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The Petrol Engine

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In the internal combustion engine, heat is generated by combustion of an inflammable charge inside a cylinder, and the heat energy is immediately converted into mechanical energy. Some heavy internal combustion engines use a gas fuel or else Diesel oil, and the fuel/air mixture may be ignited either by a spark or by compression of the mixture. However, for small i.c. engines, such as those which are used in motor-cars, the charge is a mixture of petrol and air, and is ignited by a spark from the distributor.

When the mixture is ignited, the products of combustion expand down the cylinder, which is fitted with a reciprocating piston. The downward movement of the piston is converted into a rotational movement of the crankshaft by means of a connecting rod. As the crankshaft rotates, the piston is driven upwards again, and the exhaust gases are expelled through the exhaust valve in the cylinder head. When the piston nears the top of this stroke, the inlet valve is opened and the exhaust valve closed. The piston then descends on the induction stroke, and draws a fresh charge into the cylinder. As the piston rises again on the compression stroke, the charge is compressed and ignited, and the cycle begins again. This is the four-stroke cycle which is in common use. An alternative cycle is the two-stroke cycle, which combines the exhaust and compression strokes into one.

The combustion of the mixture does not take place instantaneously. The spark is therefore timed to occur before the piston reaches top dead centre, otherwise maximum pressure would not be reached in time. By the time the piston is at top dead centre, combustion is well under way and the expansion of the gases is beginning. Once combustion starts, it should be carried through the mixture very rapidly, and this is assisted by making the clearance space above the piston as small as possible, and by careful design of the cylinder head. Rapid propagation of the flame through the compressed gas is also assisted by creating turbulence in the gas.

Most small i.c. engines in common use have four cylinders, which fire in a definite and regular sequence. This is necessary, otherwise the torque which the pistons impart to the crankshaft will be irregular and uneven. The torque is liable to be uneven in any case when the engine is running slowly, and a flywheel is fitted to the crankshaft to damp out these variations.

It is essential for the inlet and exhaust valves to open and close at exactly the appropriate moment in relation to the position of the piston. Therefore they are actuated by a cam-shaft running in phase with the crankshaft.

 

The Turbo-prop Engine

The efficiency of a turbo-jet engine varies with the speed and altitude at which it operates. Whilst it is very efficient at supersonic speeds and high altitudes, it is not suited to the low speeds involved in taking-off and landing. Under these conditions, thrust augmentors or after-burners are often required to boost the power, and this entails heavy fuel consumption and restricts the range of the aircraft. On the other hand, propeller-driven aircraft cannot attain speeds much in excess of 500 m.p.h., whereas at low speeds they have a much better performance. Since subsonic speeds are still acceptable for most civilian airliners, a type of engine known as the turbo-prop was developed, which combined some of the advantages of both jet and piston-driven engines.

In the turbo-jet, then turbine is required to develop enough power to drive the compressor only, whereas in the turbo-prop engine, it must supply power also for the propeller, to which it is coupled by means of reduction gearing. As the propeller rotates, it drives rearwards a much larger column of air than that which is expelled from the jet-tube of the turbo-jet, but at a much lower velocity. Consequently it is quieter than the turbo-jet, since the volume of noise produces by an aircraft engine increases with the velocity of the air column. Most airports are situated in or near large centers of population, with the result that any reduction in the noise level is a decided advantage. Furthermore, a large proportion of the energy of the products of combustion is needed to drive the compressor and the airscrew. As this proportion increases, so the amount of thrust developed in the jet-pipe diminishes. In consequence, the destructive blasts of hot gases which emanate from the jet-pipe of the turbo-jet while taxiing on runways or taking-off are greatly reduced.

The main disadvantage of the turbo-prop engine is of course the limitation imposed on speed by airscrew, as a result of which it is likely to become obsolete on all except short-haul aircraft.

A more recent development in jet propulsion is the ducted-fan jet, in which the turbine drives a multi-bladed fan in a duct. A certain proportion of the air which enters the engine by-passes the compressor and combustion chambers, and is impelled by the fan down the outside of the duct, so that it is expelled at considerable velocity from the rear of the engine. It amplifies the mass of hot exhaust gases, and thus serves to augment the thrust derived from them. Consequent on the more moderate speed of this ducted air, the noise level is kept reasonably low. In addition, this type of engine performs well both below and above the speed of sound, whereas the other types of engine are efficient only at certain speeds.

 

What are Sounding Rockets?

Sounding rockets take their name from the nautical term “to sound” which means to take measurements. They are basically divided into two parts – a solid fueled rocket motor and the payload. The payload is the section which carries the instruments to conduct the experiment and send the data back to Earth.

The National Aeronautics and Space Administration (NASA) currently uses 15 different sounding rockets. The rockets come in a variety of sizes from the single-stage Super Arcas which stands 7-feet (3 meters) high to the four-stage Black Brant XII which stands at 65-feet (20 meters) tall. These rockets can carry scientific payloads of various weights to altitudes from 30 miles (48 km) to more than 8000 miles (1,287 km).

Sounding rockets are low cost and the payload can be developed as quickly as six months. These rockets allow scientists to conduct investigations at specified times and altitudes. The experiments provide a variety of information on the upper atmosphere, the Sun, stars, galaxies and other planets.

NASA launches an average of 35 sounding rockets each year with a success rate of about 98%. They are launched routinely from established sites such as Wallops Island, Poker Flat Research Range as well as sites in Canada, Norway and Sweden.

Sounding rockets can also be launched from temporary launch ranges. In the past, launch programmes have been conducted from Peru, Puerto Rico, Greenland, Australia, and even from an aircraft carrier in the Pacific Ocean.

The flight profile of a sounding rocket follows a parabolic trajectory – it goes up and comes back down. Flight time is less than 30 minutes.

Following launch, as a rocket motor uses its fuel it separates from the vehicle and falls back to Earth. The payload continues into space after separating from the motor(s) and begins conducting the experiment. When the experiment is completed, the payload reenters the atmosphere and a parachute is deployed, bringing the payload gently back to Earth. The payload is then retrieved. By retrieving the payload, a tremendous savings can be achieved because the payload or parts of the payload can be refurbished and flown again.

 




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