Valve Timing Diagram of Two Stroke Engine

                   Valve Timing Diagram of Two Stroke Engine

THEORETICAL VALVE TIMING DIAGRAM

 

In theoretical valve timing diagram for a two-stroke engine, the fuel is fired at A and the expansion of gases takes place from A to B. The crankshaft revolves through approximately 120º and the piston moves from TDC towards BDC. At B both the valves open and suction and exhaust takes place from B to C. The crankshaft revolves through approximately 120º and the piston moves from first to BDC and then  little upwards. At C, both the valves close and compression takes place from C to A. The crankshaft revolves through approximately 120º and the piston moves to TDC.

ACTUAL VALVE TIMING DIAGRAM

 

The Actual Valve Timing Diagram has slight variations with respect to the Theoretical Valve Timing Diagram. The variations are made in order to maximize the engine performance.

 

In actual valve timing diagram the expansion of charge starts as the piston moves from TDC towards BDC. The exhaust port opens before the piston reaches the BDC and the burnt gases start leaving the cylinder. After a small fraction of the crank revolution, the transfer port also opens and the fresh fuel-air mixture enters into the engine cylinder. This is done as the fresh incoming charge helps in pushing out the burnt gases. Now the piston reaches BDC and then starts moving upwards. As the crank moves a little beyond BDC, first the transfer port closes and then the exhaust port also closes. This is done to suck the fresh charge through the transfer port and to exhaust the burnt gases through the exhaust port simultaneously. Now the charge is compressed with both ports closed, and then ignited with the help of a spark plug before the end of compression stroke. This is done as the charge requires some time to ignite. By the time the piston reaches TDC, the burnt gases push the piston downwards with full  force and expansion of the burnt gases takes place.

 

CENTRIFUGAL PUMP TEST RIG

 

CENTRIFUGAL PUMP TEST RIG

          OBJECTIVE:

 To obtain the efficiency of a centrifugal pump under varying speed.

 

     APPARATUS:

 The apparatus consists of a pump connected with a D.C. motor. The suction pipe is provided with a vaccum gauge for measurement of suction head. While at the discharge side a pressure gauge is fitted for measurement of the delivery head. A variable speed motor drive is provided. A tachometer is provided for the measurement of the revolution of the pump. A swinging arm field dynamometer is connected on motor for measurement of load. A collecting tank is used to find the actual discharge through the pump.

 

    SUGGESTED EXPERIMENTAL PROCEDURE:

 Step1:  Note down the area of collecting tank, position of delivery pressure gauge (Z₂) and arm distance of the spring from the centre of the shaft.

Step2:  Priming the pump-set before starting. Priming means taking the air present in the suction and pressure pipes, volute casing by filling them with water. Ensure to close the aircock / priming adaptor as the air bubbles cease appearing and continous stream of water comes from aircock/ priming adaptor.

 

NOTE: In the supplied apparatus, a self- priming pump is used so the pump does not require priming.

Step3: The speed control on the motor is set to a value and at the same time the flow control valve was adjusted to give the maximum possible discharge.

Step4: Conditions were allowed to steady before the rate of discharge Q, suction head, discharge head, load on the motor and r.p.s. value were recorded.

Step5:  The flow-rate is reduced in stages and the above procedure is repeated.

 

         SAMPLE DATA SHEET:

             Position of delivery pressure gauge (Datum head), Z₂, m     =

            Arm distance, m                                                                     =       

            rg                                                                                            =   9810

            Area of collecting tank, a, m²                                                            =       

 

Run  No

Discharge measurement

Pump Speed (r.p.s)

Suct-ion Head (m)

Delivery head (m)

Total head (m)

Load (N)

Torque Nm

Water power  Po (w)

Input power  P (w)

Effi- ciency h (%)

Initial h1 (m)

Final h2 (m)

Time (sec)

Discharge Q= (m3/         sec)

CENTRIFUGAL PUMP

 

CENTRIFUGAL PUMP

Centrifugal pump is so named because the pressure head is generated by centrifugal action. The impeller is made up of a number of curved vanes, which are supported on both sides by plates known as shrouds. It rotates inside a casing or volute. Flow enters the pump through the centre or eye of the impeller. Energy is given to the liquid as the blades of the impeller transport it outwards in a radial direction.

 

The volute is usually shaped in the form of a spiral to form a gradual increase in flow area so that the velocity energy at exit from the impeller is converted to additional pressure energy.

 

The centrifugal pump is initially primed wherein the suction pipe, casing of the pump and the portion of the delivery pipe up to the delivery valve are completely filled with the liquid to be pumped.  With the delivery valve closed, the impeller is made to rotate. As a result a forced vortex is developed which imparts a centrifugal head to the liquid. Simultaneously the angular momentum is changed resulting in an increase of the liquid pressure. When the delivery valve is opened the liquid is forced to flow in an outward radial direction thereby leaving the vanes of the impeller at the outer circumference with high velocity and pressure. The high pressure of the liquid leaving the impeller enables the liquid to rise to a high level. This action is a continuous process because the eye of the impeller is continuously supplied with replacement liquid from the sump as a result of the pressure gradient in the suction pipe (a  partial vacuum exists at the eye of the impeller and the liquid in the sump is at atmospheric pressure). The high absolute velocity at the outlet of the vanes is converted to useful pressure energy by shaping the casing such that the liquid flows through a gradually expanding passage.

 

In summary, it may be stated that a centrifugal pump lifts the liquid to a higher level as a result of a modification of the hydraulic gradient caused by centrifugal action and change in angular momentum. This is in contrast to a positive displacement pump wherein lifting action is due to pushing in a confined space.

 

It may also be noted that the action of a centrifugal pump is the reverse of a radially inward flow reaction turbine.

 

The main advantages of a centrifugal pump vis-à-vis a positive displacement pump is that its discharge capacity is much greater, it can be used to pump highly viscous liquids also, it can be operated at high speeds with less danger of separation and cavitation, and its maintenance requirements are low. However, it cannot build-up pressures as high as those that can be built up by reciprocating pumps.

 

The performance of a pump at a fixed/ variable speed may be represented as follows:

 

Let,      Inlet pressure, m                     = p₁        

Discharge pressure, m            = p₂        

Flow rate, m³/s                       = Q

Datum, m                                = Z₂     

 

(Here datum is the distance of the centre of the pressure gauge connected in the delivery line from the flange.)

 

Total head across pump H = (p₂- p₁) +Z₂ m

 

For obtaining the output of the motor (input of the pump) attached to the pump, a swinging arm field dynamometer is provided.

 

Torque T = (load x arm distance)

 

Input power P = (2p x speed in r.p.s. x T)     watts

 

Water power P_o = rg HQ        watts

(Where r is the mass density of the liquid being pumped).

 

                                        Water Power

Efficiency h% = ¾¾¾¾¾ x 100

                                        Input Power

MOTORISED GYROSCOPE

 

1.0     THEORY:

             

(A)      DEFINITIONS:

 (a)       Axis of Spin:

 

If a body is revolving about an axis, the latter is known as axis of spin.

 

(b)       Gyroscopic Effect:

 

To a body revolving (or spinning) about an axis say OX, if a couple represented by a vector OY perpendicular to OX is applied, then the body tries to precess about an axis OZ which is perpendicular both to OX and OY.  Thus the plane of spin, plane of precession and plane of gyroscopic couple are mutually perpendicular. The above combined effect is known as precessional or gyroscopic effect.

 

(c)       Precession:

 

Precession means the rotation about the third axis OZ, which is perpendicular to both the axis of spin OX and that of couple OY.

 

(d)       Axis of Precession:

 

The third axis OZ is perpendicular to both the axis of spin OX and that of couple OY is known as axis of precession.

 

(e)       Gyroscope:

 

Gyroscope is a body while spinning about an axis is free to rotate in other directions under the action of external forces. For example locomotive, automobile and aero plane making a turn. In certain cases the gyroscope forces are undesirable whereas in other cases the gyroscopic effect may be utilized in developing desirable forces. For minimizing rolling, yawing and pitching of ship or air-craft Gyroscope is used. Balloons use Gyroscope for controlling direction.

 

(B)       GYROSCOPIC COUPLE OF A PLANE DISC:

 

Let a disc of weight W and having a moment of inertia I be spinning with an angular velocity w about axis OX in an anti clockwise direction viewing from front. Therefore, the angular momentum of disc is Iw. Applying right hand screw rule, the sense of vector representing the angular momentum of disc which is also a vector quantity will be in the direction OX as shown. A couple, whose axis is OY perpendicular to OX and is in the plane XOZ, is now applied to precess the axis OX.

 

Let axis OX turn through a small angular displacement dq about axis OZ and in the plane XOY, from OX to OX’ in time dt. The couple applied produces a change in the direction of angular velocity, the magnitude remaining constant. This change is due to the velocity of precession. Therefore, ‘OX’ represents the angular momentum after time dt.

 

\ Change of angular momentum = OX’ – OX = XX’

 

                                                                                    Angular Displacement

            or rate of change of angular momentum =   ¾¾¾¾¾¾¾¾¾¾

                                                                                                Time

                                                                                   

            XX’

                                                    =   ¾¾¾  

                                                                                     dt       

But rate of change of angular momentum = Couple applied, C

 

            Where, XX’ = OX x dq in direction of XX’

                                = (Iw)dq

 

                           dq

            \ C= Iw ¾    

                           dt

 

            and in the limit, when dt is very small,

 

           dq

            C= Iw ¾        

                       dt

 

Let dq / dt = w_p, the angular velocity of precession of yoke, which is uniform and is about axis OZ.

 

            Thus, we get   C = Iw.w_p

 

The direction of the couple applied on the body is anticlockwise when looking in the direction XX’ and in the limit this is perpendicular to the axis of w and w_p.

 

In the supplied apparatus, the reaction couple exerted by the body on its frame is equal in magnitude to that C, but opposite in direction.

REVISION OF ENGINEERING MECHANICS

ENGINEERING MECHANICS

Deals with the principles of mechanics along with their application to engineering problems. Divided into two parts: 

STATICS 

DYNAMICS

Statics is the branch of Engineering Mechanics deals with force and its effects while acting upon the bodies at rest.

Dynamics is the branch of Engineering Mechanics deals with force and its effects while acting upon the bodies in motion. It is further divided into two groups.

KINETICS 

KINEMATICS

FORCE

An agent responsible for produce (tends to produce) or destroy (tends to destroy) the motion of a body.

👉 can change the motion of a body

👉 Retard the motion of a body

👉 Can balance the forces already present 

👉 Rise in internal stress in a body

CHARACTERISTICS OF A FORCE

👉 Magnitude of force

👉 Line of action of force

👉 Nature of force ( push or Pull)

👉 The point at which force is acting

RESULTANT FORCE

A single force produces the same effect as produced by all other forces acting on a body.