Knocking and Detonation MCQ Quiz in తెలుగు - Objective Question with Answer for Knocking and Detonation - ముఫ్త్ [PDF] డౌన్‌లోడ్ కరెన్

Explanation:

Factors affecting knocking in SI engine:

• Compression ratio:- Higher compression ratio will result in a higher temperature of the charge, the tendency to knock will increase. Therefore, the compression ratio should be low (6 - 10) to avoid knocking.
• Inlet air temperature:- The effect of inlet air temp is the same as the compression ratio. The high inlet temp of the air-fuel mixture will increase knocking.
• Advancing the spark:- Advancing the spark will result in more compression during sparking. The increase in temperature during the sparking will be higher. Hence tendency to knock will increase and so by retarding the spark, the tendency to knock will decrease.
• Coolant water temperature:- By increasing coolant water temperature, less heat will be carried from the engine which will increase the engine temperature. So the knocking will increase.
• Engine Speed: To avoid the knocking in SI engine the engine speed should be high.
• Octane rating: It should be as high as possible
• Self-Ignition Temperature (STI):
  • Self-Ignition Temperature is the lowest temperature at which a Diesel/Petrol will ignite itself without the presence of a spark or flame.
  • The Self ignition Temperature of Diesel is 210°C and that of Petrol varies from 247°C to 280°C.
  • Petrol engines have a compression ratio (6 –10) and they rely on spark plugs for the source of ignition.
  • So, to avoid knocking in the Petrol engine, high Self Ignition Temperature fuels are desirable.

Explanation:

Factors affecting knocking in SI engine:

Compression ratio: 

• A higher compression ratio will result in a higher temperature of the charge, the tendency to knock will increase.

Inlet air temperature:

• The effect of inlet air temperature is the same as the compression ratio. High-inlet temperature of the air-fuel mixture will increase knocking.

Advancing the spark:

• Advancing the spark will result in more compression during sparking. The increase in temperature during the sparking will be higher.
• Hence tendency to knock will increase and so by retarding the spark, the tendency to knock will decrease.

Coolant water temperature:

• By increasing coolant water temperature, less heat will be carried from the engine which will increase the engine temperature. So the knocking will increase.

Explanation:

The cooling system serves three important functions.
1. It removes excess heat from the engine;
2. It maintains the engine operating temperature where it works most efficiently; and finally,
3. It brings the engine up to the right operating temperature as quickly as possible.

Now, let us see Knocking and Pre-Ignition
Knocking:

• When the spark is advanced, burning gas is compressed by the rising piston and therefore temperature and pressure are increased, thus, the tendency to knock increases.
• In spark ignition (SI) engines, the possibility of knocking can be reduced by decreasing compression ratio as a reduction in compression ratio will result in the generation of lower peak temperature.

Pre-Ignition:

• Pre-ignition is a situation in which the fuel-air mixture in a spark ignition engine ignites before the timed spark, because of contact with a hot surface.
• Over-heated spark plugs and exhaust valves are the main causes of pre-ignition.
• Pre-ignition might be the consequence of the spark plug tip getting too hot. 
   

Thus, from the above Definitions and parameters we can conclude that Engines need to cooled to avoid pre-ignition and knocking.

∴ Option(3) is the Correct Answer.

Explanation:

Knocking in CI Engine

Knocking in CI engine occurs because of an ignition lag in the combustion of fuel between the time of injection and the time of actual burning.

As the ignition lag increases, the amount of fuel accumulated in the combustion chamber increases; and when combustion actually takes place, an abnormal amount of energy is suddenly released causing an excessive rate of pressure rise which results in a knock.

Hence a good CI engine fuel should have a short ignition lag so that will ignite more easily.

The CI engine knock can be controlled by reducing the delay period. The delay is reduced by the following :

• High charge temperature
• High fuel temperature
• Good turbulence
• Injection of fuel just before TDC
• Fuel: Lower self-ignition temperature means a lower delay period. Hence High self-ignition temperature increases knock in CI engine. A Higher Cetane number means a lower delay period and smooth engine operation.

In order to decrease the tendency of knock, it is necessary to start the actual burning as early as possible after the injection begins. In other words, it is necessary to decrease the ignition delay and thus decrease the amount of fuel present when the actual burning of the first few droplets starts

The fuel should be sufficiently volatile in the operating range of temperature to produce good mixing and combustion. Low volatile fuel causes knocking.

Explanation:

Case I:

Petrol in Diesel Engine:

When petrol is mistakenly get inserted into a diesel engine, the combustion process becomes abnormal. This happens because diesel fuel is less volatile as compared to petrol. During the compression stroke, the piston before reaching the top dead centre started burning and combustion starts, which leads to knocking.

Additional Information

Case-II:

Diesel in Petrol Engine:

Generally, petrol engine has carburettor system and as diesel is less volatile as compared to petrol, it will not mix with air in the carburettor i.e. the atomised mixture of air and diesel will not happen which will not produce any combustion inside the cylinder.

First understand the knocking in diesel engine. In normal working of diesel engines, initially air is compressed and then the fuel is injected at high pressure. Due to high compression ratio, the fuel ignites and explodes to give the power stroke. Now the requirement of this combustion model is that, the highly atomized fuel should start burning as soon as it is sprayed in the cylinder. But, if the fuel sprayed initially is not able to mix properly due to variety of reasons, it keeps on concentrating in the combustion chamber. This increases the concentration of fuel in the chamber and at some point the large quantity of fuel ignites sending pulses throughout the chamber. This leads to knocking in diesel engine.

Volatility is the ability of a liquid to change to vapor. If a liquid is less volatile then it will not immediately change to vapor in the combustion chamber and thus will increase chances of knocking.

Viscosity is the resistance to flow of the fuel. High viscous fuel will not be easily atomized and mixed. Thus knocking tendency will increase.

Similarly if the self-ignition temperature of fuel is high this will cause delay in the ignition and thus knocking will increase.

Explanation:

Knocking in SI engine:

• Knocking occurs due to the instantaneous burning of a large amount of charge.
• It occurs generally at high speeds when the fuel is not ignited properly by the spark plug and some unburnt charge is still left there in the cylinder.
• This unburned compressed charge may auto-ignite under certain temperature conditions and release energy at a very rapid rate compared to the normal combustion process in the cylinder and produce a knocking sound.
• Over a period of time, this can lead to damage to the engine as well as poor performance of the engine.

Factors affecting knocking:

Compression ratio:

• The pressure and temperature at the end of compression increase with the increase in compression ratio. This in turn increases the maximum pressure during the combustion and creates a tendency to knock.

Increasing inlet temperature:

• It results in an increase in knocking.

Increase in load:

• An increase in load results in an increase in the temperature of the cylinder, thereby rising the temperature and hence knocking tendency increasing at high loads.

Advancing the spark:

• When the spark is advanced the burning gas is compressed by the rising piston and therefore the increase in temperature will be much more and hence the knocking tendency increases. To avoid detonation spark must be retarded.

Flame travel distance:

• Flame travel distance should be small to avoid detonation.

Spark plug location:

• spark plug which is located centrally has a minimum tendency for knocking because the flame travel distance decreases.

Coolant Temperature:

• With the increase of coolant temperature, the time required for the flame to propagate through charge decreases due to higher cylinder temperature.
• Now, as there is very less time for the flame to travel, there will be some un-burnt charge remaining at the end of combustion which leads to an increase in knocking tendency.

Explanation:

• Detonation is a spontaneous combustion of end-gas within the combustion chamber after the ignition.
• When detonation occurs the end gas is under a very high amount of heat and pressure, cause spontaneous combustion, and as a result a pressure a spike within the combustion chamber.
• This can be caused various things including engine timing, too much spark advance, lower octane fuels and compression ratio.
• The flame requires a longer time to travel across the combustion chamber of a larger engine. Therefore, a larger engine has a greater tendency for knocking or detonation than a smaller engine since there is more time for the end gas to auto-ignite.

Concept:

The primary function of the spark plug is to ignite the air-fuel mixture within the combustion chamber under any operating condition.

The key to minimizing knocking is to minimize the factors which bring density difference to the air-fuel mixture. It could be a wise combustion chamber design, lowering the compression ratio or even the wise positioning of the spark plugs.

One of the fixes to engine knocking is to change the positioning of the spark plug.

Since engine knocking is a result of pre-mature burning of fuel-air mixture, the ignition of the mechanism must have correct spacing where the volume would be maximized.

In this case, the spark plug must be placed near the exhaust valve to have a greater travel distance of spark to the air-fuel mixture. This would produce less pressure to the chamber, and since the volume of the chamber is maximized, even burning of the fuel-air mixture can be observed.

Explanation:

Detonation/Knocking:

• Knocking is due to the auto-ignition of the end portion of the unburned charge in the combustion chamber.
   

Increase Engine Speed:
Increasing the engine speed reduces the time available for the end gas to reach autoignition conditions, thus decreasing the tendency to knock (detonate).

Retarding Spark:
Retarding the spark timing delays peak pressures to a point further down the power stroke and thus reduces knocking.

Reducing Pressure in the Inlet Manifold by Throttling:
Reducing the inlet manifold pressure by throttling reduces the mass of the inducted charge, thereby lowering the temperature and pressure of the charge. This reduces the tendency of knocking.

Making Air-Fuel Ratio Neither Too Lean Nor Too Rich:
While changing the air-fuel ratio can impact combustion stability, it alone is not effective in controlling detonation (knock). The most effective air-fuel ratio for preventing detonation is specific and does not fall into the "neither too lean nor too rich" category.

Additional Information

Effects of detonation or knocking:

(1) Noise and roughness:

• Knocking produces a loud noise and pressure waves.
• These waves vibrate across the cylinder.
• The presence of vibratory motion causes crankshaft vibrations and the engine runs rough.

(2) Mechanical damage:

• High-pressure waves generated during knocking can increase the rate of wear of parts of the combustion chamber.
• Severe erosion of piston crown (in a manner similar to that of marine propeller blades by cavitation), cylinder head, and pitting of inlet and outlet valves may result in complete wreckage of the engine.
• Detonation is very dangerous in engines having high noise levels. In small engines, the knocking noise is easily detected and the corrective measures can be taken but in aero-engines it is difficult to detect knocking noise and hence corrective measures cannot be taken.
• Hence high detonation may present for a long time which may ultimately result in complete damage to the piston.

(3) Carbon deposits: ​Detonation increases the number of carbon deposits.

(4) Increase in heat transfer:

• Knocking is accompanied by an increase in the rate of heat transfer to the combustion chamber walls.
• The minor reason is that the maximum temperature in a detonating engine is about 150°C higher than in a non-detonating engine, due to rapid completion of combustion.
• The major reason for increased heat transfer is the scouring away of the protective layer of inactive gas on the cylinder walls due to pressure waves. The inactive layer of gas normally reduces the heat transfer by protecting the combustion and piston crown from direct contact with flame.

(5) Decrease in power output and efficiency: Due to an increase in the rate of heat transfer the power output as well as efficiency of a detonating engine decreases