Abnormal combustion

abnormal combustion

1. Abnormal Combustion in Spark Ignition Engines S K Singh Centre for Energy Studies IIT Delhi A Performance limit due to resources & technology. 20-May-21

2. Recall what we’ve discussed on the board in class room… KNOCK IN SI ENGINES Knock is one phenomenon that is most limiting to engine efficiency. It occurs when the temperature of the unburned gas in the cylinder increases to much, and causes the fuel to self ignite. This will result in an oscillating pressure wave in the combustion chamber. For the driver, this sounds as a number of low thuds . This can be both stressing and agitating, and are therefore dangerous, since the driver will be less focused on his or her surroundings. Knock is also very strenuous for the engine . The high oscillating pressure, can cause damage that will in the end lead to a shorter lifetime for the engine. Really severe knock can damage the engine even after only one or a few self-ignitions . 20-May-21

3. Why Knocking is important aspect to discuss The reason why knock is so interesting is because the optimal operating point of the engine is often in the area where knock will occur. Therefore the ability to predict when knock will occur will make it easier to control the engine towards the optimal operating point . Running an engine at its optimum has a lot of advantages. The most obvious being that the more efficient the engine runs, the more you can get out of it for the same input. 20-May-21

4. Abnormal Combustion in SI Engine Knock is the term used to describe a pinging noise emitted from a SI engine undergoing abnormal combustion. The noise is generated by shock waves produced in the cylinder when unburned gas auto ignites . 20-May-21

5. Engine Damage From Severe Knock Damage to the engine is caused by a combination of high temperature and high pressure. Piston Piston crown Cylinder head gasket Aluminum cylinder head 20-May-21

6. MECHANICAL DAMAGE DUE TO KNOCK 20-May-21

7. Observation window for photography Spark plug Intake valve Exhaust valve Normal cycle Knock cycle

8. Engine parameters that effect occurrence of knock are: i) Compression ratio – at high compression ratios, even before spark ignition, the fuel-air mixture is compressed to a high pressure and temperature which promotes autoignition ii) Engine speed – 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 propensity to knock at high speeds while others don’t. PARAMETERS AFFECTING KNOCK 20-May-21

9. Compression Octane Number Brake Thermal Efficiency Ratio Requirement ( Full Throttle ) 5:1 72 - 6:1 81 25 % 7:1 87 28 % 8:1 92 30 % 9:1 96 32 % 10:1 100 33 % 11:1 104 34 % 12:1 108 35 % 20-May-21

10. Effect of changing the air-fuel ratio Traditionally, the greatest tendency to knock was near 13.5:1 air-fuel ratio, but was very engine specific. Modern engines, with engine management systems, now have their maximum octane requirement near to 14.5:1. For a given engine using gasoline, the relationship between thermal efficiency, air-fuel ratio, and power is complex. Stoichiometric combustion ( air-fuel ratio = 14.7:1 for a typical gasoline ) is neither maximum power - which occurs around air-fuel 12-13:1 (Rich), nor maximum thermal efficiency - which occurs around air-fuel 16-18:1 (Lean ). The air-fuel ratio is controlled at part throttle by a closed loop system using the oxygen sensor in the exhaust. Conventionally, enrichment for maximum power air-fuel ratio is used during full throttle operation to reduce knocking while providing better driveability . 20-May-21

11. iii) Spark timing – maximum compression from the piston advance occurs at TDC, increasing the spark advance makes the end of combustion crank angle approach TDC and thus get higher pressure and temperature in the unburned gas just before burnout. Factors affecting Knock contd.. P,T T Ignition x x End of combustion 20-May-21

12. x x x x x x x X crank angle corresponding to borderline knock Spark advance set to 1% below MBT (Maxm Brake torque) to avoid knock 1% below MBT Knock Mitigation Using Spark Advance 20-May-21

13. Fuel Knock Scale To provide a standard measure of a fuel’s ability to resist knock, a scale has been devised by which fuels are assigned an octane number ON (% age by volume of iso-octane in a mixture of iso-octane and normal heptane, which Exactly matches the knocking intensity of the fuel in a standard engine under a set of standard operating conditions ). 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. The higher the octane number, the higher the resistance to knock. Blends of these two hydrocarbons define the knock resistance of intermediate octane numbers: e.g., a blend of 10% n-heptane and 90% isooctane has an octane number of 90. A fuel’s octane number is determined by measuring what blend of these two hydrocarbons matches the test fuel’s knock resistance .

14. Octane Number Measurement Two methods have been developed to measure ON using a standardized single-cylinder engine developed under the auspices of the Cooperative Fuel Research (CFR) Committee in 1931. The CFR engine is 4-stroke with 3.25” bore and 4.5” stroke, compression ratio can be varied from 3 to 30. Research ON (RON) Motor ON (MON) Inlet temperature ( o C) 52 149 Speed (rpm) 600 900 Spark advance ( o BTC) 13 19-26 (varies with r ) Coolant temperature ( o C) 100 Inlet pressure (atm) 1.0 Humidity (kg water/kg dry air) 0.0036 - 0.0072 Note: In 1931 iso-octane was the most knock resistant HC, now there are fuels that are more knock resistant than isooctane. 20-May-21

15. Testing procedure : Run the CFR engine on the test fuel at both research and motor conditions. Slowly increase the compression ratio until a standard amount of knock occurs as measured by a magnetostriction knock detector. At that compression ratio run the engines on blends of n-hepatane and isooctane. ON is the % by volume of octane in the blend that produces the std. knock The antiknock index which is displayed at the fuel pump is the average of the research and motor octane numbers: Octane Number Measurement Note the motor octane number is always lower because it uses more severe operating conditions: higher inlet temperature and more spark advance. The automobile manufacturer will specify the minimum fuel ON that will resist knock throughout the engine’s operating speed and load range.

16. Knock Characteristics of Various Fuels Formula Name Critical r RON MON CH 4 Methane 12.6 120 120 C 3 H 8 Propane 12.2 112 97 CH 4 O Methanol - 106 92 C 2 H 6 O Ethanol - 107 89 C 8 H 18 Isooctane 7.3 100 100 Blend of HCs Regular gasoline 91 83 n-C 7 H 16 n-heptane 0 0 For fuels with antiknock quality better than octane, the octane number is: where m T is milliliters of tetraethyl lead per U.S. gallon 20-May-21

17. Fuel Additives Chemical additives are used to raise the octane number of gasoline. The most effective antiknock agents are lead alkyls; (i) Tetraethyl lead (TEL), (C 2 H 5 ) 4 Pb was introduced in 1923 (ii) Tetramethyl lead (TML), (CH 3 ) 4 Pb was introduced in 1960 In 1959 a manganese antiknock compound known as MMT (Methylcyclopentadienyl Tricarbonil was introduced to supplement TEL (used in Canada since 1978 and recently in australia to boost octane). About 1970 low-lead and unleaded gasoline were introduced over toxicological concerns with lead alkyls (TEL contains 64% by weight lead). Alcohols such as ethanol and methanol have high knock resistance. Since 1970 another alcohol methyl tertiary butyl ether (MTBE) has been added to gasoline to increase octane number. MTBE is formed by reacting methanol and isobutylene (not used in Canada ). 20-May-21

18. Octane Number Requirement of a Vehicle The actual octane requirement of a vehicle is called the Octane Number Requirement (ONR), and is determined by using series of standard octane fuels that can be blends of iso-octane and normal heptane ( primary reference ), or commercial gasolines. The vehicle is tested under a wide range of conditions and loads, using decreasing octane fuels from each series until trace knock is detected. The conditions that require maximum octane are not consistent, but often are full-throttle acceleration from low starting speeds using the highest gear available. They can even be at constant speed conditions, which are usually performed on chassis dynamometers. The maximum ONR is of most interest, as that usually defines the recommended fuel, however it is recognized that the general public seldom drive as severely as the testers, and so may be satisfied by a lower octane fuel . 20-May-21

19. Engine Management Systems Engine management systems are now an important part of the strategy to reduce automotive pollution. The good news for the consumer is their ability to maintain the efficiency of gasoline combustion, thus improving fuel economy. The bad news is their tendency to hinder tuning for power. A very basic modern engine system could monitor and control:- mass air flow, fuel flow, ignition timing, exhaust oxygen ( lambda oxygen sensor ), knock ( vibration sensor ), EGR, exhaust gas temperature, coolant temperature, and intake air temperature. The knock sensor can be either a nonresonant type installed in the engine block and capable of measuring a wide range of knock vibrations ( 5-15 kHz ). A resonant type that has excellent signal-to-noise ratio between 1000 and 5000 rpm .

20. A modern engine management system can compensate for altitude, ambient air temperature, and fuel octane. The management system will also control cold start settings, and other operational parameters. There is a new requirement that the engine management system also contain an on-board diagnostic function that warns of malfunctions such as engine misfire, exhaust catalyst failure, and evaporative emissions failure. The use of fuels with alcohols such as methanol can confuse the engine management system as they generate more hydrogen which can fool the oxygen sensor. The use of fuel of too low octane can actually result in both a loss of fuel economy and power, as the management system may have to move the engine settings to a less efficient part of the performance map. The system retards the ignition timing until only trace knock is detected, as engine damage from knock is of more consequence than power and fuel economy. 20-May-21