Enhanced Activities due to In Cylinder Turbulence P M V Subbarao Professor Mechanical Engineering Department Faster Actions are Necessary but Need to be in Control!?!?!

Turbulent eddy structure in an Engine Cylinder

Multi-scale Turbulent Flames Multi-scale turbulent flames are essential for operation of high speed engines. Turbulent flames are characterized by rms velocity fluctuation, the turbulence intensity, and the length scales of turbulent flow ahead of flame. The integral length scale li is a measure of the size of the larger energy containing sturctures of the flow. The Kolmogrov scale lk defines the smallest structure of the flow where small-scale kinetic energy is dissipated via molecular viscosity. Important dimensionless parameters: Turbulent Reynolds Number: Eddy turnover time:

Characteristic Chemical Reaction Time: The ratio of the characteristic eddy time to the laminar burning time is called the Damkohler Number Da.

Regimes of Turbulent Flame 108 Weak Turbulence Reaction Sheets Da 1 Distributed Reactions 10-4 108 ReT

Model of the turbulent flame speed St, m/s

Design of Engine Cylinder for Creation of A Selected Turbulent Flow The First Clue…..

The Mother of Turbulence in An Engine Cylinder The Intake process. The jet flow of gas through the inlet valve of an engine during the intake process is generally thought to be responsible for the production of the turbulent field in the engine cylinder. To test this hypothesis, Clark performed an experiment where he observed the rapid decrease in the rate of flame propagation in an engine intensity versus crank angle after the intake and exhaust processes were eliminated. Semenov repeated this experiment in a motored engine and showed the rapid decay in the instantaneous as the intake and exhaust process were eliminated.

The intake : Mother of Turbulence

Creative Ideas to Generate Turbulent Flow : Swirl based systems

Creative Ideas to Generate Turbulent Flow : Squish based GDI concepts

Creative Ideas to Generate Turbulent Flow : : Tumble based GDI systems

Creative Ideas to Generate Turbulent Flow

Eco-friendly Nature of Turbulent Combustion

Main Requirement for User Acceptability Mixture Burn Time Vs Engine Speed

Creation of Maximum Brake Torque Condition Turbulent conditions enhance the reliability of SoC and reduces the time for Complete combustion. If start of combustion is too early work is done against piston and if too late then peak pressure is reduced. Helps in selection of spark introduction timing (SIT). The optimum spark timing that gives the maximum brake torque, called MBT timing occurs when these two opposite factors cancel.

Need for Variable SIT Recall the overall burn angle (90% burn) increases with engine speed, to accommodated this you need a larger spark advance. N* Brake Torque Fixed spark advance MBT WOT CA* Brake Torque Fixed engine speed

Symptoms of Safe & Acceptable Turbulent Combustion in SI Engines

Cyclic Variation of Flame Volume

Signatures of 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 autoignites.

Physics of Knocking Combustion Knock is a phenomenon that occurs when high temperature and pressure causes the end gas to self ignite. This causes a very high local pressure and this generates pressure waves across the combustion chamber. These pressure waves excites the resonance modes of the cylinder. The frequency of the oscillations under knocking conditions depends on engine geometry, and is often in the range of 5 to 10 kHz. shock time P,T

Knock limit as a function of CR and ON for moderate and high turbulence combustion chambers.