Cams

Introduction:
A cam is a rotating machine element which gives
reciprocating or oscillating motion to another element known
as follower. The cam and the follower have a line contact
and constitute a higher pair. The cams are usually rotated at
uniform speed by a shaft, but the follower motion is predetermined
and will be according to the shape of the cam.
The cam and follower is one of the simplest as well as one
of the most important mechanisms found in modern
machinery today. The cams are widely used for operating
the inlet and exhaust valves of internal combustion engines,
automatic attachment of machineries, paper cutting machines,
spinning and weaving textile machineries, feed mechanism

of automatic lathes etc.

Classification of Followers:
The followers may be classified as discussed below :
1. According to the surface in contact. The followers,
according to the surface in contact, are as follows :
(a) Knife edge follower. When the contacting end of
the follower has a sharp knife edge, it is called a
knife edge follower, as shown in Fig. 20.1 (a). The
sliding motion takes place between the contacting
surfaces (i.e. the knife edge and the cam surface). It
is seldom used in practice because the small area of
contacting surface results in excessive wear. In knife
edge followers, a considerable side thrust exists

between the follower and the guide.
(b) Roller follower. When the contacting end of the follower is a roller, it is called a roller
follower, as shown in Fig. 20.1 (b). Since the rolling motion takes place between the
contacting surfaces (i.e. the roller and the cam), therefore the rate of wear is greatly reduced.
In roller followers also the side thrust exists between the follower and the guide. The
roller followers are extensively used where more space is available such as in stationary
gas and oil engines and aircraft engines.
(c) Flat faced or mushroom follower. When the contacting end of the follower is a perfectly
flat face, it is called a flat-faced follower, as shown in Fig. 20.1 (c). It may be noted that
the side thrust between the follower and the guide is much reduced in case of flat faced
followers. The only side thrust is due to friction between the contact surfaces of the follower
and the cam. The relative motion between these surfaces is largely of sliding nature but
wear may be reduced by off-setting the axis of the follower, as shown in Fig. 20.1 (f ) so
that when the cam rotates, the follower also rotates about its own axis. The flat faced
followers are generally used where space is limited such as in cams which operate the
valves of automobile engines.
Note : When the flat faced follower is circular, it is then called a mushroom follower.
(d) Spherical faced follower. When the contacting end of the follower is of spherical shape,
it is called a spherical faced follower, as shown in Fig. 20.1 (d). It may be noted that when
a flat-faced follower is used in automobile engines, high surface stresses are produced. In
order to minimise these stresses, the flat end of the follower is machined to a spherical
shape.

2. According to the motion of the follower. The followers, according to its motion, are of the
following two types:
(a) Reciprocating or translating follower. When the follower reciprocates in guides as the
cam rotates uniformly, it is known as reciprocating or translating follower. The followers
as shown in Fig. 20.1 (a) to (d) are all reciprocating or translating followers.
(b) Oscillating or rotating follower. When the uniform rotary motion of the cam is converted
into predetermined oscillatory motion of the follower, it is called oscillating or rotating
follower. The follower, as shown in Fig 20.1 (e), is an oscillating or rotating follower.
3. According to the path of motion of the follower. The followers, according to its path of
motion, are of the following two types:
(a) Radial follower. When the motion of the follower is along an axis passing through the
centre of the cam, it is known as radial follower. The followers, as shown in Fig. 20.1 (a)
to (e), are all radial followers.
(b) Off-set follower. When the motion of the follower is along an axis away from the axis of
the cam centre, it is called off-set follower. The follower, as shown in Fig. 20.1 ( f ), is an
off-set follower.
Note : In all cases, the follower must be constrained to follow the cam. This may be done by springs, gravity
or hydraulic means. In some types of cams, the follower may ride in a groove.
Classification of Cams:
Though the cams may be classified in many ways, yet the following two types are important

from the subject point of view :

1. Radial or disc cam. In radial cams, the follower reciprocates or oscillates in a direction perpendicular to the cam axis. The cams are all radial cams.

2. Cylindrical cam. In cylindrical cams, the follower reciprocates or oscillates in a direction parallel to the cam axis. The follower rides in a groove at its cylindrical surface. A cylindrical grooved cam with a reciprocating and an oscillating follower.

respectively.

Note : In actual practice, radial cams are widely used. Therefore our discussion will be only

confined to radial cams.

Terms Used in Radial Cams:

Fig. 20.3 shows a radial cam with reciprocating roller follower. The following terms are

important in order to draw the cam profile.

1. Base circle. It is the smallest circle that can be drawn to the cam profile.

2. Trace point. It is a reference point on the follower and is used to generate the pitch curve.

In case of knife edge follower, the knife edge represents the trace point and the pitch curve

corresponds to the cam profile. In a roller follower, the centre of the roller represents the trace point.

3. Pressure angle. It is the angle between the direction of the follower motion and a normal

to the pitch curve. This angle is very important in designing a cam profile. If the pressure angle is

too large, a reciprocating follower will jam in its bearings.

4. Pitch point. It is a point on the pitch curve having the maximum pressure angle.

5. Pitch circle. It is a circle drawn from the centre of the cam through the pitch points.

6. Pitch curve. It is the curve generated by the trace point as the follower moves relative to

the cam. For a knife edge follower, the pitch curve and the cam profile are same whereas for a

roller follower, they are separated by the radius of the roller.

7. Prime circle. It is the smallest circle that can be drawn from the centre of the cam and

tangent to the pitch curve. For a knife edge and a flat face follower, the prime circle and the base

circle are identical. For a roller follower, the prime circle is larger than the base circle by the radius

of the roller.

8. Lift or stroke. It is the maximum travel of the follower from its lowest position to the

topmost position.

Fig.

Pelton wheel

Introduction:

The Pelton wheel is an impulse type water turbine. It was invented by Lester Allan Pelton in the 1870s. The Pelton wheel extracts energy from the impulse of moving water, as opposed to water's dead weight like the traditional overshot water wheel. Many variations of impulse turbines existed prior to Pelton's design, but they were less efficient than Pelton's design. Water leaving those wheels typically still had high speed, carrying away much of the dynamic energy brought to the wheels. Pelton's paddle geometry was designed so that when the rim ran at half the speed of the water jet, the water left the wheel with very little speed; thus his design extracted almost all of the water's impulse energy which allowed for a very efficient turbine.

function:
Nozzles direct forceful, high-speed streams of water against a rotary series of spoon-shaped buckets, also known as impulse blades, which are mounted around the circumferential rim of a drive wheel also called a runner ('Old Pelton wheel'). As the water jet impinges upon the contoured bucket-blades, the direction of water velocity is changed to follow the contours of the bucket. Water impulse energy exerts the torque on the bucket-and-wheel system, spinning the wheel; the water stream itself does a "u-turn" and exits at the outer sides of the bucket, decelerated to a low velocity. In the process, the water jet's momentum is transferred to the wheel and hence to a turbine. Thus, "impulse" energy does work on the turbine. For maximum power and efficiency, the wheel and turbine system is designed such that the water jet velocity is twice the velocity of the rotating buckets. A very small percentage of the water jet's original kinetic energy will remain in the water, which causes the bucket to be emptied at the same rate it is filled, (see conservation of mass) and thereby allows the high-pressure input flow to continue uninterrupted and without waste of energy. Typically two buckets are mounted side-by-side on the wheel, which permits splitting the water jet into two equal streams. This balances the side-load forces on the wheel and helps to ensure smooth, efficient transfer of momentum of the fluid jet of water to the turbine wheel.

Because water and most liquids are nearly incompressible, almost all of the available energy is extracted in the first stage of the hydraulic turbine. Therefore, Pelton wheels have only one turbine stage, unlike gas turbines that operate with the compressible fluid. It is used for generating electricity.

Application:
Pelton wheels are the preferred turbine for hydro-power. when the available water source has the relatively high hydraulic head at low flow rates, where the Pelton wheel geometry is most suitable. Pelton wheels are made in all sizes. There exist multi-ton Pelton wheels mounted on vertical oil pad bearings in hydroelectric plants. The largest units can be over 400 megawatts. The smallest Pelton wheels are only a few inches across and can be used to tap power from mountain streams having flows of a few gallons per minute. Some of these systems use household plumbing fixtures for water delivery. These small units are recommended for use with 30 meters (100 ft) or more of the head, in order to generate significant power levels. Depending on water flow and design, Pelton wheels operate best with heads from 15–1,800 meters (50–5,910 ft), although there is no theoretical limit.

Making rules:
The specific speed parameter is independent of a particular turbine's size.
Compared to other turbine designs, the relatively low specific speed of the Pelton wheel, implies that the geometry is inherently a "low gear" design. Thus it is most suitable to being fed by a hydro source with a low ratio of flow to pressure, (meaning relatively low flow and/or relatively high pressure).
The specific speed is the main criterion for matching a specific hydro-electric site with the optimal turbine type. It also allows a new turbine design to be scaled from an existing design of known performance.
   (dimensioned parameter),
where:
• η  = Frequency of rotation (rpm)
• P  = Power (W)
• H  = Water head (m)
• ρ  = Density (kg/m3)
The formula implies that the Pelton turbine is geared most suitable for applications with relatively high hydraulic head H, due to the 5/4 exponent being greater than unity, and given the characteristically low specific speed of the Pelton Wheel.

About lean burn engine

INTRODUCTION

Lean-burn refers to the burning of fuel with an excess of air in an internal combustion engine. In lean-burn engines the air: fuel ratio may be as lean as 65:1 (by mass). The air/fuel ratio needed to stoichiometrically combust gasoline, by contrast, is 14.64:1.
Ford CVH Lean-burn engine
In 1980 Ford introduced their CVH engine in the
Escort 3, as a replacement for the old fivebearing
pushrod OHV Kent engine. The engine's designation
derives from its design — Compound Valve angle,
Hemispherical combustion chamber.

Principle

A lean burn mode is a way to reduce throttling losses. An engine in a typical vehicle is sized for providing the power desired
for acceleration but must operate well below that point in normal steady speed
operation. Ordinarily, the power is cut by
partially closing a throttle. However, the extra work done in pumping air through the throttle reduces efficiency. If the fuel/air
ratio is reduced, then lower power can be achieved with the throttle closer to fully open, and the efficiency during normal
driving (below the maximum torque capability of the engine) can be higher.
The engines designed for lean burning
can employ higher compression ratios and thus provide better performance, efficient
fuel use and low exhaust hydrocarbon emissions than those found in conventional petrol engines. Ultra lean mixtures with
very high air–fuel ratios can only be achieved by direct injection engines.
The main drawback of lean burning
is that a complex catalytic converter system is required to reduce NOx emissions. Lean burn
engines do not work well with modern 3way
catalytic converter. which require a pollutant balance at the exhaust port
so they can carry out oxidation and reduction reactions—so most modern engines run at or near the stoichiometric point.

In an ideal, 100 % efficient internal combustion engine, the fuel would burn to give just carbon dioxide and water vapour. In practice, of course,  engines are far from efficient and the combustion process. also produces carbon monoxide, oxides of nitrogen and unburnt hydrocarbons, as well as carbon dioxide and water vapour.These byproducts of combustion are expelled as part of the car's exhaust gasses into the atmosphere where they cause pollution. In recent years, public concern about atmospheric pollution, and imminentEEC pollution control laws, has led to car manufacturers trying to find ways of reducing the level of these gasses in car exhausts. 

In 1980 Ford introduced their CVH engine in the Escort 3, as a replacement for the old fivebearing pushrod OHV Kent engine. The engine's designation derives from its design — Compound Valve angle, Hemispherical combustion chamber.Now the CVH has been reengineered to make it a true lean burn engine, capable of running on air fuel ratios of over 18:1. This meant a change in the cylinder head design to incorporate a lean burn kidney-shaped combustion chamber to ensure high mixture swirl and therefore more complete fuel burning.