INTERNAL COMBUSTION ENGINES LECTURER PROF.Dr. DEMIR BAYKA

ABNORMAL COMBUSTION KNOCK (AUTOIGNITION) SURFACE IGNITION

KNOCK (AUTOIGNITION) * KNOCK OCCURS AFTER SPARK IGNITION * KNOCK OCCURS IN UNBURNED MIXTURE

DEFINITION END GAS

COMBUSTION BEGINS IN THE END GAS ZONE r T p T FLAME FRONT

ignition During flame travelincreases

r T -W -Q p T For the REACTANTS

r T -W -Q p T -ve

r T -W -Q p T -ve

r T -W -Q p T -ve +ve ATDC

-ve +ve ATDC 1. Sinceis -ve increasing it will lower Tr 2. Since is -veincreases Tr therefore increasingdecreases Tr

IGNITION LAG WHEN PRELIMINARY REACTIONS BEGIN CHAIN CARRIERS BUILD UP TO CRITICAL VALUE REACTION RATE SUDDENLY INCREASES

IGNITION LAG IS THE TIME THAT PASSES FROM THE INSTANT WHEN UNTIL CONCENTRATION OF CHAIN CARRIERS REACHES THE CRITICAL LEVEL

IGNITION LAG VARIOUS EMPIRICAL RELATIONS

IGNITION LAG VARIOUS EMPIRICAL RELATIONS

FACTORS AFFECTING KNOCK 1. OPERATIONAL FACTORS 2. FUEL FACTORS 3. DESIGN FACTORS

FACTORS AFFECTING KNOCK OPERATIONAL FACTORS 1. IGNITION TIMING 2. ENGINE COOLING 3. EQUIVALANCE RATIO 4. ENGINE SPEED AND TORQUE

FUEL FACTORS 1. PARAFFINS n-paraffinsi-paraffins SINGLE - BONDED OPEN CHAIN STRUCTURE SATURATED

1. PARAFFINS n-paraffinsi-paraffins example OCTANE H-C-C-C-C-C-C-C-C-H H H H H

1. PARAFFINS example ISO-OCTANE -C-C-C-C-C- C C C THE NEW STRUCTURE IS BY THERMAL CRACKING

1. PARAFFINS example ISO-OCTANE -C-C-C-C-C- C C C THIS STRUCTURE IS MORE RESISTANT TO SELF IGNITION

CYCLANES SINGLE BOND RING TYPE STRUCTURE UNSATURATED

OLEFINS ONE DOUBLE BOND OPEN CHAIN STRUCTURE UNSATURATED

ACETYLENES * ONE CARBON CARBON TRIPLE BOND * OPEN CHAIN STRUCTURE * UNSATURATED

AROMATICS * RING TYPE STRUCTURE * BENZENE HAS 3 DOUBLE BONDS C CC C C C

ADDITIVES * TetraEthylLead > TEL * TetraMethylLead > TML * MethylCycloPentaDienyl Manganese Tricarbonyl > MMT * Methyl and Ethyl Alcohols * TertiaryButyl Alcohol > TBA * MethylTertiaryButylEther --> MTBE

OCTANE NUMBER OF GASOLINE OCTANE NUMBER (O.N.) IS THE MEASURE OF A FUEL’S RESISTANCE TO KNOCK 2 STAR (NORMAL) FUEL : 91 O.N. 4 STAR (SUPER) FUEL : 96 O.N.

OCTANE NUMBER OF GASOLINE IS DETERMINED BY COMPARING IT WITH A REFERENCE FUEL. THE REFERENCE FUEL IS PREPARED BY MIXING ISO-OCTANE n-HEPTANE

ISO-OCTANE IS ARBITRARILY ASSIGNED AN OCTANE NUMBER OF 100. n-HEPTANE IS ARBITRARILY ASSIGNED AN OCTANE NUMBER OF 0. ISO-OCTANE HAS BRANCHED STRUCTURE & n-HEPTANE HAS CHAIN STRUCTURE

THE OCTANE NUMBER OF THE REFERENCE FUEL IS ARBITRARILY DEFINED AS THE PERCENTAGE OF ISO-OCTANE (volume base) IN THE MIXTURE

GASOLINE AND REFERENCE FUEL ARE COMPARED ON A SPECIAL ENGINE

KNOCK SENSOR COMPRESSION RATIO CAN BE VARIED DURING OPERATION. KNOCKMETER READINGS FOR REFERENCE FUELS AND GASOLINE ARE COMPARED

DEPENDING ON TEST CONDITIONS RESEARCH O.N.-----> (RON) MOTOR O.N > (MON) CAN BE FOUND. ROAD O.N. = a(RON) + b(MON) + c

DESIGN FACTORS 1. COMPRESSION RATIO 2. COMBUSTION CHAMBER SHAPE AND POSITIONING OF SPARK PLUG AND VALVES 3. COOLING OF ‘END GAS’ ZONES

SUMMARY 1. HIGHER COMPRESSION RATIO ENGINES NEED TO USE HIGHER OCTANE NUMBER FUELS 2. FOR A GIVEN ENGINE AND FUEL IGNITION TIMING IS IMPORTANT 3. AT HIGH LOADS HIGHER ENGINE SPEEDS WILL REDUCE KNOCK TENDENCY

ABNORMAL COMBUSTION KNOCK (AUTOIGNITION) SURFACE IGNITION

SURFACE IGNITION PRE IGNITION CAUSED BY SPARK PLUGS EXHAUST VALVES EDGES OF HEAD CAVITIES OR PISTON BOWL POST IGNITION AFTER THE SPARK