The main differences between a two stroke and a four stroke engine.

• Power Stroke: 
• In a four stroke engine, the thermodynamic cycle is completed in four strokes of the piston or in two revolutions of the crankshaft. Thus, one power stroke is obtained in every two revolutions of the crankshaft.
•  In a two stroke engine, the thermodynamics cycle is completed two strokes of the piston or in one revolution of the crankshaft. Thus, one power stroke is obtained in each revolution of the crankshaft.
• Power developed: 
• Because of one power stroke for two revolutions in a four stroke engine, power produced for same size of engine is less, or for the same power the engine is heavier and bulkier. 
• Because of one power stroke for every revolution, power produced for same size is twice, or for the same power the engine is lighter and more compact.
• Wear and tear: Lower cooling and lubrication requirements in a four stroke engine and hence lower rates of wear and tear. Greater cooling and lubrication requirements in a two stroke engine and hence higher rates of wear and tear.
• Valves: Four stroke engines have valves and valve actuating mechanisms for opening and closing of the exhaust valves. Two stroke engines have no valves but only ports.
• Initial cost: Initial cost of a four stroke engine is higher than a two stroke engine because of bulky nature of the cylinders and complicated nature of the valve mechanisms.
• Efficiency: Volumetric and thermal efficiency of a four stroke engine is higher than a two stroke engine.
• Applications:
•  Four stroke engines are used where efficiency is important as in cars, buses, trucks, tractors, power generation, etc. 
• Two stroke engines are used where low cost, compactness, and light weight are required as in scooters, motor cycles, etc.

Screw and screw threads

Screws form an important part of any machinery. Screws are present almost everywhere, ranging from a wristwatch to an automobile. In this post, the manufacturing of a screw thread, and the types  of screw thread will be discussed.

First of all one needs to know the term 'Helix'.
A helix is a 3D structure which is obtained when a particle rotates with a constant angular velocity along a fixed axis along with a linear motion in a direction parallel to the axis with a constant linear velocity.
The screw thread also forms a helix.

Helix

Now knowing what a helix is, when we have a path carved on a cylindrical metal piece in the form of a helix, we get a screw.

Now let us see various terms related to a screw.

• While holding a screw horizontally, a wave kind of pattern can be seen made by the threads. The upper extreme position is called 'Crest' and the lowermost position is called 'Trough' (or 'Root'). 
• The diameter of the screw including the threads is called 'Major Diameter' and the diameter of the cylinder formed by not considering the threads is called 'Minor Diameter'.
• The distance between two consecutive crests ( which is same as the distance between two consecutive troughs) is called the 'Pitch'.
• The angle formed between the threads is called 'Thread Angle'.
• The mean of Major Diameter and Minor Diameter is called 'Pitch Circle Diameter'.

 

Now to discuss the classification:

1. Based on orientation of threads, we have right hand threads and left hand threads.
When the screw is held vertically, if the threads incline from right at top to left at bottom, we have right-hand threads.
Similarly, if the threads incline from left at top to right at bottom, we have left-hand threads.

2.  Based on the start of threads:
[Note: The linear distance vertically upwards or downwards the screw travels during one complete rotation in known as the 'Lead' of the Screw.]
If a single thread is present, it is known as a single start screw. If two threads start simultaneously we have a double start screw. Similarly we can have a triple start screw and so on.

The main point is that for a fixed pitch, as the number of starts increases, the lead of the screw increases i.e. one revolution the lead of a double start screw will be more than single start screw.

So, for one complete revolution, a triple start screw will travel more linear distance than a double start screw, which in turn would be more than a single start screw.

 3. Based on the shape of the threads:

The controlling factor is the thread angle.

Square threads give  higher friction than v-threads and also the load bearing surface is higher.

But they are difficult to manufacture and the efficiency is low.

V-threads are easy to manufacture, are highly efficient but provide lower friction and lesser bearing surface. Both square and v-threads are used for power transmissions.

Buttress threads are ideally used for power transmission in only one direction, and are easy to manufacture, provide high friction, high bearing surface along with good efficiency.

The methods of manufacturing screw threads are:

• Die and tapping
• Threading on Lathe
• Milling 
• Rolling, Forming

Rolling, forming, etc. are used for mass production of screws and bolts.Whereas lathe may be used where a part of the work-piece needs to be threaded, and not for small diameter screws.

To watch manufacturing of threads on a bolt, click here

To watch threading operation on a Lathe machine, click here

 

Wireless Charging

Hi guys !
In this post I am going to explain how a wireless charger works.

The basic working principle of a wireless charger is Electromagnetic Induction (EMI).
When a varying magnetic field, magnetic flux in particular, is applied across a closed loop, emf is induced across the loop.
The emf induced can be found by using the Faraday's law of Electromagnetic Induction.
A coil of wires carrying current produces magnetic flux and when supplied with Alternating current produces variable flux. This variable flux when linked to another coil, emf is induced in that coil.
 To refer more on Faraday's law of EMI, click here.

The working is similar to that of a transformer, the only difference being that the flux is linked through the air, instead of the core.

The transmitter part consists of a 220/12 V or 120/12 AC transformer which reduces the voltage to 12 V.
It also consists of coil of conductors. When current passes through this coil, a magnetic flux is created. The receiver part (phone) also has a coil.
When the phone is placed close enough to the transmitter, the flux is linked through air and emf is induced across the receiver coil.
(DC cannot be used to transmit the power, as the flux created by a DC current carrying coil will not be fluctuating, and emf will not be induced in the secondary. )
This induced EMF is alternating in nature. It is rectified and by using filters, a Direct Current is obtained which is supplied to the battery.

Here, the transmitter and receiver act as a transformer but with air as a core. As air is not a ferromagnetic material, most of the flux will be leakage flux ( useless flux). Hence it is not an efficient process. Even the range of the coil is less as air can link flux only for short distances.
This was Inductive charging.

Sometimes a capacitor is placed in parallel to the inductor, to create a resonance circuit. This helps in creating flux of variable frequency. Air being the core, the problems of core magnetization and power transmitting at a particular frequency will not occur. But, here the receiver circuit must have the inductor coil and the capacitor in parallel such that their resonance frequency s the same as that of the transmitting frequency. Then and then only the power transmission will take place. Then the AC is rectified and filtered into DC which is then supplied to the battery.

Advantages of Wireless Charging:

• Free from wires. No need to plug in the wire every time you need to charge the phone and to remove it after being charged.
• Many coffee shops and cafes are giving wireless port hubs where you can charge your phone.
• One can charge multiple devices from a single transmitter. 
• Not harmful, unlike microwaves.

Disadvantages of Wireless Charging:

• As air being the core, much energy is lost. Not an efficient process.
• One cannot charge a phone that has a receiver coil set to a frequency which is different from the frequency of the transmitter.
• The whole package is costlier than the wired charger.
• The charging process takes more time to completely charge the battery than a wired charger. Even the phone gets heated up quickly.
• Not used Everywhere.  

This technology is gaining speed at a considerable speed and will be available everywhere in a few years.

Please go through this video. It beautifully explains and describes the pros and cons of this process :
https://www.youtube.com/watch?v=-_3L4Hm_2ZI