and knocking which confuses us. Pre ignition refers to the event of uncontrolled combustion happening in the combustion chamber before applying the spark. Detonation refers to the event of uncontrolled combustion happening in the combustion chamber after applying the spark. Knocking is the noise that we hear if detonation happens. Knocking is also called as knock or pinging. In modern day vehicles, you can able to see a knock sensor. Generally what manufacturers do is, they'll tune a microphone to hear a specific frequency to detect this knock. I hope, now you're clear about the terms pre ignition, detonation and knocking. When someone talks about knocking, they actually talks about detonation. Now here in this post, let us see something about detonation and knocking.
Knock mechanism
The above diagram represents the normal combustion chamber of a single cylinder 4 stroke petrol engine. Imagine the 3rd stroke of the combustion process where the sparkplug ignites the compressed air fuel mixture is happening. At power stroke, the flame front travels from top to bottom i.e piston crown and from center to walls of the cylinder. The flame front compresses the unburnt fresh charge thereby the temperature increases due to compression and by heat transfer from burnt charge. If the surrounding temperature is not high, the flame front will burn all the remaining fresh charge thus making the normal combustion process.
Imagine, somehow the temperature of the cylinder walls is very high. Due, to this, the temperature of combustion process will be slightly higher. At this scenario, the temperature of the charge inside the chamber will also increase. Because of this, two instant sparks will be created inside the combustion chamber at different places. A new flame front will be developed and travels opposite to the direction of main flame front. When this two flame fronts collide, a huge pressure pulse or vibration will be created and it makes a direct impact on head, cylinder and in piston crown. This collide between two flame fronts can be detected by weird sound from the engine and that's why modern day vehicle comes with knock sensor. The duty of this knock sensor is to inform the ECU to alter the fuel air mixture if it experience any different vibrations.
Factors affecting
Time
Increase in engine speed reduces the chance for pre flame reactions. Because in high speeds, there'll be no time for the new flame to be developed. So, knocking is reduced in higher speeds. But, when you continuously tend to ride in high speeds for long time, the combustion chamber will get more hot and there are chances for the formation of hotspots inside the chamber which again leads to knock. So, it is suggested not to ride in high speeds for long time. And, incorrect spark timing and valve timing also leads to pre or post ignition.
Temperature
Temperature of the cylinder wall plays a important role here. The sparkplug area and the area of exhaust port will be the most hottest regions. To reduce knocking tendency, development of hotspots should be reduced. In some bikes, you can see fins in the exhaust's header pipe to reduce the temperature there. In some Yamaha motorbikes, YRCS system is implemented which cools the area of sparkplug. To reduce the temperature around cylinder walls, manufacturers are moving towards oil and liquid cooling.
Others
Redesign of the combustion chamber, creating more turbulence, adding triple sparkplugs like in DTS-I have been designed and developed to reduce the tendency of knock and to get max power from the engine. High octane petrol have high self igniting temperature thereby reducing the chance for knock.
Effects
* Damage in piston crown
* Heavy noise and vibration
* More fuel consumption
* Release of unburned hydrocarbons
Causes
* Bad quality of fuel
* Bad engine condition
* Improper combustion process
* Improper cooling process
Recognition
* Drop in economy and power
* High engine noise
* More carbon deposits
Prevention
* Using good quality of fuel.
* Give breaks during long ride. High operating temperature leads to knock.
* Make sure whether your cooling system is working perfectly.
* Make sure whether your valve timing and ignition timing is kept properly.
Knock mechanism
The above diagram represents the normal combustion chamber of a single cylinder 4 stroke petrol engine. Imagine the 3rd stroke of the combustion process where the sparkplug ignites the compressed air fuel mixture is happening. At power stroke, the flame front travels from top to bottom i.e piston crown and from center to walls of the cylinder. The flame front compresses the unburnt fresh charge thereby the temperature increases due to compression and by heat transfer from burnt charge. If the surrounding temperature is not high, the flame front will burn all the remaining fresh charge thus making the normal combustion process.
Imagine, somehow the temperature of the cylinder walls is very high. Due, to this, the temperature of combustion process will be slightly higher. At this scenario, the temperature of the charge inside the chamber will also increase. Because of this, two instant sparks will be created inside the combustion chamber at different places. A new flame front will be developed and travels opposite to the direction of main flame front. When this two flame fronts collide, a huge pressure pulse or vibration will be created and it makes a direct impact on head, cylinder and in piston crown. This collide between two flame fronts can be detected by weird sound from the engine and that's why modern day vehicle comes with knock sensor. The duty of this knock sensor is to inform the ECU to alter the fuel air mixture if it experience any different vibrations.
Factors affecting
Time
Increase in engine speed reduces the chance for pre flame reactions. Because in high speeds, there'll be no time for the new flame to be developed. So, knocking is reduced in higher speeds. But, when you continuously tend to ride in high speeds for long time, the combustion chamber will get more hot and there are chances for the formation of hotspots inside the chamber which again leads to knock. So, it is suggested not to ride in high speeds for long time. And, incorrect spark timing and valve timing also leads to pre or post ignition.
Temperature
Temperature of the cylinder wall plays a important role here. The sparkplug area and the area of exhaust port will be the most hottest regions. To reduce knocking tendency, development of hotspots should be reduced. In some bikes, you can see fins in the exhaust's header pipe to reduce the temperature there. In some Yamaha motorbikes, YRCS system is implemented which cools the area of sparkplug. To reduce the temperature around cylinder walls, manufacturers are moving towards oil and liquid cooling.
Others
Redesign of the combustion chamber, creating more turbulence, adding triple sparkplugs like in DTS-I have been designed and developed to reduce the tendency of knock and to get max power from the engine. High octane petrol have high self igniting temperature thereby reducing the chance for knock.
Effects
* Damage in piston crown
* Heavy noise and vibration
* More fuel consumption
* Release of unburned hydrocarbons
Causes
* Bad quality of fuel
* Bad engine condition
* Improper combustion process
* Improper cooling process
Recognition
* Drop in economy and power
* High engine noise
* More carbon deposits
Prevention
* Using good quality of fuel.
* Give breaks during long ride. High operating temperature leads to knock.
* Make sure whether your cooling system is working perfectly.
* Make sure whether your valve timing and ignition timing is kept properly.
Tags
Random talks