CHAPTER 2 LITERATURE REVIEW
2.4 In-cylinder Flow and Mixture Formation
Details of in-cylinder flow, turbulence generation and mixture preparation were discussed in [14], [17], [40]. The large scale swirl and tumble flows formed during intake stroke have significant contribution in air-fuel mixture preparation and formation of turbulence at the time of ignition and combustion. The in-cylinder flow is basically composed of a highly turbulent flow with variable length and time scales that can be derivatives of the decomposition of the macro flow structure. The common characteristic of the flow for different kinds of engines are that it becomes unsteady because of the piston motion. In addition, cyclic variability and turbulent flow can be observed at any engine speed.
The induction process during the opening of the intake valves is one of the sources of turbulence in the engine cylinder. The intake flow generally have two flow structures when it is introduced into the cylinder; swirl and tumble as can be shown in Figure 2.10. A swirling intake is the flow which rotates about the cylinder axis, whereas a tumble flow has a rotation axis that is perpendicular to cylinder axis. In most cases practical engine induction creates a combined swirl and tumble flow condition in the cylinder. The magnitude of the generated swirl and/or tumble intake flows are highly dependent on engine geometry such as intake port design, valve geometries, bore/stroke ratio and cylinder head shape.
Figure 2.10 Schematic of intake flow structure in engine cylinder
The intake opening in a four-stroke engine is the minimum area for the induction such that the gas flow has the highest velocity at the valve during the intake process.
The gas which flows into the cylinder as a conical jet has a high axial and radial velocity components that can be about 10 times of the mean piston speed as mentioned in [14]. The evolution of turbulent in the intake stroke and in the compression stroke was discussed in [41] and the schematic is shown in Figure 2.11.
With the continuous valve lift, the turbulence energy increases starting from the valve opening. This can be sources for the turbulence created in the cylinder. Basically, in a turbulent fluid flow, viscous shear stresses perform deformation of fluid layers, and the break-up of eddies convert the turbulent kinetic energy into internal energy of the fluid in an energy cascade process [42]. This makes turbulent flow always dissipative, and so unless otherwise there is a continuous source of energy to generate turbulence, it eventually decays. The turbulent energy starts to decay even before the inlet valves closed as discussed in [41]. Nonetheless, the decaying rate is very much dependant on the intake flow structure. Due to the favorable geometry of the cylinder, the swirling structure of the in-cylinder flow experiences a lesser rate of viscous dissipation than the tumble structure. Hence, the energy of a swirling intake can be preserved longer during compression process compared to tumble flows [40]. In practical engine, the intake flow structure can be complex depending on the position of the intake valves, intake port geometry and the swirl angle. Fuel direct injection, especially during compression process, can have its own impact on the in-cylinder turbulence variation.
Figure 2.11 Typical representation of turbulence in the cylinder during intake and compression process [41]
There are different mechanisms to create a swirling intake into the engine cylinder. Modifying the intake valves to be shrouded to one side or masking cylinder head around the valve periphery so that the flow will be discharged into the cylinder tangentially in a side way deflection manner [14] or applying swirl control valve (SCV) in the port that employs adjustable butterfly valves to vary swirl are the most common techniques. Shrouding of the intake valves is not a good choice for swirling flow generation due to its negative effect on volumetric efficiency. In modern research engines SCVs can be used to produce swirling flow at the valve stem inside the port. This kind of SCV is advantageous to create different levels of swirl intake without changing or modifying the intake port. Application of helical ports is also able to create swirling intake flow inside the port before it is introduced into the cylinder [14], [43].
On the other side, the level of tumble induction can be varied by varying the entry angles of the port. Lee et al. [38] used tumble induction at three different port entry angles (15o, 20o, 25o) to study the effects of tumble strength on combustion. They found that tumble generated turbulence was best at 20o port entry, and this was found to enhance lean burn performance.
During compression process the intake flow structures are subjected to significant change by decomposition of the large scale flow into smaller scale turbulent eddies.
Heywood [14] discussed this process that an increase in turbulent intensity due to compression and combustion was noticed toward the end of compression stroke in some in-cylinder flow patterns. With the compression of the large scale flows, angular
velocity increases to conserve angular momentum. This phenomenon generates turbulence due to shear flow.
The other important flow structure created during compression process is squish flow, which can be defined as radially inward flow that occurs toward the end of compression process when the piston approaches TDC [14]. Zhao et al. [40]
highlighted that squish flow can enhance a swirl in-cylinder flow and increase turbulence intensity in the early portion of the combustion period.