SOME TYPES OF JET ENGINES

 JET ENGINES

                Jet engines date back to the invention of the aeolipile before the first century AD. This device directed steam power through two nozzles to cause a sphere to spin rapidly on its axis. So far as is known, it did not supply mechanical power and the potential practical applications of this invention did not receive recognition. Instead, it was seen as a curiosity.

                Jet propulsion only took off, literally and figuratively, with the invention of the gunpowder-powered rocket by the Chinese in the 13th century as a type of fireworks, and gradually progressed to propel formidable weaponry. However, although very powerful, at reasonable flight speeds rockets are very inefficient and so jet propulsion technology stalled for hundreds of years.

                A jet engine is a reaction engine discharging a fast moving jet that generates thrust by jet propulsion in accordance with Newton's laws of motion. This broad definition of jet engines includes turbojets, turbofans, rockets, ramjets, and pulse jets. In general, jet engines are combustion engines but non-combusting forms also exist. The term jet engine loosely refers to an internal combustion air breathing jet engine. These typically consist of an engine with a rotary (rotating) air compressor powered by a turbine ("Brayton cycle"), with the leftover power providing thrust via a propelling nozzle. Jet aircraft use these types of engines for long-distance travel. Early jet aircraft used turbojet engines which were relatively inefficient for subsonic flight. Modern subsonic jet aircraft usually use high-bypass turbofan engines. These engines offer high speed and greater fuel efficiency than piston and propeller aero engines over long distances.

APPLICATIONS

i. Jet engines power aircraft, cruise missiles and unmanned aerial vehicles.

ii. In the form of rocket engines they power  model rocketry, spaceflight, and military missiles.

iii. Jet engines have propelled high speed cars, particularly drag racers, with the all-time record held by a rocket car.

iv. Jet engine designs are frequently modified for non-aircraft applications, as industrial gas turbines or marine power plants.

v. These are used in electrical power generation, for powering water, natural gas, or oil pumps, and providing propulsion for ships and locomotives. Industrial gas turbines can create up to 50,000 shaft horsepower.

vi. Jet engines are also sometimes developed into, or share certain components such as engine cores, with turbo shaft and turboprop engines, which are forms of gas turbine engines that are typically used to power helicopters and some propeller-driven aircraft.

VARIOUS TYPES OF JET ENGINES

Water jet

For propelling boats; squirts water out the back through a nozzle.

Motor jet

Most primitive air breathing jet engine. Essentially a supercharged piston engine with a jet exhaust. Higher exhaust velocity than a propeller, offering better thrust at high speed.

Turbojet

A tube with a compressor and turbine sharing a common shaft with a burner in between and a propelling nozzle for the exhaust. Uses a high exhaust gas velocity to produce thrust. Has a much higher core flow than bypass type engines. Simplicity of design, efficient at supersonic speeds.

Turbofan

A turbofan is a type of jet engine, similar to a turbojet. It essentially consists of a large ducted fan with a smaller diameter turbojet engine mounted behind it that provides propulsion and also powers the fan. Part of the airstream from the ducted fan passes through the turbojet, providing oxygen to burn fuel to power the turbojet. But part, usually most, of the flow bypasses the turbojet, and is accelerated by turbine blades acting like a propeller. The combination of these two processes produces thrust more efficiently than other jet designs. Turbofans have a net exhaust speed that is much lower than a turbojet. This makes them much more efficient at subsonic speeds than turbojets, and somewhat more efficient at supersonic speeds up to roughly Mach 1.6.

All of the jet engines used in currently manufactured commercial jet aircraft are turbofans. They are used commercially mainly because they are highly efficient and relatively quiet in operation. Turbofans are also used in many military jet aircraft.

Low-bypass Turbofan

One- or two-stage fan added in front bypasses a proportion of the air through a bypass chamber surrounding the core. Compared with its turbojet ancestor, this allows for more efficient operation with somewhat less noise. This is the engine of high-speed military aircraft, some smaller private jets, and older civilian airliners such as the Boeing 707, the McDonnell Douglas DC-8, and their derivatives.

High-bypass Turbofan

First stage compressor drastically enlarged to provide bypass airflow around engine core, and it provides significant amounts of thrust. Compared to the low-bypass turbofan and no-bypass turbojet, the high-bypass turbofan works on the principle of moving a great deal of air somewhat faster, rather than a small amount extremely fast. Most common form of jet engine in civilian use today- used in airliners like the Boeing 747, most 737s, and all Airbus aircraft.

Scramjet

Similar to a ramjet without a diffuser; airflow through the entire engine remains supersonic. Few mechanical parts, can operate at very high Mach numbers (Mach 8 to 15) with good efficiencies.

Ramjet

Intake air is compressed entirely by speed of oncoming air and divergent shape, and then it goes through a burner section where it is heated and then passes through a propelling nozzle. Very few moving parts, Mach 0.8 to Mach 5+, efficient at high speed (> Mach 2.0 or so), lightest of all air-breathing jets (thrust / weight ratio up to 30 at optimum speed), cooling much easier than turbojets as no turbine blades to cool.

Pulsejet

Air is compressed and combusted intermittently instead of continuously. Some designs use valves. Very simple design, commonly used on model aircraft.

Pulse detonation engine

Similar to a pulsejet, but combustion occurs as a detonation instead of a deflagration, may or may not need valves.

Rocket

Carries all propellants and oxidants on-board, emits jet for propulsion. Very few moving parts, Mach 0 to Mach 25+, efficient at very high speed (> Mach 10.0 or so), thrust / weight ratio over 100, no complex air inlet, high compression ratio, very high speed (hypersonic) exhaust, good cost / thrust ratio, fairly easy to test, works in a vacuum-indeed works best exo-atmospheric which is kinder on vehicle structure at high speed, fairly small surface area to keep cool, and no turbine in hot exhaust stream

Air-augmented rocket

Essentially a ramjet where intake air is compressed and burnt with the exhaust from a rocket.

Turbo-rocket

A turbojet where an additional oxidizer such as oxygen is added to the airstream to increase maximum altitude. Very close to existing designs, operates in very high altitude, wide range of altitude and airspeed.

Pre-cooled jets

Intake air is chilled to very low temperatures at inlet in a heat exchanger before passing through a ramjet and / or turbojet and / or rocket engine. Easily tested on ground. Very high thrust / weight ratios are possible (~14) together with good fuel efficiency over a wide range of airspeeds, mach 0-5.5+; this combination of efficiencies may permit launching to orbit, single stage, or very rapid, very long distance intercontinental travel.

Brief idea on diesel engine

DIESEL ENGINE

                The diesel engine (also known as a compression-ignition or 'CI' engine) is an internal combustion engine in which ignition of the fuel that has been injected into the combustion chamber is initiated by the high temperature which a gas achieves when greatly compressed (adiabatic compression). This contrasts with spark-ignition engines such as a petrol engine (gasoline engine) or gas engine (using a gaseous fuel as opposed to gasoline), which use a spark plug to ignite an air-fuel mixture.

                The diesel engine has the highest thermal efficiency (engine efficiency) of any practical internal or external combustion engine due to its very high compression ratio and inherent lean burn which enables heat dissipation by the excess air. A small efficiency loss is also avoided compared to two-stroke non-direct-injection gasoline engines since unburnt fuel is not present at valve overlap and therefore no fuel goes directly from the intake/injection to the exhaust. Low-speed diesel engines (as used in ships and other applications where overall engine weight is relatively unimportant) can have a thermal efficiency that exceeds 50%.

WORKING

Diesel engines work by internal combustion. First, air is allowed into the cylinder and the piston compresses it—but much more than in a petrol engine. In a petrol engine, the fuel-air mixture is compressed to about a tenth of its original volume. But in a diesel engine, the air is compressed by anything from 14 to 25 times. If you've ever pumped up a bicycle tire, you'll have felt the pump getting hotter in your hands the longer you used it. That's because compressing a gas generates heat. Imagine, then, how much heat is generated by forcing air into 14-25 times less space than it normally takes up. So much heat, as it happens, that the air gets really hot—usually at least 500°C (1000°F) and sometimes very much hotter. Once the air is compressed, a mist of fuel is sprayed into the cylinder typically (in a modern engine) by an electronic fuel-injection system, which works a bit like a sophisticated aerosol can. The air is so hot that the fuel instantly ignites and explodes without any need for a spark plug. This controlled explosion makes the piston push back out of the cylinder, producing the power that drives the vehicle or machine in which the engine is mounted. When the piston goes back into the cylinder, the exhaust gases are pushed out through an exhaust valve and, the process repeats itself.

REASONS FOR MANUFACTURING

Diesel engines are manufactured in two-stroke and four-stroke versions. They were originally used as a more efficient replacement for stationary steam engines. Since the 1910s they have been used in submarines and ships. Use in locomotives, trucks, heavy equipment and electricity generation plants followed later. In the 1930s, they slowly began to be used in a few automobiles. Since the 1970s, the use of diesel engines in larger on-road and off-road vehicles in the USA increased. According to the British Society of Motor Manufacturing and Traders, the EU average for diesel cars account for 50% of the total sold, including 70% in France and 38% in the UK.

The world's largest diesel engine is currently a Wärtsilä-Sulzer RTA96-C Common Rail marine diesel, which produces a peak power output of 84.42 MW (113,210 hp) at 102 rpm.

 APPLICATION

i. Diesel engines are commonly used as mechanical engines, power generators and in mobile drives.

ii. They find wide spread use in locomotives, construction equipment, automobiles, and countless industrial applications.

iii. Industrial diesel engines and diesel powered generators have construction, marine, mining, hospital, forestry, telecommunications, underground, and agricultural applications, just to name a few.

iv. Power generation for prime or standby backup power is the major application of today's diesel generators.

ADVANTAGES

i. Diesel engines have several advantages over other internal combustion engines:

ii. They burn less fuel than a petrol engine performing the same work, due to the engine's higher temperature of combustion and greater expansion ratio.

iii. The longevity of a diesel engine is generally about twice that of a petrol engine due to the increased strength of parts used.

iv. Diesel fuel has better lubrication properties than petrol as well.

v. Diesel fuel is considered safer than petrol in many applications.

vi. The low vapour pressure of diesel is especially advantageous in marine applications, where the accumulation of explosive fuel-air mixtures is a particular hazard.

vii. For any given partial load the fuel efficiency (mass burned per energy produced) of a diesel engine remains nearly constant.

viii. They generate less waste heat in cooling and exhaust.

ix. Diesel engines can accept super- or turbo-charging pressure without any natural limit, constrained only by the strength of engine components.

x. Biodiesel is an easily synthesized, non-petroleum-based fuel.

DISADVANTAGES

i. Diesel engines are expensive than petrol engines.

ii. Higher maintenance cost.

iii. Higher engines noise.

iv. Higher engine vibration.

v. Diesel engine emits more harmful gases to the environment.

vi. Sluggish acceleration.

Brief idea on petrol engine.

PETROL ENGINES

                A petrol engine (known as a gasoline engine in American English) is an internal combustion engine with spark-ignition, designed to run on petrol (gasoline) and similar volatile fuels. It was invented in 1876 in Germany by German inventor Nikolaus August Otto. The first petrol combustion engine (one cylinder, 121.6 cm³ displacement) was prototype in 1882 in Italy by Enrico Bernardi. In most petrol engines, the fuel and air are usually pre-mixed before compression (although some modern petrol engines now use cylinder-direct petrol injection). The pre-mixing was formerly done in a carburetor, but now it is done by electronically controlled fuel injection, except in small engines where the cost/complication of electronics does not justify the added engine efficiency. The process differs from a diesel engine in the method of mixing the fuel and air, and in using spark plugs to initiate the combustion process. In a diesel engine, only air is compressed (and therefore heated), and the fuel is injected into very hot air at the end of the compression stroke, and self-ignites.

WORKING

i. Suction of air (is also known as breathing or aspiration).

ii. Mixing of the fuel with air after breaking the liquid fuel into highly atomised / mist form.

iii. Igniting the air-fuel mixture with an electric spark using spark plug.

iv. Burning of highly atomised fuel particles; which results in releasing / ejection of heat energy.

TYPES OF FUEL INJECTION SYSTEM IN PETROL ENGINES

Continuous type:-

In this type petrol is injected in to the inlet manifold continuously when the engine is running.

Intermittent type:-

In this type petrol is injected during the suction stroke only it is also known as timed or jerk type.

Direct type:-

In this type petrol is feed directly in to the glider.

Indirect type:-

In this type petrol is injected in to the inlet port or in to the inlet manifold.

Single point:-

In this type the petrol is injected through single injector.

Multi point :-

In this type petrol is injected by number of injector it is more advance and recent development it is computer control system it is having high fuel efficiency.

Electronic petrol injection system(commonly known as fuel injection):-

- In this system electronically controlled metering valve is used.

- The metering valve meters the desired quantity of petrol and supplied to injector.

- The opening injector is also controlled by electronically controlled so that electronic unit known as ECU which consider computer and sensor.

- The sensors sense the various engine conditions [tempter load speed air pressure] and sends the singles to the computer [E.C.U/E.C.M] the computer reads the singles to the sensor to operate the pump and nozzle.

Some disadvantage of using carburetor:

i.  With single carburetor it is difficult to supply to mixture uniformly to all cylinder.

ii. Ventura throat of carburetor restricts the smooth flow of mixture.

iii. Chock restricts the flow of mixture.

To solve the above said problem, electronic injection is necessary.

WORKING CYCLES

Petrol engines may run on the four-stroke cycle or the two-stroke cycle. For details of working cycles see:

·         Four-stroke cycle

·         Two-stroke cycle

·         Wankel engine

CYLINDER CONFIGURATION

Common cylinder arrangements are from 1 to 6 cylinders in-line or from 2 to 16 cylinders in V-formation. Flat engines – like a V design flattened out – are common in small airplanes and motorcycles and were a hallmark of Volkswagen automobiles into the 1990s. Flat 6sare still used in many modern Porsches, as well as Subarus. Many flat engines are air-cooled. Less common, but notable in vehicles designed for high speeds is the W formation, similar to having 2 V engines side by side. Alternatives include rotary and radial engines the latter typically have 7 or 9 cylinders in a single ring, or 10 or 14 cylinders in two rings.

COOLING

Petrol engines may be air-cooled, with fins (to increase the surface area on the cylinders and cylinder head); or liquid-cooled, by a water jacket and radiator. The coolant was formerly water, but is now usually a mixture of water and either ethylene glycol or propylene glycol. These mixtures have lower freezing points and higher boiling points than pure water and also prevent corrosion, with modern antifreezes also containing lubricants and other additives to protect water pump seals and bearings. The cooling system is usually slightly pressurized to further raise the boiling point of the coolant.

IGNITION

Petrol engines use spark ignition and high voltage current for the spark may be provided by a magneto or an ignition coil. In modern car engines the ignition timing is managed by an electronic Engine Control Unit.

ADVANTAGES

i. Cheaper than a diesel engine.

ii. Less maintenance cost.

iii. Easy to construct and repair.

iv. Can be used in light weight vehicles.

v. Smooth, less vibration and better acceleration than a diesel engine.

DISADVANTAGES

i. Less mileage.

ii. Supply of is decreasing and one will petrol supplies will be exhausted.

iii. Price of petrol is increasing everyday.

iv. Burning of petrol affects the environment as it produces carbon.

v. Transport of petrol is dangerous.      

vi. The volatile components of petrol cause smog.
vii. Less durable than diesel engine.

CONCLUSION

                Petrol engines run at higher speeds than diesels, partially due to their lighter pistons, connecting rods and crankshaft (a design efficiency made possible by lower compression ratios) and due to petrol burning more quickly than diesel. Because pistons in petrol engines tend to have much shorter strokes than pistons in diesel engines, typically it takes less time for a piston in a petrol engine to complete its stroke than a piston in a diesel engine. However the lower compression ratios of petrol engines give petrol engines lower efficiency than diesel engines.

Discussion on Internal Combustion Engines

INTERNAL COMBUSTION ENGINE

WHAT IS AN INTERNAL COMBUSTION (IC) ENGINE?

An internal combustion engine (ICE) is a heat engine where the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal combustion engine the expansion of the high-temperature and high-pressure gases produced by combustion apply direct force to some component of the engine. The force is applied typically to pistons, turbine blades, or a nozzle. This force moves the component over a distance, transforming chemical energy into useful mechanical energy. The first commercially successful internal combustion engine was created by Étienne Lenoir around 1859^[1] and the first modern internal combustion engine was created in 1864 by Siegfried Marcus.

The term internal combustion engine usually refers to an engine in which combustion is intermittent, such as the more familiar four-stroke and two-stroke piston engines, along with variants, such as the six-stroke piston engine and the Wankel rotary engine. A second class of internal combustion engines use continuous combustion: gas turbines, jet engines and most rocket engines, each of which are internal combustion engines on the same principle as previously described.

HOW THEY ARE DIFFERENT FROM EXTERNAL COMBUSTION ENGINE?

Internal combustion engines are quite different from external combustion engines, such as steam or Stirling engines, in which the energy is delivered to a working fluid not consisting of, mixed with, or contaminated by combustion products. Working fluids can be air, hot water, pressurized water or even liquid sodium, heated in a boiler. ICEs are usually powered by energy-dense fuels such as gasoline or diesel, liquids derived from fossil fuels. While there are many stationary applications, most ICEs are used in mobile applications and are the dominant power supply for vehicles such as cars, aircraft, and boats.

Typically an ICE is fed with fossil fuels like natural gas or petroleum products such as gasoline, diesel fuel or fuel oil. There's a growing usage of renewable fuels like biodiesel for compression ignition engines and bioethanol for spark ignition engines. Hydrogen is sometimes used, and can be made from either fossil fuels or renewable energy.

IC ENGINES CAN BE CLASSIFIED AS:-

In terms of number of stroke

i. Two stroke engine- A two-stroke, or two-cycle, engine is a type of internal combustion engine which completes a power cycle with two strokes (up and down movements) of the piston during only one crankshaft revolution. This is in contrast to a "four-stroke engine", which requires four strokes of the piston to complete a power cycle. In a two-stroke engine, the end of the combustion stroke and the beginning of the compression stroke happen simultaneously, with the intake and exhaust functions occurring at the same time.

Two-stroke engines often have a high power-to-weight ratio, usually in a narrow range of rotational speeds called the "power band". Compared to four-stroke engines, two-stroke engines have a greatly reduced number of moving parts, and so can be more compact and significantly lighter.

ii. Four Stroke- A four-stroke engine (also known as four-cycle) is an internal combustion (IC) engine in which the piston completes four separate strokes while turning a crankshaft. A stroke refers to the full travel of the piston along the cylinder, in either direction. The four separate strokes are termed:

1.      Induction: This stroke of the piston begins at Top Dead Center (T.D.C.) and ends at Bottom Dead Center (B.D.C.). In this stroke the intake valve must be in the open position while the piston pulls an air-fuel mixture into the cylinder by producing vacuum pressure into the cylinder through its downward motion.

2.      Compression: This stroke begins at B.D.C, or just at the end of the suction stroke, and ends at T.D.C. In this stroke the piston compresses the air-fuel mixture in preparation for ignition during the power stroke (below). Both the intake and exhaust valves are closed during this stage.

3.      Power: This is the start of the second revolution of the four stroke cycle. At this point the crankshaft has completed a full 360 degree revolution. While the piston is at T.D.C. (the end of the compression stroke) the compressed air-fuel mixture is ignited by a spark plug (in a gasoline engine) or by heat generated by high compression (diesel engines), forcefully returning the piston to B.D.C. This stroke produces mechanical work from the engine to turn the crankshaft.

4.      Exhaust: During the exhaust stroke, the piston once again returns to T.D.C from B.D.C while the exhaust valve is open. This action expels the spent air-fuel mixture through the exhaust valve.

iii. Six strokes- The six-stroke engine is a type of internal combustion engine based on the four-stroke engine, but with additional complexity intended to make it more efficient and reduce emissions. Two types of six-stroke engine have been developed since the 1890s:

In the first approach, called the single piston design, the engine captures the heat lost from the four-stroke Otto cycle or Diesel cycle and uses it to power an additional power and exhaust stroke of the piston in the same cylinder. Designs use either steam or air as the working fluid for the additional power stroke. The pistons in this type of six-stroke engine go up and down three times for each injection of fuel. There are two power strokes: one with fuel, the other with steam or air.

The second approach, called the opposed piston design, uses a second opposed piston in each cylinder that moves at half the cyclical rate of the main piston, thus giving six piston movements per cycle. Functionally, the second piston replaces the valve mechanism of a conventional engine but also increases the compression ratio.

                                                                            

APPLICATION OF INTERNAL COMBUSTION ENGINES:-

i. Internal combustion engines are most commonly used for mobile propulsion in automobiles, equipment, and other portable machinery.

ii. In mobile scenarios internal combustion is advantageous, since it can provide high power to weight ratios together with excellent fuel energy-density.

iii. These engines have appeared in almost all automobiles, motorcycles, boats, and in a wide variety of aircraft and locomotives. Where very high power is required, such as jet aircraft, helicopters, and large ships, they appear mostly in the form of turbines.

iv. They are also used for electric generators and by industry.