Operations Of An Internal Combustion Engine Engineering Essay

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Internal combustion is an engine where combustion of fuel like fossil fuel takes place with oxidizer; usually air takes place in a combustion chamber. In internal combustion engine, the expansion of high pressure gases and temperature takes place that is produced by combustion applying direct force to movable components in the engine, such as turbine blades and pistons by moving them in a certain distance generating useful mechanical energy.

The term "internal combustion" usually is referred to engine where combustion takes place at irregular intervals like the more familiar 4 stroke and 2 stroke piston engines. A second class of internal combustion engines uses repetitive combustion. Gas turbine, jet engines and almost all rocket engines made of internal combustion engines work on same principle described.

Figure : IC Engine

Classifcation:

Engines can be classified in different ways:

By the cycle of the engine used, source of energy, the use of engine, the engine layout, or by the cooling system.

Principles of operation

Reciprocating:

Two-stroke cycle

Four-stroke cycle

Six-stroke engine

Diesel engine

Atkinson cycle

SECTION 2

Four Stroke Engine:

Operation

Four-stroke cycle

Intake

Compression

Power

Exhaust

This operation has four main steps which repeat after every two revolutions:

Intake

Combustible mixtures enter into the combustion chamber.

Compression

The mixtures are pressurised with piston.

Power

The mixture is burnt instantly due to spark causing rapid expansion pushing the piston down and moving parts of the engine to perform necessary work.

Exhaust

The cooled combustion gas escapes into the atmosphere.

Combustion

IC engines depend on exothermal chemical process of combustion. The reaction of fuel takes place with oxygen from air (though injecting nitrous oxide in order could gain power boost and do same thing). The resultant of combustion process is production of heat in great quantity and also production of carbon dioxide and steam and other chemicals at very high temperature; the maximum temperature is ascertained by chemical makeup of oxidisers and the fuel.

Most common modern fuels are made of derived mostly from fossil fuels like petrol, diesel and also hydrocarbons. Fossil fuels are gasoline, petroleum gas and diesel fuel. Rarer usage of propane can also be seen. Except for those fuel components, most internal combustion engines designed for gasoline usage can also run on liquefied petrol gases and natural gas without many modifications. Large diesel engines run with air mixed in gas and a diesel fuel ignition injection. Liquefied and gas biofuels like ethanol and biodiesel (which is a form of diesel fuel produced from crops such as soybean oil) can also be used. Few engines with apt modifications can also run on hydrogen gases.

IC engines require mixture of ignition by compression ignition or spark ignition. But before the invention of these things, flame methods and hot tubes were used. [1]

Gasoline Ignition Process

A gasoline engine ignition system usually depends on combination of induction coil and acid-lead battery so as to provide high voltage electric spark to ignite air-fuel mixture in engine's cylinder. The battery is then recharged while car is running which uses a device to generate electricity. They can be a generator or alternator that is driven by the engine. Gas engine first takes in mixture of air and gas to compress it to 1.28 MPa, then a spark plug is used to ignite the mixture after the mixture is compressed by the piston head inside each cylinder

Diesel Ignition Process

Diesel engine relies only on pressure and heat produced by engine during compression process for gasoline ignition. The compression level that occurs is usually three times or more times than gasoline engine. Diesel engine takes in only air and just before total compression a small amount of diesel fuel is flowed into the cylinder with the help of fuel injector to allow the fuel ignite instantly. HCCI type engines takes in both air and fuel, but such engines continue to rely on an unassisted auto combustion process because of high heat and higher pressures. This is also the reason why diesel and HCCI engines are more vulnerable to cold-starting issues, although they run just fine in cold weather once they start. Light duty diesel engine that are equipped with indirect injection in lighter truckes and automobiles use glow plugs that pre-heat combustion chambers just before they start so as to reduce no-start conditions during winter. Most diesel engines also have a battery and charging system; however, this system is not primary and it is added by manufacturers as a luxury for starting comfort and to turn on and off fuel (which can also be done through mechanical apparatus or switch), and to run auxiliary electric accessories. Most of the new engines depend on electronic and electrical control systems which helps to reduce emissions and also to control combustion process for efficiency increase.

Four-stroke

The above diagram shows Ideal pressure/volume diagram of Otto cycle with combustion heat input, denoted as Qp and waste exhaust output, denoted as Qo. The bottom curved line is compression stroke and the power stroke is top curved line.

Engines that are based on the four-stroke ("Otto cycle") have one power stroke for every four strokes (up-down-up-down) and employ spark plug ignition. Combustion occurs quickly, and during combustion the volume varies little ("constant volume"). They are used in larger boats, cars, some motorcycles, and many light aircraft. They are generally quieter, more efficient, and larger than their two-stroke counterparts. [2]

The steps involved are:

Intake stroke: Vapour fuel and air are sucked in.

Compression stroke: Air and fuel vapour are compressed and ignited.

Combustion stroke: Fuel burns up and piston is pushed downwards.

Exhaust stroke: It is driven outside. In 1st, 2nd and 4th stroke, the piston depends on the power and the momentum generated by other pistons. Comparatively, a six or eight cylinder engine would be more powerful than four cylinder engine.

Otto Cycle

The first human to build a car with an engine was German engineer Nicolaus Otto. Today, four-stroke engine cycle is commonly known as Otto cycle (named after the german engineer) and four-stroke engines which uses spark plugs are called Otto engines. The Otto cycle has four steps. They are

Adiabatic compression

Heat addition

Adiabatic expansion

Rejection of heat at constant volume

In modern world, internal combustion engines in trucks, cars, aircrafts, motorcycles construction vehicles, trailers and many others automobiles commonly use four-stroke cycle engine. The four stroke refers to intake, compression, combustion, and exhaust which occurs during two crankshaft rotations per working cycle of a gasonline engine. A easier and more understandable description of the four-stroke cycle is, "Suck, Squeeze, Bang and Blow".

The cycle begins at top dead center, when the piston is furthest away from the crankshaft axis. A stroke refers to the total travelling of the piston from Top Center (TDC) to Bottom Center (BDC).

Stroke 1 of 4 "Suck": After the intake of the piston, the piston descends from the top of the cylinder to the bottom of the cylinder, which reduces the pressure inside the cylinder. An air- fuel mixture is forced into the cylinder by the atmospheric pressure through intake port. The intake valve(s) closes afterwards.

Stroke 2 of 4 "Squeeze": With both exhaust and intake valves closed, the piston moves to the top of the cylinder and compresses the fuel-air mixture. This is called as the compression stroke.

Stroke 3 of 4 "Bang": when the piston is at Top Dead Center, the compressed mixture of air and fuel ignites, usually by a spark plug or by the compression of head and pressure of air-fuel mixture. The high pressure result from compressed air-fuel mixture combustion pushes the piston down to bottom dead center with heavy force. It is the main source of engine's torque and power and is also called power stroke.

Stroke 4 of 4 "Blow": In this step, the piston returns back to its top dead center position while exhaust valve opens. Because of exhaust valve opening, the product of air-fuel combustion mixture is pushes the spent fuel-air mixture out of the cylinder through exhaust valves.

Figure 2: Starting Position Figure 3: Intake Stroke

Figure 4: Compression Stroke Figure 5: Ignition of Fuel

Figure 6: Power Stroke Figure 7: Exhaust Stroke

Energy balance

Otto cycle engines are around 34% efficient. 34% of the energy generated in the engine by combustion is converted into valuable rotational energy at the output shaft of the engine, while the remaining appears as waste heat. By contrast, a six-stroke engine which is still under research and development may possibly convert more than 50% of the energy of combustion into valuable rotational energy.

Modern engines are often deliberately built to be slightly less efficient than they otherwise be. This is necessary to control emissions such as catalytic converters that reduce smog and other atmospheric pollutants and exhaust gas recirculation. Efficiency reduction may be countered with an engine control unit by using lean burn techniques.

Diesel cycle

P-v Diagram of Ideal Diesel cycle.

The image above shows P-v diagram of ideal diesel cycle; where v is specific volume and P is pressure. Below are the 4 distinct steps that follow ideal diesel cycle:

1 to 2 is isentropic compression

2 to 3 is reversible constant pressure heating

3 to 4 is isentropic expansion

4 to 1 is reversible constant volume cooling

The diesel is a heat engine which converts heat energy into work.

Work (Win) is done by the compression of the working fluid by piston.

Heat(Qin) is done by the fuel combustion

Work out (Wout) is done by the working fluid expanding on to the piston, this produces torque for movement of parts

Heat out (Qout) is done by venting the air out

Most trucks and automotive diesel engines use a cycle that is comparable to four stroke cycle, but they have compression heating ignition system. They don't need separate ignition system. This difference makes it give a different name, diesel cycle. Diesel fuel is injected in diesel cycle directing in the cylinder for the combustion process to occur at constant pressure.

SECTION - 3

Engine Parts

Almost all of Compression ignition and spark ignition components are similar. Important components of the engine are

Piston

Cylinder

Cylinder block

Crankshaft

Connecting rod

Crankcase

There are two main types of internal combustion engines. The spark ignition is also called the petrol engine, and compression ignition is also called the diesel engine. Almost all the components of both the engines are same, but only their fuel burning process differs. In CI engines the burning of fuel occurs by its compression to high pressures but in SI engines the burning of fuel occurs by the spark generated by the spark plug while.[3]

Here are the important components of the IC engines:

1. Cylinder block: Cylinder block is the main body of the engine, this structure supports all the other components of the engine. In this case of the single cylinder engine the cylinder block stocks the cylinder while in the case of multi-cylinder engine, number of cylinders are arranged in a row together to form the cylinder block. The cylinder head is mounted at the topmost part of the cylinder block.

When the vehicle runs, large amounts of heat is generated inside the cylinder block. To remove the heat the cylinder head and the cylinder block are cooled down by water that is flowing through the water jackets inside the larger engines that are found in cars and trucks. For smaller vehicles like motorcycles, fins are rendered on the cylinder head and on the cylinder block to cool them. The bottom portion of the cylinder block is called crankcase. Inside the crankcase is lubricating oil, which is used for lubricating various moving parts of the engine stored.

2. Cylinder: As the name says, it is a cylindrical shaped vessel that is fitted in the cylinder block. This cylinder can be removed from the cylinder block and machined whenever required. It is also called a liner or sleeve. Inside the cylinder the piston moves up and down, which is called the reciprocating motion of the piston. Burning of fuel occurs at the top of the cylinder, due to which the reciprocating motion of the piston is produced. The surface of the cylinder is finished to a high finish, so that there is minimal friction between the piston and the cylinder.

In smaller vehicles like motorcycles there is only one cylinder. In larger more powerful vehicles like high cc motorcycles, trucks, cars, etc., there is more than one cylinder in the engine. These engines are called multi-cylinder engines. The number of cylinders in these engines can be 2, 3, 4, 6, 8. In very large engines such as those of ships and submarines, the number of cylinders can be 12 to 16. The displacement volume of engine solely depends on the size of the cylinder.

3. Piston: The piston is a round cylindrical component that performs a reciprocal motion in the cylinder. The piston has to fit perfectly inside the cylinder. Piston rings are fitted over and on the piston. The gap between the cylinder and the piston is filled by the lubricating oil and piston rings. The piston is generally made up of aluminium.

4. Piston rings: Piston rings are thin rings in nature that are fitted in the slots made along the piston surface. These rings provide tight seal between cylinder walls and piston thus helping prevent leakage of combustion gases from either side of piston.

5. Combustion chamber: The burning of fuel takes place in combustion chamber. The combustion chamber is top portion of cylinder enclosed by piston and cylinder head. When the fuel burns, thermal energy is produced. It generates very high pressure in the combustion chamber causing reciprocating motion of piston push down with intense force.

6. Inlet manifold: Mixture of air and fuel or air is drawn into the cylinder through the inlet manifold.

7. Exhaust manifold: The exhaust manifold is used to discharge all the exhaust gases generated inside the cylinder due to fuel burn into the atmosphere.

8. Inlet and exhaust valves: The function of inlet valve is to allow the intake of fuel during suction stroke. It closes after the fuel is entered into the cylinder. Exhaust valve opens fully to discharge the gases burnt inside the cylinder into the atmosphere. Both the valves allow the flow of gas and fuel only in one direction.

9. Spark plug: Spark plug is used to produce spark. The spark plug produces spark all the time causing instant burning of pressurized fuel in the cylinders.

10. Connecting rod: Connecting rod is a link between crankshaft and piston. It helps perform rotary motion. There are 2 ends for the connecting rod. Both have holes of different diameters. The bigger hole is connected to the crankshaft with the help of crank pin.

11. Crankshaft: The function of crankshaft is to perform rotatory motion. It is connected to wheel's axle which moves when crankshaft rotates. With the help of a connecting rod, reciprocating motion of the piston is converted to rotatory motion. In the bushings, the crankshaft rotates in the crankcase

All the above mentioned components are IC engine components. In modern times, automotive companies have advanced very much in manufacturing the precise components for IC engine. That is the reason engine lifespans increases over years. Many changes are being made in the engine designing to increase fuel efficiency and total power output of the vehicle.

Today, engines are designed to work not just on petrol. They can also run on gasoline fuel or LPG gas.

SECTION - 4

Oxidizers and Fuels

Hydrogen

In future, internal combustion engines could eventually be replaced by Hydrogen fuels. Alternative fuel cell technology will slowly replace existing internal combustion engines.

Although there are many ways of producing free hydrogen, all those methods require converting combustion molecules into consuming electric energy or hydrogen. The energy produced for the above process should be from renewable source to avoid energy crisis. Often storage of hydrogen is a major issue. Hydrogen gas requires heavy tankage as it does not have high density whereas liquid hydrogen has very low density. Compared to water, it has density 14 times lower due to which it requires extensive insulation. Hydrogen has higher specific energy even after liquefaction, however petrol has five times higher volumetric energetic storage than hydrogen. The energy density of hydrogen is significantly higher than electric batteries, making it a strong candidate as an energy carrier which can replace fossil fuels in future. The 'Hydrogen on Demand' process creates hydrogen as needed, but has many other issues such as production of highly priced sodium borohydride, a raw material.

Oxidizers

As air is abundant on the earth's surface, there is no need for oxidizer, which is atmospheric oxygen to store in the vehicle which unnecessarily increases power to volume ratio and power to weight. Other materials can be used for special purposes, to increase output power.

Compressed air was usually used in torpedoes. Compressed oxygen and some compressed air were used in the Japanese Type 93 torpedo. Certain submarines are specifically designed to carry pure oxygen. Rockets often use liquid oxygen.

To increase power and control combustion Nitro methane is used to some racing and model fuels.

In tactical aircrafts and in specially equipped cars, Nitrous oxide was used to increase power which can also be called short bursts added from the engine while they usually run on air and gasoline mixture.It is also used in rocket spacecraft.

Hydrogen peroxide power was under development for submarines in 1943 and may have also been used in some non-nuclear submarines. It was also used in some rocket engines like Black Arrow and Me-163 rocket plane.

Other chemicals like chlorine and fluorine have been used experimentally but weren't found to be useful practically.

SECTION - 5

Engine Problems

There are few fundamental reasons for the failure of the engine early morning especially during winter. They are

Bad fuel mix

Lack of compression

Lack of spark

Beyond that, there are few more minor causes that might create problems apart from those big three. Based on the simple engine, below are the details how those problems affects the engine.

Bad fuel mix - A bad fuel mix can occur due to several reasons:

When the engine is out of gas, which makes it to burn only air.

Clogging of air intake creating imbalance between fuel and air

Too much fuel supply to the tuel system mix leading to improper combustion.

There might be an impurity in the fuel that makes the fuel not burn[4].

Lack of compression - If the charge of fuel and air cannot be compressed suitably, the combustion process doesn't work like the way it has to. Lack of compression might also occur for the below reasons:

When piston rings are worn (allowing fuel/air leak past the piston during compression).

When the exhaust valves or inlet valves are not sealed properly, which might leak during compression.

When a hole is formed in the cylinder.

The hole in the cylinder might occur at the top of the cylinder. Generally, cylinder head bolt and cylinder are fitted together with a gasket pressed between them. Small holes develop between the cylinder and cylinder head if the gasket breaks down making a hole which can cause leaks.

Figure 8: Maintenance Work

Regular engine maintenance can help avoid frequent repairs.

Lack of spark - The spark might be weak or non-existent for a number of reasons:

If the spark plug or the wire leading to it is worn out, the spark will be too weak.

If the system that sends a spark down the wire is not working properly there will be no spark.

If the spark occurs too late in the cycle, fuel doesn't ignite at the right time.

The below mentioned reasons can also be included:

If the battery is dead, engine cannot be started.

Engine cannot run if the bearings worn out that allows crankshaft to turn freely cannot turn.

Engine can't run if valves open or close at right time

If the tailpipe is blocked due to numerous reasons, exhaust cannot exit the cylinder. The engine will not run.

If the oil is run out, piston cannot move freely.

SECTION - 6

Volumetric Efficiency

Performance: The performance of an Otto cycle can be assessed under varying volumetric efficiency. Finite time thermodynamics is used to derive relations between thermal efficiency and power output at different volumetric efficiency and compression ratio for an air-standard Otto Cycle. The effect of volumetric efficiency on the irreversible cycle performance is important. The effect of volumetric efficiency on cycle performance is obvious, and practice cycle analysis should be considered.

SECTION 6

Conclusion

In this report we have covered so far the history of IC engine, types of engine cycles and their problems. This being the mid-semester report has described everything briefly. For the final report I will cover the designing and analysis of engine and its parts and how efficiency differs by varying the length and cross sectional area of engine cylinders.

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