Combustion in Compression Ignition Engines - Essential features

The CI engine combustion process is extremely complex

− It depends on

•  the characteristics of the fuel
•  the design of the engine's combustion chamber and
•  fuel-injection system
•  the engine's operating conditions

The process is an unsteady, heterogeneous,three-dimensional combustion process. 

 Some important consequences of diesel combustion are as follows:

•  Injection commences just before the combustion starts, so there is no knock limit as in SIE – hence
• a higher compression ratio can be used improving its fuel conversion efficiency.
•  
• Since injection timing is used to control combustion timing, delay period between the start
• of injection and the start of combustion must be kept short (and reproducible)
• A short delay is also needed to hold the maximum cylinder gas pressure below the maximum theengine can tolerate. 

− The spontaneous ignition characteristics of the fuel-air mixture must be held within a specified  range

•  Thus requiring the diesel fuel to have a cetane number above a certain value. 
• Since engine torque is varied by varying the amount of fuel injected per cycle with the engine air flow essentially unchanged, the engine can be operated unthrottled – thereby, reducing pumping requirement, and improving part-load mechanical efficiency relative to SIE. 
• As the amount of fuel injected per cycle is increased, problems with air utilization during combustion lead to formation of excessive amounts of soot which cannot be burned up prior to exhaust. 

− Excessive soot or black smoke in the exhaust constrains the F/A ratio at maximum engine power to values 20% (or more) lean of stoichiometric.

Hence the maximum imep (in a n.a. engine) is lower than values for an equivalent SIE Because the diesel always operates with lean F/A ratio (more so at part load), the effective value of over the expansion process is higher than in SIE

− This gives a higher fuel conversion efficiency than in an SIE, for a given expansion ratio.

• The major problem in diesel combustion chamber design is achieving sufficiently rapid mixing between fuel and air to complete combustion in the appropriate crank angle interval (40 to 50 ) close to TC
• − Mixing rate controls the fuel burning rate
•  The mean piston speed at maximum rated engine speed is approximately constant over a large size range (bore dia 70 to 900 mm), so the maximum rated engine speed will be inversely proportional to the stroke

Please read :
Comparison of ignition systems 
Principles of similitude in engine design 

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Principles of Similitude Applied to Engine Design

Similitude Applied to Engine Design 

• For a new design the number, and, therefore, the size, of cylinders to be used for a given power output is an extremely important decision
• In practice, the greatest differences in cylinder design are caused by the differing requirements of various types of service.
• Within each service category, however, a surprising degree of similitude in cylinder design is found
• Thus the principles governing similar designs can be used to obtain qualitative comparisons of engine design and performance within a given category.
• The use of reduced-scale models to simulate the behavior of full-scale prototypes is necessary when physical testing constraints limit the feasibility of full scale prototype testing
• Engine performance is predicted based on cylinder diameter, stroke length, fuel supply, combustion methods, and speed and compression ratio.
• Designing of a new engine is based on experience about the existing engine similarities and performance.
• Cylinder-size effects
• Diesel engines are built in a wide range of cylinder sizes, and for a given type of service the cylinder designs used are quite similar
• The mean values of bmep, piston speed, and specific output all tend to fall as bore increases – the trends are explained as
• Small cylinders are generally used in applications in which high output in proportion to size and weight is very important
• In general, large cylinders are used only in services in which there is great emphasis on reliability, durability, and fuel economy – encourages low ratings in terms of bmep and piston speed.
• Stresses due to temperature gradients increase with increasing cylinder size unless gas temperatures are reduced – leads toward lower F/A ratios, hence lower rated bmep as the bore increases
• Extensive development work, and especially destructive testing, become less practicable as cylinder size increases  – thus, with little development testing, both bmep and piston-speed ratings must be low in order to insure proper reliability.
• With increasing cylinder size, it becomes necessary to build up such elements as crankcases, crankshafts, and cylinders out of many parts fastened together, whereas, with small cylinder sizes, one-piece construction is generally used.
• The foregoing  factors appear sufficient to account for the fact that average rated bmep and piston speed grow smaller as cylinder size increases
• In spite of this fact, rated power is much more nearly proportional to piston area than to piston displacement
• Similar engines will, of course, have weights proportional to their piston displacements
• If power is proportional to piston area, for similar cylinders weight per horsepower increases in direct proportion to the bore. 

Please read :
Combustion in CI Engine .
Comparison of ignition systems 

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Types of Molding sand in Casting Process

Types of molding sand:

 

  Green sand:

                                 Natural sand with moisture

 Dry sand:

                                Not suitable for large castings

Facing sand:

This sand is used directly next to the surface of the pattern and comes into contact with the molten metal when the mould is poured.

As a result, it is subjected to the severest conditions and must possess, therefore, high strength and refractoriness. This sand also provides a smoother casting surface and should be of fine texture. It is made of silica sand and clay, and some additives without the addition of used sand.

Facing sand is always used to make dry sand moulds while system sand is frequently used for green sand molding.

Parting sand:

This sand is used to prevent adhering of two halves of mould surfaces in each molding box when they are separated. Thus, to ensure good parting, the mould surface (at contact of cope and Drag) should be treated with parting sand or some other parting material.

It is also sprinkled or applied on the pattern surface (before the molding sand is put over it) to avoid its sticking and permit its easy withdrawal from the mould. The parting sand is fine dry sand.

Backing or floor or black sand:

This is the sand which is used to back up the facing sand and to fill the whole volume of the flask. Old, repeatedly used molding sand is mainly employed for this purpose.

              

  Core sand:

                                The core sand mainly consists of silica sand and an organic binder, with very little, if any, clay content. The presence of clay in core sand reduces its permeability and collapsibility. The core sand may contain small percentages of other constituents also, to enhance its properties.

               

 Loam sand:

                                50 % of clay and dried hard and using for large castings

 

               

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Comparison of The Ignition Systems in SI Engine

The CPI system or Commence Primary Ignition 

Includes a battery, switch, resistor, coil, distributor, spark plugs, and the necessary wiring.

When ignition  is required, the breaker points are opened by the action of the distributor cam,
interrupting the primary current flow.

The resulting decay of magnetic flux in the coil induces a voltage in both the primary and secondary winding.

The major limitations of the breakeroperated inductioncoil system are the decrease in available voltage as engine speed increases due to limitations in the current switching capability of the
breaker system, and the decreasing time available to build up the primary coil stored energy.

 

The TCI or Transistor Controlled Ignition System

Use electronic triggering to maintain the required timing without wear or adjustment.

In addition to higher voltage, it provides longer spark duration (about 2 ms).

Much reduced ignition system maintenance, extended sparkplug life, improved ignition of lean and
dilute mixtures, and increased reliability and life.

The CDI or Capacitor Discharge Ignition System 

A capacitor, rather than an induction coil, isused to store the ignition energy. 

The ignition transformer steps up the primary voltage, generated at the time of spark by the discharge of the capacitor through the thyristor, to the high voltage required at the spark plug. 

The CDI or Capacitor Discharge System 

A capacitor, rather than an induction coil, is used to store the ignition energy.

The ignition transformer steps up the primary voltage, generated at the time of spark by the discharge of the capacitor through the thyristor, to the high voltage required at the spark plug.

The CDI trigger box contains the capacitor, thyristor power switch, charging device, pulse shaping unit, and control unit.

It is insensitive to electrical shunts in the high-voltage  ignition circuit that result from spark plug fouling.
 Because of the fast capacitive discharge, the spark is strong but short (0.1 to 0.3 ms).

Please Read :
Principles of similitude used in Engine design .
Essential features of combustion in CI engine

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