Importance Of Turbulence ,swirl , Squish , Tumble Related to CI Engine
Importance of Turbulence in CI Combustion Engines:
(a) It is essential to have well organised movement of air within the combustion chamber for –
(i) speedy evaporation of fuel
(ii) to enhance air fuel mixing
(iii) to increase combustion speed and
(iv) to increase efficiency. Due to high velocities involved the flow of air within the combustion chamber is turbulent. Due to turbulence the transfer rates like vaporisation of fuel, heat transfer rates, mixing and combustion rates are greatly increased.
(b) The main aim of the CI combustion chamber design is to ensure proper mixing of fuel with the air in a very short time. To achieve this well organised movement of air is essential. Turbulence in the CI engines is quite systematic and built into the design. It would be desirable to understand various types of movements of the air within the combustion chamber.
(i) Turbulence is quite high during suction and recedes towards BDC.
(ii) Again during compression the turbulence increases as swirl, squish and tumble.
The rotational motion of air within the cylinder is called swirl. The swirl is achieved by a variety of methods like shaping the intake manifold, valve, valve port and at times by suitably contouring the piston. Swirl enhances mixing and makes the flue air mixtures homogeneous. Swirl is the main mechanism to spread the flame within the combustion chamber.
The swirl ratio used to indicate the extent of rotational motion within the combustion chamber. If the ratio of average angular speed to the engine RPM.
Following are the main types of swirls:
(a) Induction Swirl:
In this air flow is directed in a particular path while entering the cylinder. This method is generally employed for open combustion chamber.
(b) Compression Swirl:
Here the swirl is produced during the compression stroke air is forced through a passage into swirl chamber made separately during the compression stroke
(c) Combustion Induced Swirl:
This swirl is produced by using the phenomenon of high initial pressure rise due to partial combustion.
Radial inward movement of air while the piston reaches the TDC is called Squish. “Squish” can be defined as radically inward flow of air towards the combustion recess by squeezing it out from between the piston and cylinder head during the end of compression stroke. This movement of air is due to the fact that most of the modern chambers have clearance volume in the central portional only. While the piston goes moves the TDC the air from the outer edges where there remains no space rushes towards the central portional giving rise to inwards radial motion called squish.
As the piston reaches close to the TDC the squish generates secondary motion about circumferential axis near the outer edges. This motion is called ‘tumble’.
To achieve this either the fuel is directed towards air or air is directed towards fuel.
Basically there are three methods to create air movement or swirl during:
(ii) Compression or
(iii) Partial combustion.
(a) Open Combustion Chambers:
The combustion chambers in which swirl is produced during induction are called ‘open combustion chambers’ or Direct injection chambers.
(b) Turbulent Combustion Chambers:
The combustion chamber in which compression stroke is used to produce swirl are called ‘Turbulent or swirl combustion chambers’.
(c) Pre-Combustion Chambers:
The combustion chambers in which partial combustion is used to create swirl are ‘Turbulent or air-cell combustion chambers’.
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