Turbo-jet engines | Jet Propulsion System

Jet propulsion

Jet propulsion is based on newton’s second law and third law of motion. Newton’s second law states that the rate of change of momentum in any direction is proportional to the force acting in that direction. Newton’s third law of motion states that for every action there is an equal and opposite reaction. Turbo-jet engine a type of jet engine is explained below.

Turbo-jet engines

The Turbo-jet engine is a modified form of gas turbine engine. A Turbo-jet consists of a diffuser, rotary compressor combustion chamber, turbine and nozzle.


Air from the surrounding atmosphere is drawn by the compressor through the diffuser slows down the entering the speed of the air and compresses it. This is known as ram compression. Inside the rotary compressor the air is further compressed to a pressure of 4 atm. This pressurized air then passes into the combustion chamber. Inside the combustion chamber the fuel is sprayed over the pressurized air. The air fuel mixture is burnt. Thus heat is applied at a constant pressure.


The temperature of the air increases rapidly. The amount of air supplied is about 60 times the amount of fuel burnt. The excess air produces sufficient mass for the propulsion of the jet and at the same time prevents the turbine blades from high temperature gases.

The products of combustion process enter into the turbine and expand partially. The power generated by the turbine is only sufficient to drive the compressor, fuel pump and other auxiliaries. The hot gases from the turbine are at a pressure which is above the atmospheric pressure. These gases then enter into the nozzle where it is expanded which converts pressure energy into kinetic energy. Thus the gases come out from the unit with a very high velocity. Due to the increased velocity of gases coming out of the unit, a reaction or thrust is produced in the opposite direction. This thrust propells the air craft. For initial starting, some starting device is provided. This method of propulsion is best suitable for air crafts which travels at or about 800 kmhr.


Advantages of turbo-jet

· Simple construction.

· Less moving parts. Less wear the tear. Hence maintenance cost is less.

· Thrust is directly applied. Hence no loss of power in transmission.

· Unit runs smoothly without vibration since continuous thrust is produced by continuous combustion of fuel.

· Lighter construction of the unit since low working pressure.

· It can operate at higher speeds than turbo-propeller air crafts since the speed of turbo-jet is not limited by the propeller.

· Low grade fuels can be used. This reduces fuel cost.

Disadvantages of turbo-jet:

· A poor starting characteristic since less power is produced during take-off.

· Costly materials are used.

· Life of the unit is shorter than reciprocating engine.

· Produces more noise than reciprocating internal combustion engines.

· High fuel consumption.

Gear Generating Process | Methods Used to Generate a Spur Gear

Gear generating process

This method of gear manufacturing is based on the fact that any two involute gears of the same module will mesh together. In this one of the meshing gears is made as the cutter. The other gear rotates and also reciprocates along the width of the gear blank. Because of the relative rolling motion of the cutter and the blank, gear teeth are generated on the gear blank.

The gears may be generated by a rack cutter, pinion cutter, or a hob. Using the generating method, profile of the gear teeth can be very accurately produced.

The following methods are generally used to produce gear

Gear shaping

Gear shaping is done on a special type of machine called as gear shaper.

In gear shaping, a pinion type cutter is used. The cutter teeth are ground with a top rake and clearance. The cutter is mounted on a vertical spindle. The axes of a cutter and blank are parallel. The cutter and the blank are made to rotate together as two gears which are in mesh. The apparent speeds of the cutter and the blank are the same. The cutter reciprocates in vertical direction along the width of the blank.


The pinion cutter is fed radially into the gear blank to give the depth of cut. The cutter and blank slowly rotate together till all the teeth are generated on the blank. During each return stroke of the cutter, the blank is withdrawn. This is done to prevent rubbing of the cutting edges and damage to gear teeth being cut.


The different movements are given below:

1. Rotary motion of the cutter and the blank.

2. Vertical reciprocating motion of the cutter.

3. Radial feed of the cutter towards the blank.

4. Withdrawal motion of the blank away from the cutter during return stroke.

Gear Planing

Here the cutter is in the form of an involute rack. The rake and clearance angles are cut on it. The cutter is mounted on a reciprocating slide. The blank is mounted a vertical arbor. The cutter is fed radially into the gear to give the depth of cut. The rack cutter reciprocates across the face of the blank.


The blank rotates. While reciprocating, the cutter also moves in a longitudinal direction, i.e. the cutter and the blank roll together as rack and pinion.

Gear hobbing

Hobbing is a process of generating a gear by means of a rotating cutter called hob. The hob has helical threads. Grooves are cut in the threads parallel to the axis. This will provide the cutting edges. Proper rake and clearance angles are ground on these cutting edges. The rotating hob acts like a continuously moving rack as it cuts.


The gear blank is mounted on a vertical arbor. The hob is mounted in a rotating arbor. The hob axis is tilted through the hob lead angle so that its teeth are parallel to the axis of the gear blank.

The hob is rotated at a suitable cutting speed. It is fed towards the gear blank. The hob and the gear blank are made to rotate in a correct relationship with each other. for one rotation of the hob the gear is moved by one tooth.