ABSTRACT
The chemical potential energy of a fuel released through combustion oxidation reactions can be considered in principle to be a much more versatile and valuable commodity for the production of useful work than its equivalent amount in the form of heat energy� This is due to the fact that the chemical energy of the fuel can be converted more readily to other forms of energy or work than heat�
This utilization could represent the direct conversion of the chemical energy of the fuel into work without the need for converting it into heat first� Hence, ideally, the corresponding work production efficiency under the appropriate conditions can be very high and in principle may even approach 100%� The combustion energy of a fuel when converted directly into heat becomes degraded and is less amenable to universal conversion into useful work energy (Figure 4�1)�
The conversion of heat energy into useful work is limited by the constraints of the Second Law of Thermodynamics� Depending on the prevailing temperature only a fraction of the heat energy can be converted directly into work, which is a more organized and universally convertible useful energy form� The maximum ideal extent of heat conversion to work by a device depends on the temperatures of the heat supplied and rejected, and it is established via the wellknown Carnot Cycle Efficiency consideration� Other forms of non-heat energy, for example, electrical, chemical, high pressure, and so on, are more amenable to conversion into useful work� Ideally, if no conversion into heat and associated losses are involved then they are mutually convertible� An example of these is the functioning in principle of ideal fuel cells where the chemical energy ideally may be converted to electrical work or the stored electrical energy of a battery, which can be converted to electrical work, or the high-pressure energy of compressed air when used to provide pneumatic work� Alternatively, of course, energy of the work type can be readily converted fully and wastefully into heat, a lower quality form of energy, such as in the conversion of the fuel chemical form of energy through combustion to heat energy or the wastage of mechanical work into friction or electrical energy through conductor resistances� On this basis, it can be seen that in the case of fuels through their specific chemical structure, there is the advantage of converting in principle their chemical energy fully into useful work�
To convert the potential chemical energy of the fuel into heat does represent in principle a loss of opportunity to optimize fuel usage for the production of work� For example, as presented in Figure 4�2, to burn the fuel and use the heat liberated by combustion to produce the steam employed in a heat engine
by operating on the Rankine cycle type it is possible to convert only a fraction of the fuel energy into useful work because of the limitations associated with the conversion of the heat produced into work� However, if the fuel can be employed in a suitable process directly to produce work such as in fuel cells or an internal combustion type device, then a much higher conversion factor of the fuel energy into work production becomes potentially achievable�
Figures 4�2 and 4�3 show schematic representations of the two approaches to produce work from fuel energy, showing a Carnot limited conversion initially via heat in a heat engine and a conversion of the fuel chemical energy directly into work through chemical reactions of a flow process�
The term efficiency is universally employed for the evaluation of the performance of fuel-consuming energy devices and processes� Care is needed to avoid confusion in its application or understanding its significance, as it may be used loosely in the energy and fuel application fields� Generally, efficiency is used to relate the output to the corresponding input of a commodity or the yield to the cost involved� In engineering-type applications, it is commonly used to indicate the net output relative to the ideal or maximum output, which is represented by the corresponding energy change� The definition employed will depend on the specific field of application and the objective of the intended evaluation�
The following is a listing of some of the definitions used for efficiency in relation to the performance of thermal and work-producing devices burning fuels:
Efficiency
output input
=
=
η
or
=
yield cost
or
=
net output ideal maximum output
For devices of the heat engines type where some of the heat supplied is converted to work and the left over is rejected to a lower temperature medium:
Efficiency work output
heat input =
For internal combustion engines, strictly it is incorrect to use the same definition of efficiency as the above for heat engines, since there is strictly no cycle nor an external heat energy input in the process� Instead the following term needs to be used:
Work production efficiency work output
max w =
ork output
or
=
work output energy change
or
=
work output Gibbs function change
which for the combustion of most common fuels approximates to
= × ∼
work output fuel mass heating value
In principle, the value on the basis of this type of efficiency ideally may reach up to 100%�
For the efficiency of heating devices through the combustion of fuels such as in furnaces, comparison is merely made between the actual heat energy output and the maximum heat energy that can be produced� The efficiency then becomes:
Combustion efficiency = heat output
fuel mass
× heating value
Often the term specific energy or rate of fuel consumption per power produced, kgfuel/kJ or kgfuel/kW·h, is used instead of efficiency when evaluating the performance of fuel combustion devices�
Energy release via the combustion of fuels has been employed for the production of heat energy and work in a very wide range of devices employing a variety of processes� A broad classification of such devices is presented as a listing in Table 4�1� Figure 4�4 shows the interrelated engine-type devices that manage to a varying degree of success for converting the fuel chemical energy into useful work through combustion�
4.4.1 Internal Combustion Engines
The internal combustion engine may be considered as the most significant invention that changed human life through the provision of prompt and simple control of power generation at the individual level while consuming a variety of common fossil fuels� Over the years billions of units have been produced� Enormous research effort and resources have been increasingly expended, particularly in recent years to improve their performance and reduce their undesirable exhaust gas emissions� Accordingly, the prime devices at present for the production of work through fuel combustion are of the internal combustion engine type� Such devices as shown in Figure 4�4, can be mainly reciprocating employing intermittent combustion or rotary with continuous steady combustion�
Their distinctly different modes of operation and associated combustion processes often dictate a restrictive choice of the types of fuels that can be used by them� The prime examples of the reciprocating type are the spark ignition and compression ignition diesel engines� These are shown schematically in Figures 4�5 through 4�11�
4.4.1.1 Various Types of Reciprocating Engines
As will be shown in Chapter 13, the spark ignition engine, shown schematically in Figure 4�5, operates by employing an electric spark to ignite at a predetermined time a homogeneous mixture of fuel and air� However, the diesel engine, shown schematically in Figure 4�6, operates on the basis of autoigniting a spray of injected liquid fuel at a sufficiently high temperature and pressure of the surrounding air medium provided�
There are some major variations in these types of engines, such as the dual fuel engine, shown schematically in Figure 4�7, which aims at utilizing gaseous fuels as the prime source of its energy while using only a small amount of liquid diesel fuel injection, mainly to provide ignition� More recently, much research and development efforts are being expended to develop successfully an engine that operates on the principle of having homogeneously
fed fuel-air mixtures undergo compression ignition controllably� This type of device, if it could be made to work successfully, has the potential for improved efficiency and reduced emissions� Figure 4�8 shows a schematic of a homogeneous charge compression ignition (HCCI) engine�
The main rotary type internal combustion engine is the gas turbine, shown schematically for aviation application to provide jet propulsion (Figure 4�9) and for land applications (Figure 4�10)� The reliance of gas turbines on steady continuous combustion tends to make them more amenable to consume a wider range of different fuels than the reciprocating type engines� Figure 4�11 shows a schematic representation of a vapor power cycle operating as a heat engine of the Rankine cycle type� The bulk of thermal power stations employed for the production of electricity use nowadays variations of this type�
Of course modern engine installations tend to be more complex than what has been shown schematically in this group of figures� This is mainly motivated by the continuing need to improve their specific power output and production efficiency and increase flexibility of operation and control while reducing to a minimum undesirable exhaust emissions� The need to be
able to burn effectively a wider range of types of fuel is also an increasingly important consideration�
Figure 4�12 shows a schematic arrangement of an internal combustion engine installation where a turbocharger is fitted� It utilizes usefully a significant fraction of the thermal energy of the exhaust gases to provide compression work for the incoming engine intake charge, thereby increasing the intake mass and the specific power output� More recently, some controlled exhaust gas recirculation, EGR is being increasingly employed, mainly for the control of exhaust emissions, especially those of nitrogen type oxides� This recirculation may involve cooled or only partially warm exhaust gas�
Figure 4�13 shows schematically a fuel cell of the proton exchange membrane (PEM) type where through the catalytic oxidation of a fuel like hydrogen, electrical energy is produced directly under virtually isothermal conditions� Fuel cells are usually made of an appropriate number of modules� Such a device, in principle has many attractive potential features, such as high efficiency, very low NOx emissions, and silent running� However, they do suffer from some serious limitations such as the need to provide ultra pure hydrogen, being expensive, low specific power output, electrochemical losses, and incomplete combustion of the fuel� Further research and development are still needed to make it widely available and successfully competitive with the other traditional combustion power producing devices�
The internal combustion engine can manage to convert at present only a relatively moderate fraction (typically on the average around a third) of the chemical energy of its fuel released through combustion into useful mechanical or electrical work� The bulk of the energy is dissipated in the form of environmental thermal pollution via the high-enthalpy exhaust gases discharged and the heat transferred to the external environment of the engine such as through the circulating cooling water and air, lubricating oil, and radiation� The proportional distribution of this wastage of energy between the various paths possible can vary significantly depending on the engine design, fuel used, and operating conditions� Of course, much effort is expended at all stages of engine design and operation to increase its work production capacity and its associated efficiency� However, there are practical and theoretical barriers that limit such increases� Accordingly, there is an increasingly greater effort expended to utilize this rejected heat energy� This is especially employed in power generation stationary engine installations to provide whenever relatively low-temperature heat is required to produce hot water, low-temperature steam, or hot air� This simultaneous production of power and utilization of waste exhaust gas heat, which is known as cogeneration, is being increasingly employed to cut down on fuel costs and reduce the overall discharge of exhaust emissions, particularly carbon dioxide, the greenhouse gas� A common example of cogeneration in automobiles is the heating of the interior of the vehicle by using some of the thermal energy rejected by the engine� Another example is improving the power output by fitting exhaust gas turbochargers, as shown in Figure 4�12�
Figure 4�14 shows a typical representation of a fuel fired furnace using its hot exhaust gas as a means for preheating the incoming cold air so as
to improve the combustion process and minimize the specific energy consumption of the whole installation�
There are also increasingly a wide range of cogeneration installations with an assortment of designs and complexities depending on the type of fuel used, engine power, heating demands, and the heat recovery conditions required to be provided� These combined supplies expectedly add much to the capital and operational costs of the installation and increase the complexity of its optimum control (Figure 4�15)�
The gas-diesel compression ignition engine commonly known as the dual fuel engine (Figure 4�7), is an example of a power device that is well-suited for cogeneration applications� It has been used over the years, for example, in sewage works to produce electric power using the sewage gas produced and provide hot water to improve the effectiveness of the anaerobic sewage
digestion process and enhance the overall gas yield� Also, cogeneration is widely used in some countries in the heating of horticultural greenhouses� They provide artificial electrical lighting over long periods of up to 24 hours per day, while increasing the concentration of carbon dioxide in the local atmosphere so as to promote more vigorous growth of plants while reducing greenhouse emissions� There are also applications where engines can be made to provide cooling for air conditioning through the use of chillers and engine waste heat�
Figure 4�15 shows a heating power system widely used in recent years where high-intensity gas turbines are coupled thermally to a vapor power Rankine cycle system� In these installations the sufficiently hot exhaust gas of the gas turbine is used to produce the steam required to drive the steam turbine and produce additional power� It may also be employed to produce process steam that may be needed for industrial applications such as in some enhanced oil recovery thermal processes� Such types of arrangements can be associated, expectedly, with very high overall energy utilization efficiency, but with increased complexity and capital and operational costs�
It is becoming increasingly essential to reduce the fuel consumption of energy devices in general and the internal combustion engine in particular� This is not only for economic reasons but also to reduce total exhaust and greenhouse gas emissions when using fossil fuels� Some of the general practical methods that may be implemented to reduce the fuel consumption for heating and air conditioning in buildings, for example, may include the following:
• Optimize the air conditioning load and the comfort levels of temperature and humidity both for cooling as well as for heating�
• Have windows that are made to reduce to a minimum heat losses in winter and heat gains in summer and improve the air tightness�
• Use instead of incandescent light bulbs, the electrically more efficient fluorescent lighting�
• Add occupancy sensors in areas where occasional or intermittent usage is expected�
• Optimize the operation of pumps and fans when installed according to demand via variable speed drives and reduce frictional losses further such as through the use of improved lubricants�
• Use heating equipment that is of optimum efficiency at the most common part-load conditions, where much of the demand usually is�
• Have two boilers or furnaces installed instead of a large one so that when there is partial demand for energy only one needs to operate at a higher load more efficiently�
Some of the main measures that have been used to reduce the fuel consumption of combustion engines include the following:
• Run the engine at the lowest speed at which the required power output can be produced�
• Incorporate design and operational features that will reduce the need for throttling and frictional losses�
• Reduce auxiliary power requirements and usage such as air conditioning�
• Reduce heat losses and exploit usefully the exhaust energy such as through cogeneration and exhaust gas turbocharging�
• Use lightweight and high-strength materials while employing design and operational features to minimize the engine size�
• Choose design and operating conditions that take every possible advantage of the specific fuel and lubricants being used, such as employing sufficiently high compression and expansion ratios with optimum spark timing�
• Improve the progress of the combustion process while avoiding the incidence of knock�
• Keep all fuel energy consuming equipments in well-maintained condition�
Some of the measures that may potentially improve the performance and output of fuel combustion engines are the following:
• Vary lean mixture operation while not needing to maintain stoichiometric operation to control the extent of emissions released into the atmosphere�
• Develop the optimum operation of a variety of new versions of engines such as those of the homogeneous charge ignition, stratified charge mixture operation, dual fuel, and quasi-adiabatic combustion engines�
• Improve and optimize engine controls through methods such as employing variable compression ratio, fuel injection, variable
valve timing, enhanced turbo-charging, and exhaust gas recirculation�
• Develop a variety of possible hybrid operations� • Keep all fuel energy consuming equipments in well-maintained
condition�
The internal combustion engine, when installed in a vehicle, is expected to satisfy numerous requirements as it functions on a variety of operating conditions both steady and transient� Often, these would have the engine not confined to operate over a limited set of operating conditions that would permit optimal performance, especially in relation to efficiency and reduced emissions� Accordingly, the concept of hybrid vehicle operation relates to an engine coupled to an electric motor/generator/storage system that provides controllable power to the wheels via these components� This hybrid approach has been developed over the years in a variety of forms� A well-established example of a hybrid operation is the diesel electric locomotive, where the diesel engine is coupled to an electric generator that transfers its output to electric motors that drive the wheels� Figure 4�16 shows schematically a relatively simple hybrid arrangement for a vehicle� Such approaches have the potential to improve overall efficiency and emissions� However, there are a number of limitations to such potential benefits� These include increased capital cost, difficulties in control, increased vehicle mass, and potential increased fire and safety hazards� Figure 4�17 shows a typical representation of a corresponding arrangement for a vehicle driven entirely electrically�
To chose an energy system to perform a specific task, such as generating electricity in relatively small units, requires the consideration of numerous factors beyond those of thermodynamic and combustion origins� Often, there are other overriding factors that must be catered to� For example, the following are some of the factors that need to be considered in choosing a fuel-consuming energy system and their impact on output, cost, and emissions evaluated:
• Total costs such as those relating to equipment, design, related civil engineering, land acquisition, insurance, legal, and infrastructure
• Current requirements and future potential changes in social acceptance, workers’ safety, environmental impact, noise, and vibration
• Availability and potential changes in markets, demand for products and by-products, taxes, subsidies, and incentives
• Manpower quality and requirements and whether there are manufacturers’ representatives available readily
• Time needed for conception, design, construction, commissioning, and production, and ease of starting and shut down
• Fuel and materials quality required, availability of alternative sources, current supply costs, and future trends
• Durability, working life expected, maintenance frequency and costs, material compatibility, proneness to corrosion and wear, ease of
replacement of parts and equipment, retrofitting possibilities, recycling, and second-hand value
• Complexity of instillation, operation, and control, and whether remote control and compatibility with other similar installations are possible
• The availability and costs of any auxiliary power and water of the right quality needed
1� Derive a relationship between the specific fuel consumption in kg of fuel per kWh and the thermal load production efficiency of an internal combustion engine�
2� Explain briefly why devices of the internal combustion type are in principle capable of converting higher fractions of the fuel chemical energy usefully into work in comparison to those of the heat engine types, where work is generated via heat energy produced through the combustion of the fuel�
3� In a typical natural gas-fired domestic heating furnace installation, list some measures in the design and operation of the installation you may consider necessary to ensure superior efficiency of operation�
4� What do you consider to be the main obstacles for the wider use of solar energy as a significant replacement to our current dependence on fossil fuels for the production of power?
