AERO ENGINES

Well this is the topic most of the aircraft lovers want to know or must know. I, myself being one, was greatly interested in this topic. Many think understanding this stuff is difficult, but in reality it's very simple.

Let's start :

First let's look at the types of engines -

• Reciprocating type - Also known as piston engines. These are mostly used in trainer aircrafts and in flying clubs.
• Air Breathing engines - Most commercial jets and Fighter jets use these engines.

Reciprocating (Piston) engines -

These engines use a normal assembly of cylinders fitted with pistons which compress the air and fuel mixtures. Most common are

• IC (Internal combustion) engines, used extensively in petrol cars.
• CI (Compression Ignition) engines used in diesel cars.

Internal combustion (IC) engines :-

The above image shows a general four stroke IC engine.

E = Exhaust camshaft

S = Spark plug

I = Inlet camshaft

W = Waterjacket for coolant flow

V = Valves

P = Piston

C = Crankshaft

R = Connecting Rod

Working :-

As we see from the above diagram, there are four strokes. Let's discuss the four strokes of this engine

• STROKE 1 : Intake stoke. In this the piston moves down. This action leads to the opening of the inlet valve, letting the air and fuel volume inside the cylinder.
• STROKE 2 : Compression stroke. The piston moves upwards compressing the air-fuel mixture. During the compression both the valves (I and E) remain closed. During compression the temperature and pressure increases to a great extent.
• STROKE 3 : Power stroke. The sparkplug creates a spark in the compressed mixture. This spark creates a combustion pushing the piston down with a great force.
• STROKE 4 : Exhaust stroke. The exhaust formed after combustion moves out of the exhaust line. The exhaust valve opens up letting the residual air out.

This is a general working of a 4-stroke engine.

Compression Ignition (CI) engine :-

The working Is similar to that of the IC engine, only difference is that it does not have a sparking mechanism.
In a diesel engine, only air is introduced into the combustion chamber. The air is then compressed with pressures reaching 40-bar. This high compression heats the air. Fuel is injected directly into the compressed air in the combustion chamber. The fuel injector ensures that the fuel is broken down into small droplets and is distributed evenly. The heat of the compressed air vaporizes fuel. The vapour is then ignited by the heat from the compressed air in the combustion chamber, the droplets continue to vaporise from their surfaces and burn, getting smaller, until all the fuel in the droplets has been burnt. As the vapour reaches ignition temperature it causes an abrupt increase in pressure above the piston. The rapid expansion of combustion gases then drives the piston downward, supplying power to the crankshaft.

AIR BREATHING ENGINES -

Most commercial aircrafts use this type of engine. There are many sub-categories in this type

Let's look at the two main types -

• Turbine powered - 
• Turbojet
• Turbofan
• Turboprop
• Ram powered -
• RAMJET
• SCRAMJET

Starting with Turbine engines -

Turbojet engines -

The above image shows the schematic of a turbojet engnie

As shown in the above diagram, there are two sections - Hot section
                                                                                        - Cold section

The cold section contains two types of axial compressors -

1. Low pressure axial compressor - This compressor rotates so that maximum air comes in through the intake opening. It also compresses the air but since the volume is more in the chamber, more air can be trapped, so the output pressure is low too.

2. High pressure axial compressor - This compressor again compresses the air coming through the low pressure axial compressor. But this time the volume is less than previous one. But the amount of air trapped in the chamber is same as previous. This leads to high pressure compression.

Due to both of the above compression actions, the temperature and pressure of the resulting air increases to a much large extent.

• eg - Let inlet temp and pressure be = 10°C, 40 bar resp
•        After the final cold section compression temp and pressure = 400°C, 350 bar resp

Now comes the Hot section consisting of - Combustion chamber
                                                                   - Exhaust turbine

1. Combustion chamber -

The above image shows the combustion chamber of a turbojet engine.

• The holes in the front side are given for fuel inlet.
• The holes on the sides of the entire tube structure is for the compressed gas coming through the compressors.

As discussed above, the temperature of the air entering the compression chamber is very high. As the fuel comes in drop by drop through the fuel nozzle gets heated up. This causes the air-fuel mixture to heat up and combustion occurs.

2. Exhaust turbine - The combusted heated air rotates another turbine on the rear end. This rotating action cools down the combusted exhaust gas. The exhaust turbines are internally connected to the front intake turbines.

Turbofan engine - 

FIG. A - LOW BYPASS TURBOFAN ENGINE

FIG. B - HIGH BYPASS TURBOFAN ENGINE

The above image shows the schematic of a turbofan engine.

The working of this engine is same as that of the turbojet engine.

• Only exception is that it has a fan attached to the front turbine.
• The fan creates a bypass.
• The air flowing towards the engine is divided into two sections -
• One which flows near the centre of the fan, passes through the compressor, combustor and finally comes out through the exhaust.
• Other part of the air which is flowing towards the upper part of the fan flows above the compressor-combustion assembly in the exhaust.
• This ratio of the air flowing out into the exhaust to the air into the gas combustion chamber is called the bypass ratio.
• eg - Bypass ratio of 10:1 denotes that for 10kg of air passing directly into the exhaust, 1kg of air passes into the gas combustion chamber.
• Depending on the bypass ratio, the turbofan engine is further divided into -
• High bypass turbofan - 
• Shown in fig. B
• These engines produce maximum thrust through the bypassed air.
• The thrust produced is low than low bypass engines.
• Mostly used in commercial aircrafts. 
• Low bypass turbofan - 
• Shown in fig. A
• These engines produce maximum thrust through the combustion.
• So the thrust produced is much much higher than high bypass engines.
• They are normally used in combat aircrafts due to the need of supersonic speeds.

The above video shows the working of the turbofan engine.

Turboprop engine -

The above image shows the schematic of a turboprop engine.

• Working is same as that of turbojet.
• A propeller rotates which is controlled by the gear box.

Exception -

• A part of the engine thrust is provided by the peopellers rather than completely relying on the jet thrusts.
• The performance of the turboprop is best than turbojets or turbofans at low altitudes.
• Mostly used in show aircrafts and trainer aircrafts.

Ram powered engine -

These engines are known as air breathing engines. They do not contain any mechanical moving parts. Entire functioning is dependent on the design of the engine and the amount of air the engine breathes, Ram powered engines can never start moving the aircraft at zero airspeed.
Ram engines are divided into two parts -

• Ramjet
• Scramjet

Ramjet engines - 

• As shown above, ramjet engine takes in the entire air coming into it.
• The design is such that the volume in the upper part of the engine is small ie the air gets compressed at high pressure.
• Fuel is then sprayed over the compressed air.
• The compressed air gains supersonic speed when it comes out of the exhaust.
• Speeds of upto Mach 3 can be gained.

Scramjet engine - 

Working of the Scramjet is similar to that of ramjet engines

Exceptions - 

• Scramjet engine forcefully compresses the incoming air before combustion. While ramjet engine decelerates the air to subsonic speed.
• Airflow in scramjet is supersonic through out the entire operation, which is not the case for ramjets.
• So projected (not tested) speeds of scramjets may reach upto Mach 12. !!!

WORKING OF A RAM POWERED ENGINE

Thank you !!!!

Any improvements or adding extra info is welcome.