Question Corner

Engine difference

QUESTION: How is a four-stroke engine of a two-wheeler more eco- friendly than a two-stroke engine?

Y. Rajesh, Pendurthy, A.P.

ANSWER 1: There are 3 major reasons for a four-stroke engine of a two-wheeler being more eco-friendly than a two-stroke engine:

In a two stroke engine, a part of the incoming air and fuel is short-circuited directly through the exhaust port of the cylinder and is carried away with the exhaust gases. This pollutes the atmosphere. But, the design of four-stroke engine is different (there are valves (inlet and exhaust).

Scavenging is effectively done in a four-stroke engine than in a two-stroke. (`scavenging' is a process of removal of burnt gases from the engine cylinder.) If scavenging is not done properly, the residual burnt gas that inside the cylinder will be combusted again; this leads to smoky emission in two-stroke engines.

The lubrication system of a two-stroke engine is different from that of a four-stroke engine. In two-stroke engines, 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 partially pushed through the exhaust port, along with the exhaust gases. This results in smoky emission.K. Magesh & R. Senthil Deepak, Coimbatore

ANSWER 2: The suction, Compression, expansion and exhaust strokes takes place in four strokes of the piston or two revolution of the crank shaft, in a four stroke engine.

In a two stroke engine compression and suction takes place in one stroke of piston and expansion and exhaust strokes also takes place in one stroke of piston. Hence in two strokes of piston all the four processes are completed, which makes scavenging less efficient.

Scavenging is the process in which the products of combustion (i.e. burnt exhaust gases) are cleared from the cylinder and filling it with more or less completely with a fresh mixture of air and fuel. The incoming fresh mixture entering into the engine cylinder at certain velocity sweeps out the products of combustion from the cylinder.

The fresh mixture entering acts as a scavenging or sweeping agent. But instead of acting as a sweeping agent, the fresh mixture also escapes through the exhaust port, as unburnt hydrocarbons which is a pollutant.

J. John Silas,Chennai

Spherical shape

QUESTION: Why are all the planets in the universe spherical in shape?

J. Balaji, Chennai

ANSWER: Dip a finger into a bowl of water and then take it out. Now watch the drop of water as it trickles down the finger and drops down. You will notice that the drop of water acquires a spherical shape. Place a drop of mercury on a smooth surface. It attains a spherical shape, this phenomena is due to the cohesive force (force of attraction between the molecules of same element) which brings the molecules closer.

But then why does it take a spherical shape, why not a cubic one? This is because, out of all multidimensional geometric figures, sphere has the least surface area. Thus all particles try their best to attain a spherical shape in order to reduce their surface area. It is for the same reason that when you pour any liquid in space they attain a spherical shape. But this is not the case with solids since their molecules are compactly packed and cannot be displaced. When a new star is born the gas clouds around it condenses to form planets. The dense gas passes through three states, the gaseous state, the molten liquid state and finally the rigid solid state. When a planet is in the first two states it attains a spherical structure due to the cohesive force as mentioned in the illustration. Later the molten planet solidifies into a rigid planet like Earth.

T. Prakash, Tanjore

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This week's questions

Is drinking milk from a cow injected with oxytocin harmful ?

N.Sukumar, A.P.

Why does the earth attract the stone by gravitation and not otherwise?

J.Srividya,Bavani, Coimbatore

How is it possible for the dragonfly to keep afloat in the air without flapping its wings?

K.AnandKumar, Mancherial, A.P.

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