Chapter One ICE

8/18/2019 Chapter One ICE

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Chapter One: Introduction

1.1 DEFINITION

Internal-combustion engine, one in which combustion of the fuel takes place in aconfined space, producing expanding gases that are used directly to providemechanical power. Such engines are classified as reciprocating or rotary, sparkignition or compression ignition, and two-stroke or four-stroke; the most familiarcombination, used from automobiles to lawn mowers, is the reciprocating, spark-ignited, four-stroke gasoline engine. Other types of internal-combustion enginesinclude the reaction engine (jet propulsion, rocket), and the gas turbine. Engines arerated by their maximum horsepower, which is usually reached a little below thespeed at which undue mechanical stresses are developed.

Internal Combustion engines

Positive Displacement Ramjet Gas Turbine

Wankel ‘rotary’ Piston

Spark-ignition Compression-ignition

2-stroke 4-stroke 2-stroke 4-stroke

Figure 1.1: The various type of ICEs.

The purpose of internal combustion engines is the production of mechanical powerfrom the chemical energy contained in the fuel. In internal combustion engines, asdistinct from external combustion engines, this energy is released by burning oroxidizing the fuel inside the engine. The fuel-air mixture before combustion and theburned products after combustion are the actual working fluids. The work transferswhich provide the required power output occur directly between these working fluidsand the mechanical components of the engine.

Continuous combustion

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Figure 1.2 (a): Ramjet in blackbird.

Figure 1.2 (b): Turbojet engine as vehicle propulsion.

Figure 1.2 (c): Turbofan engine as aircraft propulsion


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A ram jet engine is a device from which usefuI thrust can be obtained by creating avelocity difference between the atmosphere entering the ram jet body and the samequantity of air leaving the ram jet body. This velocity difference between entranceand exit air is accomplished by the addition of heat to that portion of the airstreamflowing through the ram jet body. The engine is composed of three major

components: a body structure, fuel injection system, and a flame stabilizationsystem.

Figure 1.3 Example of a ram jet.

For high supersonic or hypersonic flight, the ideal propulsion system is a ramjet. Aramjet uses the forward speed of the aircraft to compress the incoming air and,therefore, has fewer moving parts than a turbine engine.

A turbojet, or "straight jet" engine, consists of the four stages of thrust generationarranged in a straight line within a tube. At the leading edge of the tube are one ormore compressor fans that compress the airstream; then, fuel injector nozzles mixatomized fuel with the compressed air; then, the fuel-air mixture is ignited from acontinuous flame housed in a flame eddy; this combustion accelerates exhaustgases out the back of the tube, producing thrust. An exhaust turbine is situatedbehind the combustion stage; this turbine is driven by the exhaust gases, and drivesthe compressor fan.

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Figure 1.4: Turbo-jet, cross-sectional view.

A turbofan however consists of a turbojet engine (the "core") surrounded by a largertube (the "bypass"). A larger, slower fan, positioned upwind of the compressorsection in the core, pushes air (relatively) slowly through both the core and thebypass. The air that makes it to the core is compressed and undergoes combustionas described above. The air that makes it to the bypass is ducted around the core,and mixes with the exhaust gases when it exits the back of the engine. Although theair in the bypass is moving slower than the exhaust gases ejected out the back of thecore, it is still much faster than the ambient air that does not enter the engine at all.

Figure 1.5: Turbofan, cross-sectional view.

Ramjets, turbojets and turbofan engines are of continuous combustion and they arenot meant for automotive applications but are for larger mode of transport i.e. planes,cruise missiles, tanks, ship and power plants.


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1.2 ROTARY ENGINES

The Wankel engine is a type of internal combustion engine. It uses rotors instead ofpistons inside its combustion chamber and relies on eccentric rotary design to

convert pressure into rotating motion. Over the commonly used reciprocating pistondesigns, the Wankel engine delivers advantages of: simplicity, smoothness,compactness, high revolutions per minute, and a high power-to-weight ratio. Theengine is commonly referred to as a rotary engine, although this name applies alsoto other completely different designs. All parts rotate moving in one direction asopposed to the common piston engine which has pistons violently changingdirection. The four-stroke cycle occurs in a moving combustion chamber between theinside of an oval-like epitrochoid-shaped housing, and a rotor that is similar in shapeto a Reuleaux triangle with sides that are somewhat flatter.

The engine was invented by German engineer Felix Wankel. Wankel received hisfirst patent for the engine in 1929, began development in the early 1950s at NSU, and completed a working prototype in 1957.[1] NSU subsequently licensed the designto companies around the world, which have continually added improvements.

The Wankel engine has the unique advantages of compact design and low weightover the most commonly used internal combustion engine employing reciprocatingpistons. These advantages have given rotary engine applications in a variety ofvehicles and devices, including: automobiles, motorcycles, racing cars, aircraft, go-karts, jet skis, snowmobiles, chain saws, and auxiliary power units.

The Wankel engine has 48% fewer parts and about a third the bulk and weight of areciprocating engine. Its main advantage is that advanced pollution control devicesare easier to design for it than for the conventional piston engine. Another advantageis that higher engine speeds are made possible by rotating instead of reciprocatingmotion, but this advantage is partially offset by the lack of torque at low speeds,leading to greater fuel consumption.

Figure 1.6 (a): Wankel – Internal structure, components and arrangement.

http://en.wikipedia.org/wiki/Internal_combustion_enginehttp://en.wikipedia.org/wiki/Eccentric_%28mechanism%29http://en.wikipedia.org/wiki/Rotary_combustion_enginehttp://en.wikipedia.org/wiki/Piston_enginehttp://en.wikipedia.org/wiki/Revolutions_per_minutehttp://en.wikipedia.org/wiki/Pistonless_rotary_enginehttp://en.wikipedia.org/wiki/Four-stroke_cyclehttp://en.wikipedia.org/wiki/Epitrochoidhttp://en.wikipedia.org/wiki/Reuleaux_trianglehttp://en.wikipedia.org/wiki/Germanshttp://en.wikipedia.org/wiki/Felix_Wankelhttp://en.wikipedia.org/wiki/NSU_Motorenwerke_AGhttp://en.wikipedia.org/wiki/Wankel_engine#cite_note-a-1http://en.wikipedia.org/wiki/Wankel_engine#cite_note-a-1http://en.wikipedia.org/wiki/Wankel_engine#cite_note-a-1http://en.wikipedia.org/wiki/Piston_enginehttp://en.wikipedia.org/wiki/Automobilehttp://en.wikipedia.org/wiki/Motorcyclehttp://en.wikipedia.org/wiki/Racing_carhttp://en.wikipedia.org/wiki/Aircrafthttp://en.wikipedia.org/wiki/Kart_racinghttp://en.wikipedia.org/wiki/Kart_racinghttp://en.wikipedia.org/wiki/Personal_water_crafthttp://en.wikipedia.org/wiki/Snowmobilehttp://en.wikipedia.org/wiki/Chainsawhttp://en.wikipedia.org/wiki/Auxiliary_power_unithttp://en.wikipedia.org/wiki/Auxiliary_power_unithttp://en.wikipedia.org/wiki/Chainsawhttp://en.wikipedia.org/wiki/Snowmobilehttp://en.wikipedia.org/wiki/Personal_water_crafthttp://en.wikipedia.org/wiki/Kart_racinghttp://en.wikipedia.org/wiki/Kart_racinghttp://en.wikipedia.org/wiki/Aircrafthttp://en.wikipedia.org/wiki/Racing_carhttp://en.wikipedia.org/wiki/Motorcyclehttp://en.wikipedia.org/wiki/Automobilehttp://en.wikipedia.org/wiki/Piston_enginehttp://en.wikipedia.org/wiki/Wankel_engine#cite_note-a-1http://en.wikipedia.org/wiki/NSU_Motorenwerke_AGhttp://en.wikipedia.org/wiki/Felix_Wankelhttp://en.wikipedia.org/wiki/Germanshttp://en.wikipedia.org/wiki/Reuleaux_trianglehttp://en.wikipedia.org/wiki/Epitrochoidhttp://en.wikipedia.org/wiki/Four-stroke_cyclehttp://en.wikipedia.org/wiki/Pistonless_rotary_enginehttp://en.wikipedia.org/wiki/Revolutions_per_minutehttp://en.wikipedia.org/wiki/Piston_enginehttp://en.wikipedia.org/wiki/Rotary_combustion_enginehttp://en.wikipedia.org/wiki/Eccentric_%28mechanism%29http://en.wikipedia.org/wiki/Internal_combustion_engine


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Figure 1.6 (b): The processes involved in a cycle i.e. induction, compression,combustion and exhaust.

Figure 1.6 (c): The application of wankel engine (Renesis) in Mazda 8 platform.


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Figure 1.7: Areas of applications i.e. automotive, motorcycle, light airplane and marines.

1.3 RECIPROCATING ENGINES

The most common internal-combustion engine is the piston-type gasoline engineused in most automobiles. The confined space in which combustion occurs is calleda cylinder. The cylinders are now usually arranged in one of four ways: a single rowwith the centre lines of the cylinders vertical (in-line engine); a double row with thecentre lines of opposite cylinders converging in a V (V-engine); a double zigzag rowsomewhat similar to that of the V-engine but with alternate pairs of opposite cylindersconverging in two vs (W-engine); or two horizontal, opposed rows (opposed,pancake, flat, or boxer engine). In each cylinder a piston slides up and down. Oneend of a connecting rod is attached to the bottom of the piston by a joint; the otherend of the rod clamps around a bearing on one of the throws, or convolutions, of acrankshaft; the reciprocating (up-and-down) motions of the piston rotate the

crankshaft, which is connected by suitable gearing to the drive wheels of theautomobile. The number of crankshaft revolutions per minute is called the engine


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speed. The top of the cylinder is closed by a metal cover (called the head) boltedonto it. Into a threaded aperture in the head is screwed the spark plug, whichprovides ignition.

Figure 1.8: Reciprocating Mechanism.

Two other openings in the cylinder are called ports. The intake port admits the air-gasoline mixture; the exhaust port lets out the products of combustion. A mushroom-shaped valve is held tightly over each port by a coil spring, and a camshaft rotatingat one-half engine speed opens the valves in correct sequence. A pipe runs fromeach intake port to a carburetor or injector, the pipes from all the cylinders joining toform a manifold; a similar manifold connects the exhaust ports with an exhaust pipeand noise muffler. A carburetor or fuel injector mixes air with gasoline in proportionsof weight varying from 11 to 1 at the richest to a little over 16 to 1 at the leanest. Thecomposition of the mixture is regulated by the throttle, an air valve in the intake

manifold that varies the flow of fuel to the combustion chambers of the cylinders. Themixture is rich at idling speed (closed throttle) and at high speeds (wide-openthrottle), and is lean at medium and slow speeds (partly open throttle).


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Figure 1.9: Reciprocating mechanism in a two-stroke engine.

Figure 1.10: The structure of a diesel engine.

The other main type of reciprocating engine is the diesel engine, invented by RudolfDiesel and patented in 1892. The diesel uses the heat produced by compressionrather than the spark from a plug to ignite an injected mixture of air and diesel fuel (aheavier petroleum oil) instead of gasoline. Diesel engines are heavier than gasolineengines because of the extra strength required to contain the higher temperaturesand compression ratios. Diesel engines are most widely used where large amountsof power are required: heavy trucks, locomotives, and ships.

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1.4 ENGINE OPERATION

1.4.1 The Four-Stroke Cycle

In most engines a single cycle of operation (intake, compression, power, andexhaust) takes place over four strokes of a piston, made in two engine revolutions.When an engine has more than one cylinder the cycles are evenly staggered forsmooth operation, but each cylinder will go through a full cycle in any two enginerevolutions. When the piston is at the top of the cylinder at the beginning of theintake stroke, the intake valve opens and the descending piston draws in the air-fuelmixture.

At the bottom of the stroke the intake valve closes and the piston starts upward onthe compression stroke, during which it squeezes the air-fuel mixture into a smallspace at the top of the cylinder. The ratio of the volume of the cylinder when thepiston is at the bottom to the volume when the piston is at the top is called thecompression ratio. The higher the compression ratio the more powerful the engineand its efficiency will be increased. However, in order to accommodate air pollutioncontrol devices, manufacturers have had to lower compression ratios.

Just before the piston reaches the top again, the spark plug fires, igniting the air-fuelmixture (alternatively, the heat of compression ignites the mixture). The mixture onburning becomes a hot, expanding gas forcing the piston down on its power stroke.Burning should be smooth and controlled. Faster, uncontrolled burning sometimesoccurs when hot spots in the cylinder pre-ignite the mixture; these explosions arecalled engine knock and cause loss of power. As the piston reaches the bottom, theexhaust valve opens, allowing the piston to force the combustion products—mainly

carbon dioxide, carbon monoxide, nitrogen oxides, and unburned hydrocarbons—outof the cylinder during the upward exhaust stroke.

Figure 1.11 and 1.12 are the representation of the actual and theoretical cyclesdepicted by the two types of the most common combustion engines known today.While they look similar but they are not quite exact, especially in the way of heat isbeing supplied.

Figure 1.11: Actual and theoretical cycles of a four-stroke gasoline engine.


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Figure 1.12: Actual and theoretical cycles of a four-stroke diesel engine.

1.4.2 The Two-Stroke Cycle

The two-stroke engine is simpler mechanically than the four-stroke engine. The two-stroke engine delivers one power stroke every two strokes instead of one every four;thus it develops more power with the same displacement, or can be lighter and yetdeliver the same power. For this reason it is used in lawn mowers, chain saws, smallautomobiles, motorcycles, and outboard marine engines.

However, there are several disadvantages that restrict its use. Since there are twice

as many power strokes during the operation of a two-stroke engine as there areduring the operation of a four-stroke engine, the engine tends to heat up more, andthus is likely to have a shorter life. Also, in the two-stroke engine lubricating oil mustbe mixed with the fuel. This causes a very high level of hydrocarbons in its exhaust,unless the fuel-air mixture is computer calculated to maximize combustion. A highlyefficient, pollution-free two-stroke automobile engine is currently being developed byOrbital Engineering, under arrangements with all the U.S. auto makers.

Figure 1.13: Actual and theoretical cycles of a two-stroke gasoline engine.


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1.5 ENGINE CLASSIFICATION

There are many different types of internal combustion engines. They can beclassified as:

1. Appl icat ion . Automobile, trucks, locomotive, light aircraft, marine portablepower system, power generation.

2. Basic engine design . Reciprocating engines (e.g. Inline, radial, V), rotaryengines (Wankel and others).

3. Working cyc le . Four-stroke: naturally aspirated, supercharged andturbocharged, two-stroke cycle: crankcase scavenged supercharged andturbocharged.

4. Valve and port d esign and loc at ion . Overhead (or I-head) valves, underhead (or L-head) valves, rotary, valve cross-scavenged porting (inlet andexhaust ports on same sides of cylinder at one end), loop-scavenged porting(inlet and exhaust ports on same side of cylinder at one end), through –oruniflow-scavenged (inlet ad exhaust ports or value at different ends ofcylinder).

5. Fuel . Gasoline, fuel oil, natural gas, liquid petroleum gas, alcohols hydrogen,dual fuel.

6. Method of m ixture preparation . Carburetion fuel injection into the intakeports or intake manifold fuel injection into the engine cylinder.

7. Method of ignit ion . Spark ignition (in conventional engines where the mixtureis uniform and in stratified-charge engine where the mixture is non-uniform),compression ignition (in conventional diesel, as well as ignition in gas enginesby pilot injection of fuel oil.

8. Combust ion ch amber design . Open chamber (many designs: e.g. disc,wedge, hemisphere, bowl-in-piston), divided chamber (small and largeauxiliary chambers e.g. swirl chamber, pre-chamber).

9. Method of load contro l . Throttling of fuel and air flow together so mixturecomposition is essentially unchanged control of fuel flow alone, a combinationof these.

10.Method of cool ing . Water cooled, air-cooled, uncooled (other than by naturalconvection and radiation).


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Table 1.1: Classification of reciprocating engines by application.


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Figure 1.14: Engine configuration and timing.


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1.6 ENGINE COMPONENTS

Inside the engine are several fascinating parts as shown in Figure 1.11(a). At the topare the cyl inder h eads . These contain the mechanisms that allow the valves to

open and close, letting the fuel/air mixtu re into the cyl inders and allowing the burntexhaust gases to leave. Below the cylinder heads is the engine block itself. Thispiece contains the cyl inders , which contain the pis tons.

Figure 1.15 (a): Main parts of an engine.

Figure 1.15(b): Engine components in a multi-cylinder gasoline engine.


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Figure 1.11(b) shows the major components to be expected in a typical four-strokegasoline engine. The spark plug is mounted onto the engine cylinder head. The inletand the exhaust valve are fitted onto the engine cylinder head together with theengine spark plug mounting. Also fitted onto the cylinder head would be the camshaft(s) and the cylinder head cover. The piston, rings and connecting rod are

positioned in the engine block, whilst the engine crankshaft is mounted between theengine block and crankcase. Intricate passages are made to link between thecylinder head and the engine block to provide the coolant passage necessary toallow the heat produced during combustion to be extracted and dissipated to theatmosphere via a heat exchanger called radiator.

Figure 1.15(c): Exploded view of an engine parts.

Figure 1.15 (c) shows the actual components associated the cylinder head and theengine block. These are precision components that contributes towards the ultimateoutput (brake power, torque), efficiency (fuel economy), robustness and reliability of

the engine in use.


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1.7 USUAL TERMINOLOGY AND ABBREVIATIONS

The following terms and abbreviations are most commonly used among enginetechnologist, engineers and researchers, and will be used in this lecture. They

should be learned and used widely in the subsequent chapters.

Air-fuel ratio – Ratio of the mass of air to that of fuel in a cycle.Brake power – The engine shaft output (kW) at a certain speed (rpm).Torque – The turning ability of an engine (Nm) at certain speed (rpm)Equiv alence ratio - is the ratio of actual AFR to stoichiometry for a givenmixture. λ= 1.0 is at stoichiometry, rich mixtures λ < 1.0, and lean mixtures λ >1.0. There is a direct relation-ship between λ and AFR.Spark ignit ion (SI) – an engine in which combustion process in each cycle isstarted by use of a spark plug.Comp ression ratio (CR) – Ratio of maximum volume to minimum volume inone complete engine rotation.Compress ion ign i t ion (CI) – An engine in which the combustion processstarts when the air-fuel mixture self-ignites due to high temperature in thecombustion chamber caused by high compression. CI engines are often calledDiesel engines, especially in the non-technical community.Ignit ion delay (ID) – Time interval between ignition initiation and the actualstart of combustion.Top-dead centre (TDC) – The position of the piston when it stops at the

furthest point away from the crankshaft.Bottom -dead centre (BDC) – The position of the piston when it stops at theclosest point away from the crankshaft.Clearance v olume (CVol) – the minimum volume trapped between the flatsurface of the piston and the cylinder head.Connect ing rod – Linkage connecting piston with rotating crankshaft, usuallymade of steel, alloy forging, or aluminium.Cool ing f ins – Metal fins on the outside surfaces of cylinders and head of anair-cooled engine. These extended surface cool the cylinders by conductionand convection.Crankcase – Part of the engine block surrounding the rotating crankshaft. In

many engines the oil pan makes up of the crankcase housing.Crankshaft – Rotating shaft through which engine work output is supplied toexternal systems. The crankshaft is connected to the engine block with themain bearings. It is rotated by the reciprocating pistons through connecting rodsconnected to the crankshaft, offset from the axis of rotation. This offset issometime called crank throw or crank radius. Most crankshafts are made offorged steel, while some are made of cast iron.Cylinders – The circular cylinders in the engine block in which the pistonsreciprocate back and forth. The walls of the cylinder have highly polishedsurfaces. Cylinders may be machined directly in the engine block, or a hardmetal (drawn steel) sleeves may be pressed into the softer metal block.

Sleeves may be dry sleeves which do not contact the liquid in the water jacket,or wet sleeves, which form part of the water jacket. In a few engines, the


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cylinder walls are given a knurled surface to help hold a lubricant film on thewalls. In some very rare cases, the cross section of the cylinder is not round.Direct in ject ion (DI) – Fuel injection into the main combustion chamber of anengine. Engine have either one main combustion chamber (open chamber) or adivided combustion chamber made up of a main chamber and a smaller

secondary chamber.Indirect in ject ion (IDI) – Fuel injection into the secondary chamber of anengine with a divided combustion chamber.Bore – Diameter of the cylinder or diameter of the piston face, which is thesame minus a small clearance volume.Stroke – movement distance of the piston from one extreme position to theother, e.g. TDC to BDC or vice versa.Displacement or displacement volum e – Volume displaced by the piston asit travels through one stroke. Displacement can be given for one cylinder or forthe entire engine (one cylinder multiply the number of cylinders). Somereferences quote it as swept volume.

Gasol ine direct in ject ion (GDI) – Spark ignition engine with fuel injectorsmounted in combustion chambers. Gasoline is injected directly into cylindersduring compression stroke.Wide open throt t le (WOT) – Engine operated with throttle valve fully openwhen maximum power and/or speed is desired.

1.8 Important Engine Characteristics

Basic geometrical relationships and the parameters commonly used to characteriseengine operation have been developed over the years of engine evolution. Thefactors important to an engine user are:

1. The engine’s performance over its operating range 2. The engine’s fuel consumption within this operating range and the cost of the

fuel3. The engine’s noise and air pollutant within this operating range 4. The initial cost of the engine and its installation5. The reliability and durability of the engine, its maintenance requirements, and

how these will affect engine availability and operating costs.

These factors control engine operating costs – usually the primary consideration ofthe user – and whether the engine in operation can satisfy environmentalregulations. The emphasis of this subject is primarily with the performance,efficiency, and emissions characteristics.

To be precise, engine performance is defined as follows:

1. The maximum power (maximum torque) available at each speed within theuseful engine operating range

2. The range of speed and power over which engine operation is satisfactory


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The following definitions are frequently used to define performance:

Maximum rated power . The highest power and engine is allowed to develop forshort periods of operation.Normal rated power . The highest power and engine is allowed to develop in

continuous operation.Rated speed. The crank angle rotational speed at which rated power is developed.

Figure 1.16 (a): Examples of engine performance curve.

Figure 1.16 (b): Defining the curves in an engine performance graph.


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Figure 1.16 (c): Instrument cluster showing the normal rated speed theoperator is allowed to ramp the engine.

Figure 1.16 (a) and (b) illustrate examples of graph showing the important measuringparameters i.e. brake power, torque and specific fuel consumption. Figure 1.16 (c)shows an instrument cluster indicating the speed zone in which vehicle operators arenot allowed to exceed while driving.

1.9 RECIPROCATING ENGINE – GEOMETRICAL PROPERTIES

The following are parameters which define the basic geometry of a reciprocatingengine:

Compression ratio r c :

=

=

+

(1.1)

Where V d is the displaced or swept volume and V c is the clearance volume.

Ratio of cylinder bore to piston stroke:

=

(1.2)


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Ratio of Connecting rod Length to Crank Radius:

=

(1.3)

Stroke and crank radius are related by

= 2 (1.4)

Note: Typical values of these parameters are: r c is between 8 to 12 for SI enginesand r c is between 12 to 24 for CI engines; B/L is between 0.8 to 1.2 for small-and medium-size engines, decreasing to about 0.5 for large slow-speed CIengines; R = 3 to 4 for small – and medium-size engines, increasing to 5 to 9for large slow-speed CI engines.

The cylinder volume V at any crank position Ɵ is written as:

= +

4 + (1.5)

Figure 1.17: Geometry of cylinder, piston, connecting rod and crankshaft whereB =bore, L = stroke, l = connecting rod length, a =crank radius, Ɵ =

crank angle.


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Where s is the distance between the crank axis and the piston pin axis (refer Figure1.13), and is given by

= c o s Ɵ + (l2 –a2 sin2Ɵ)0.5 (1.6)The angle Ɵ is called the crank angle. Equation (1.4) with the above definitions canbe rearranged:

= 1 + 0 . 5 1[ + 1 Ɵ

Ɵ.]

(1.7)

The combustion chamber surface area A at any crank position Ɵ is given by:

= + + + (1.8)

Where Ach is the cylinder head surface area and A p is the piston crown surface are.For flat-topped pistons, A p = πB2/4. Using Eq. (1.5), Eq (1.7) can be rearranged:

= + +

2 [ + 1 Ɵ

Ɵ.

]

(1.9)

Figure 1.18: Instantaneous piston speed/mean piston speed as a functionof crank angle for R = 3.5.


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An important characteristic speed is the mean speed S p:

=2

(1.10)

where N is the rotational speed of the crankshaft. Mean piston speed is often amore appropriate parameter than crank rotational speed for correlating enginebehaviour as a function of speed. As an example, gas-flow velocities in the intakeand the cylinder all scale with S p. The instantaneous piston velocity S pins is obtainedfrom

=

(1.11)

The piston velocity is zero at the beginning of the stroke, reaches a maximum nearthe middle of the stroke, and decrease to zero at the end of the stroke. Differentiationof Eq (1.5) and substituting will produce

=

sinƟ[1+

Ɵ− Ɵ.]

(1.12)

Figure 1.14 shows how S p varies over each stroke for R =3.5.

Resistance to gas flow into the engine or stresses due to the inertia of the movingparts limit the maximum mean piston speed to within the range of 8 to 15 m/s. Automobile engines operate at the higher end of this range, the lower end is typicalof large marine diesel engines.

1.10 BRAKE TORQUE AND POWER

Engine torque is normally measured with a dynamometer. The engine is clamped ona test bed and the shaft is connected to the dynamometer rotor. Figure 1.19illustrates the operating principle of a dynamometer. The rotor is coupledelectromagnetically, hydraulically, or by mechanical friction to a stator, which issupported in low friction bearing. The stator is balanced with the rotor stationary. Thetorque exerted on the stator with the rotor turning is measured by balancing thestator with weights, springs, or pneumatic means.


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Figure 1.19(a): Engine dynamometer test arrangement.

Figure 1.19(b): An example of a hydraulic dynamometer.


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Figure 1.19 (c): Schematic of principle of operation of dynamometer.

Using the notation in Figure 1.19 (c), if the torque exerted by the engine is T:

= (1.13)

The power P delivered by the engine and absorbed by the dynamometer is theproduct of torque and angular speed:

= 2 (1.14a)where N is the crankshaft rotational speed. In SI units:

P (kW) = 2πN (rev/s)T (N.m) x 10-3 (1.14b)

Note: Torque is a measure of an engine’s ability to do work whereas power is therate at which work is done. The value of engine power measured is called

brake power, P b. This power is the usable power delivered by the engine tothe load i.e. brake.

1.11 INDICATED WORK PER CYCLE

Pressure data for the gas in the cylinder over the operating cycle of the engine canbe used to calculate the work transfer from the gas to the piston. The cylinderpressure and corresponding cylinder volume throughout the engine cycle can beplotted on a p-V diagram as shown in Figure 1.16. The indicated work per cycle W c ,i (per cylinder) is obtained by integrating around the curve to obtain the area enclosedon the diagram:


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,= ∮ (1.15)

Figure 1.20: Examples of p-V diagrams for (a) a two-stroke cycle engine, (b) a four-stroke cycle engine; (c) a four-stroke cycle spark-ignition engineexhaust and intake strokes (pumping loop) at part load.

With two-stroke cycles, the application of Eq. (1.14) is straightforward. With theaddition of inlet and exhaust strokes for the four-stroke cycle, some ambiguity isintroduced as two definitions of indicated output are in common use. These will bedefined as:

Gross indicated work per cycle, W c ,ig . Work delivered to the piston over thecompression and expansion strokes only.

Net indicated work per cycle, W c ,in. Work delivered to the piston over the entire four-stroke only.

In Figure 1.20 b and c, W c ,ig is (area A + area C) and W c ,in is (area A + area C) – ((area B + area C), which equals (area A- area B), where each of these areas isregarded as a positive quantity. Area B + area C is the work transfer between thepiston and the cylinder gases during the inlet and exhaust strokes and is called thepumping work, W p. The pumping work transfer will be to the cylinder gases if thepressure during the intake stroke is less than the pressure during the exhaust stroke.This is the situation with naturally aspirated engines. The pumping work transfer willbe from the cylinder gases to the piston if the exhaust stroke pressure is lower thanthe intake pressure, which is normally the case with highly loaded turbochargedengines.


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The power per cylinder is related to the indicated work per cycle by,

=,

(1.16)

where nr is the number of crank revolutions for each power stroke per cycle percylinder. For four-stroke cycles, nr equals 2; for two-stroke cycles, nr equals 1. Thispower is the indicated power; i.e., the rate of work transfer from the gas within thecylinder to the piston. It differs from the brake power by the power absorbed inovercoming engine friction, driving engine accessories, and (in the case of grossindicated power) the pumping power.

The terms brake and indicated are used to describe other parameters such as meaneffective pressure, specific fuel consumption, and specific emissions in a mannersimilar to that for work per cycle and power.

1.12 Mechanical Efficiency

Part of the gross indicated work per cycle or power is used to expel exhaust gasesand induct fresh charge. An additional portion is used to overcome the friction of the

bearings, pistons and other mechanical components of the engines, and to drive theengine accessories. All of these power requirements are grouped together andcalled friction power P f . Therefore:

P ig = P b +P f (1.17)

Friction power is difficult to determine accurately. One common approach for high-speed is to drive or motor the engine with a dynamometer and measure the powerwhich has to be supplied by the dynamometer to overcome all these frictional losses.

The ratio of the brake power delivered by the engine to the indicated power is calledthe mechanical efficiency hm:

ℎ = = 1

(1.18)

Since the friction power includes the power required to pump gas into and out of theengine, mechanical efficiency depends on throttle position as well as engine designand engine speed. Typical values for a modern automotive engine at wide open


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throttle (WOT) are 90 percent at speeds below 1800 to 2400 rpm, decreasing to 75percent at maximum rated speed. As the engine is throttled, mechanical efficiencydecrease, eventually to zero at idle operation.

1.13 MEAN EFFECTIVE PRESSURE

While torque is a valuable measure of a particular engine’s ability to do work, itdepends on engine size. A more useful relative engine’s performance measure isobtained by dividing the work per cycle by the cylinder volume displaced per cycle.The parameter so obtained has units of force per unit area and is called the meaneffective pressure (mep). Since from Eq. (1.16),

Work per cycle = PnR /N

(1.19)

Where nR is the number of crank revolutions for each power stroke per cylinder (2 forfour-stroke; 1 for two-stroke cycles), then

mep = PnR /V d N (1.20a)

= × 1033

(1.20b)

Mean effective pressure can also be expressed in terms of torque by using Eq.(1.14):

= 6.28.3

(1.21)

Typical values for bmep are as follows:

For naturally aspirated spark-ignition engines, maximum values are in the range of850 to 1050 kPa at the engine maximum speed where maximum torque is obtained(about 3000 rpm). At the maximum rated power, bmep is in the 900 to 1400 kParange. For naturally aspirated four-stroke diesels, the maximum bmep is in the 700

to 900 kPa range, with the bmep at the maximum rated power of about 700 kPa.Turbocharged four-stroke diesel maximum bmep values are typically in the range of


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1000 to 1200 kPa; for turbocharged aftercooled engines this can rise to 1400 kPa. Atmaximum rated power, bmep is about 850 to 950 kPa. Two-stroke cycle dieselshave comparable performance to four-stroke cycle engines. Large low-speed cycleengines can achieve bmep values of about 1600 kPa.

1.14 SPECIFIC FUEL CONSUMPTION

The fuel consumption (volumetric or gravimetric) per power output of an engine iscalled specific fuel consumption. It is a measure of how efficient an engine is usingthe fuel supplied to produce work:

= (1.22)

With units,

( ) =

(1.23a)

or . ℎ =

(1.23b)

Low values of sfc are obviously desirable. For SI engines typical best values of brakespecific fuel consumption are about 75µg/J = 270 g/kW.h. For CI engines, bestvalues are lower and in large engines can go below 55 µg/J = 200 g/kW.h.

The measure of an engine’s efficiency, which will be called the fuel conversion

efficiency hf , is given by,

h = / =

(1.24)

Where mf is the mass of fuel inducted per cycle. Substitution for P/mf from Eq. 1.22gives


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ℎ = (1.25a)

Or with units:

ℎ = (1.25b)

ℎ = 3 (1.25c)

Typical heating values for the commercial hydrocarbon fuels used in engines are in

the range of 42 to 44MJ/kg. Thus, specific fuel consumption is inversely proportionalto fuel conversion efficiency for normal hydrocarbon fuels.

1.15 Air-fuel ratio

In engine testing, both the air mass flow rate ma and the fuel flow rate mf arenormally measured. The ratio of these flow rates is useful in defining engineoperating conditions:

Air/fuel ratio (A/F) =

(1.26)

Fuel/air ratio (F/A) =

(1.27)

The normal operating range for a conventional SI engine using gasoline fuel is 12 ≤ A/F ≤ 18; for CI engines with diesel fuel, it is 18 ≤ A/F ≤ 70.

1.16 VOLUMETRIC EFFICIENCY

The intake system – the air filter, carburettor, and throttle plate (in SI engine),intrake manifold, intake port, intake valve – restrict the amount of air which an engineof given displacement can induct. The parameter used to measure the effectiveness

of an engine’s induction process is the volumetric efficiency hv . Volumetric efficiency

is only used with four stroke cycle engines which have a distinct process. It is

defined as the volume flow rate of air into the intake system divided by the rate atwhich volume is displaced by the piston


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hv = (1.28a)

where rai is the inlet air density. An alternative equivalent definition for volumetric

efficiency is,

hv=

(1.28b)

Where ma is the mass of air inducted into the cylinder per cycle.

Typical maximum values of volumetric efficiency for natural aspirated engines are inthe range of 80 to 90 percent.

1.17 GENERAL ENGINE PERFORMANCE DATA

Engine ratings usually indicate the highest power at which manufacturer expectstheir products to give satisfactory economy, reliability, and durability under serviceconditions. Maximum torque and the speed at which it is achieved, is usually givenalso. Since both of these quantities depend on displaced volume, for comparativeanalyses between engine of different displacements in a given engine categorynormalised performance parameters are more useful. The flowing measures, at the

operating points indicated, have most significance.

Table 1.2: Table of engine design and typical operating data


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1. At maximum or normal rated point:

Mean pis ton s peed . Measures comparative success in handling loads due toinertia of the parts and/or engine friction.Brake mean effect ive pressure . In natureally aspirated engines bmep is not

stress limited. It then reflects the product of volumetric efficiency (ability to inductair), fuel/air ratio (effectivelness of air utilization in combustion), andfuelconversion efficiency. In supercharged engines bmep indicates the degree ofsuccess in handling higher gas pressure and thermal loading.Power per unit piston area . Measures the effectivenss with which the pistonare is used, regardless of cylinder size.Specif ic weight . Indicates relative economy with which materials are used.Speci f ic vo lum e . Indicates relative effectiveness with which engine space hasbeen utilised.

2. At all speeds at which the engine will be used with full throttle or with maximumfuel-pump setting:

Brake mean effect ive pressure . Measures ability to obtain/provides high air flowand use it effectively over the fuel range.

3. At all useful regimes of operation and particularly in those regimes where theengine is run for long periods of time:

Brake specif ic fuel cons umpt ion or fuel conversion efficiency .Brake specif ic emiss ions .

Typical performance data for SI and CI engines over the normal production sizerange are summarised in Table 1.2. The four-stroke cycle dominates except in thesmallest and largest engine size. The larger engines are turbocharged orsupercharged. The maximum rated engine speed decreases as engine sizeincreases, maintaining the maximum mean piston speed in the range of 8 to 15 m/s.The maximum brake mean effective pressure for turbocharged and superchargedengine is higher than for naturally aspirated engines. Because the maximum fuel/airratio for SI engines is higher than for CI, their naturally aspirated maximum bmeplevels are higher. As engine size increases, brake specific fuel consumptiondecreases and fuel conversion efficiency increases, due to reduced importance ofheat losses and friction. For the largest diesel engines, brake fuel consumptionefficiencies of about 50 percent and indicated fuel conversion efficiencies of over 55percent can be obtained.

1.18 ENVIRONMENTAL CONSIDERATIONS IN ENGINE DEVELOPMENT

Generally internal combustion engines, produce moderately high pollution levels,due to incomplete combustion of carbonaceous fuel, leading to carbon dioxide andsome soot along with oxides of nitrogen and sulphur and some unburnedhydrocarbons depending on the operating conditions and the fuel-air ratio. Theprimary causes of this are the needs to operate near to the stoichiometric ratio forgasoline engines in order to achieve combustion and the quench of the flame by the

relatively cool cylinder walls.


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In order to meet numerous legislations and restrictions on exhaust emissions,automobile manufacturers have had to make various modifications in the operationof their engines.

Diesel engines produce a wide range of pollutants including aerosols of manyparticles (PM10) that are believe to penetrate deeply into human lungs. Enginesrunning on liquefied natural gas petroleum gas (LPG) burn very clean and does notcontain sulphur and lead.

Many fuels contain sulphur leading to sulphur dioxide (SOx) in the exhaust,promoting acid rain.

The high temperature of combustion creates greater proportions of nitrogenoxides (NOx), demonstrated to be hazardous to both plants and animals.

Net carbon dioxide production is not a necessary feature of internalcombustion engines. Since most engines are run from fossil fuels this usuallyoccur.

Hydrogen engines need only produce water, but when air is used as theoxidiser nitrogen oxides are also produced.

To reduce the emission of nitrogen oxides, one modification involves sending acertain proportion of the exhaust gases back into the air-gasoline mixture going intothe engine. This cuts peak temperatures during combustion, lessening the amount ofnitrogen oxides produced. In the stratified charge piston engine two separate air-fuelmixtures are injected into the engine. A small, rich mixture that is easily ignited isused to ignite an exceptionally lean mixture that drives the piston. These results inmuch more efficient burning of the gasoline and diesel fuels further reduce tail pipe

emissions. Another device, the catalytic converter, is connected to the exhaust pipe;exhaust gases travel over bars or pellets coated with certain metals that promotechemical reactions, reducing nitrogen oxide and burning hydrocarbons and carbonmonoxide.

1.19 Internal Combustion Engine Efficiency

The efficiency of various types of internal combustion engines vary. It is generally

accepted that most gasoline fuelled internal combustion engines, even when aided

with turbochargers and stock efficiency aids, have a mechanical efficiency of about

20 percent. Most of the internal combustion engines waste about 36 percent of the

energy in the gasoline as heat loss to the cooling system and another 38 percent to

the exhaust. The rest of about 6 percent is lost to friction. Most engineers have not

been able to successfully harness wasted energy for any meaningful purposes,

although there are various add on devices and systems that can greatly improve

combustion efficiency.

Chapter One ICE

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