Physics in practice on thermodynamics

In a two wheeler, how is a four stroke engine more eco-friendly than a two stroke engine?


The two stroke engine has no valves but uses ‘ports’, holes on the cylinder, which are opened or closed as the piston moves. On a power stroke, the piston compresses the petrol and air mixture which has entered the crank case. This is forced into the upper cylinder as the transfer port opens and helps to push out the exhaust gas through the open exhaust port. In a two stroke engine, compression and suction takes place in one stroke of the piston and expansion and exhaust strokes also takes place in one stroke. Thus in a two stroke of the piston all the four processes are completed which does not burn the gases completely and hence makes the scavenging less effective.

In a two stroke engine, oil is mixed with the fuel to provide lubrication for moving surfaces such as piston skirt and roller bearings. It is subsequently carried into the combustion chamber by the air stream, where it is partially burnt and pushed through the exhaust port along with the exhaust gases. This results in pollution.

In a four stroke engine, the suction, compression, expansion and exhaust strokes take place in four stroke of the piston or two revolution of the crank shaft. Thus the scavenging is more effective in four stroke engine and hence four stroke engine is more eco-friendly.

Why two wheelers are not normally fitted with Diesel engines?


The two wheelers are not normally fitted with Diesel engines because the power output from the Diesel engine is high and more over nose and vibrations are at high level. The problem also lies in power to weight ratio of the two wheelers. Two wheelers that use Diesel as fuel are heavy in size. Diesel can not be used with 50, 100 and 150 cc engines because of high power output.


What is the difference between internal and external combustion engines?

External combustion engines burn fuel outside the engine and produce a hot working fluid that powers the engine. The classic example of an external combustion engine is a steam engine.

Internal combustion engines burn fuel inside the engine and use the fuel and the gases resulting from its combustion as the working fluid that powers the engine. An automobile engine is a fine example of an internal combustion engine.

How does a steam engine work?

A steam engine is a type of heat engine that converts the heat flowing from a hotter object to a colder object into useful work. The fraction of heat that can be converted to work is governed by the laws of thermodynamics and increases with the temperature difference between the hotter and colder objects. In the case of the steam engine, the hotter the steam and the colder the outside air, the more efficient the engine is at converting heat into work.

1. A typical steam engine has a piston that moves back and forth inside a cylinder. Hot, high-pressure steam is produced in a boiler and this steam enters the cylinder through a valve. Once inside the cylinder, the steam pushes outward on every surface, including the piston.

2. The steam pushes the piston out of the cylinder, doing mechanical work on the piston and allowing the piston to do mechanical work on machinery attached to it. The expanding steam transfers some of its thermal energy to this machinery, so the steam becomes cooler as the machinery operates.

3. But before the piston actually leaves cylinder, the valve stops the flow of steam and opens the cylinder to the outside air.

4. The piston can then re-enter the cylinder easily. In many cases, steam is allowed to enter the other end of the cylinder so that the steam pushes the piston back to its original position. Once the piston is back at its starting point, the valve again admits high-pressure steam to the cylinder and the whole cycle repeats.

Overall, heat is flowing from the hot boiler to the cool outside air and some of that heat is being converted into mechanical work by the moving piston.

Why achieving absolute zero is impossible? What will happen to objects at this temperature (i.e., solid, liquid, and gas)?


Absolute zero can not be reached for the reason that any perfect order is impossible. For an object to reach absolute zero, every single bit of thermal energy and every aspect of disorder must leave the object. If the object is a crystalline material, then its crystal structure must become absolutely perfect. This sort of perfection is essentially impossible. Reducing the temperature of an object towards absolute zero requires great effort and ends up creating a great disorder elsewhere. The closer the approach to absolute zero, the more disorder is created elsewhere. Hence achieving absolute zero is impossible.

NANO

NANOPARTICLES

A nanometer is Ǻ, so particles having a radius of ≤100 nm can be considered to be nanoparticles.

Nanostructured materials are having a characteristic length less than 100 nm. The nanoparticles have different electronic, optical, electrical, magnetic, chemical and mechanical properties which are different from the properties of bulk materials.

A number of metals, alloys, intermetallics, ceramics, polymers, semiconductors have been synthesized in nanoform. Apart from nanoparticles or nanocrystals made of metals, semiconductors or oxides, the special forms of nanomaterials are nanocomposites, nanotubes, quantum wires, quantum dots and quantum wells.

Nanocomosites are special type of materials originating from suitable combination of more nanoparticles by some appropriate techniques resulting in materials having unique physical properties.

Nanotubes are a one dimensional fullerence with a cylindrical tube. These cylindrical carbon molecules have novel properties and make them potentially useful in many applications in nanotechnology, electronics, optics and other fields.

Semiconductor structures in the nanometer size range exhibiting the characteristics of quantum confinement are commonly referred to as quantum dots when the confinement is in a three dimensions quantum wires and when the confinement is in two dimensions and quantum wells when the confinement is in one dimension. These can develop intense, long lasting colours due to excitation by UV and visible light, LEDs, lasers etc.

ORIGIN OF NANOTECHNOLOGY

While the topic nanotechnology seems to be relatively new, the existence of functional devices and structure of nanometer dimensions is not new and in fact nanostructures exists on earth as long as life exists.

In the fourth century, Roman glass makers fabricated glasses containing nanosized metals. They produced cups made up of soda lime glass containing silver and gold nanoparticles. The colour of the cup changes from green to deep red when a light source is placed in it.

In eighteenth century British scientists Thomas wedgewood and Sir Humprey davy were able to provide images using silver nitrate and chloride, but their images were not permanent.

Photography which has been developed in the nineteenth centuries depends on production of silver nanoparticles sensitive to light. Photographic film is an emulsion, a thin layer of gelatin containing silver halides such as silver bromide producing nanoparticles of silver which are pixels of the image.

In 1883, the American scientist Geroge Eastman produced a film containing a large paper strip coated with an emulsion containing silver halides. He later developed this into a flexible film that could be rolled which made photography very simple. Hence, colour films are named after him as Eastman colour film.

In 1960, Richard Feynman presented a visionary and prophetic lecture at a meeting of American physical society entitled “there are plenty of room at the bottom” where he speculated on the possibility and potent of nanosized particles. Richard Feynman

QUANTUM CONFINEMENT

Regarding the electronic, magnetic and optical properties, the internal electronic structure of the nanoparticles is of particular importance. Indeed the electronic properties of nanoparticles are special and in general summarized as quantum size effect. Metals in general are characterized by their unique electronic bonding system differing from non metals and semiconductors. However if the dimension decreases to a certain extent, this special behaviour is no longer possible. The electrons are trapped in a too small volume and they behave very differently and consequently change their chemical and physical properties.

In small nanocrystals the electronic energy levels are not continuous as in the bulk but are discrete (finite density of states), because of the confinement of the electronic wavefunction to the physical dimensions of the particles. This phenomenon is called quantum confinement and therefore nanocrystals are also referred to as quantum dots. In any material substantial variation of fundamental electrical and optical properties with reduced size will be observed when the energy spacing between the electronic levels exceeds the thermal energy.