Means & Methods of Homogeneous Charge Combustion P M V Subbarao Professor Mechanical Engineering Department A Sudden Combustion, Yet Needs A Care & takes some time ….

Mixture Burn Time vs Engine Speed The time for an overall burn is: If we take a typical value of 50 o crank angle for the overall burn N (rpm) t 90% (ms) Standard car at idle Standard car at max power4, Formula car at max power 19, Note: To achieve such high engine speeds a formula car engine has a very short stroke and large bore.

Onset of A Successful Combustion in Homogeneous Charge The initiation of the spark and its characteristics are important. The most crucial part of the combustion process is that of the early stages of flame propagation. A parametric study of the phenomena controlling the initial behaviour of spark-ignited flames confirmed that combustion starts as self-ignition. This self ignition occurs in the volume of very hot gases (spark kernel) behind the expanding, spark-created, shock wave. Spark-ignited flames pass through a non-steady propagation period before reaching a steady speed. This transient period is relatively important, compared to the total time available for combustion, in an engine cycle.

Phases of Combustion in Homogeneous SI Engine Crank Angle,  Ignition Start of Combustion End of Combustion Statistical analysis of groups of pressure distributions demonstrated that events early in the development of the flame kernel largely dictate the subsequent rate of combustion and pressure development

Ignition of the mixture in SI engines WHEN DOES THE MIXTURE GETS IGNITED ?!?!?! Spark energy must be higher than the minimum energy of ignition of the mixture, Distance between electrodes is larger than the extinguishing distance for a given mixture, Local gradient of velocity is smaller than the critical for a given mixture. Comment: In the cylinder the mixture is moving, which makes ignition difficult.

Spark Ignition The electrical discharge produced between spark plug electrodes starts the combustion process A high-temperature plasma kernel created by the spark develops into a self-sustaining and propagating flame front A spark is caused by applying a sufficiently high voltage between two electrodes separated by explosive gas in the gap. When the spark energy is increased, that is, when the voltage across the electrodes is raised above a certain critical value (below which a spark may not even occur), a threshold energy is eventually obtained at which the spark ignites the charge This minimum ignition energy is a function of properties of the explosive gas and the configuration of the spark gap.

Spark ignition of the mixture

Voltage & Current Waveforms during Ignition

The ignition process and initial flame propagation The spark that initiates combustion may be considered in three phases. Prebreakdown : Before the discharge occurs, the mixture in the cylinder is a perfect insulator. As the spark pulse occurs, the potential difference across the plug gap increases rapidly (typically kV/ms) This causes electrons in the gap to accelerate towards the anode. With a sufficiently high electric field, the accelerated electrons may ionize the molecules they collide with. This leads to the second phase avalanche breakdown.

Break Down Phase The breakdown causes a very rapid temperature and pressure increase. Temperatures of K give rise to pressures of several hundred bars. These high pressures cause an intense shock wave as the spark channel expands at supersonic speed. Expansion of the spark channel allows the conversion of potential energy to thermal energy, and facilitates cooling of the plasma. Prolonged high currents lead to thermionic emission from hot spots on the electrodes and the breakdown phase ends as the arc phase begins.

Arc discharge Phase The characteristics of the arc discharge phase are controlled by the external impedances of the ignition circuit. Typically, the burning voltage is about 100 V and the current is greater than 100 mA, and is dependent on external impedances. The arc discharge is sustained by electrons emitted from the cathode hot spots. This process causes erosion of the electrodes, with the erosion rate increasing with the plug gap. Depending on the conditions, the efficiency of the energy- transfer process from the arc discharge to the thermal energy of the mixture is typically between 10 and 50 per cent.

With currents of less than 100 mA, this phase becomes a glow discharge, which is distinguished from an arc discharge by the cold cathode. Electrons are liberated by ion impact, a less efficient process than thermionic emission. Even though arc discharges are inherently more efficient, glow discharges are more common in practice, due to the high electrode erosion rates associated with arc discharges.