ABSTRACT
Diesel engines rely on autoignition through compression ignition of an atomized liquid fuel jet injected into the high-temperature and-pressure air toward the end of the compression stroke of a high compression ratio unthrottled reciprocating piston engine� To understand the specific requirements of liquid fuels for use in compression ignition diesel-type engines, a list of the main specific features of these engines and how they are different from the gasoline spark ignition type are the following:
• They rely on the compression ignition of fuel-air mixtures and need to have sufficiently high compression ratios to affect reliable and controlled autoignition�
• They involve nonhomogenous mixtures leading mainly to heterogeneous diffusion-type combustion�
• Excess air operation is employed throughout with intense turbulence and swirling action, mainly to aid in the atomization, vaporization, and mixing of the fuel and air�
• The liquid fuel is injected into the cylinder toward the end of compression using very high injection pressures�
• They produce rapid energy releases requiring robust engine construction to withstand the resulting high mechanical and thermal loading rates�
• The fuels needed are more prone to autoignition and are of a high cetane number and a low, even negative, octane number�
• The engines in comparison to spark ignition engines tend to be of lower speeds, unthrottled, and are harder to start unaided under cold weather conditions�
• They have superior work production efficiency and torque characteristics� They can be made extremely large in size and power capacity�
• The engines are highly suitable for a high degree of turbocharging� • Their emissions tend to show low CO and unburned hydrocarbons
but relatively high NOX and particulates�
Continuous improvements in diesel fuel injection systems have produced significant improvements in driving dynamics, fuel consumption, and reduced exhaust emissions�
In diesel engine operation, the injected liquid fuel cannot ignite instantaneously despite the very high temperatures, pressures, and availability of excess air� A certain time period needs to elapse from the commencement of fuel spray injection before the ignition can take place� This time lag to ignition is described as the ignition delay (Figure 13�1), which results from the fact that certain physically and chemically based requirements need to be satisfied before autoignition can take place� As shown in Figure 13�2, these are due to the time needed for the injection, atomization, and evaporation of some of the injected liquid fuel and mixing of the resulting vapor with the air, before chemical gas phase reactions begin to proceed seriously� These take their own time to reach the ignition stage and begin releasing energy� It is essential to keep the length of this delay as short as possible so as to control effectively the progress of the combustion process through controlled fuel injection rates� Much delayed ignition will lead to low power output and efficiency, increased
emissions, and, as can be seen in Figure 13�1, undesirably very high rates of pressure rise� These arise from the sudden autoignition of the relatively large amount of fuel that continued to be injected throughout the delay period� Figure 13�3 shows a schematic representation of the two main types of diesel engines widely used, where direct or indirect liquid fuel injection is employed�
The degree of suitability of a liquid fuel to satisfactory operation in a compression ignition type diesel engine is determined mainly through its rating via the empirically devised cetane number scale� This rating is essentially based on
the test fuel having the same value of ignition delay as that of a specific binary liquid fuel blend of cetane (n-hexadecane) and isohexadecane (heptamethylnonane), when tested in the same standard engine under the same standard operating conditions� In this rating scale, pure cetane is given a rating of 100 cetane number, whereas isohexadecane is given a rating of 15 cetane number, that is
Cetane no� % n - cetane 0�15 % hepta - methyl= ( ) + nonane( ) On this basis, the fuel tested is given a cetane number rating depending on the relative concentration on a liquid volume basis of the standard reference fuel mixture composition�
It can be seen that a fuel having a low cetane number indicates that it is difficult to ignite within the available time in the engine settings� Accordingly, a fuel with a high octane number, which indicates a resistance to autoignition, will have a very low cetane number value� Most common commercial diesel fuels have a cetane number within the range of 45 to 60� Also, long chain normal hydrocarbons tend to have high values of cetane numbers whereas aromatic fuels are less reactive and have very low cetane numbers� This is because the higher the normal hydrocarbons are, the more they are reactive with air� Suitable additives are often employed to improve this ignitability of fuels destined for diesel engine applications and hence increase their cetane number values� Ignition improvers are used to increase the cetane number (Figure 13�4)� Common examples of these are
alkyl-nitrates and nitrate-esters� Generally, high-cetane-number fuels have a greater response to additives than fuels of a low cetane number� Flow improvers, which are long-chain polymers, are employed to help reduce the viscosity of diesel fuels�
In majority of the diesel engines under normal operating conditions involving high power output, the ignition delay is shorter than the total combustion period� This latter period is considered to be made up of the following consecutive stages (Figures 13�1 through 13�5) ignition delay, followed by a rapid pressure-rise stage, which later becomes a moderate pressure rise and ends with some combustion energy release during the early
part of the expansion stroke� It is important to keep the length of the delay period low and avoid having excessively high rates of pressure rise� The continued release of the combustion energy during expansion needs also to be reduced, as it affects adversely engine efficiency, power output, exhaust emissions, extent and rates of heat transfer, and overall engine reliability and durability�
The injection of some of the highly reactive ether into the intake of a diesel engine for starting under cold weather conditions was a practice that does not see much application nowadays, because it is a rather dangerous practice that may result in uncontrolled excessive rates of early pressure rise leading to potential engine damage� Other approaches such as electric heating are widely used in the form of glow plug aiding in ignition and switched on within the combustion chamber whenever required�
The suitability of a fuel for diesel engine applications is identified through a number of properties that are specified according to standard specifications� These include the following:
• Cetane number • Density (commonly using the empirically developed American
Petroleum Institute [API] gravity, which is defined as 141�5/specific gravity-131�5, at 15oC)
• Volatility/distillation characteristics • Viscosity • Heating value • Flash, cloud, and pour points • Sulfur and fuel nitrogen contents • Carbon and ash residues • Water, sediment contents, and overall composition
Most of these are defined briefly within the Glossary� Significant research and development have taken place to improve
fuel injection characteristics so as to reduce exhaust emissions without undermining engine performance� This has been achieved through the increased incorporation of the common rail system, which employs exceptionally high fuel line pressures with an electronic valve actuation system� This arrangement ensures precise control of fuel injection and the production of very fine fuel atomization, aiding in the rapid vaporization of the resulting ultra-small-diameter fuel droplets and mixing fuel vapor with the air�
Reduction of sulfur levels in diesel fuels down to ultra-low sulfur concentrations reduces the emission levels of sulfur oxides and particulates� It
would also enable the use of advanced exhaust after-treatment methods, especially to reduce oxides of nitrogen and particulates�
There are numerous important properties diesel fuels must possess for acceptable engine operation� These include ignition quality, density, heat of combustion, volatility, viscosity, surface tension, cleanliness, and noncorrosiveness� In addition, the following requirements are closely associated with diesel fuels:
• The aniline point is an empirical test that provides some information about the hydrocarbon composition of the fuel and provides a basis for estimating its ignition quality as a diesel fuel and its smoking tendency� The test determines the lowest temperature at which aniline, an oily liquid, and diesel fuel oil are completely soluble with one another�
• The carbon to hydrogen atomic ratio of a hydrocarbon fuel may be employed in empirically derived correlating equations to estimate some of the fuel properties such as the heating value and possibly some of its other combustion characteristics�
• The cloud point of a fuel is a temperature slightly above the pour point at which waxy components in the fuel crystallize, giving a cloudy appearance to the fuel�
• Water and sediments are very undesirable contaminants and their presence ought to be limited to very small quantities� Water corrodes tanks, fuel-handling equipment, and burner parts� It may lead to poor and erratic combustion� This is exaggerated under cold weather conditions� Sediments accumulated, in tanks may block fuel lines and valves, plug filters and nozzles, and cause undue wear in various parts of the system�
• The carbon residue test is an empirical indication of the tendency of a fuel to form undesirable deposits� Because most light fuel oils have very low carbon residue, meaningful measurements can be obtained usually by distilling first the lighter 90% of the fuel and then determining the carbon residue of the heavier 10% that remains�
• The distillation volatility test, as indicated in the previous chapter, determines the temperatures at which various percentages of the fuel are vaporized� Test results are frequently plotted to give distillation curves such as that shown in Chapter 12� Two points are of particular significance, the 10% point that determines how easily the fuel can be ignited and the 90% point that determines whether it can be vaporized completely within the time available during combustion, before forming hard-to-burn deposits�
Current technologies to reduce diesel engine emissions include improving the following:
• Fuel quality with ultra-low sulfur content • Fuel delivery and spray systems and the resulting spray characteristics • Turbocharging and employing optimized appropriate modifications
to the combustion chamber
Much of the current and future efforts are also focused on the exhaust gas after treatment� These would include the provision of oxidation catalyst converters, particulate matter (PM) traps and oxidizers, selective catalytic reduction of NOX, and employing optimum control exhaust gas recirculation�
Elaborate methods have been devised more recently to deal with diesel engine emissions� These include the fitting of exhaust particulate filters to trap and burn or oxidize the particulates, thus regenerating or cleaning the filter periodically� Diesel PM is usually found in sizes of the order of less than 1 μm and consists of mainly two primary constituents as follows:
1� Unburned carbon particles (soot), which make up the largest portion of the total PM
2� Soluble organic fractions (SOF), which consist of unburned hydrocarbons that have condensed into liquid droplets or have condensed on the soot particles
With exhaust gases of diesel engines that contain high concentrations of oxygen, the oxides of nitrogen may be removed through the addition of the reducing agent ammonia to sufficiently hot exhaust gas temperatures, in the presence of a catalyst according to the following overall reaction equation:
aNO bNH a b N b H O dO2 2 2X + → +( ) ( ) + +3 2 3 2/ / At a high b/a ratio, a high extent of removing the oxides of nitrogen is
obtained, but with an undesirable increase in the emission of the unused ammonia� Obviously, this adds to the complexity of controls and both operating and capital costs� The reactor also needs to be equipped with means for preheating the system during start-up�
Accordingly, controlling the emission of NOX in diesel engines is somewhat a harder task than in spark ignition engines, because diesel combustion is of the diffusion type, where localized stoichiometric combustion operation continues to dominate irrespective of what the overall equivalence ratio is� Resorting to the injection into the exhaust gas of some urea is being
increasingly introduced to reduce the amount of oxides of nitrogen discharged into the atmosphere� Since the exhaust gas contains much unconsumed oxygen, the common automotive three-way catalytic converter universally used with spark ignition engines under these conditions tends to be relatively less effective�
Figure 13�6 shows a schematic diagram of an arrangement for removing the particulates in the exhaust gas of a diesel engine with the aid of an oxidizing catalyst�
Biodiesel fuels, which are derived in principle from renewable resources, are not new and some of the earliest diesel engines were developed to run on vegetable oils� These fuels are blends of conventional diesel fuel with a fatty acid methyl ester produced from vegetable oils or animal fats obtained from a variety of sources� Their use is growing steadily around the world, particularly with the increasing demand and cost of conventional diesel fuel and the gradual decline in its availability globally� For example in Europe, the vegetable component may be derived from grape seeds, whereas in East Asia, palm or coconut oil may be used� Only small amount of biodiesel are normally mixed with conventional diesel fuels, typically at present very much less than 20% by mass�
Biofuels may produce lower particulate emissions and have improved cetane numbers and fuel lubricity, but so far they tend to be of limited supply and expensive� A wide variety of additives are employed to reduce the impact of some of their negative features� These include high viscosity, poor cold-weather performance, higher NOX emissions, slight power loss, and poor materials
compatibility including corrosion of parts of the fuel system with increased wear�
The production of biodiesel from biological material is usually done by reacting it with an alcohol in the presence of a catalyst� A biodiesel blend with petroleum-derived diesel fuel, for example B10, indicates that it is made up of 10% biodiesel and the rest is diesel fuel on a liquid volume basis�
Biofuels have been championed to reduce energy reliance, boost farm revenues, and contribute toward reducing global warming� However, there are some views that indicate that such usage in fact may stress the environment and increase food prices worldwide� According to the Organization for Economic Cooperation and Development (OECD), when factors such as acidification, fertilizer use, biodiversity loss, and toxicity of agricultural pesticides are taken into account, the overall environmental impacts of ethanol and biodiesel can very easily exceed those of gasoline and mineral diesel�
Fischer-Tropsch Diesel (FTD) is a synthetic diesel fuel made from synthesis gas, a mixture of primarily hydrogen and carbon monoxide that is produced commonly by the catalytic reforming with steam or partial oxidation of coal or natural gas� The product of FT synthesis is a mixture of hydrocarbons of different-size molecules, which are cracked to produce diesel fuel� FT diesel is considered a cleaner-burning fuel and of better quality than conventional diesel fuels� However, its production remains somewhat costly� Diesel fuel produced from synthesis gases made out of coal tends to be wasteful energy, more expensive, and more complex than if it were produced from natural gas� FT diesel is almost identical in its relevant properties to regular diesel� The combustion energy and viscosity of synthetic diesel are rather similar to regular diesel fuels� It can be used without modifying existing engines and has quite a high cetane number (e�g�, 70 versus 50)� This is mainly because it has a high fraction of straight-chain high-molecular-weight components that are easily reactive and require lower ignition temperatures� It also has virtually no sulfur� However, in the production of synthetic diesel, significantly more carbon dioxide is produced as compared to the refining of petroleum and production of regular liquid fuels�
Dual-fuel engines are compression ignition engines of the diesel type that can be operated to consume usefully gaseous fuels, notably natural gas, while retaining many of the positive features of diesel operation� Such an approach involves the introduction of a gaseous fuel component either at the intake stage (fumigated into the air) or injected from a high pressure supply directly into the cylinder sometime after intake valve closure during compression (Figure 13�7)� The gaseous fuel-air mixture is then ignited through diesel fuel injection in the usual way� The ignition center of the liquid fuel initiates rapid combustion� This would result from the strong and multiple energy sources for ignition and combustion of the gaseous fuel-air mixture, and it can lead to the combustion of very lean fuel mixtures as well as fuel mixtures of very low heating values� By reducing the amount of diesel fuel injection, the bulk of the energy release comes from the combustion of the gaseous fuel component�
Dual-fuel engines represent a versatile approach for employing high compression ratio conventional diesel operation on gaseous fuels� Such an operation can produce superior efficiencies and lower undesirable emissions than the corresponding gas-fueled spark ignition engine, while consuming a very wide range of gaseous fuels and economizing on the amount of the more expensive liquid fuel�
Most modern large-capacity aircraft are powered with gas turbines via jet propulsion that require distinct fuels with special properties� Most of the much smaller aircraft are still powered by piston-type spark ignition engines� Accordingly, there are two main classes of fuels to power modern aircraft: aviation gasoline (avgas) and aviation turbine jet fuels� Aviation gas is a very high octane number and relatively high-density gasoline used primarily in piston engine aircraft, whereas jet fuels, which are classified in a number of different categories, are used in civil and military jet propulsion gas turbine engine applications�
Aviation fuels are an important class of liquid fuels derived from the refining of petroleum� Aviation jet fuels are based on the kerosene family and have properties ranging between those for gasoline and diesel fuels� The reason why kerosene was used in jet engines is mainly due to the fact that it was widely available from the refining of petroleum, while gasoline and diesel fuels were needed mainly for engines in road-transport applications� The apparent specifications of aviation fuels may vary slightly from one country to another and from civilian aircraft to military applications� High octane numbers of special specification aviation gasolines are still being used for piston engine propelled aircraft�
Due to the severe conditions that aircraft experience during flight, their fuels must meet stringent specifications worldwide� The requirements mainly include the following:
1� High availability in different locations around the world with the required properties
2� Low fire risk, particularly in the event of an accident 3� High thermal stability, irrespective of wide changes in temperature 4� High heating value, both on volume and mass bases 5� Low vapor pressure, especially because of high-altitude operation 6� High specific heat, so that no high temperatures will be formed
within the liquid fuel 7� High lubricating properties 8� Contaminant-free with low carbon formation within engine parts
When the aircraft climbs to a high altitude where low ambient pressures prevail, some of the air dissolved in the fuel comes out of the solution and is expelled from the fuel tank via the venting system� This loss is an important concern because the liberated gases contain fuel vapors, which escape from the tank� One approach is to pressurize the tank to reduce such fuel vapors escaping� Potential warming of the fuel due to aerodynamic heating can
occur, and the fuel needs to have high thermal stability and low tendency to produce electrostatic charges�
The ratio of carbon to hydrogen has a significant influence on the extent of carbon formation� Carbon deposited on the walls of the combustion chamber not only impedes heat transfer but also can affect the combustion process itself and may lead to wall damage�
Jet fuels of the kerosene type have a carbon number ranging from C8 to C12� There are several types of jet fuels� A common one is designated as Jet B,
which is used for commercial aircraft capable of flights into cold regions of the world� Aviation jet fuel, like other processed fuels, is being continuously optimized for different fields of application by increasing its energy content, combustion quality, thermal stability, storage stability, lubricity, fluidity, volatility, noncorrosiveness, cleanliness, and safety�
One of the main requirements of jet fuels is to have a high flash point needed to ensure greater safety against the likelihood of igniting or exploding in case of an accident or catching fire in the presence of a heated surface or hot spot� The electrical conductivity also needs to be high to reduce the buildup of static electricity significantly, such as during the agitation and sloshing of the liquid fuel in its tanks during flight maneuvers and thunderstorms�
Another important fuel property required is a high thermal stability, because jet fuel is often used as a coolant in aircraft� When the fuel is heated, it may undergo decomposition that leads to the undesirable formation of gums and particulates that can clog filters, reduce heat exchange effectiveness, and undermine injection characteristics, fuel handling, and the ultimate operation and reliability of the aircraft� Fuels with high naphthenes and aromatics are more likely to form carbonaceous particles and smoke� Stability during storage is particularly important for military aircraft, where they usually remain fully fueled and ready for prompt action when needed� Antioxidant additives are employed to improve the stability of the fuel during storage�
Jet fuel needs to flow at all operating conditions and temperatures� The fuel must stay liquid and must not freeze over the expected operating temperature ranges� Moreover, any dissolved water in the fuel, however little, must be inhibited from freezing, usually with the use of special additives� The right volatility characteristics are needed to ensure a sufficiently high volatility to aid in effective combustion, yet it needs to be low enough to avoid vapor lock in the fuel lines� Viscosity is also an important characteristic that affects fuel spray and combustion�
Jet fuels do need and contain numerous fuel-soluble additives, usually in very small concentrations that are often merely a few parts per million, with some that are propriety� They are selected and designed depending on the field of application of the aircraft, to suitably modify the fuel properties so as to improve fuel performance and handling� Many of the additives for military applications are different from those for civilian applications�
Typical additives to jet fuels are system corrosion inhibitors, thermal stability improvers, antioxidants, metal deactivators, lubricity improvers, leak detectors, and biocides� Additives to render jet fuels clean from the action of solid particulates and water are needed because water freezes at low temperatures and may cause corrosion� Also, water in the fuel permits microbial and fungal growth that can generate acidic compounds that cause corrosion and generate growths that can clog fuel filters, especially in inactive military aircraft during periods of peace�
Boiler fuels associated with steam generation for process heating or power applications are widely used, such as in the bulk production of electric power with high efficiencies� These applications, in principle, operate on a wide range of fuels with varying properties, including those of low grade where corrosion and deposit problems are confined to the stationary boiler components, while other components such as those of the rotary turbine are exposed throughout to only high-purity treated steam� Such units have the potential to be operated in linkages with other utilization processes� However, they tend to be of high capital cost, requiring numerous auxiliary components, and they are usually confined to very large units operated regularly at high load and have a very long operational life� Controls are being increasingly applied to ensure acceptable levels of sulfur and nitrogen oxide emissions� Elaborate measures are also being increasingly enacted to ensure low particulates emissions�
Rocket engines use thrust as their means of propulsion� They are fundamentally simple devices that operate on the basis of Newton’s third law of motion� The thrust produced is dependent on the momentum of the fluid leaving the system� Rocket engines can employ many unconventional forms of fuels� Propellants are usually chemical mixtures that are burned to generate thrust within rocket-type devices� They can be classified in a variety of ways such as the state, number of components, method of ignition, or type of oxidizer used� Rocket fuels are typically used in either liquid or solid states� Solid propellants are categorized as homogeneous mixtures, made typically of a fuel and oxidizer components�
A major advantage of using liquid rocket motors is that they can produce a much higher specific impulse than solid fuels� Hence, powerful rocket engines tend to use liquid fuels that can also be controlled to throttle, shut down, or restart the engine� A major requirement of a good propellant is to have a high specific impulse� This is directly related to the temperature of combustion, as higher temperatures provide more energy to drive the exhaust gases through the nozzle at a higher velocity� Safety is also critical because toxic and highly reactive and corrosive compounds are often involved�
There is a wide range of liquid fuels that have been developed specifically to be employed in providing the propulsion of rocket motors of various sizes and thrusts� One of the liquid propellants is a highly refined type of kerosene known as refined petroleum (RP1), with liquid oxygen as the oxidizer� RP1 is relatively cheap and can be stored easily at ambient temperature�
Cryogenic propellants are gases at normal temperatures but become liquid by cooling to sufficiently low cryogenic temperatures� The most common mixture is liquid hydrogen the fuel, with liquid oxygen as the oxidizer� Of course, there are many challenges in maintaining and dealing with the very low temperatures involved in such systems� They are not used in rockets or missiles that need to be stored for long periods� In addition, hydrogen, even when it is in a liquid state, has a very low density that requires very large tanks to contain sufficient mass to generate the thrust needed�
Liquid rocket engines store the fuel and oxidizer in separate tanks, until they are required in the combustion chamber� The complexity of such a design lies in the system of pumps, valves, and pipes that are required to transfer the propellants from storage tanks to the combustion chamber effectively, irrespective of the temperature of the fluids involved� Liquid propellants are either petroleum, cryogenic, or hypergolic� Hybrid systems consist of solid and liquid propellants together�
There is another group of propellants that are known as hypergolic� These are fuels and oxidizers that will spontaneously combust on contact with one another� The most common hypergolic fuel is hydrazine that is used in combination with nitrogen tetroxide as an oxidizer� Hydrazine can also act as a monopropellant through exothermic decomposition, eliminating the need for an oxidizer�
Many liquid propellants are composed of a separate fuel and oxidizer and are called bipropellants because they do not mix until pumped into the combustion chamber�
A dual-fuel engine operates at an atmospheric intake pressure of 90 kPa and a temperature of 310 K on diesel fuel assumed to be cetane (C16H34) and natural gas assumed to be methane� The engine consumes 0�225 kg/h diesel, 0�123 kg/h natural gas, and 19�53 kg/h of air� What are the amounts of excess air used and the total equivalence ratio of the intake charge? What is the ideal composition of the dry exhaust products? Assume ideal gases throughout�
Answer: Find the moles of the components used�
M 12 16 34 1 226 kg kmolC H16 34 = × + × = /
Diesel fuel used = 0�225/226 = 0�001 kmol
M 12 4 1 16 kg kmolCH4 = + × = /
Methane used = 0�123/16 = 0�0077 kmol
M 21 32 79 28 28 88 kg kmolair = × + × =0 0� � � /
Per hour of supply, the overall equation becomes:
0 00 0 0 0 0� � , � �1 C H 77 CH 6762 21O 79 N16 34 4 2 2+ + +( ) → + + +
aCO bH O
dO FN 2 2
By solving the set of elemental mass balance equations: d = 0�1021 kmol and the excess air = 0�1021 × 28�88/0�21 = 14�05 kg Supplied air = 19�53 kg/h and excess air = 14�05 kg Stoichiometric air = 19�53 − 14�05 = 5�48 kg Equivalence Ratio = stoichiometric air/air supplied = 5�48/19�53 = 0�2806
CO dry 237 1 237 1 21 53422 % � / � � �= × + +( ) =0 0 00 0 0 0 0 0 0 0 0 0� / � � %237 66 3 6=
O dry2 % � / � � %= × =0 1021 199 0 660 15 45
N dry2 % � / � � %= × =0 5342 100 0 660 80 90
1� The suitability of a fuel for diesel engine applications is identified through a number of properties according to standard specifications� What do you consider the five most important properties
2� Indicate whether an increase in the value of each of the following operating design parameters will reduce the minimum value of the cetane number required by a compression ignition engine of the diesel type: (a) intake air temperature, (b) engine speed, (c) cylinder diameter,
(d) viscosity of the fuel, (e) compression ratio, and (f) water jacket
temperature� 3� Contrast and compare briefly between the cetane and octane num-
bers as empirical characteristics for the utilization of common liquid fuels in power-producing devices� Why is the gas turbine considered to be more tolerant to variations in the properties of the fuel than piston-type engines?
