WILLARD W. PULKRABEK
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`.
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`Page 1 of 59
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`r I
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`Engineering Fundamentals
`of the
`Internal Combustion Engine
`
`Willard W. Pulkrabek
`University of Wisconsin-Platteville
`
`Prentice Hall
`Upper Saddle River, New Jersey 07458
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`Library of Congress Cataloging·in·Pnblication D»ta
`
`Pulkrabek, Willard W.
`Engineering fundamentals of the internal combustion engine I by
`Willard W. Pulkrabek.
`p. em.
`Includes bibliographical references and index.
`ISBN 0-13-570854-0
`1. Internal combustion engines.
`TJ785.P78 1997
`621.43---"-<ic21
`
`I. Title.
`
`96-54259
`CIP
`
`Acquisitions editor: William Stenquist
`Production editor: Rose Kernan
`Editor-in-chief" Marcia Horton
`Copy editor: Sharyn Vitrano
`Cover designer: Bruce Kenselaar
`Cover photo: Cutaway view of General Motors L82 V6 engine.
`Copyright General Motors Corp.; used with permission.
`Director of production and manufacturing: David W. Riccardi
`Managing editor: Bayani Mendoza De Leon
`Manufacturing buyer: Julia Meehan
`Compositor: Prepare Inc. I Ernilcomp sri
`
`© 1997 by Prentice-Hall, Inc.
`Simon & Schuster I A Viacom Company
`Upper Saddle River, New Jersey 07458
`
`All rights reserved. No part of this book may be
`reproduced, in any form or by any means,
`without permission in writing from the publisher.
`
`The author and publisher of this book have used their best efforts in preparing this book. These efforts
`include the development, research, and testing of the theories and programs to determine their
`effectiveness. The author and publisher make no warranty of any kind, expressed or implied, with
`regard to these programs or the documentation contained in this book. The author and publisher shall
`not be liable in any event for incidental or consequential damages in connection with, or arising out of,
`the furnishing, performance, or use of these programs.
`
`Printed in the United States of America
`
`10 9 8 7 6
`
`ISBN 0-13-570854-0
`
`Prentice-Hall International (UK) Limited, London
`Prentice-Hall of Australia Pty. Limited, Sydney
`Prentice-Hall Canada Inc:; 1"oronto
`Prentice-Hall Hispanoamericana, S.A., Mexico
`Prentice-Hall of India Private Limited, New Delhi
`Prentice-Hall of Japan, Inc., Tokyo
`Simon & Schuster Asia Pte. Ltd., Singapore
`Editora Prentice-Hall do Brasil, Ltda., Rio de Janeiro
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`1
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`Introduction
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`This chapter introduces and defines the internal combustion engine. It lists ways of
`classifying engines and terminology used in engine technology. Descriptions are
`given of many common engine components and of basic four-stroke and two-stroke
`cycles for both spark ignition and compression ignition engines.
`
`1-1 INTRODUCTION
`
`The internal combustion engine (IC) is a heat engine that converts chemical energy
`in a fuel into mechanical energy, usually made available on a rotating output shaft.
`Chemical energy of the fuel is first converted to thermal energy by means of com(cid:173)
`bustion or oxidation with air inside the engine. This thermal energy raises the
`temperature and pressure of the gases within the engine, and the high-pressure gas
`then expands against the mechanical mechanisms of the engine. This expansion is
`converted by the mechanical linkages of the engine to a rotating crankshaft, which is
`the output of the engine. The crankshaft, in turn, is connected to a transmission
`and/or power train to transmit the rotating mechanical energy to the desired final
`use. For engines this will often be the propulsion of a vehicle (i.e., automobile, truck,
`locomotive, marine vessel, or airplane). Other applications include stationary
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`2
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`Introduction
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`Chap. 1
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`engines to drive generators or pumps, and portable engines for things like chain
`saws and lawn mowers.
`Most internal combustion engines are reciprocating engines having pistons
`that reciprocate back and forth in cylinders internally within the engine. This book
`concentrates on the thermodynamic study of this type of engine. Other types of IC
`engines also exist in much fewer numbers, one important one being the rotary
`engine [104]. These engines will be given brief coverage. Engine types not covered
`by this book include steam engines and gas turbine engines, which are better classi(cid:173)
`fied as external combustion engines (i.e., combustion takes place outside the
`mechanical engine system). Also not included in this book, but which could be clas(cid:173)
`sified as internal combustion engines, are rocket engines, jet engines, and firearms.
`Reciprocating engines can have one cylinder or many, up to 20 or more. The
`cylinders can be arranged in many different geometric configurations. Sizes range
`from small model airplane engines with power output on the order of 100 watts to
`large multicylinder stationary engines that produce thousands of kilowatts per
`cylinder.
`There are so many different engine manufacturers, past, present, and future,
`that produce and have produced engines which differ in size, geometry, style, and
`operating characteristics that no absolute limit can be stated for any range of engine
`characteristics (i.e., size, number of cylinders, strokes in a cycle, etc.). This book will
`work within normal characteristic ranges of engine geometries and operating para(cid:173)
`meters, but there can always be exceptions to these.
`Early development of modern internal combustion engines occurred in the lat(cid:173)
`ter half of the 1800s and coincided with the development of the automobile. History
`records earlier examples of crude internal combustion engines and self-propelled
`road vehicles dating back as far as the 1600s [29]. Most of these early vehicles were
`steam-driven prototypes which never became practical operating vehicles. Technol(cid:173)
`ogy, roads, materials, and fuels were not yet developed enough. Very early examples
`of heat engines, including both internal combustion and external combustion, used
`gun powder and other solid, liquid, and gaseous fuels. Major development of the
`modern steam engine and, consequently, the railroad locomotive occurred in the lat(cid:173)
`ter half of the 1700s and early 1800s. By the 1820s and 1830s, railroads were present
`in several countries around the world.
`
`HISTORIC-ATMOSPHERIC ENGINES
`
`Most of the very earliest internal combustion engines of the 17th
`and 18th centuries can be classified as atmospheric engines. These were
`large engines with a single piston and cylinder, the cylinder being open
`on the end. Combustion was initiated in the open cylinder using any of the
`various fuels which were available. Gunpowder was often used as the
`fuel. Immediately after combustion, the cylinder would be full of hot
`exhaust gas at atmospheric pressure. At this time, the cylinder end was
`closed and the trapped gas was allowed to cool. As the gas cooled, it ere-
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`Sec. 1-1
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`Introduction
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`3
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`Figure 1-1 The Charter Engine made in 1893 at the Beloit works of Fairbanks,
`Morse & Company was one of the first successful gasoline engine offered for sale in
`the United States. Printed with permission, Fairbanks Morse Engine Division,
`Coltec Industries.
`
`ated a vacuum within the cylinder. This caused a pressure differential
`across the piston, atmospheric pressure on one side and a vacuum on the
`other. As the piston moved because of this pressure differential, it would
`do work by being connected to an external system, such as raising a
`weight [29].
`Some early steam engines also were atmospheric engines. Instead
`of combustion, the open cylinder was filled with hot steam. The end was
`then closed and the steam was allowed to cool and condense. This cre(cid:173)
`ated the necessary vacuum.
`
`In addition to a great amount of experimentation and development in Europe
`and the United States during the middle and latter half of the 1800s, two other tech(cid:173)
`nological occurrences during this time stimulated the emergence of the internal
`combustion engine. In 1859, the discovery of crude oil in Pennsylvania finally made
`available the development of reliable fuels which could be used in these newly
`developed engines. Up to this time, the lack of good, consistent fuels was a major
`drawback in engine development. Fuels like whale oil, coal gas, mineral oils, coal,
`and gun powder which were available before this time were less than ideal for
`engine use and development. It still took many years before products of the petro(cid:173)
`leum industry evolved from the _first crude oil to gasoline, the automobile fuel of the
`20th century. However, improved hydrocarbon products began to appear as early
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`4
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`Introduction
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`Chap. 1
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`1:
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`•
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`Figure 1-2 Ford Taurus SHO 3.4 liter (208 in. 3), spark ignition, four-stroke cycle
`engine. The engine is rated at 179 kW at 6500 RPM (240 hp) and develops 305 N-m
`of torque at 4800 RPM (225lbf-ft). It is a 60° V8 with ~20 em bore (3.23 in.), 7.95 em
`stroke (3.13 in.), and a compression ratio of 10:1. The engine has four chain driven
`camshafts mounted in aluminum heads with four valves per cylinder and coil-on(cid:173)
`plug ignition. Each spark plug has a separate high-voltage coil and is fired by Ford's
`Electronic Distributorless Ignition System (EDIS). Courtesy of Ford Motor
`Company.
`
`as the 1860s and gasoline, lubricating oils, and the internal combustion engine
`evolved together.
`The second technological invention that stimulated the development of the
`internal combustion engine was the pneumatic rubber tire, which was first marketed
`by John B. Dunlop in 1888 [141]. This invention made the automobile much more
`practical and desirable and thus generated a large market for propulsion systems,
`including the internal combustion engine.
`During the early years of the automobile, the internal combustion engine com(cid:173)
`peted with electricity and steam engines as the basic means of propulsion. Early in
`the 20th century1 electricity and steam faded from the automobile picture-electricity
`because of the limited range it provided, and steam because of the long start-up time
`needed. Thus, the 20t~ century is the period of the internal combustion engine and
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`Sec. 1-3
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`Engine Classifications
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`5
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`the automobile powered by the internal combustion engine. Now, at the end of the
`century, the internal combustion engine is again being challenged by electricity and
`other forms of propulsion systems for automobiles and other applications. What
`goes around comes around.
`
`1-2 EARLY HISTORY
`
`During the second half of the 19th century, many different styles of internal com(cid:173)
`bustion engines were built and tested. Reference [29] is suggested as a good history
`of this period. These engines operated with variable success and dependability using
`many different mechanical systems and engine cycles.
`The first fairly practical engine was invented by J.J.E. Lenoir (1822-1900) and
`appeared on the scene about 1860 (Fig. 3-19). During the next decade, several hun(cid:173)
`dred of these engines were built with power up to about 4.5 kW (6 hp) and
`mechanical efficiency up to 5%. The Lenoir engine cycle is described in Section
`3-13. In 1867 the Otto-Langen engine, with efficiency improved to about 11%, was
`first introduced, and several thousand of these were produced during the next
`decade. This was a type of atmospheric engine with the power stroke propelled by
`atmospheric pressure acting against a vacuum. Nicolaus A. Otto (1832-1891) and
`Eugen Langen (1833-1895) were two of many engine inventors of this period.
`During this time, engines operating on the same basic four-stroke cycle as the
`modern automobile engine began to evolve as the best design. Although many peo(cid:173)
`ple were working on four-stroke cycle design, Otto was given credit when his
`prototype engine was built in 1876.
`In the 1880s the internal combustion engine first appeared in automobiles [ 45].
`Also in this decade the two-stroke cycle engine became practical and was manufac(cid:173)
`tured in large numbers.
`By 1892, Rudolf Diesel (1858-1913) had perfected his compression ignition
`engine into basically the same diesel engine known today. This was after years of
`development work which included the use of solid fuel in his early experimental
`engines. Early compression ignition engines were noisy, large, slow, single-cylinder
`engines. They were, however, generally more efficient than spark ignition engines. It
`wasn't until the 1920s that multicylinder compression ignition engines were made
`small enough to be used with automobiles and trucks.
`
`1-3 ENGINE CLASSIFICATIONS
`
`Internal combustion engines can be classified in a number of different ways:
`
`1. Types of Ignition
`(a) Spark Ignition (SI). An SI engine starts the combustion process in each
`cycle by use of a spark plug. The spark plug gives a high-voltage electrical
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`6
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`Introduction
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`Chap. 1
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`Figure 1-3 1955 Chevrolet "small block" V8 engine with 265 in. 3 ( 4.34 L) displace(cid:173)
`ment. The four-stroke cycle, spark ignition engine was equipped with a carburetor
`and overhead valves. Copyright General Motors Corp., used with permission.
`
`discharge between two electrodes which ignites the air-fuel mixture in the
`combustion chamber surrounding the plug. In early engine development,
`before the invention of the electric spark plug, many forms of torch holes
`were used to initiate combustion from an external flame.
`(b) Compression Ignition (CI). The combustion process in a CI engine starts
`when the air-fuel mixture self-ignites due to high temperature in the com(cid:173)
`bustion chamber caused by high compression.
`2. Engine Cycle
`(a) Four-Stroke Cycle. A four-stroke cycle experiences four piston move(cid:173)
`ments over two engine_revolutions for each cycle.
`(b) Two-Stroke Cycle. A two-stroke cycle has two piston movements over one
`revolution for each cycle.
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`24
`
`Introduction
`
`Chap.1
`
`HISTORIC-STARTERS
`
`Early automobile engines were started with hand cranks that
`connected with the crankshaft of the engine. This was a difficult and dan(cid:173)
`gerous process, sometimes resulting in broken fingers and arms when the
`engine would fire and snaj)'back the hand crank. The first electric starters
`appeared on the 1912 Cadillac automobiles, invented by C. Kettering, who
`was motivated when his friend was killed in the process of hand starting
`an automobile [45].
`
`Supercharger Mechanical compressor powered off of the crankshaft, used to com(cid:173)
`press incoming air of the engine.
`
`Throttle Butterfly valve mounted at the upstream end of the intake system, used
`to control the amount of air flow into an SI engine. Some small engines and
`stationary constant-speed engines have no throttle.
`
`Turbocharger Turbine-compressor used to compress incoming air into the engine.
`The turbine is powered by the exhaust flow of the engine and thus takes very
`little useful work from the engine.
`
`Valves Used to allow flow into and out of the cylinder at the proper t:hne in the
`cycle. Most engines use poppet valves, which are '!;pring loaded closed and
`pushed open by camshaft action (Fig. 1-12). Valves are mostly made of forged
`steel. Surfaces against which valves close are called valve seats and are made of
`hardened steel or ceramic. Rotary valves and sleeve valves are sometimes used,
`but are much less common. Many two-stroke cycle engines have ports (slots) in
`the side of the cylinder walls instead of mechanical valves.
`
`Water jacket System of liquid flow passages surrounding the cylinders, usually
`constructed as part of the engine block and head. Engine coolant flows
`through the water jacket and keeps the cylinder walls from overheating. The
`coolant is usually a water-ethylene glycol mixture.
`
`Water pump Pump used to circulate engine coolant through the engine and radia(cid:173)
`tor. It is usually mechanically run off of the engine.
`
`Wrist pin Pin fastening the connecting rod to the piston (also called the piston pin).
`
`1-6 BASIC ENGINE CYCLES
`
`Most internal combustion engines, both spark ignition and compression ignition,
`operate on either a four-stroke cycle or a two-stroke cycle. These basic cycles are
`fairly standard for all engines, with only slight variations found in individual designs.
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`Sec. 1-6
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`Basic Engine Cycles
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`25
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`Four-Stroke Sl Engine Cycle {Fig. 1-16)
`
`1. First Stroke: Intake Stroke or Induction The piston travels from TDC to
`BDC with the intake valve open and exhaust valve closed. This creates an increasing
`volume in the combustion chamber, which in turn creates a vacuum. The resulting
`pressure differential through the intake system from atmospheric pressure on the
`outside to the vacuum on the inside causes air to be pushed into the cylinder. As the
`air passes through the intake system, fuel is added to it in the desired amount by
`means of fuel injectors or a carburetor.
`2. Second Stroke: Compression Stroke When the piston reaches BDC, the
`intake valve closes and the piston travels back to TDC with all valves closed. This
`compresses the air-fuel mixture, raising both the pressure and temperature in the
`cylinder. The finite time required to close the intake valve means that actual com(cid:173)
`pression doesn't start until sometime aBDC. Near the end of the compression
`stroke, the spark plug is fired and combustion is initiated.
`3. Combustion Combustion of the air-fuel mixture occurs in a very short but
`finite length of time with the piston near TDC (i.e., nearly constant-volume com(cid:173)
`bustion). It starts near the end of the compression stroke slightly bTDC and lasts
`into the power stroke slightly aTDC. Combustion changes the composition of the
`gas mixture to that of exhaust products and increases the temperature in the cylin(cid:173)
`der to a very high peak value. This, in turn, raises the pressure in the cylinder to a
`very high peak value.
`4. Third Stroke: Expansion Stroke or Power Stroke With all valves closed,
`the high pressure created by the combustion process pushes the piston away from
`TDC. This is the stroke which produces the work output of the engine cycle. As the
`piston travels from TDC to BDC, cylinder volume is increased, causing pressure and
`temperature to drop.
`5. Exhaust Blowdown Late in the power stroke, the exhaust valve is opened
`and exhaust blowdown occurs. Pressure and temperature in the cylinder are still
`high relative to the surroundings at this point, and a pressure differential is created
`through the exhaust system which is open to atmospheric pressure. This pressure
`differential causes much of the hot exhaust gas to be pushed out of the cylinder and
`through the exhaust system when the piston is near BDC. This exhaust gas carries
`away a high amount of enthalpy, which lowers the cycle thermal efficiency. Opening
`the exhaust valve before BDC reduces the work obtained during the power stroke
`but is required because of the finite time needed for exhaust blowdown.
`6. Fourth Stroke: Exhaust Stroke By the time the piston reaches BDC,
`exhaust blowdown is complete, but the cylinder is still full of exhaust gases at
`approximately atmospheric pressure. With the exhaust valve remaining open, the
`piston now travels from BDC to IDC in the exhaust stroke. This pushes most of the
`remaining exhaust gases out of the cylinder into the exhaust system at about atmos(cid:173)
`pheric pressure, leaving only that trapped in the clearance volume-when the piston
`reaches TDC. Near the end of the exhaust stroke bTDC, the intake valve starts to
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`26
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`Introduction
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`Chap. 1
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`0
`
`(a)
`
`(b)
`
`(c)
`
`0
`
`(d)
`
`(e)
`
`(f)
`
`Figure 1-16 Four-stroke SI engine operating cycle. (a) Intake stroke. Ingress of air(cid:173)
`fuel as piston moves from TDC to BDC. (b) Compression stroke. Piston moves from
`BDC to TDC. Spark ignition occurs near end of compression stroke. (c) Combus(cid:173)
`tion at almost constant volume near TDC. (d) Power or expansion stroke. High
`cylinder pressure pushes piston from TDC towards BDC. (e) Exhaust blowdown
`when exhaust valve opens near end of expansion stroke. (f) Exhaust stroke.
`Remaining exhaust pushed from cylinder as piston moves from BDC tt/TDC.
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`Sec. 1-6
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`Basic Engine Cyc~s
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`27
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`open, so that it is fully open by TDC when the new intake stroke s~ the next
`cycle. Near TDC the exhaust valve starts to close and finally is fully closed sometime
`aTDC. This period when both the intake valve and exhaust valve are open is called
`valve overlap.
`·
`
`Four-Stroke Cl Engine Cycle
`
`1. First Stroke: Intake Stroke The same as the intake stroke in an SI engine
`with one major difference: no fuel is added to the incoming air.
`2. Second Stroke: Compression Stroke The same as in an SI engine except
`that only air is compressed and compression is to higher pressures and temperature.
`Late in the compression stroke fuel is injected directly into the combustion chamber,
`where it mixes with the very hot air. This causes the fuel to evaporate and self-ignite,
`causing combustion to start.
`3. Combustion Combustion is fully developed by TDC and continues at
`about constant pressure until fuel injection is complete and the piston has started
`towards BDC.
`4. Third Stroke: Power Stroke The power stroke continues as combustion
`ends and the piston travels towards BDC.
`5. Exhaust Blowdown Same as with anSI engine.
`6. Fourth Stroke: Exhaust Stroke Same as with an Sl engine.
`
`Two-Stroke Sl Engine Cycle (fig. 1-17)
`
`1. Combustion With the piston at TDC combustion occurs very quickly, rais(cid:173)
`ing the temperature and pressure to peak values, almost at constant volume.
`2. First Stroke: Expansion Stroke or Power Stroke Very high pressure cre(cid:173)
`ated by the combustion process forces the piston down in the power stroke. The
`expanding volume of the combustion chamber causes pressure and temperature to
`decrease as the piston travels towards BDC.
`3. Exhaust Blowdown At about 75° bBDC, the exhaust valve opens and
`blowdown occurs. The exhaust valve may be a poppet valve in the cylinder head, or
`it may be a slot in the side of the cylinder which is uncovered as the piston
`approaches BDC. After blowdown the cylinder remains filled with exhaust gas at
`lower pressure.
`4. Intake and Scavenging When blowdown is nearly complete, at about 50°
`bBDC, the intake slot on the side of the cylinder is uncovered and intake air-fuel
`enters under pressure. Fuel is added to the air with either a carburetor or fuel injec(cid:173)
`tion. This incoming mixture pushes much of the remaining exhaust gases out the
`open exhaust valve and fills the cylinder with a combustible air-fuel mixture, a
`process called scavenging. The piston passes BDC and very quickly covers the
`intake port and then the exhaust port (or the exhanst valve closes). The higher pres-
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`Combustion
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`Chap.7
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`bustion chamber containing no intake, ignition, or special swirl. When combustion
`occurs in the main chamber, high-pressure gases are forced through the very small
`orifice into the secondary chamber. When the pressure in the main chamber falls
`during the power stroke, these high-pressure gases flow slowly back into the main
`chamber, slightly increasing the pressure pushing on the piston face and producing
`more work. Depending on the design, these backflowing gases may contain a com(cid:173)
`bustible mixture and extend combustion time (and, consequently, work output).
`Many combinations and variations of divided chambers and/or stratified
`charge engines have been tried, and a number of these exist in modern automobiles.
`
`7-3 ENGINE OPERATING CHARACTERISTICS
`
`Power Operation
`
`For maximum power at WOT (fast startup, accelerating up a hill, an airplane taking
`off), fuel injectors and carburetors are adjusted to give a rich mixture, and ignition
`systems are set with retarded spark (spark later in cycle). This gives maximum
`power at a sacrifice of fuel economy. The rich mixture burns faster and allows the
`pressure peak to be more concentrated near TDC, with the probable compromise of
`rougher operation. At high engine speeds, there is less time for heat transfer to
`occur from the cylinders, and exhaust gases and exhaust valves will be hotter. To
`maximize flame speed at WOT, no exhaust gas is recycled, resulting in higher levels
`ofNOx.
`Interestingly, another way of obtaining added power from an engine is to oper(cid:173)
`ate with a lean mixture. Race cars are sometimes operated this way. In a lean
`mixture, flame speed is slow and combustion lasts well past TDC. This keeps the
`pressure high well into the power stroke, which produces a greater power output.
`This way of operation produces very hot exhaust gases due to the late combustion.
`This hot exhaust, combined with the unused oxygen of the lean mixture, oxidizes
`the exhaust valves and seats very quickly. This requires changing of the exhaust
`valves quite often, something unacceptable except maybe for race cars. Ignition
`·
`timing must be set specially for this kind of operation.
`
`Cruising Operation
`
`For cruising operation such as steady freeway driving or long-distance airplane
`travel, less power is needed and brake-specific fuel consumption becomes impor(cid:173)
`tant. For this type of operation a lean mixture is supplied to the engine, high EGR is
`used, and ignition timing is advanced to compensate for the resulting slower flame
`speed. Fuel usage efficiency (miles/liter) will be high, but thermal efficiency of the
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`Sec. 7-3
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`Engine Operating Characteristics
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`247
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`engine will be lower. This is because the engine will be operating at a lower speed,
`which gives more time per cycle for heat losses from the combustion chamber.
`
`Idle and low Engine Speed
`
`At very low engine speeds the throttle will be almost closed, resulting in a high
`vacuum in the intake manifold. This high vacuum and low engine speed generate a
`large exhaust residual during valve overlap. This creates poor combustion, which
`must be compensated for by supplying a rich mixture to the engine. The rich mixture
`and poor combustion contribute to high exhaust emissions of HC and CO. Misfires
`and cycles where only partial combustion occurs in some cylinders are more com(cid:173)
`mon at idle speeds. A 2% misfire rate would cause exhaust emissions to exceed
`acceptable standards by 100-200%.
`
`Closing Throttle at High Engine Speed
`
`When quick deceleration is desired and the throttle is closed at high engine speed, a
`very large vacuum is created in the intake system. High engine speed wants a large
`inflow of air, but the closed throttle allows very little air flow. The result is a high
`intake vacuum, high exhaust residual, a rich mixture, and poor combustion. Misfires
`and high exhaust emissions are very common with this kind of operation.
`Engines with carburetors give especially poor combustion under these condi(cid:173)
`tions. Due to the high vacuum, the carburetor gives a large fuel flow through both
`the normal orifice and the idle valve. This, combined with the restricted air flow
`rate, creates an overrich mixture with poor combustion and high exhaust pollution
`of HC and CO. The controls on engines with fuel injectors shut the fuel flow down
`under these conditions, and this results in much smoother operation.
`
`Starting a Cold Engine
`
`When a cold engine is started, an overrich supply of fuel must be supplied to assure
`enough fuel vapor to create a combustible gas mixture. When the walls of the intake
`system and cylinders are cold, a much smaller percentage of the fuel will vaporize
`than in normal steady-state operation. The fuel is also cold and does not flow as
`readily. The engine turns very slowly, being driven only by the starting motor, and a
`greater amount of the compressive heating during compression is lost by heat trans(cid:173)
`fer to the cold walls. This is made worse by the cold viscous lubricating oil that
`resists motion and slows the starting speed even more. All of these factors con(cid:173)
`tribute to the need for a very rich air-fuel ratio when starting a cold engine. Air-fuel
`ratios as rich as 1:1 are sometimes used.
`Even when everything is very cold, a small percentage of fuel vaporizes and a
`combustible air and vapor mixture can be obtained. This mixture is ignited, and
`after only a few cycles of combustion, the engine begins to heat up. Within a few
`
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`Combustion
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`Chap.7
`
`seconds it starts to operate in a more normal mode, but it can take many minutes
`before fully warmed steady-state operation is reached. Once the engine starts to
`warm, all of the excess fuel that was originally input vaporizes and a short period of
`overrich operation is experienced. During this period, there is a large excess of HC
`and CO emissions in the exhaust. To compound this problem, the catalytic converter
`is also cold at startup and does not remove these excess emissions. This problem of
`excess air pollution at cold startup is addressed in Chapter 9.
`Special starting fluids can be purchased for aiding engine startup in extremely
`cold temperatures. Substances like diethyl ether with very high vapor pressures
`evaporate more readily than gasoline and give a richer air-fuel vapor mixture for
`initiating combustion. These fluids generally are obtained in pressurized containers
`and are sprayed into the engine air intake before starting.
`
`7-4 MODERN FAST-BURN COMBUSTION CHAMBERS
`
`The combustion chamber for a modern high-speed SI engine must be able to burn
`the contained air-fuel mixture very rapidly without creating excess exhaust emis(cid:173)
`sions. It must provide a smooth power stroke, low specific fuel consumption, and
`maximum thermal efficiency (a high compression ratio). Two general designs for
`such a combustion chamber are shown in Fig. 6-4. Many modern engines have com(cid:173)
`bustion chambers that are a variation of one or bot~ of these designs. As a
`comparison, Fig. 7-14 shows the general design of a combustion chamber found in
`historic, L head, valve-in-block engines.
`It is desirable to have the minimum combustion time possible without actually
`having an instantaneous constant-volume reaction (detonation). If the combustion
`time is less than the ignition delay time of the air -fuel mixture after the temperature
`has been raised above self-ignition temperature, knock is avoided (see Chapter 4).
`
`0
`
`Figure 7-14 Combustion chamber of L
`head, valve-in-block engine. For several
`decades from the 1910s to the 1950s, this
`was the standard geometry of many engines.
`With a few exceptions to the general design,
`this type of combustion chamber generally
`did not promote high levels of swirl, squish,
`or tumble, considered very desirable in mod(cid:173)
`ern combustion philosophy. Flame travel
`distance-was also long compared to modem
`combustion chambers. All of this restricted
`these early engines to much lower compres(cid:173)
`sion ratios than what is common today.
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`Modern Fast-Burn Combustion Chambers
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`249
`
`The faster the burn time, the higher the allowable compression ratio and/or the
`lower the octane number required in the fuel.
`For the fastest burn time, a minimum flame travel distance is desired with
`maximum turbulence, swirl, and squish. The two chambers in Fig. 6-4 satisfy these
`requirements, while the older chamber in Fig. 7-14 does not. As the piston
`approaches TDC in the chambers in Fig. 6-4, the air-fuel mixture is compressed
`towards the centerline of the cylinder. Conservation of angular momentum will
`cause a large increase in swirl rotation as the average mass radius is decreased. Some
`momentum will be lost via viscous friction with the walls. This inward compression
`also causes a large squish velocity in the radial direction towards the cylinder cen(cid:173)
`terline. Both these motions greatly increase flame front speed and decrease
`combustion time. There is also a reverse outward squish which further increases the
`spread of the flame front. This occurs early in the power stroke when the piston
`starts to move away from TDC. In a modern combustion chamber these motions,
`along with turbulence, increase flame velocity by a factor of about 10 over a flame
`passing through a stagnant ai