Lecture 5. Abnormal Combustion

Transcription

1 Lecture 5 Abnormal Combustion

2 Abnormal Combustion The Abnormal Combustion:- When the combustion gets deviated from the normal behavior resulting loss of performance or damage to the engine. It is happened when the charge inside the cylinder may be ignite before the flame front reach it. The abnormal combustion: 1. Auto ignition:- During the combustion of charge, the flame front make pressure and temperature of unburned mixture at the end gas very high, so that its value be reach the self ignition condition so it burn. 2. Surface ignition:- if a spot inside the combustion chamber as spark plug, valves, carbon deposit be at high temperature it may be do as igniter source for the charge.

3 Abnormal Combustion Auto ignition:- Surface ignition:-

4 Abnormal Combustion

5 Abnormal Combustion Pre-ignition: It is the combustion of the charge during the compression process before the appearance of the timed spark due to hot spots.

6 Abnormal Combustion Effect of Pre-ignition:- It increase the tendency of detonation in engines. It increases the heat transfer to the cylinder walls because high temperature gases remain in contact with the cylinder for a longer period. The load on crankshaft during compression is abnormally high, this may cause crank failure. Pre-ignition in single cylinder engine will result in a steady reduction in speed and power output.

7 Abnormal Combustion Note:- Pre ignition is not responsible for abnormally high cylinder pressure, but there can be a slight pressure rise above the normal due to the ignition point and therefore, the peak pressure creeping forward to the TDC position where maximum pressure occurs. Over head spark plug and exhaust valves which are the main causes of pre ignition should be carefully avoided in the engines. If pre ignition occurs at the same time as the timed of spark plug fire, combustion will appear as normal. And if the ignition switched off, the engine would continue to operate at the same speed as if it were controlled by the conventional time spark, provide the self ignition temperature continues to occur at the same point. Normal Ignition Pre-Ignition

8 Abnormal Combustion

9 Detonation:- Abnormal Combustion It s a sudden combustion of the end gas ahead of the flame front due to increase in charge pressure and temperature higher than self ignition conditions of the charge causing a pressure wave leading to a vibration of cylinder walls. As the flame propagates away from the spark plug the pressure and temperature of the unburned gas increases. Under certain conditions the end-gas can auto ignite and burn very rapidly producing shock waves. The end-gas auto ignites after a certain induction time which is dictated by the chemical kinetics of the fuel-air mixture. If the flame burns all the fresh gas before auto ignition in the end-gas can occur then knock is avoided.

10 Abnormal Combustion

11 The effect of detonation on engine:- 1. Noise and Roughness. Knocking produces a loud pulsating noise and pressure waves. These waves which vibrates back and forth across the cylinder. The presence. The presence of vibratory motion causes crankshaft vibrations and the engine runs rough. 2. Mechanical Damage. (a)high pressure waves generated during knocking can increase rate of wear rate of parts of combustion chamber. Sever erosion of piston crown ( in a manner similar to that of marine propeller blades by cavitations), cylinder head and pitting of inlet and outlet inlet and outlet valves may result in complete wreckage of the engine. (b) Detonation is very dangerous in engines having high noise level. 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 severe detonation may persist for a long time which may ultimately result in complete wreckage of the piston. Abnormal Combustion

12 Abnormal Combustion 3. Carbon deposits : Detonation results in increased 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 increase heat transfer is due to two reasons: 1. 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. 2. The major reason for increased heat transfer is the scouring away of protective layer of inactive stagnant 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.

13 Abnormal Combustion Knock Damage

14 Factors affecting auto ignition delay or engine detonation Factors Affecting Engine Detonation Abnormal Combustion Design parameters Engine operating conditions Fuel properties Compression ratio Spark advance Octane number Combustion Champer shape Spark plug position and numbers Air-Fuel ratio Engine speed Relative Humidity Volatility Cooling water temperature Inlet temperature and pressure

15 Factors affecting auto ignition delay or engine detonation 1. Fuel properties a. Octane number The octane number determines whether or not a fuel will knock in a given engine under given operating conditions. By definition, Normal Heptane (n-c 7 H 16 ) has an octane value of zero and Isooctane (C 8 H 18 ) has a value of 100. A fuel s octane number is determined by measuring what blend of these two hydrocarbons matches the test fuel s knock resistance. Note: The higher the octane number, the higher the resistance to knock. Alcohols such as ethanol and methanol have high knock resistance. b. Volatility Abnormal Combustion The volatility is the ability of fuel to evaporate at normal condition. When the volatility of fuel increases the burning rate increase and the auto ignition delay increase so that the detonation intensity decreases.

16 Abnormal Combustion 2. Design factors a. Compression ratio P 2 = P 1 (r) k and T 2 = T 1 (r) k-1 Pressure r 1 > r 2 > r 3 r 1 r 1 > r 2 > r 3 r 2 r 3 Ignition Motoring curve TDC TDC TDC CAD

17 b. Effect of combustion chamber shape: Abnormal Combustion The best combustion chamber which burn more than 90% of charge in minimum time at the beginning of combustion. A V V u o V V b o B 1 C 1 A C B D D X L or d D 1

18 Abnormal Combustion c. Spark plug position and numbers The best position of spark plug be in the center of the combustion chamber or offset from the center to the exhaust valve. It can be use more than one spark plug in the same cylinder but the fire order

19 Pressure 3. Operating conditions a. Spark Time: Abnormal Combustion The time of ignition has an effect on the detonation intensity, so that: Spark advance :- increase the pressure and temperature inside cylinder so detonation intensity increases. Spark retard :- decrease the pressure and temperature inside cylinder so detonation intensity decreases. SA 1 SA 2 SA 3 Ignition 3 Ignition 2 Ignition 1 TDC CAD

20 Abnormal Combustion b. Fuel-Air ratio At low engine speeds the flame velocity is slow and thus the burn time is long, this results in more time for auto ignition However at high engine speeds there is less heat loss so the unburned gas temperature is higher which promotes auto ignition These are competing effects, some engines show an increase in its probability to knock at high speeds while others don t. c. Engine speed d. Inlet temperature and pressure Due to the increase of inlet temperature and pressure, the combustion chamber temperature increase and the end gas temperature increase and the detonation intensity increase. P 2 = P 1 (r) k T 2 = T 1 (r) k-1

21 Abnormal Combustion e. Cooling water temperature Due to the increase of the amount of cooling water, the heat transfer from the engine increase so that engine parts temperature decrease and detonation intensity decrease. Due to the decrease of the cooling water temperature, the heat transfer from the engine increase so that engine parts temperature decrease and detonation intensity decrease. f. Relative humidity

22 Knock detection and control strategy Knock sensors can be classified in two broad categories: I. Direct measurements Pressure Transducer Ionic Current technique Direct measurements Remote measurements. Abnormal Combustion

23 Abnormal Combustion II. Remote measurements Today, knock detection is generally performed using an Accelerometer mounted on the engine block. Problems related to Knock sensors Noises originated from mechanical sources Comparing the signal by already designed values

24 In-Cylinder Fluid flow :- Three parameters are used to characterize large-scale in-cylinder fluid motion: swirl, squish, and tumble. Swirl is the rotational flow about the cylinder axis. Swirl is used to: i) Promote rapid combustion in SI engines. ii) Rapidly mix fuel and air in gasoline direct injection engines. iii) Rapidly mix fuel and air in CI engines. The swirl is generated during air induction into the cylinder by either: i) Tangentially directing the flow into the cylinder, or ii) Pre-swirling the incoming flow by the use of helical ports.

25 iii) Many engines have a wedge shape cylinder head cavity or a bowl in the piston where the gas ends up at TC. During the compression process as the piston approaches TC more of the air enters the cavity and the air cylinder moment of inertia decreases and the angular velocity (and thus the swirl) increases. Squish is the radial flow occurring at the end of the compression stroke in which the compressed gases flow into the piston or cylinder head cavity.

26 Squish is the radial flow occurring at the end of the compression stroke in which the compressed gases flow into the piston or cylinder head cavity. As the piston reaches TC the squish motion generates a secondary flow called tumble, where rotation occurs about a circumferential axis near the outer edge of the cavity.