INTRODUCTION AND HISTORICAL PERSPECTIVE | ENGINE CLASSIFICATIONS

1.1 INTRODUCTION AND HISTORICAL

PERSPECTIVE

The purpose of internal combustion engines is to produce mechanical power

from the chemical energy contained in the fuel. In internal combustion

engines, as distinct from external combustion engines, this energy is released

by burning or oxidizing the fuel inside the engine. The fuel-air mixture before

combustion and the burned products after combustion are the actual working

fluids. The work transfers that provide the desired power output occur

directly between these working fluids and the mechanical components of the

engine. The internal combustion engines that are the subject of this book are

spark-ignition (SI) engines (sometimes called Otto engines, or gasoline or

petrol engines, though other fuels can be used) and compression-ignition (CI)

or diesel engines. a Because of their simplicity, ruggedness, high power to

weight ratio, efficiency, and low cost, these two types of engine have found

wide application in transportation (land, sea, and air) and power generation.

It is the fact that combustion takes place inside the work-producing part of

these engines that makes their design and operating characteristics

fundamentally different from those of other types of engine.

Power-producing engines have served human beings for over two and a

half centuries. For the first 150 years, water, converted to steam, was

interposed between the combustion gases produced by burning the fuel and

the work-producing piston-in-cylinder expander. It was not until the 1860s

that the internal combustion engine became a practical reality. 1, 2 The early

engines developed for commercial use burned coal-gas air mixtures at

atmospheric pressure—there was no compression before combustion. J. J. E.

Lenoir (1822–1900) developed the first marketable engine of this type. Gas

and air was drawn into the cylinder during the first half of the piston stroke.

The charge was then ignited with a spark, the pressure increased, and the

burned gases then delivered power to the piston for the second half of the

stroke. The cycle was completed with an exhaust stroke. Some 5000 of these

engines were built between 1860 and 1865 in sizes up to six horsepower.

Efficiency was at best about 5%.

A more successful development—an atmospheric engine introduced in

1867 by Nicolaus A. Otto (1832–1891) and Eugen Langen (1833–1895)—

used the pressure rise resulting from combustion of the fuel-air charge early

in the outward stroke to accelerate a free piston and rack assembly so its

momentum would generate a vacuum in the cylinder. Atmospheric pressure

then pushed the piston inward, with the rack engaged through a roller clutch

to the output shaft. Production engines, of which about 5000 were built,

obtained thermal efficiencies of up to 11%. A slide valve controlled intake,

ignition by a gas flame, and exhaust.

To overcome this engine’s shortcomings of low thermal efficiency and

excessive size and weight, Otto proposed an engine cycle with four piston

strokes: an intake stroke, then a compression stroke before ignition, an

expansion or power stroke where work was delivered to the crankshaft, and

finally an exhaust stroke. He also proposed incorporating a stratified-charge

induction system, though this was not achieved in practice. His prototype

the four-stroke engine first ran in 1876. A comparison between the Otto engine

and its atmospheric-type predecessor indicates the reason for its success (

Table 1.1): the enormous reduction in engine weight and volume. This was

the breakthrough that effectively founded the internal combustion engine

industry. By 1890, almost 50,000 of these engines had been sold in Europe

and the United States.

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Comparison of Otto’s early four-stroke cycle and Otto-Langen’s engines

In 1884, an unpublished French patent issued in 1862 to Alphonse Beaude

Rochas (1815–1893) was found that described the principles of the four stroke

cycle. This chance discovery cast doubt on the validity of Otto’s own

patent for this concept, and in Germany, it was declared invalid. Beau de

Rochas also outlined the conditions under which maximum performance and

efficiency in an internal combustion engine could be achieved. These were:

1. The largest possible cylinder volume with the minimum boundary

surface

2. The greatest possible working speed

3. The greatest possible expansion ratio

4. The greatest possible pressure at the beginning of expansion

The first condition holds heat losses from the charge to a minimum. The

the second condition increases the power output from a given size engine. The

third condition recognizes that the greater the expansion of the

post combustion gases, the greater the amount of work extracted. The fourth

condition recognizes that higher initial pressures make greater expansion

possible and give higher pressures throughout the process, both resulting in

greater work transfer. Although Beau de Rochas’ unpublished writings

predate Otto’s developments, he never reduced these ideas to practice. Thus

Otto, in the broader sense, was the inventor of the modern internal

combustion engine as we know it today.

Further developments followed fast once the full impact of what Otto had

achieved became apparent. By the 1880s, several engineers (e.g., Dugald

Clerk, 1854–1913, James Robson, 1833–1913, in England, and Karl Benz,

1844–1929, in Germany) had successfully developed two-stroke cycle

internal combustion engines where the exhaust and intake processes occur

during the end of the power stroke and the beginning of the compression

stroke. James Atkinson (1846–1914) in England made an engine with a

longer expansion than compression stroke, which had a high efficiency for

the times but mechanical weaknesses. It was recognized that efficiency was a

direct function of expansion ratio, yet compression ratios were limited to

less than four if serious knock problems were to be avoided with the

available fuels. Substantial carburetor and ignition system developments

were required, and occurred, before high-speed gasoline engines suitable for

automobiles became available in the late 1880s. Stationary engine progress

also continued. By the late 1890s, large single-cylinder engines of 1.3-m

a bore fueled by low-energy blast furnace gas produced 600 bhp at 90 rev/min.

In Britain, legal restrictions on volatile fuels turned their engine builders

toward kerosene. Low compression ratio “oil” engines with heated external

fuel vaporizers and electronic ignition were developed with efficiencies

comparable with those of gas engines (14 to 18%). The Hornsby-Ackroyd

engine became the most popular oil engine in Britain, and was also built in

large numbers in the United States. 2

In 1892, the German engineer Rudolf Diesel (1858–1913) outlined in his

patent a new form of internal combustion engine. His concept of initiating

combustion by injecting a liquid fuel into the high-temperature air in the

cylinder produced by compression permitted a doubling of efficiency over

the other internal combustion engines then available. Much greater

compression and expansion ratios, without detonation or knock, were now

possible. However, even with the efforts of Diesel and the resources of

M.A.N. in Augsburg combined, it took 5 years to develop a practical engine.

Engine developments, perhaps less fundamental but nonetheless important

to the steadily widening internal combustion engine markets, have continued

ever since. 2-4 There has always been an interest in engine geometries

different from the standard reciprocating piston-in-cylinder, connecting rod,

and crankshaft arrangement. Especially, there has been an interest in rotary

internal combustion engines. Although a wide variety of experimental rotary

engines have been proposed over the years, 5 the first practical rotary

internal combustion engine, the Wankel, was not successfully tested until

1957. That engine, which evolved through many years of research and

development, was based on the designs of the German inventor Felix Wankel.

6, 7 While the Wankel engine has been used in niche markets, its advantages of

compactness and smoother operation have not been sufficient to overcome its

high manufacturing cost.

Fuels have also had a major impact on engine development. The earliest

engines used for generating mechanical power burned gaseous fuels.

Gasoline, and lighter fractions of crude oil, became available in the late

1800s, and various types of carburetors were developed to vaporize the fuel

and mix it with air. Before about 1905, there were few issues with gasoline;

though compression ratios had to be low (4 or less) to avoid knock, the

highly volatile fuel made starting easy and gave good cold weather

performance. However, a serious crude oil shortage developed, and to meet

the fivefold increase in gasoline demand between 1907 and 1915, the yield

from crude had to be raised. Through the work of William Burton (1865–

1954) and his associates of Standard Oil of Indiana, a thermal cracking

process was developed whereby heavier oils were heated under pressure

and decomposed into less complex, more volatile compounds. These

thermally cracked gasolines satisfied demand, but their higher boiling point

range created cold weather starting problems. Fortunately, electrically driven

starters, introduced in 1912, came along just in time.

On the farm, kerosene was the logical fuel for internal combustion engines

since it was used for heat and light. Many early farm engines had heated

carburetors or vaporizers to enable them to operate with such a fuel.

The period following World War I saw a tremendous advance in our

understanding of how fuels affect combustion, and especially the problem of

knock. The antiknock effect of tetraethyl lead was discovered at General

Motors, 4 and it became commercially available as a gasoline additive in the

United States in 1923. In the late 1930s, Eugene Houdry found that vaporized

oils passed over an activated catalyst at 450 to 480°C were converted to

high-quality gasoline in much higher yields than was possible with thermal

cracking. These advances, and others, permitted fuels with ever better

antiknock properties to be produced in large quantities; thus engine

compression ratios steadily increased, improving power and efficiency.

During the past several decades, new factors for change have become

important and now significantly affect engine design and operation. These

factors are, first, the need to control the automotive contribution to urban air

pollution and, second, the need to achieve significant improvements in

automotive fuel consumption.

The automotive air-pollution problem became apparent in the 1940s in the

Los Angeles basin. In 1952, it was demonstrated by Prof. A. J. Haagen-Smit

that the smog problem there resulted from reactions between oxides of

nitrogen and hydrocarbon compounds in the presence of sunlight. 8 In due

course it became clear that the automobile was a major contributor to

hydrocarbon and oxides of nitrogen emissions, as well as the prime cause of

high carbon monoxide levels in urban areas. Diesel engines are a significant

source of small soot or smoke particles, as well as hydrocarbons and oxides

of nitrogen. Table 1.2 outlines the dimensions of the problem. As a result of

these developments, emission standards for automobiles were introduced

first in California, then nationwide in the United States, starting in the 1960s.

Emission standards in Japan and Europe, and for other engine applications,

have followed. Substantial reductions in emissions from spark-ignition and

diesel engines have been achieved. Both the use of catalysts in SI engine

exhaust systems for emissions control and concern over the toxicity of lead

antiknock additives have resulted in the reappearance of unleaded gasoline

as the dominant part of the automotive fuels market. These emission-control

requirements and fuel developments have produced significant changes in the

way internal combustion engines are now designed and operated.

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The automotive urban air-pollution problem: typical vehicle emissions

Internal combustion engines are also an important source of noise.

There are several sources of engine noise: the exhaust system, the intake system, the

fan used for cooling, and the engine block surface. The noise may be

generated by aerodynamic effects, may be due to forces that result from the

combustion process, or may result from mechanical excitation by rotating or

reciprocating engine components. Vehicle noise legislation to reduce this

impact on the ambient environment (and thus on people) was first introduced

in the early 1970s.

During the 1970s, the price of crude petroleum rose rapidly to several

times its cost (in real terms) in 1970. In the 1980s, the price of crude oil fell,

and then fluctuated at relatively low levels until the early 2000s when it rose

too close to its late 1970s values. The price then fell rapidly, and then rose

again. Currently, the growth in oil demand in the developing world, the

uncertainty in future extraction from established fields and discovery of new

sources of oil, and the nonuniform concentration of petroleum reserves in a

few nations, suggest that the balance between global oil production and

transportation fuel demand will be tight over the next few decades. This

uncertainty regarding the longer-term availability of adequate supplies of

petroleum-based fuels is creating substantial pressures for significant

improvements in internal combustion engine efficiency (in all the engine’s

many applications). Much work is being done to develop the supply and use

of alternative fuels to gasoline and diesel. Of the non petroleum-based fuels,

natural gas, methanol (methyl alcohol), and biomass-derived fuels such as

ethanol (ethyl alcohol) and biodiesel are receiving significant attention.

Synthetic gasoline and diesel are being made from tar (oil) sands, and could

be produced from shale oil or coal. Hydrogen is being considered as a

longer-term zero carbon containing possibility.

The growing consumption of fossil fuels has raised the concern that the

greenhouse gas (GHG) emissions from our energy supply and use are causing

global warming that could lead to changes in our climate. Emissions of

carbon dioxide, along with other GHGs—methane, nitrous oxide, three

groups of fluorinated gases (sulfur hexafluoride, hydrofluorocarbons, and

perfluorocarbons), ozone—will need to be significantly reduced over the

next several decades. Thus, internal combustion engines will need to become

more efficient, and low GHG emitting sources of energy will need to be

developed so that consumption of petroleum-based fuels—gasoline and

diesel—can be significantly reduced. Transportation is estimated to be the

source of about one-quarter of the world’s GHG emissions.

Table 1.3 lists the CO2 emissions of various fuels and other sources of

energy that might be used in transportation. Emissions from the various fossil

fuels listed vary by about a factor of two. Emissions from biofuel production

are generally lower (and could be significantly lower), depending on the

biomass feedstock, the choice of fuel produced, and the process used to

produce that fuel.

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CO2 emissions per unit chemical energy from various fuels or energy sources

The lower value given for hydrogen (which contains no carbon) is based

on the current industrial hydrogen production process—steam reforming of

natural gas.

The electricity carbon dioxide-emissions intensity value depends

on the mix of coal, natural gas, nuclear, hydro, wind (and solar) used to

generate the electricity. While this electricity generating mix varies country

to country, the major roles of coal and natural gas are common to most

regions.

What would such fuel changes mean for internal combustion engines?

With appropriate changes in engine design and operation, natural gas and the

liquid fuels listed in Table 1.3 can be effectively utilized; indeed engines

using these fuels are in use today. While the potential for hydrogen as a major

transportation energy source (actually an energy storage medium) is partly

based on large-scale use of highly efficient fuel cell technology, it can be

used effectively in suitably designed SI engines. Vehicle propulsion systemṣ

electrification is already occurring through the use of hybrid electric vehicle

(HEV) technology—a combination of a battery, electric motor, internal

combustion engine, and generator. The next step in vehicle electrification is

to expand the battery’s energy storage capacity and recharge (in part) from

the electricity supply grid: deployment of this plug-in hybrid (PHEV)

technology is occurring. HEV and PHEV propulsion systems require an

internal combustion engine, albeit with specific characteristics that improve

its efficiency (see Sec. 1.7.2 ). Some view the pure battery electric vehicle is

the final step in this electrification process. Whether, and how far into the

future complete electrification might occur is currently unclear.

This brings us back to internal combustion engines. It might be thought that

after over a century of development, the internal combustion engine has

reached its peak and little potential for further improvement remains. Such is

not the case. As spark-ignition and diesel engine technology evolves, these

engines continue to show substantial improvements in efficiency, power

density, degree of emission control, and operational capacity. Changes in

engine operation and design are steadily improving engine performance in its

broadest sense. New materials becoming available and more knowledgebased

design offer the potential for continuing to reduce engine weight, size,

and cost, for a given power output, and for different and more efficient

internal combustion engine concepts. Emissions control technologies, in both

the engine and the exhaust system, are becoming more effective and robust.

Variable valve control is replacing fixed valve control approaches, with

performance and efficiency benefits. Direct-injection gasoline engines,

which offer improved dynamic engine control relative to port fuel injection,

are now in large-scale production. These technologies are enabling

increasing deployment of more highly boosted turbocharged gasoline and

diesel engines. 10 Looking ahead, the engine development opportunities of the

future are many and substantial. While they present a formidable challenge to

automotive engineers, they will be made possible in large part by the

enormous expansion of our knowledge of engine processes that the last

several decades have witnessed.

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1.2 ENGINE CLASSIFICATIONS

There are many different types of internal combustion engines. They can be

classified by:

1. Application. Automobile, truck, bus, locomotive, light aircraft,

marine, portable power system, power generation

2. Basic engine configuration. Reciprocating engines (in turn

subdivided by arrangement of cylinders: e.g., in-line, V, radial,

opposed), rotary engines (Wankel and other geometries)

3. Working cycle. Four-stroke cycle: naturally-aspirated (admitting

atmospheric air), supercharged (admitting pre compressed air), and

turbocharged (admitting air compressed in a compressor driven by an

exhaust turbine). Two-stroke cycle: crankcase scavenged,

supercharged, and turbocharged

4. Valve or port design and location. Four-stroke cycle: Overhead (or

I-head) valves, underhead (or L-head) valves, with two, three, or

four valves per cylinder, and fixed or variable valve control (timing,

opening and closing points, and lift), rotary valves. Two-stroke

cycle: cross-scavenged porting (inlet and exhaust ports on opposite

sides of cylinder at one end), loop-scavenged porting (inlet and

exhaust ports on same side of cylinder at one end), through- or

uniflow-scavenged (inlet and exhaust ports or valves at different ends

of cylinder)

5. Fuel. Gasoline (or petrol), fuel oil (or diesel fuel), natural gas,

liquid petroleum gas (LPG), alcohols (methanol, ethanol), hydrogen,

dual fuel

6. Method of mixture preparation. Carburetion or single-point fuel

injection upstream of the throttle, fuel injection into the intake ports,

fuel injection directly into the engine cylinder

7. Method of ignition. Spark ignition in engines where the in-cylinder

fuel-air mixture is uniform and in stratified-charge engines where the

mixture is nonuniform; compression ignition locally of the evolving

in-cylinder fuel-air mixture in diesel engines, as well as ignition in

natural gas engines by pilot injection of fuel oil)

8. Combustion chamber design. Open chamber (many designs: e.g.,

disc, wedge, hemisphere, pent-roof, bowl-in-piston), divided

chamber (small and large auxiliary chambers; many designs: e.g.,

swirl chambers, prechambers)

9. Method of load control. Varying fuel and air flow together so

mixture composition is essentially unchanged, control of fuel flow

alone, a combination of these

10. Method of cooling. Water cooled, air cooled, uncooled (other than

by natural convection and radiation)

All these distinctions are important and they illustrate the breadth of

engine designs available. Because this book approaches the operating and

emissions characteristics of internal combustion engines from a fundamental

point of view, method of ignition has been selected as the primary classifying

feature. From the method of ignition—SI or CI c—follow the important

characteristics of the fuel used, method of mixture preparation, method of

load control, combustion chamber design, details of the combustion process,

engine emissions, and operating characteristics. Some of the other

classifications are used as subcategories within this basic classification. The

engine operating cycle—four-stroke or two-stroke is next in importance; the

principles of these two cycles are described in the following section.

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