KNOCKING : REASON AND SOLUTION EXPLAINED !!!

Knocking is a phenomenon of generating unwanted pressure wave which create unpleasant sound and can damage engine wall during combustion in engines. Knocking is largely associated with SI engines. It occurs due to self-ignition of unburned charge into combustion chamber.

When two flame fronts in SI engines collide, they produce an unwanted pressure wave which generates unpleasant sound (knock-knock) and can damage engine wall. Learn more about how Engine knocking works in this article.

Engine Knocking : 

Process:

Knocking is mainly associated with SI engines. It depends on auto ignition quality of fuel. The higher auto ignition temperature causes lower down knocking tendency. To understand whole concept of knocking let’s look out the combustion process.

When the piston reaches at TDC after compression, the spark plug produces spark which ignite the compressed mixture and starts the combustion process. It ignites only those part of mixture witch is in contact with spark plug and generates a main flame front which further ignites the whole mixture. This will generate a high temperature and pressure force inside the cylinder. This burnt part of mixture (combustion products) separate the fresh mixture from spark plug to the other end of the cylinder. As this flame front expands, it compressed the unburned part of the charge. This compression increases the temperature and pressure of unburned part of mixture. If the temperature of this part reaches its auto ignition temperature, it will ignite from other end and a new flame front starts moving towards opposite direction of main flame. When both of these flame fronts collide, it will generate a high pressure wave which produces an unpleasant sound and also damage the cylinder wall.

Factor affecting knocking:

Any factor which tends to decrease auto ignition temperature of charge or tends to ignite the charge except spark plug will affect knocking. Main affecting factor are given below.

Density factor:

The lower dense charge generates low energy and lower rise in temperature thus avoid knocking. If the density of charge decrease, will decreases the knocking tendency. The factors which affect the density of charge are as follows:

Compression Ratio

Increase in compression ratio will increase density of charge thus increase knocking tendency.

Mass of the charge induced:

The mass of charge depends on throttling or supercharging. A reduction in mass of induced charge, reduce the knocking tendency.

Time Factor:

The lower the time available to auto ignition will decrease the knocking tendency. The most affecting time factors are:

Engine Turbulence:

Turbulence reduces the ignition lag or time available for auto ignition. So increase in turbulence will increase the main flame speed and reduce ignition lag, thus reduces knocking tendency.

Spark Plug Location:

To reduce time available for auto ignition, spark plug should locate in order to have minimum flame travel for proper combustion. Generally spark plug located at center to reduce the tendency of knocking.

Engine Size:

As the engine size increase will increase the flame travel thus increases knocking tendency.

Combustion Chamber:

Combustion chamber design play main role in flame travel. The more compact the combustion chamber has minimum flame travel distance thus reduce knocking tendency. Also, the hot spot in combustion chamber should be avoided which can ignite the charge.

Temperature Factor:

Any factor which increases the temperature of inside gases will increase the knocking tendency. These are

Inlet Temperature of Mixture:

More the inlet temperature of mixture, increase tendency of auto ignition thus increase knocking tendency.  

Supercharging:

Supercharging increase both mass and temperature of inside the cylinder thus increases tendency of knocking.

Temperature of Combustion Wall

It will play main role in knocking. The higher the temperature of wall, more chance of auto ignition, thus more chance of knocking.

How to fix knocking:

We have known about knocking phenomenon and factor affecting it. The most methods are used to avoid knock are as follows.

• Spark plug location should select to minimize the flame travel distance.
• By proper cooling of Engine wall.
• Combustion chamber is so design which has less hot spot and higher turbulence.
• Uses higher auto ignition temperature fuels or add additive in gasoline to increase its knocking temperature.
• Higher engine speed will reduce knocking.
• Reduce compression ratio.
• Reduce supercharging to avoid knocking. 

MODES OF HEAT TRANSFER : CONDUCTION,CONVECTION AND RADIATION EXPLAINED !!

Heat, energy that is transferred from one body to another as the result of a difference in temperature.Heat will always be transferred from higher temperature to lower temperature independent of the mode. The energy transferred is measured in Joules (kcal or Btu). The rate of energy transfer, more commonly called heat transfer, is measured in Joules/second  (kcal/hr or Btu/hr). 

Heat is transferred by three primary modes:

• Conduction (Energy transfer in a solid)
• Convection (Energy transfer in a fluid)
• Radiation (Does not need a material to travel through)

CONDUCTION :

Conduction is the transfer of heat between substances that are in direct contact with each other. The better the conductor, the more rapidly heat will be transferred.If one body is at a higher temperature than the other, the motion of the molecules in the hotter body will vibrate the molecules at the point of contact in the cooler body and consequently result in increase in temperature.   The amount of heat transferred by conduction depends upon the temperature difference, the properties of the material involved, the thickness of the material, the surface contact area, and the duration of the transfer. 

Metals are good conductors of heat, while gaseous substance, having low densities or widely spaced molecules, are poor conductors of heat. Poor conductors of heat are usually called insulators. The measure of the ability of a substance to insulate is its thermal resistance. This is commonly referred to as the R-value (RSI in metric).  The R-value is generally the inverse of the thermal conductivity, the ability to conduct heat.

Typical units of measure for conductive heat transfer are:

Per unit area (for a given thickness)

Metric (SI) :  Watt per square meter (W/m)

Overall 

Metric (SI) :  Watt (W)  or kilowatts (kW)

CONVECTION :

When a fluid, such as air or a liquid, is heated and then travels away from the source, it carries the thermal energy along. This type of heat transfer is called convection. The fluid above a hot surface expands, becomes less dense, and rises.There are two types of convection: natural and forced. In case of natural convection, the fluid in contact with or adjacent to a high temperature body is heated by conduction. As it is heated, it expands, becomes less dense and consequently rises. This begins a fluid motion process in which a circulating current of fluid moves past the heated body, continuously transferring heat away from it. In the case of forced convection, the movement of the fluid is forced by a fan, pump or other external means.  A centralized hot air heating system is a good example of forced convection.  

Units of measure for rate of convective heat transfer are:

Metric (SI) : Watt (W) or kilowatts (kW)

RADIATION:

Radiation is a method of heat transfer that does not rely upon any contact between the heat source and the heated object as is the case with conduction and convection. Heat can be transmitted through empty space by thermal radiation often called infrared radiation. This is a type electromagnetic radiation . No mass is exchanged and no medium is required in the process of radiation. Examples of radiation is the heat from the sun, or heat released from the filament of a light bulb.

Typical units of measure for rate of radiant heat transfer

Metric (SI) ——Watt per square meter 

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ENGINE CONTROL UNIT-SIMPLE AND PRECISE INFORMATION !!

ECU-Engine control unit, is a control unit that controls your engine. It determines the amount of fuel, ignition timing and other parameters whether a bike or a car needs to keep running smoothly.

Your bike or car equipped with it has got many sensors that continuously monitors the engine. The ECU reads the input values and decides what is the correct value required at the given condition. The input values can be collected from various sensors like crankshaft position sensor, air temperature sensor, oxygen sensor, Throttle position sensor and gives the data values to the ECU.

So a bike equipped with an ECU don't have a carburetor instead it uses a fuel injector to deliver the fuel. Suddenly if you raise the throttle, the ECU can sense it and can deliver the optimum amount of air/fuel required. 

Suppose If you ride your machine in a hilly terrain on higher altitudes more than 4000 metres, the oxygen level will be marginally less when compared to the sea level. The bike which uses an ECU will be more efficient when compared to a carburetted one, since it optimizes the right amount of fuel to be delivered for the changing conditions in environment. Optimum performance and efficiency - both can be obtained by using an ECU.