Analysis & Control of Knock in SI Engines

The Knock in SI Engines Knock in gasoline engines is one of the major challenges to design an engine with higher thermal efficiencies. Without knock, an engine can be designed to have a higher compression ratio, giving higher efficiency and power output. The demand to design engines closer to the allowable knock limit with consequential reductions in safety limits is highly appreciated. The phenomenon is characterized by excessively high pressure amplitudes with stochastic occurrence. The phenomenon occurs independent of the mixture formation process in both natural aspirated and turbo charged engines. This phenomenon is a fundamental issue of modern SI engine design methods.

The Reason for the Birth of Knock The end-gas autoignites 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. Therefore knock is a potential problem when the burn time is long.

Fuel : The Resource is the Culprit : 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. The octane number determines whether or not a fuel will knock in a given engine under given operating conditions. By definition, normal heptane (n-C7H16) has an octane value of zero and isooctane (C8H18) 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

The higher the octane number, the higher the resistance to knock. 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. The octane number determines whether or not a fuel will knock in a given engine under given operating conditions. By definition, normal heptane (n-C7H16) has an octane value of zero and isooctane (C8H18) 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.

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 Motor Inlet temperature (oC) 52 149 Speed (rpm) 600 900 Spark advance (oBTC) 13 19-26 (varies with CR) Coolant temperature (oC) 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.

Octane Number Measurement 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 stand. Knock The antiknock index which is displayed at the fuel pump is the average of the research and motor octane numbers: 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.

Knock Characteristics of Various Fuels Formula Name Critical r RON MON CH4 Methane 12.6 120 120 C3H8 Propane 12.2 112 97 CH4O Methanol - 106 92 C2H6O Ethanol - 107 89 C8H18 Isooctane 7.3 100 100 Blend of HCs Regular gasoline 91 83 n-C7H16 n-heptane 0 0 For fuels with antiknock quality better than octane, the octane number is: where mT is milliliters of tetraethyl lead per U.S. gallon

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), (C2H5)4Pb was introduced in 1923 (ii) Tetramethyl lead (TML), (CH3)4Pb was introduced in 1960 In 1959 a manganese antiknock compound known as MMT was introduced to supplement TEL (used in Canada since 1978). 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).

Future Antiknocking Additives The aromatics, toluene and xylene are the most likely candidates for a good solvent to use as an antiknock additive/octane booster. They are already present in gasoline and no adverse effects due to adding more are apparent. Organo Silicon Compounds – Under Study

Octane Number Requirement of a Vehicle The actual octane requirement of a vehicle is called the Octane Number Requirement (ONR). This 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 full-throttle acceleration from low starting speeds using the highest gear available.

Engine Design Parameters Causing the Knock The end-gas temperature and the time available before flame arrival are the two fundamental symptoms that determine whether or not knock will occur. Engine parameters that effect these two fundamental variables are: Compression ratio, spark advance, speed, inlet pressure and temperature, coolant temperature, fuel/air ratio.

Important Engine Variables 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 autoignition. However at high engine speeds there is less heat loss so the unburned gas temperature is higher which promotes autoignition. These are competing effects, some engines show an increase in propensity to knock at high speeds while others don’t.

Effect of Initial Mixture Temperature on Available Combustion Time to Avoid Knocking

Most Useful Engine Parameter to Control Knocking Spark timing – maximum compression from the piston occurs at TC. Increasing the spark advance makes the end of combustion crank angle approach TC and thus get higher pressure and temperature in the unburned gas just before burnout. P,T T Ignition x End of combustion

Knock Mitigation Using Spark Advance Spark advance set to 1% below MBT to avoid knock x X crank angle corresponding to borderline knock 1% below MBT

Auto Sparking Strategy

Effect of Fuel-air Dilution Set spark timing for MBT, leaner mixture needs more spark advance since burn time longer. Along MBT curve as you increase excess air reach partial burn limit (not all cycles result in complete burn) and then ignition limit (misfires start to occur). Ignition limit Partial burn limit Complete burns in all cycles MBT spark timing Partial burn regime

Why Damage due to Knocking There are several theories about what it is that causes the damage on the engine during knocking conditions. The most accepted is that it is caused by heat transfer . When knocking conditions occur, the piston and the walls of the combustion chamber are exposed to a great deal of additional heat which results in overheating of these parts. As a result, the thermal boundary layer at the combustion chamber wall can be destroyed. This causes increased heat transfer which might lead to certain surfaces causing pre-ignition . Substantial knock can damage the engine and is stressful to the driver and is therefore the most important limitation for SI engines. In order to control the knock it is sometimes necessary to regulate away from the most efficient operating point.

Knock Behavior and Conceptual Formulation The knock phenomenon to be investigated is characterized by excessively high pressure amplitudes nearby or direct at the knock limit. Due to this damaging knocking cycles, an efficient engine operation at the knock limit is impossible, due to the risk of severe engine damage. To characterize the knock behavior of an SI engine with wide open throttle (WOT) the control range (CR) of a knock control system will subsequently be introduced as an index. The CR is defined as the advance ignition angle between the knock limit (KL) and the damage limit (DL) of a specific engine operation point

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.

Knock Sensor Knock Sensors generate a voltage when vibration is applied to them utilizing the piezoelectric effect. Generated voltage is proportional to the acceleration . Due to the vibration, a counter weight inside the sensor is applying pressure on the piezo element, this pressure creates an electric charge in the piezo element which is the output signal of the sensor. Tuned to engine knock frequency (typically 6-8kHz).

Location of Knocking Sensor The knock sensor is located on the engine block, cylinder head, or the intake manifold. This is because the function of this sensor is to sense vibrations an engine creates. The PCM uses this signal to alter the ignition timing and prevent detonation. It will compare this information with its preset tables to identify an engine knock or ping. If a ping is sensed it will retard the timing to protect the engine from this damaging pre-ignition.

Knock Sensor Voltage Generation

Knock Sensor Circuit Once signs of detonation are detected (i.e. knocking), the knock sensor sends a voltage signal to the engine management computer which retards the spark timing slightly to avoid detonation.

Knock Control

Benefits Vehicle engines work more efficiently and produce more power when operating near the detonation limit. Although simple, knock sensors allow optimum engine performance and protect the engine from potential damage caused by detonation.