Appendix C: Internal Combustion Engine Basic Operating Principles and Technology Trends
This Appendix outlines the basic operation of petrol and diesel engines, and how this relates to basic fuel characteristics. The impact of engine design factors and condition on fuel consumption and emissions is presented. The trends and technologies employed to minimise fuel consumption and emissions, and the impacts on fuel quality requirements of these technologies are reviewed.
C.1 Basic Operating Principles
Internal combustion engines convert chemical energy contained in fuel into mechanical power. The combustion process is, however, quite different between a petrol engine and diesel engine, and this impacts on the required properties of the fuel for each type of engine. Although it is common to refer to engines as either petrol or diesel, the more correct distinction is between the nature of the ignition and combustion. The relative quantities of the various exhaust emission materials that are formed vary as a result of the difference in the combustion process.
In petrol engines the air and fuel are usually premixed before initiation of combustion, and the mixture then burns progressively as a flame front moves across the combustion chamber. In diesel engines the fuel burns primarily as a diffusion flame as the injected fuel mixes with air to produce a combustible mixture. The respective fuels have developed to suit the requirements of each type of combustion process, rather than by their nature defining the combustion process.
In addition to classification by fuel/combustion type, engines may further be described by the number of piston strokes to complete a full cycle. Thus a four cycle, or four stroke engine, performs the four basic phases of a cycle, induction, compression, combustion and expansion, and exhaust in four piston strokes. A two stroke engine performs the four basic phases of operation condensed into two piston strokes. Two stroke spark ignition engines have historically had significantly higher fuel consumption and exhaust emissions, but higher power to weight ratio, and are therefore primarily used in small mobile equipment and motor cycles.
C.1.1 Diesel Engines (Compression Ignition)
Light-duty (LD) diesel engines are generally defined as of displacement less than 4.0 litres and power output of up to 100 kW, and are characterised by relatively high engine speeds of up to 5000 rpm. LD engines would normally be found in passenger vehicle and light commercial vehicle applications.
Heavy duty (HD) diesel engines may be generally defined as of displacement greater than 8 litres and power outputs of greater than 150 kW with maximum engine speeds of less than 3000 rpm. HD engines are found in heavy road transport, industrial and marine applications. Medium duty engines fill the gap in the middle and are found in medium size trucks, buses and light industrial equipment.
A diesel engine is referred to as a compression ignition engine. Air alone is drawn into the cylinder on the downward induction stroke of the piston. No throttling of the intake is used; therefore, for a non-turbocharged engine, the amount of air drawn into the cylinder at a certain engine speed is relatively independent of the engine output, or load. The air is compressed and therefore heated in the combustion chamber before injection of the fuel which, as a result of the temperature and pressure within the combustion chamber, spontaneously ignites shortly after it is injected. The amount of fuel injected is varied to change the output power of the engine.
Fuel may be injected either directly into the main combustion chamber formed between the piston and the cylinder head, termed direct injection (DI), or into a smaller separate prechamber located in the cylinder head, and connected to the main combustion chamber by a small passage. This second configuration is termed indirect injection (IDI). The IDI system achieves high mixing rates as the fuel injected into the prechamber ignites and the expanding gases pass through the orifice in a high-speed jet. This allows higher engine speeds to be utilised and thus IDI is used extensively for small light-duty engines, while DI engines are used almost exclusively in larger heavy-duty engines that operate at lower engine speeds.
The fuel is injected under high pressure (of order 30-100 MPa) through fine holes to form a spray of fuel droplets that are typically of order 10-50µm in diameter. This must mix with the air in the combustion chamber until the local air-fuel ratio is within flammability limits before combustion can occur. The time taken from the start of injection to the start of combustion is termed the ignition delay.
Once ignition occurs the portion of the fuel injected that has mixed to a flammable air-fuel ratio burns rapidly in premixed combustion. This rapid combustion of the initial portion of the charge causes a rapid pressure rise and causes the characteristic "diesel knock". It is desirable to minimise the ignition delay and resulting proportion of premixed combustion in order to reduce the intensity of diesel knock. The autoignition characteristic of the diesel fuel, specified by the cetane number, is an important parameter contributing to the time taken to initiate combustion after fuel injection. A higher cetane number reduces the ignition delay and the premixed combustion.
The rate of subsequent diffusion combustion is primarily controlled by the rate of injection of fuel and the rate of mixing of the fuel in the air. The rate of mixing is in turn dependent on the motion of the air in the combustion chamber and fuel injection parameters such as injection pressure and number and size of injector holes. These are therefore important design parameters in order to maximise the efficiency of diesel engine combustion and reduce emissions.
C.1.2 Petrol Engines (Spark Ignition)
In contrast, a petrol engine is defined as a spark ignition engine. Air and fuel are normally mixed together in the intake, and compressed in the combustion chamber. As the air-fuel ratio needs to be kept within certain flammability limits, the intake of the mixture is throttled to change the engine output, while the air-fuel ratio is kept relatively constant. The correct amount of fuel is metered into the incoming airstream by either a carburettor or by fuel injection. A timed spark ignites the air-fuel mixture which then burns progressively but rapidly across the combustion chamber. The timing of the spark is adjusted over the range of operating conditions, and the optimum timing is dependent on factors such as the combustion chamber geometry, compression ratio and fuel octane rating. Spark timing and the air-fuel ratio have strong effects on the efficiency and emissions produced by spark ignition engines.
The fuel is required to evaporate in order to be mixed thoroughly with the air prior to combustion, and needs to resist autoignition (or "knock") as the mixture is compressed. Therefore essential characteristics of the fuel include appropriate volatility and resistance to premature ignition (measured by the octane rating). A higher octane fuel is more resistant to knock. In terms of engine performance, carburettor equipped engines are generally more sensitive to fuel volatility.
Two stroke engines may have lower octane requirements than four stroke, but are also more prone to spark plug fouling and combustion chamber deposit formation. Petrol specification impacts on this problem, however any changes (improvements) in petrol specifications are more likely to provide benefit in this area rather than cause additional problems.
C.2 Relative Efficiency of Diesel and Petrol Engines
The overall efficiency of internal combustion engines may be defined as the ratio of useful work output to the amount of fuel energy consumed.
As the diesel engine operates on the self-ignition principle (achieved by the high pressure and temperature in the chamber) the compression ratio is much higher than in the case of a petrol engine. The requirement to prevent uncontrolled combustion (commonly experienced as "knock") in a petrol engine limits the compression ratio. The compression ratio for a diesel engine is normally around 15-20 whereas for a petrol engine it is in most cases between 8 and 10. The high compression ratio of the diesel engine is one reason for its higher efficiency. The octane rating of the petrol has a direct influence of the maximum compression ratio that can be used in petrol engines.
The unthrottled intake of the diesel engine also contributes to an efficiency advantage over the spark ignition engine, by decreasing the work required to "pump" the charge in and out of the engine. Heat losses from diesel engines are less at part loads also, as the overall gas temperature in the combustion chamber is lower, and this also contributes to the efficiency advantage over petrol engines.
These factors combine to give diesel engines approximately a 15 to 25% increase in efficiency over spark ignition engines, and about the same percentage improvement in fuel economy over comparable sized vehicles powered by petrol engines whilst sacrificing some acceleration and maximum speed performance.
C.3 Engine Design
Over the history of the internal combustion engines, the design of both petrol and diesel engines has evolved, initially to meet demands for increased performance, fuel consumption and driveability, but in the past few decades primarily driven by legislated requirements to reduce emissions. Current and future European and United States legalisation requires large reductions in emissions, while increasing concerns of global warming continue to drive the need for improved fuel efficiency and hence reduced CO2 emissions.
The regulated emissions species of internal combustion engines are carbon monoxide (CO), unburnt hydrocarbons (HC), oxides of nitrogen (NOx), and for diesel engines particulate matter (PM). In the ideal situation, all of the carbon in the fuel would be oxidised to CO2, while all the hydrogen would be oxidised to H2O. CO, HC and PM are products of incomplete combustion which may occur due to a rich mixture and/or inefficient combustion. NOx is formed in the high temperature burnt gases in the combustion chamber by reactions between oxygen and atmospheric nitrogen. NOx formation is highly sensitive to temperature, time and oxygen concentration.
For diesel engines the current and future challenge is to significantly reduce emissions of PM and NOx, while for petrol engines significant reductions of all regulated emissions are required. Section C.4 and C.5 describe some of these technical developments and their impact on fuel efficiency and emissions for diesel and petrol engines respectively. Future developments to meet the proposed large reduction in emissions required are also discussed.
C.4 Diesel Engine Technology
Engine design technologies which impact on the fuel efficiency and emissions of diesel engines are summarised below.
Table C.1: Diesel Engine Technology
| Mechanical Design Features / Technology | Fuel Sensitivity20 | Status21 |
|---|
| Light-duty | Heavy-duty |
|---|
| Multi-valve heads | None | Production | Production |
| Variable valve timing | None | Emerging | N/A |
| Turbocharging and after / intercooling | Low | Production | Production |
| Combustion chamber / charge air motion optimisation | Low | Production | Production |
| Engine Control Technology | | | |
| Electronic engine management | Low | Emerging | Production |
| Common rail injection | Low | Production | Production |
| Exhaust gas recirculation | Low | Production | Emerging |
| Exhaust Aftertreatment | | | |
| Oxidation catalysts | High (S<500 ppm) | Production | Production |
| Particulate filters / traps | High (S<30 ppm) | Emerging | Emerging |
| NOx reduction technology | High (S<30 ppm) | Emerging | Emerging |
C.4.1 Mechanical Design
In general, all the mechanical design developments discussed here do not have significant impacts on fuel specification requirements.
Friction
Continuing small incremental improvements are being made to reduce the energy losses to friction within engines. Modern designs are highly evolved in this area, and hence future gains are likely to be limited to of order a few percent. Future developments for diesel engines are likely to include the increased use of roller cam followers and overhead camshafts, and low friction bearing and piston design.
Volumetric Efficiency
Volumetric efficiency is the measure of how completely an engine cylinder is filled with air or air-fuel mixture in each operating cycle. A volumetric efficiency of 100% means that in each operating cycle the engine cylinders are completely filled with mixture at atmospheric pressure. Improved volumetric efficiency and air availability improve the performance of diesel engines in several ways.
Pumping losses are typically reduced with features that improve volumetric efficiency, resulting in small improvements in overall engine efficiency. The increased air availability means that more fuel may be burnt, thus increasing power density. Hence engine size for a specific application may be reduced, which typically will result in higher overall efficiency when operating at the same loads. The greater air availability may also be used to increase excess air to ensure complete combustion and lower combustion chamber temperatures, leading to reduced emissions of all regulated species. The term excess air refers to air availability in the cylinder in excess to that theoretically required to burn all HC to CO2 and H2O. Where the generalisation "reduced emissions" is used in this discussion, there is some beneficial impact on all species, but the magnitude may vary between species.
Several technologies have developed to increase volumetric efficiency:
Multi-valve cylinder heads have become common on heavy-duty diesel engines and are starting to become more prevalent in light-duty applications also.
Variable valve timing offers increased volumetric efficiency and torque increases across the entire speed range. Control of residual burnt gas remaining in the cylinder from the previous cycle is possible by control of valve overlap, and this can reduce NOx emissions in the same manner as exhaust gas recirculation (EGR). This technology offers greater potential to light-duty engines, where the higher speed range over which they operate results in a greater compromise when fixed valve timing is used, than to heavy-duty.
Turbocharging is now common with both heavy-duty and light-duty engines. This provides significantly increased air for combustion resulting in higher power outputs, and can contribute to reduced emissions and specific fuel consumption through increasing excess air. Significant advances have been made in turbocharging technology resulting in higher boost pressures, greater efficiency and improved engine response and hence driveability. Current and future development is in variable geometry turbochargers and electronic boost control.
Associated with turbocharging, aftercooling and air to air intercooling of the boosted intake air has become almost universal in modern engines. This increases air availability for combustion by increasing the density of the intake air. This also results in higher power density and improved efficiency. NOx emissions are decreased by the reduction in combustion temperatures.
C.4.2 In-Cylinder Air Motion and Combustion Chamber Design
The design of the inlet tract and combustion chamber has a major influence on the performance and particularly, emissions, of diesel engines. A high level of air motion and turbulence is required within the combustion chamber in order to promote rapid and complete mixing of the injected fuel with air to combustible proportions. It is desired to optimise the in-cylinder air motion to provide smooth, complete combustion within the time available, and to minimise resulting emissions. However there are often volumetric efficiency tradeoffs in some design approaches to generating the desired air motion.
The combustion chamber contributes to the air motion achieved, and is also required to be as compact as possible with minimal crevices and quench areas where fuel may escape complete combustion. There are many ways of achieving this and many types of air motion including swirl, tumble, reverse tumble, and swish, and combinations of these.
Continual developments in design techniques have contributed significantly to reduced fuel consumption. Emissions have decreased by an order of magnitude over the last 20 years as a result of improved air motion and chamber design, in conjunction with improved fuel injection. Extensive research efforts continue and ever more sophisticated modelling techniques are developing in this area. Small fuel consumption benefits will continue to be gained along with larger emission reductions.
Recent significant developments in the area of combustion chamber design include the application of direct injection (DI) designs to light-duty engines. Historically high (engine) speed light-duty engines have been required to be of indirect injection (IDI) design in order to provide high enough levels of charge air motion to mix and combust the fuel in the short time available. DI engines offer reductions of 10-20% in fuel consumption and improved emission control over IDI engines. Heavy-duty engines are almost exclusively of DI design.
C.4.3 Engine Control Technology
Fuel Injection Systems
The design of the fuel injection system is an area where significant improvements have been gained in the last 10-15 years. Diesel injection pressures have increased to of order 150-200 MPa using electronically controlled unit injectors (HD engines) and injectors have tended towards multiple smaller orifices. This results in improved fuel atomisation and mixing with the compressed air within the cylinder, and ability to vary injection timing to a greater degree than mechanically controlled systems. This has resulted in significant reductions in PM mass emissions of up to 60%, and smaller reductions in HC and NOx. However the size of particles generated has decreased and there is growing concern that the number of ultra fine particles may be more important than particle total mass in determining health impacts.
Electronic Injection Controls and Common Rail Injection Systems
Greater control over injection timing through electronic control has become common in modern HD engines, and this technology is now being applied to LD engines also. Much research is being conducted in systems to provide injection rate shaping and multiple injections to further control the combustion process. The latest technology currently emerging is that of common rail systems. In these systems high pressure diesel is supplied to all injectors continuously via a "common rail" and the individual injectors are electronically actuated. Very high speed actuation allows for precise timing of the injection and for techniques such as pilot injection and multiple injections.
The decoupling of the generation of the injection pressure from engine speed allows high pressures to be consistently available even at low engine speeds. This technology provides significant decrease in combustion noise, NOx and PM emissions, and allows for "post" injection late in the combustion process to enable the use of emission aftertreatment technologies such as passively regenerating PM traps and NOx adsorbers. These systems are now entering the market in LD vehicles but are also applicable to HD applications.
Exhaust Gas Recirculation
Exhaust gas recirculation (EGR) has been used for many years in both petrol and diesel LD engines to reduce NOx emissions by diluting the charge air/fuel mixture and thereby reducing peak gas temperatures and decreasing oxygen concentrations. A portion of the exhaust gas is recirculated to the combustion chamber through a control valve and mixed with the incoming air in the inlet system. The application of EGR is being continuously refined with more sophisticated electronic systems to give greater control over EGR ratios, and thereby further reducing NOx. In order to meet future emission regulation levels this technology is being extended to HD applications, in particular using cooled EGR systems. As well as NOx reductions of up to 50%, fuel consumption reductions of 2% have also been achieved.
C.4.4 Exhaust Aftertreatment
Although advances in mechanical design, advanced fuel injection systems and electronic controls have made large reductions in emissions, and small incremental gains in fuel consumption, in order to meet future emission regulations aftertreatment devices are considered indispensable. In general, all of these devices are very sensitive to fuel specification, in particular requiring a very low sulphur content.
Oxidation Catalysts
Oxidation catalysts have been fitted to European LD diesel vehicles for several years to reduce emissions of HC, the SOF content of PM (soluble organic fraction; the HC adsorbed onto the surface of particles), and CO. Oxidation catalysts promote the oxidation of HC and CO with oxygen in the exhaust to form CO2 and H2O. Fuel sulphur levels of maximum 500 ppm are required to avoid excessive production of sulphate based PM and to minimise catalyst deactivation by sulphur poisoning. Lower levels of sulphur (50 ppm) can increase the effectiveness of oxidation catalysts by up to 50% and contribute to greater durability. Oxidation catalysts have not generally been used in HD vehicles with the exception of urban buses, and are not considered necessary to meet HC and CO requirements of future HD emission regulations.
Particulate Traps
Much development has been directed towards filtration systems for trapping diesel PM. Most systems developed use filters based on ceramic monoliths, but the key to achieving commercial practicality is the development of regeneration methods to either periodically or continuously "burn" off the PM trapped to avoid build up of excessive exhaust back pressure. Very high PM reduction efficiencies of >90% are possible. This technology is considered essential for attaining future PM emissions standards.
Several variations of these traps are near to or have reached commercial production status (such as the Johnson and Mathey Continuously Regenerating Trap (CRT)). Most are based on catalyst technology and require fuel sulphur levels of less than 30 ppm.
NOx Reduction Catalysts
Similarly to PM traps, NOx catalyst or adsorbers are considered necessary to attain the large reductions of NOx in future regulations. As diesel engines run on excess air, the highly efficient three way catalysts used on petrol engines are not effective to reduce NOx. Much work is ongoing to develop lean NOx catalysts using a variety of catalytic technologies.
Selective Catalytic Reduction
This uses reducing agents such as urea or ammonia to catalytically reduce NOx to N2. This highly effective technology has been used for many years in industrial processes and stationary engines, but has been limited in application to vehicles due to the large size and complexity of the technology. There are also safety concerns using ammonia. However, it is beginning to emerge on some light duty vehicles (Peugeot) and systems developed by Siemens are being trialled on HD also.
NOx Adsorbers
Unlike catalysts, which continuously convert NOx to N2, NOx adsorbers are materials which store NOx under lean conditions and release and catalytically reduce the stored NOx under rich conditions. NO and NO2 (together referred to as NOx) are acidic oxides and can be trapped on basic oxides. Periodic rich operation of the engine is used to generate the necessary conditions to convert the stored NOx at programmed intervals. Common rail fuel systems offer the potential of "post" (combustion) injection to produce the required reductants for adsorber regeneration.
Lean NOx Catalysts
These use catalyst formations to reduce NOx with HC. Due to the low levels of HC available in diesel exhaust, passive systems are limited in their reduction capability, and active systems using periodic rich running or post injection using common rail injection technology has the greatest potential, although NOx reduction is still limited to around 35-50%. This strategy also carries a small fuel consumption penalty. Low sulphur fuel (<30 ppm) is required to limit sulphate generation and catalyst poisoning.
C.5 Petrol Engine Technology
Engine design technologies which impact on the fuel efficiency and emissions of petrol engines are summarised below.
Table C.2 : Petrol Engine Technology
| Mechanical Design Features / Technology | Fuel Sensitivity22 | Status23 |
|---|
| Multi-valve heads | None | Production |
| Variable valve timing | None | Production |
| Turbocharging and after / intercooling | Low | Production |
| Combustion chamber / charge air motion optimisation | Low | Production |
| Engine Control Technology | | |
| Electronic engine management | Low | Production |
| Exhaust gas recirculation | Low | Production |
| Direct Fuel Injection | High (S<30 ppm) | Production | |
| Lean burn | High (S<30 ppm) | Emerging |
| Exhaust Aftertreatment | | |
| Advanced catalysts | High (S<30 ppm) | Production |
C.5.1 Mechanical Design
In general, all the mechanical design developments discussed here do not have significant impacts on fuel specification requirements.
Friction
The same general comments as for diesel engines apply, see Section C.4.1.
Volumetric Efficiency
Similar comments to those for diesel engines in Section C.4.1 apply, with the exception that excess air is not used in homogeneous charge spark ignition applications.
Several technologies have developed to increase volumetric efficiency:
Multi-valve cylinder heads are now almost standard in automotive applications.
Variable valve timing offers increased volumetric efficiency and torque increases across the entire speed range. Control of residual burnt gas remaining in the cylinder from the previous cycle is possible by control of valve overlap, and this can reduce NOx emissions in the same manner as exhaust gas recirculation (EGR).
Variable Intake Geometry
By using variable length inlet tracts, the air flow can be improved across the engine speed range. This results in a flatter torque curve and higher power output. Thus smaller displacement engines may be possible.
Turbocharging while common in performance based petrol vehicles is not generally utilised for any fuel efficiency or emissions benefit. However, by increasing the specific power output, smaller engines can be used which may give some vehicle mass benefit and associated fuel savings.
C.5.2 Combustion Chamber Design
While the main focus and the biggest reductions in emissions for spark ignition engines has come from catalyst technology, development of combustion chamber geometry has contributed much to improved engine out emissions and fuel consumption. Minimising crevice volumes where fuel can escape primary combustion, and achieving stable robust combustion with low variability are the main objectives. More robust combustion allows higher levels of EGR to be used to reduce NOx emissions and also provides some small fuel consumption benefits. Future impacts of further refinement in conventional spark ignition engines are however, limited in further gains attainable.
C.5.3 Engine Control Technology
Carburettor Fuel Systems
A large proportion of the New Zealand vehicle fleet use carburettors for fuel metering. Due to the generally poorer fuel atomisation of carburettors than fuel injectors, carburettor engines may be more sensitive to fuel parameters such as high-end volatility and heavy aromatics content. Although carburettors were developed intensely through the late 70's and 80's, they are unable to provide the close air-fuel ratio control required to effectively reduce CO, HC and NOx using three way catalyst technology. They were thus phased out rapidly overseas as this technology became prominent. Many Japanese cars of the 1980's were still produced with carburettors and were fitted with oxidation catalysts to effectively reduce CO and HC levels.
Fuel Injection Systems
The control of air-fuel ratio and spark timing is critical to minimising both fuel consumption and emissions production in spark ignition engines. Multi-point fuel injection (MPFI) systems are now standard and sequential fuel injection (SFI) is becoming more common, initially on higher specification vehicles. SFI allows individual optimising for each combustion event and more precise control of fuelling, particularly during transient events.
Gasoline Direct Injection (GDI)
Direct fuel injection in spark ignition engines allows fuel injection directly into the cylinder (rather than into the manifold as with conventional fuel injection systems) during the compression stroke to create a stratified, or non-homogeneous charge. This allows ultra lean burn conditions under part load conditions (AF ratio ~50 compared to stoichiometric AF ~14.5) which give large fuel consumption and emissions benefits. Pumping losses are greatly decreased by reducing the degree of throttling required at part loads. At high loads the GDI engine can operate in a homogeneous mode in the same manner as a conventional engine. Although engine out NOx is reduced compared to a conventional homogeneous charge engine, the lean operation renders conventional 3-way catalysts ineffective, and NOx adsorber type catalysts with periodic stoichiometric operation regeneration may be required to meet Euro IV limits. This requires a very low sulphur fuel (<30 ppm) to enable this technology to meet this NOx level. Both Mitsubishi and Toyota have GDI powered vehicles in production.
Homogeneous Charge Lean Burn Engines
Lean burn engines offer high fuel efficiency and low engine out HC and CO emissions. However, engine out NOx emissions are higher than allowable. Difficulty in effective aftertreatment to reduce NOx in lean burn engines has limited their development in favour of stoichiometric 3-way catalyst technology. Development of this technology has continued however, and improved high swirl combustion chamber design leading to better combustion stability has extended the lean limit to air/fuel ratios of around 25:1. Further developments in NOx adsorber and lean NOx reduction technologies (see section C.4) may see this technology enter the market. As for diesel engines, a low sulphur fuel is required to enable the NOx aftertreatment technologies. Toyota produced a lean burn vehicle in 1994 which was claimed to reduce fuel consumption by as much as 20% over certification test cycles. Subaru and Hyundai have also produced lean burn engines in production vehicles.
Exhaust Gas Recirculation
Exhaust gas recirculation (EGR) has been used for many years in petrol automotive engines to reduce NOx emissions by diluting the charge air/fuel mixture and thereby reducing peak gas temperatures. Pumping losses are decreased by use of EGR in spark ignition engines, as wider throttle openings are required to obtain the desired power. The application of EGR is being continuously refined with more sophisticated electronic systems to give greater control over EGR ratios, and thereby lower NOx. Improved combustion chamber designs allow higher EGR ratios without causing combustion instability. Future developments will also extend EGR to higher engine operating loads. As well as large NOx reductions, fuel consumption benefits also results from EGR.
C.5.4 Exhaust Aftertreatment
Although advances in mechanical design, advanced fuel injection systems and electronic controls have made large reductions in emissions, and small incremental gains in fuel consumption, in order to meet future emission regulations aftertreatment devices are considered indispensable. In general, all of these devices are sensitive to fuel specification, in particular requiring a very low sulphur content.
Oxidation Catalysts
Oxidation catalysts were the first application of catalyst exhaust aftertreatment in automobiles. These catalysts promote oxidation of CO and HC in the exhaust to CO2 and H2O. As oxygen is required for this process, a stoichiometric or leaner mixture is required. Initial engines fitted with oxidation catalysts had carburettor fuel systems which provide adequate air-fuel ratio control for this technology. Oxidation catalysts were fitted to many of the 1980's Japanese vehicles that have been imported second hand into New Zealand, but this technology is largely outdated now in application to petrol engines.
Advanced Three Way Catalysts
As emission limits have reduced, the proportion of emission produced in the cold operation phase before the catalyst reaches "light-off" has become increasingly significant. Catalysts having faster warm-up and lower light-off temperatures are necessary. Close coupling of catalysts to the exhaust manifold decreases warm up time, but increases thermal stress on the catalyst under high load conditions which may result in accelerated catalyst ageing and failure. Other approaches, such as electrically heated catalysts and adsorbers, have proved feasible to reduce cold start emissions, but are not yet production technology partly due to cost implications.
Increased tolerance to fuel sulphur has also been a significant focus of development of advanced catalysts to provide high conversion efficiencies over an extended life, to meet durability regulations. Reduction of sulphur levels in petrol will aid this area. European work has shown that reductions in sulphur from 380 ppm to 18 ppm decreased CO, HC and NOx emissions by 10%. Similar work in the United States showed reduction of between 10-20% in the various species using 1983-89 technology vehicles with sulphur reductions from 450 to 50 ppm.
Much of the development work of existing catalyst technology is focussing on improved formulations including optimum washcoats and the type and concentrations of the precious metals used, in order to provide improved performance in the areas discussed above.
NOx Reduction Catalysts
These have been discussed in the diesel section and also in the lean burn and GDI petrol engine sections. Again, low levels of sulphur are required to enable this technology, which for petrol engines is in turn a technology enabler for lean burn and GDI, with their associated fuel efficiency benefits.
C.6 Bibliography
Coffey Geosciences Pty Ltd. Review of Fuel Quality Requirements for Australian Transport, Environment Australia, 2000.
Degobert P. Automobiles and Pollution, SAE, 1995.
DieselNet Technology Guide, www.dieselnet.com
EFRU. Recent and Expected Technical Developments for the Road Transport Sector, Auckland UniServices report 6943.17, 1988.
Heisler, H. Vehicle and Engine Technology, SAE, 1999.
Heywood J.B. Internal Combustion Engine Fundamentals, MacGraw-Hill International Editions, 1988
Ministry of Transport. Vehicle Fleet Emissions Control Strategy - Final Report, 1998
C.7 Acknowledgements
This Appendix was prepared by:
Energy and Research Fuels Unit (ERFU)
Auckland UniServices Limited
University of Auckland
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