COMBUSTION ENGINE

By atap_aje

1.0 INTRODUCTION

This study ( laboratory/lab.) describes the basic components found in the internal combustion engine; two-stroke engine, four-stroke engine and rotary engine, thus explains how their are operations. The internal combustion engine is a heat engine in which the burning of a fuel occurs in a confined space called a combustion chamber.

As already learned that an engine is classified according to the number of strokes its piston takes to complete one full engine cycle. In order for any engine to operate, it must run through four stages of operation: intake, compression, power and exhaust.

The four-stroke engine accomplishes these four stages in four piston strokes-one stroke for each stage. As known, in a two-stroke engine the piston takes only two strokes to complete one full operational cycle. When the piston in a two-stroke engine moves in an upward direction, it completes the intake and compression stages. When the piston moves downward, it completes the power and the exhaust stages.

A rotary engine works in a completely different way than the conventional piston engine. In a piston engine, the same volume of space (the cylinder) alternately does four different jobs; intake, compression, combustion and exhaust. A rotary engine does these same four jobs, but each one happens in its own part of the housing. It's kind of like having a dedicated cylinder for each of the four jobs, with the piston moving continually from one to the next. Like a piston engine, the rotary engine uses the pressure created when a combination of air and fuel is burned. In a piston engine, that pressure is contained in the cylinders and forces pistons to move back and forth. The connecting rods and crankshaft convert the reciprocating motion of the pistons into rotational motion that can be used to power a car.

Internal combustion engines are most commonly used for mobile propulsion systems. In mobile scenarios internal combustion is advantageous, since it can provide high power to weight ratios together with excellent fuel energy-density. These engines have appeared in almost all automobiles, motorbikes, many 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 gas turbines. They are also used for electric generators and by industry.

2.0 OBJECTIVE

After the Laboratory, student should be able to do the following:

Understand the operational of the internal combustion engine.
Understand the process of converting reciprocating motion to rotating motion.
Identify the four-strokes, two-stroke ad rotary engine of the internal combustion engine.
Identify the differences between the petrol engine and the Diesel engine.

3.0 APPARATUS

The model of internal combustion engine ( two-stroke, four-stroke and rotary engine ) shown by lecturer.

Petrol4Stroke871_M

Model of four-stroke ( Petrol )

4.0 PRINCIPLE

The combustion process in the petrol engine and the diesel engine differ in the following significant features: in the petrol engine the petrol and air mixture is drawn into the cylinder, compressed (compression ratio ranging from 4:1 to 10:1), and ignited by a spark introduced by an electrical system. In the diesel engine, on the other hand, air alone is drawn into the cylinder and is compressed to a much higher ratio (14:1 to 25:1) than in the petrol engine. As a result of this compression the air is heated to a temperature of 700 – 900 ^oC. Only then is a certain quantity of diesel fuel injected in to the cylinder and because of the prevailing high temperature the fuel ignites spontaneously. Hence the petrol engine is often referred to as the spark ignition (SI) engine and the diesel as the compression ignition (CI) engine.

Secondly, there is the sub-division according to cycle type; the two stroke or four stroke cycle. This categorization differentiates between engines, which have an ignition phase on every revolution of the crankshaft or every other revolution. The method for mixing and injecting air and fuel is different for the two cycle types.

In a rotary engine, the pressure of combustion is contained in a chamber formed by part of the housing and sealed in by one face of the triangular rotor, which is what the engine uses instead of pistons. The rotor follows a path that looks like something you'd create with a Spiro graph. This path keeps each of the three peaks of the rotor in contact with the housing, creating three separate volumes of gas. As the rotor moves around the chamber, each of the three volumes of gas alternately expands and contracts. It is this expansion and contraction that draws air and fuel into the engine, compresses it and makes useful power as the gases expand, and then expels the exhaust.

In these three cases ; the intake valve opens at a precise time to allow the air/fuel mixture to enter the cylinder. The exhaust valve opens at a precise time to allow the burned gases to leave the cylinder. The spark plug ignites the air/fuel mixture in the cylinder, which creates an explosion. The force of the explosion is transferred to the piston. The piston travels up and down in a reciprocation motion. The force from the piston is then transferred to the crankshaft through the piston rod (connecting rod). The piston rod converts the reciprocating motion of the piston, to the rotating motion of the crankshaft.

5.0 RESULT AND EXPLAINATION

5.1 FOUR-STROKE

Engines based on the four-stroke cycle or Otto cycle have one power stroke for every four strokes (up-down-up-down). These engine-operating cycle of most petrol and diesel engines. In a petrol engine the cycle begins with the induction of a fuel mixture as the piston goes down on its first stroke. On the second stroke (up) the piston compresses the mixture in the top of the cylinder. An electric spark then ignites the mixture, and the gases produced force the piston down on its third, power, stroke. On the fourth stroke (up) the piston expels the burned gases from the cylinder into the exhaust.

These engine always applications in cars, larger boats and many light aircraft. They are generally quieter, more efficient and larger than their two-stroke counterparts. Most truck and automotive diesel engines use a four-stroke cycle, but with a compression heating ignition system. This variation is called the diesel cycle.

5.1.1 DIESEL ENGINE

Internal-combustion engine that burns a lightweight fuel oil. The diesel engine operates by compressing air until it becomes sufficiently hot to ignite the fuel. It is a piston-in-cylinder engine, like the petrol engine, but only air (rather than an air-and-fuel mixture) is taken into the cylinder on the first piston stroke (down). The piston moves up and compresses the air until it is at a very high temperature. The fuel oil is then injected into the hot air, where it burns, driving the piston down on its power stroke. For this reason the engine is called a compression-ignition engine.

Diesel engines have sometimes been marketed as ‘cleaner’ than petrol engines because they do not need lead additives and produce fewer gaseous pollutants. However, they do produce high levels of the tiny black carbon particles called particulates, which are believed to be carcinogenic and may exacerbate or even cause asthma.

5.1.1.1 DIESEL ENGINE CYCLE

image024a The Intake Stroke.

On the intake stroke, the intake valve has opened.

The piston is moving down, and a mixture of air

and vaporized fuel is being pushed by atmospheric

pressure into the cylinder through

the intake valve port.

The Compression Stroke.

image024b

The piston is at bottom dead center

at the beginning of the compression

stroke, and, as the piston moves upward,

the air compresses. As the piston

reaches top dead center, the compression

stroke ends.

The Power Stroke.

image024d The piston begins the power stroke at

top dead center. The air is compressed to

as much as 500 psi and at a compressed

temperature of approximately 1000°F. At

this point, fuel is injected into the

combustion chamber and is ignited by the

heat of the compression. This begins the

power stroke. The expanding force of the

burning gases pushes the piston downward, providing power to the crankshaft. The

diesel fuel will continue to bum through the entire power stroke (a more complete

burning of the fuel).

image024e

The Exhaust Stroke.

As the piston reaches bottom dead center

on the power stroke, the power stroke ends

and the exhaust stroke begins. The exhaust

valve opens and as the piston rises towards

top dead center, the burnt gases are pushed

out through the exhaust port. As the piston

reaches top dead center, the exhaust valve

closes and the intake valve opens. The engine is now ready to begin another

operating cycle.

5.1.2 PETROL ENGINE

The most commonly used source of power for motor vehicles, introduced by the German engineers Gottlieb Daimler and Karl Benz in 1885. The petrol engine is a complex piece of machinery made up of about 150 moving parts. It is a reciprocating piston engine, in which a number of pistons move up and down in cylinders. A mixture of petrol and air is introduced to the space above the pistons and ignited. The gases produced force the pistons down, generating power. The engine-operating cycle is repeated every four strokes (upward or downward movement) of the piston, this being known as the four-stroke cycle. The motion of the pistons rotate a crankshaft, at the end of which is a heavy flywheel. From the flywheel the power is transferred to the car's driving wheels via the transmission system of clutch, gearbox, and final drive.

The parts of the petrol engine can be subdivided into a number of systems. The fuel system pumps fuel from the petrol tank into the carburetor. There it mixes with air and is sucked into the engine cylinders. (With electronic fuel injection, it goes directly from the tank into the cylinders by way of an electronic monitor.) The ignition system supplies the sparks to ignite the fuel mixture in the cylinders. By means of an ignition coil and contact breaker, it boosts the 12-volt battery voltage to pulses of 18,000 volts or more. These go via a distributor to the spark plugs in the cylinders, where they create the sparks. (Electronic ignitions replace these parts.) Ignition of the fuel in the cylinders produces temperatures of 700°C/1,300°F or more, and the engine must be cooled to prevent overheating.

Most engines have a water-cooling system, in which water circulates through channels in the cylinder block, thus extracting the heat. It flows through pipes in a radiator, which are cooled by fan-blown air. A few cars and most motorcycles are air-cooled, the cylinders being surrounded by many fins to present a large surface area to the air. The lubrication system also reduces some heat, but its main job is to keep the moving parts coated with oil, which is pumped under pressure to the camshaft, crankshaft, and valve-operating gear.

5.1.2.1 PETROL ENGINE CYCLE

4stroke1 The Intake Stroke.

On the intake stroke, the intake valve has

opened. The piston is moving down, and a

mixture of air and vaporized fuel is being

pushed by atmospheric pressure into the

cylinder through the intake valve port.

4stroke2a The Compression Stroke.

After the piston reaches the lower limit of

its travel, it begins to move upward. As this

happens, the intake valve closes. The exhaust

valve is also closed, so the cylinder is sealed.

As the piston moves upward, the air/fuel

mixture is compressed. On some small high

compression engines, by the time the piston

reaches the top of its travel, the mixture is compressed to as little as one-tenth its original volume. Thus, the compression of the air/fuel mixture increases the pressure in the cylinder. The compression process also creates the air/fuel mixture to increase in temperature.

4stroke3a The Power Stroke.

As the piston reaches the top of its travel on the

compression stroke, an electric spark is produced

at the spark plug. The ignition system delivers

a high voltage surge of electricity to the spark

plug to create the spark. The spark ignites the

air/fuel mixture. The mixture burns rapidly and

cylinder pressure increases to as much as (600psi).

All of this pressure against the piston forces it

down in the cylinder. The power impulse is transmitted down through

the piston, through the piston rod (connecting rod) and to the crankshaft. The crankshaft

is rotated due to the force.

4stroke4a The Exhaust Stroke.

As the piston reaches the bottom of its travel, the

Exhaust valve opens. Now, as the piston moves up

on the exhaust stroke, it forces the burned gases out

of the cylinder through the exhaust port. When the

piston reaches the top of its travel, the exhaust valve

closes, and the intake valve opens. The cycle repeats

again with the intake stroke. The four strokes are continuously

repeated during the operation of the engine.

5.2 TWO-STROKE

Two-stroke Basics

This is what a two-stroke engine looks like:

two-stroke-parts

Engines based on the two-stroke cycle use two strokes (one up, one down) for every power stroke. Since there are no dedicated intake or exhaust strokes, alternative methods must be used to scavenge the cylinders. The most common method in spark-ignition two-strokes is to use the downward motion of the piston to pressurize fresh charge in the crankcase, which is then blown through the cylinder through ports in the cylinder walls. Spark-ignition two-strokes are small and light (for their power output), and mechanically very simple.

Common applications include snowmobiles, lawnmowers, chain saws, jet skis, mopeds, outboard motors and some motorcycles. Unfortunately, they are also generally louder, less efficient, and far more polluting than their four-stroke counterparts, and they do not scale well to larger sizes. Interestingly, the largest compression-ignition engines are two-strokes, and are used in some locomotives and large ships. These engines use forced induction to scavenge the cylinders

5.2.1 TWO-STROKE CYCLE

Remember, the two-stroke engine must go through the same four stages of engine operation as any internal-combustion engine—intake, compression, power, and exhaust. However, where the four-stroke engine uses one piston stroke to accomplish each stage, the two-stroke engine accomplishes the four stages in just two piston

strokes. Each time the piston moves upward, it completes the intake and compression stages. Each time the piston moves downward, it completes the power and exhaust stages. Because two stages of engine operation occur for each piston stroke, the operation of the two-stroke engine is more complex.

In a two-stroke engine, as the piston moves upward, it completes the intake and compression stages of operation. As the piston moves downward, it completes the power and exhaust stages of operation.

5.3 COMPARISON

A water-cooled two-stroke and four-stroke engine having equal displacements are pictured here for comparison.

5.4 ROTARY ENGINE

rotary-engine-200

A rotary engine is an internal combustion

engine, like the engine in your car, but it works in

a completely different way than the conventional

piston engine.

In a piston engine, the same volume of

space (the cylinder) alternately does four different

jobs -- intake, compression, combustion and exhaust

A rotary engine does these same four jobs, but each one happens in its own part of the housing. It's kind of like having a dedicated cylinder for each of the four jobs, with the piston moving continually from one to the next.

Like a piston engine, the rotary engine uses the pressure created when a combination of air and fuel is burned. In a piston engine, that pressure is contained in the cylinders and forces pistons to move back and forth. The connecting rods and crankshaft convert the reciprocating motion of the pistons into rotational motion that can be used to power a car.

The rotor follows a path that looks like something you'd create with a Spiro graph. This path keeps each of the three peaks of the rotor in contact with the housing, creating three separate volumes of gas. As the rotor moves around the chamber, each of the three volumes of gas alternately expands and contracts. It is this expansion and contraction that draws air and fuel into the engine, compresses it and makes useful power as the gases expand, and then expels the exhaust.

5.4.1 ROTARY ENGINE CYCLE

Intake
The intake phase of the cycle starts when the tip of the rotor passes the intake port. At the moment when the intake port is exposed to the chamber, the volume of that chamber is close to its minimum. As the rotor moves past the intake port, the volume of the chamber expands, drawing air/fuel mixture into the chamber.

When the peak of the rotor passes the intake One of the two end pieces of

port, that chamber is sealed off and compression begins. a two-rotor

Compression
As the rotor continues its motion around the housing, the volume of the chamber gets smaller and the air/fuel mixture gets compressed. By the time the face of the rotor has made it around to the spark plugs, the volume of the chamber is again close to its minimum. This is when combustion starts.

The part of the rotor housing

that holds the rotors
(Note the exhaust port location.)

Combustion
Most rotary engines have two spark plugs. The combustion chamber is long, so the flame would spread too slowly if there were only one plug. When the spark plugs ignite the air/fuel mixture, pressure quickly builds, forcing the rotor to move.

The pressure of combustion forces the rotor to move in the direction that makes the chamber grow in volume. The combustion gases continue to expand, moving the rotor and creating power, until the peak of the rotor passes the exhaust port.

The center piece

another intake port for each rotor.

Exhaust
Once the peak of the rotor passes the exhaust port, the high-pressure combustion gases are free to flow out the exhaust. As the rotor continues to move, the chamber starts to contract, forcing the remaining exhaust out of the port. By the time the volume of the chamber is nearing its minimum, the peak of the rotor passes the intake port and the whole cycle starts again.

6.0 DISCUSSION

* How does a petrol engine work?

Stroke 1. The downward moving piston sucks a mixture of air and petrol vapour into the cylinder through the inlet valve.

Stroke 2. The piston then moves upwards, compressing the gas mixture.

Stroke 3. Just before the piston reaches the top of the cylinder a spark from the spark plug explodes the gas mixture. The pressure from the rapidly expanding gas pushes the piston down and causes a flywheel that it is connected to by the crankshaft to rotate. It is this rotation that is used to drive the wheels of the car.

Stroke 4. The piston moves upwards in the cylinder again to push out the gases through the exhaust valve into the exhaust system of the vehicle. As the piston moves down, it pulls more fuel/air mixture in to begin the cycle again.

** The knocking problem

Some hydrocarbons that have the potential to be a component of petrol suffer from a problem called 'knocking'. When they are compressed by the upward moving piston in the cylinder, they tend to auto ignite rather than waiting to catch fire when the spark occurs. There are therefore two explosions, one caused by the compression and the other caused by the spark. This produces a knocking sound from the engine. It also reduces the engine performance and can damage the piston and cylinder.

7.0 CONCLUSION

The four strokes of the internal combustion engine are as follows (and in order): Intake, Compression, Power, and Exhaust. These four strokes require two revolutions of the crankshaft. The process continuously repeats itself during the operation of the engine. So, if a four-cylinder engine requires two complete revolutions of the crankshaft to ignite all of its cylinders, how many revolutions does an eight-cylinder engine require.

It only takes two revolutions of the crankshaft to fire all of the cylinders of any four-stroke engine. The connecting rod converts the reciprocating motion (up and down) of the piston to the rotating motion of the crankshaft. The Diesel engine differs from the gasoline engine in that the intake stroke only pulls in air, not air and fuel. The fuel is injected into the cylinder at the end of the compression stroke. The fuel burns immediately (without the use of a spark plug) because of the high temperature of air in the cylinder.

The rotary engine is that each of the three faces of the rotor is always working on one part of the cycle -- in one complete revolution of the rotor, there will be three combustion strokes. But remember, the output shaft spins three times for every complete revolution of the rotor, which means that there is one combustion stroke for each revolution of the output shaft.

8.0 REFERENCES

THERMODYNAMICS – An Engineering Approach

( Fifth Edition In SI Units ), Mc Graw Hill, 2006,

Yunus A.Cengel and Michael A.Boles.

http://auto.howstuffworks.com/engine.htm

http://www.rotarynews.com/

http://auto.howstuffworks.com/diesel.htm

http://science.howstuffworks.com/two-stroke2.htm

http://auto.howstuffworks.com/rotary-engine1.htm

http://auto.howstuffworks.com/ignition-system.htm

http://en.wikipedia.org/wiki/Internal_combustion_engine

http://www.schoolscience.co.uk/content/4/chemistry/petroleum/knowl/4/2index.htm?knock.html