An Overview of Effect of Automotive Diesel Engines in Future

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1 An Overview of Effect of Automotive Diesel Engines in Future K. M. Venkatesh & E. Britto Sebastiraj Rajalakshmi Engineering College, Rajalakshmi Nagar, Thandalam, Chennai srtvenkat215@gmail.com,ebrittosebastiraj@gmail.com Abstract The roll of the vehicle for the transportation of people and goods will become more important all over the world. The reciprocating engine, burning petroleum, will continue to be demanded in the future as the most practical power plant to power the vehicle. The diesel engine, which has the highest thermal efficiency among engines, will become more valuable, considering the increasing threat of the limited energy resources and global warming due to CO2 emission. Therefore, diesel engine technology must be one of the most important technological fields for the future. The current status of performance, fuel economy and exhaust emissions of vehicle diesel engines is summarized in this paper, and the possibility of further technological advancement is discussed. In this discussion, various technologies focusing on the simultaneous reduction of fuel consumption and exhaust emissions by combustion and cycle efficiency improvement are reviewed. Direct injection passenger car diesel engines incorporating those technologies are built and achieved very low fuel consumption and exhaust emissions. The result of these studies shows the diesel engines high potential of further improvement in fuel economy and exhaust emissions in the future, meeting social demand of the world. Keywords current status, engines, emissions, new approaches, technologies I. INTRODUCTION In 1897, Rudolf Diesel recorded as much as 26.2 percent thermal efficiency with his 19.6-liter single cylinder engine, widely announcing the diesel engine s amazing fuel economy. Since then, diesel engines have provided efficient means of people and cargo transportation, and made significant contributions to society with their high thermal efficiency, or low fuel consumption. Since 1936, Isuzu Motors Limited has manufactured 12 million diesel engines and is producing over 800 thousand units annually for a wide variety of vehicles from passenger cars to heavy-duty trucks. In the face of inevitable local and global environmental problems and possible energy shortage in the future, automobiles and their power plants must be designed to meet social needs arising from these problems in the 2000s. In view of this, many countries are pursuing projects to develop fuel-efficient, low-emission vehicles and engines. Well known among such projects are 3- liter (3L/100 km) cars in Europe and the PNGV project in the U.S. (80 mpg), both intended to drastically reduce passenger car fuel consumption. For heavy-duty engines, the U.S. has a project to achieve 55% thermal efficiency (30% fuel economy improvement) and significantly lower emissions (2 g/bhp-hr NOx and 0.05 g/bhp-hr PM) [1]. At the same time, a variety of alternative automotive power plant systems are being developed in many countries. Even though alternative systems could play some important roles in limited localities or applications, gasoline and diesel engines are likely to stay for several decades ahead as the most popular and useful power plants, and thus further efforts to refine these engines are essential. Especially, the roles of diesel engines with high thermal efficiency are certain to grow even more important, and so will the importance of developing diesel engine technology. In view of this situation, we have reviewed current trends and major technological possibilities of the future covering a wide range of diesel engines from those for passenger cars to those for heavy-duty trucks. The focus of this study has been on further improvement of thermal efficiency, that is, reduction of fuel consumption and CO2, and further reduction of pollutant emissions focusing on NOx and PM. Some prototype engines incorporating new technology have been tested, and test results have been reviewed to obtain some insight into potentials of diesel engines for future automobiles. 1. PECIFIC POWER II. CURRENT STATUS Fig. 1 indicates changes in BMEP ranges for heavyduty diesel engines in the world, as well as projection for future improvement. By applying turbocharging and intercooling, the maximum BMEP is already in excess of 20 bars, making it possible to significantly improve thermal efficiency and reduce maximum engine speed, 119

2 which in turn makes it possible to reduce noise and increase durability. With these improvements, engine size and weight per output power have steadily reduced. The increase in BMEP ranges is likely to continue. Fig 1: Maximum BMEP trend of heavy-duty diesel engine The trends in specific power of passenger car diesel engines sold in Europe, where diesel passenger cars are widely used, are shown in Fig. 2. The zone of European gasoline passenger cars is also shown for comparison. As shown here, turbocharging had triggered a sharp rise in the specific power of diesel engines recently to a level close to that of gasoline engines. which are critical in diesel engine operation. Public concern about NOx have been especially intense in Japan, and the regulation expected to be enforced until 1999 requires that commercial vehicles exceeding 2.5 ton in GVW should achieve 74% reduction from the unregulated level, and that diesel passenger cars exceeding 1250 kg in inertia weight should achieve 83% reduction. In the U.S. and Europe, priority has been more on PM control, and NOx is often controlled in combination with HC. Thus regulations and test procedures are not comparable worldwide. Even so, as shown in Fig. 3, the direction of control in the future is common in the three areas of the world. As it is well known, NOx and PM emissions are in tradeoff relation and it is difficult to simultaneously reduce these two pollutants. This difficult theme has been dealt with to date by controlling fuel injection timing, raising injection pressures, introducing turbocharging and intercooling, and modifying combustion chamber design and, in some applications, applying EGR and oxidizing catalytic converters. Developing new technologies is further required to reduce NOx and PM emissions by half beyond the year Fig 2: Specific power trend of passenger-car engine In fact, the maximum specific power of diesel engines almost doubled since the late 1970s and early 1980s, when the world was in the midst of the oil crisis which boosted the demand for diesel passenger cars temporarily in the US, and this improvement enhanced diesel passenger cars acceptance in Europe. In addition to what is shown in the graph, car buyers feel that diesel cars are even more acceptable in performance because diesel engines develop high torques at low speed. As will be discussed later, advents of new fuel injection and air charging systems in the future are likely to add even more advancement to diesel engines. 2. EXHAUST EMISSIONS Efforts have been under way mostly in the U.S., Japan and Europe to reduce NOx and PM emissions Fig 3: Exhaust emissions regulations trend 3. FUEL ECONOMY The tradeoff between fuel economy and exhaust emissions, especially NOx emission, is also a difficult challenge. Fig. 4 shows how heavy-duty diesel engines minimum specific fuel consumption and NOx have been reduced simultaneously in Japan, and this traces the efforts to meet that challenge. Major techniques used for that purpose are shown in the figure. Needless to say, other techniques for improving overall engine design have also been tried, including those for reducing friction and pumping losses. Further simultaneous improvement of fuel economy and exhaust emissions must be done by the development of new devices and combustion technology, combined with steady efforts for reducing losses. The target for the next step should be 50% thermal efficiency (approximately 170g/kW h) and 55% for the second step for heavy-duty diesel engines. 120

3 Fig 4: Fuel consumption improvement while reducing NOx (Japanese heavy-duty engines) Fuel economy of gasoline and diesel passenger cars in Europe is compared in Fig.5. Though the advantage of diesel cars is obvious, further drastic fuel economy improvement can be achieved by introducing a 4-valve direct injection diesel engine in which the injection nozzle is arranged at the center of the combustion chamber as in a heavy-duty diesel engine. This lightduty DI diesel engine can be a key to produce a low fuel consumption passenger car in the future. 3L/100km fuel consumption seems to be a good challenge. a). COMBUSTION CONTROL More precise control of air/fuel mixture and temperature inside the cylinder must be realized to control combustion for the simultaneous reduction of fuel consumption and emissions. Fig. 6 is a schematic graph indicating how combustion should be controlled within short combustion duration. Although the shape and duration of combustion shown in Fig.6 change with load and speed, the directions for combustion control are as shown by arrows in the figure. Essential to such a control are an injection system capable of controlling the injected fuel quantity precisely during the injection period, a high pressure injection system capable of improving fuel atomization and mixing with air, an air motion control, an EGR capable of controlling combustion velocity and temperature, and an intake air temperature control..fig 5: Fuel consumption of passenger-car in Europe III. TECHNOLOGIES Discussed in this section are technologies for simultaneously reducing fuel consumption and exhaust emissions. These technologies are being pursued now and must be worked on in the future. Table 1 shows the major technology areas to be considered and components to be developed for both heavy-duty and light-duty diesel engines. In order to realize a socially beneficial and commercially acceptable engine, pursuing a wide front of technologies and selecting the most suited technologies for each application must be carried out. Even though reducing friction and heat losses and improvement of fuel quality are essential to improving fuel economy and exhaust emissions, the discussion here is mainly focused on combustion and cycle efficiency. Fig 6: Direction towards clean and efficient combustion b). HIGH PRESSURE INJECTION It is well known that a combination of a high pressure fuel injection pump system and a reduced orifice area of the injector improves air/fuel mixture and drastically reduces PM, especially smoke emission. Injection pressure combined with injection rate shape must be optimized in all operating ranges to achieve desirable torque characteristics, reduced smoke emission and fuel consumption. Currently a variety of jerk type and common rail type high pressure injection systems are available as shown in Fig.7. In order to reduce PM further with reduced NOx emission, injection pressure must be raised further in the future to reduce PM in higher EGR gas content atmosphere in the cylinder. For the market where high torque and low smoke at low engine speed is demanded, like Japanese market, the capability of high pressure injection in lower speed range is preferable. The parasitic loss of power due to pumping up the surplus fuel to the injection system must be minimized for the efficient system. 121

4 Fig 7: Schematic graph showing high pressure fuel injection pump characteristic. c). INJECTION RATE CONTROL Injection rate control is one of the means in designing combustion patterns shown in Fig. 6. Fig. 8 shows an example of test results obtained from a simple pilot injection intended to control pre-mixed combustion [2].This example shows a significant NOx reduction at part load, because of reduced rate of heat release in the early part of combustion. This reduced combustion noise also. Essential to controlling PM, which consists of soot and SOF, and HC is to cut off fuel injection instantly at the end of injection. This is another example of the importance of injection rate control. In addition, when a de-nox catalytic converter is introduced in the future, practical need will arise to supply the necessary amount of fuel to the catalyst as a reducing agent by positively using so-called after-injection. Development of injection systems capable of controlling injection rates is under way but is still far from being sufficient. In the case of pilot injection in Fig. 8, as an example, if much less pilot fuel volume at closer timing to the main injection could be realized, fuel consumption penalty due to pilot injection would have been avoided. Therefore, the injection system which can control injection rate much more freely and precisely is highly demanded. d). COMBUSTION CHAMBER Combustion chamber shape controls the air motion and mixing of air and fuel during combustion. A reentrant type combustion chamber which, by experiment, produced a fast burn is compared to a conventional combustion chamber in terms of how much swirl intensity remains in the piston cavity during expansion stroke by simulated calculation. [3] This result explains one of the reasons of better performance of welldesigned reentrant type combustion chamber. It also gives us some guidance for determination of good chamber shape which can maintain swirl intensity during the expansion stroke. Fig. 10 shows the effect on smoke and HC of matching of the fuel spray length and combustion chamber diameter as tested on various small size engines whose smoke at high load and HC at low load are the critical conflicting items. The fuel spray length is the measured length of the liquid phase of fuel spray in the experimental engine [4]. Although smoke and HC emissions are influenced by various other factors, Fig. 10 suggests that 0.7 to 0.8 of L/R value would be a good compromise for light-duty diesel engines. Those two results indicate the importance of the air motion in the combustion chamber and optimum matching of the spray and combustion chamber dimensions. Fig 8: Effect of pilot injection on combustion (At 20% load, 50% speed) But it should also be emphasized that analytical methods have become a strong tool to develop engine combustion systems. Combustion chamber shape design has been a typical trial and error item or the art of an engineer. Further development of analytical technologies and their practical use are necessary in the future. e). EGR Fig 10: Matching of fuel spray to combustion chamber diameter With low sulfur fuels becoming available, the use of EGR on diesel engines is likely to expand. Cooled EGR is an especially effective means to deal with the tradeoff between further reduction of NOx emission and fuel economy improvement. Fig. 11 shows the result of a test in which deterioration of fuel economy and smoke is avoided at high load because of less sacrifice of induced fresh air in the cylinder by cooling the recirculated gas. 122

5 Development of the cooling system and high EGR rate system of turbocharged engines without sacrificing turbocharger efficiency is one of the most important tasks in the near future. element by sensing emitting substance from the exhaust manifold or what is taking place inside the cylinder. With such a control, exhaust emissions can be controlled strictly to the mandatory requirements while optimizing other performance items such as fuel economy. Development of reliable sensors, such as NOx sensors, is necessary. Fig 11: Effect of cooled EGR on emissions and fuel consumption (at 80% load, 60% speed) f). OPTIMIZED CONTROL Optimized control of various factors that affects fuel economy and exhaust emissions will become more important. This area depends largely on the clever design of hardware and some examples are given below. Effect of the turbocharger on torque and SFC. In this case, expanded low fuel consumption zone and increased low-speed torque contribute to improving operating fuel economy. In addition, this system can be a tool for efficient EGR control to adjust exhaust back pressure of a turbocharged engine. Various kinds of variable orifice injectors are being proposed. Fig. 14 shows an example of injection nozzle with variable orifice area under development [6]. Orifice openings are controlled by a rotary valve, actuated by a motor on the top of the injector assembly. If such a device becomes available, variable orifice areas provides large freedom in selecting injection pressures, injection duration and spray characteristics, and thus promises a high potential for future engine improvements. Fig. 15 shows the result of calculation in which how, by changing intake valve closing timing, the effective compression ratio varies. For a high speed engine having a fixed intake valve closing timing of 55 degree ABDC, effective compression ratio can be raised by approximately 2 at low speed and load by a variable timing device. This is likely to help optimize compression ratio at start and during low speed and load operation for minimum HC emission. At high loads and speeds, the reduced compression ratio can improve smoke and fuel consumption. The variable valve timing device can be a tool for various other controls for future diesel engine. In the future, more advanced control systems will be required, which will feed-back control each engine g). CYCLE EFFICIENCY Fig 14: Variable orifice injector Improving cycle efficiency is basic to reducing engine fuel consumption. Even the already fuel-efficient diesel engine has to be further improved in this respect. Fig. 16 shows the calculated indicated thermal efficiency as the duration up to the end of combustion is varied, while assuming the same amount of fuel and the pattern of rate of heat release. Significant gain in thermal efficiency is evident as the centroid of the rate of heat release is advanced. Fig 15: Change of Effective compression ratio by Variable intake valve timing Important factors in improving the bottom part of the P-V diagram are improvement in turbocharging system efficiency and introduction of the turbocompound system. Fig. 17 shows the result of a study on the effect of turbine and compressor efficiency improvement on thermal efficiency of a turbo compound heavy-duty diesel engine by simulation calculation, assuming 10% increase of efficiency of power turbine 123

6 and turbocharger turbine (Yt) and turbocharger compressor (Yc) as a case study [7].This figure shows that the improvement of turbine and compressor efficiency makes a significant contribution to reducing fuel consumption of an engine. such a catalyst makes quite a contribution to the simultaneous reduction of exhaust emissions and fuel consumption. The solid fine in Fig. 18 shows the fuel consumption penalty as a reducing agent against NOx reduction under Japanese 13 mode test schedule, using a de-nox catalyst having peak NOx conversion efficiency of 50%. If the target line is attained, either 60% reduction of NOx under mode testing with 3% fuel consumption penalty will be realized, or 30% NOx reduction without fuel consumption penalty can be achieved by 3% fuel consumption gain by means of advancing fuel injection timing utilizing 30% NOx allowance. Fig 16: Effect of combustion duration on indicated thermal efficiency Various attempts may be needed such as the addition of variable turbine nozzle and compressor diffuser, and even the reduction of turbine speed (increase of wheel diameter) allowing size and weight increase of turbocharger, in addition to steady efforts for improvement. Heat insulation around the combustion chamber must be evaluated in terms of changes in the combustion, increase in NOx emission, reduction of intake air, all resulting from temperature rise inside the combustion chamber due to heat insulation. Fig 17: Effect of turbine and compressor efficiency Improvement on thermal efficiency by simulation h). AFTER TREATMENT The most important technological theme in the near future is the commercial application of the de-nox catalyst. Although no report of this catalyst actually being used has been made, it may not be very long until Fig 18: NOx-fuel consumption tradeoff of De-NOx catalytic converter IV. ENGINES For the purpose of proving the possibility of future low fuel consumption and low emission vehicles, Isuzu Motors Limited and Isuzu Advanced Engineering Center, Ltd. have developed prototype engines that incorporate various new technologies discussed here. The discussion hereafter focuses on diesel engines for passenger cars. a). 1.7L DIESEL ENGINE A 1.5L IDI turbocharged diesel engine for a production passenger car of 2500 lbs. inertia weight equipped with manual transmission has been modified into a 1.7L DI prototype engine[8].its injection pump is of Bosch VE type with the maximum injection pressure of 70 MPa at the injector inlet, which is not a so-called high-pressure injection system. EGR controls NOx with a conventional duty control system without cooling. PM is controlled with a re-entrant type combustion chamber which provides good swirl retention during the diffusion combustion process, combined with an oxidation catalytic converter for after-treatment. This engine s performance curves in Fig.20 show significant improvement in fuel economy and low-speed torques with the 4-valve DI arrangement. With low sulfur fuel(about 0.05%), its exhaust emissions levels are

7 g/km NOx+HC, and 0.05 g/km PM, both well lower than European EURO2 IDI diesel standards. Fig 20: Performance curve of 1.7L DI proto-type Engine Under the European EURO-MIX test procedures, its fuel consumption is 4.6L/100 km, a 16-percent reduction from that recorded with the original IDI diesel engine. With modifications to the vehicle by a vehicle manufacturer for further reduction of fuel consumption, which included reducing such items as the CD value, vehicle weight, rolling resistance, and reduction of gear ratio, fuel consumption of 3.4L/100 km was achieved, which in fact realized a prototype of what is called a "3- litercar". b). 2L DIESEL ENGINE A prototype has been built of a 2L in-line 4-cylinder 4-vaive DI turbocharged and intercooled diesel engine having bore/stroke of 80mm/97.2mm, intended for a passenger car of 3000 ibs inertia weight class. This engine is a modified prototype from 1.7 L IDI production diesel engine and its structure is similar to that shown in Fig. 19. Even more techniques than those used for the aforementioned 1.7L diesel engine are used for reducing fuel consumption and exhaust emissions simultaneously. Its injection pump is a prototype of in-line type, variable injection rate, and high pressure pump with the maximum injection pressure of 160 MPa at the injector side. An air-cooled EGR system is used with its maximum EGR rate of 30%.The combustion chamber is again a re-entrant type with 2.0 swirl ratios and matching of the fuel spray to the combustion chamber dimensions is carefully done. An oxidation catalytic converter is employed. This engine was installed on a European vehicle with 1360kg (3000 lbs.) inertia weight and 1150kg vehicle weight, with no modification to production specifications except changing the engines..exhaust emission levels of this prototype vehicle are shown in Fig. 21, and fuel consumption in Fig.22. Thanks to new technologies, its emission levels indicate a high potential to meet the European standards expected to be enforced in the future. It should be noted that what are shown in this graph as the effects of each individual action are rough breakdowns of the total achievement. It also includes the aforementioned trading off of NOx reduction to fuel economy. The fuel economy graph Fig.22 is basically the same as Fig.5. For passenger cars with 1150-kg vehicle weight, reading the median values of these zones, one gets 8L/100 km for gasoline vehicles, and 6L/100 km for IDI diesel vehicles. When the data from this prototype, 4.5L/100 km, is compared with those of the European vehicles plotted in this graph, the prototype has 44- percent fuel saving or 78-percent mileage gain in the case of gasoline vehicles, and 25-percent fuel saving or 33-percent mileage gain in the case of IDI diesel vehicles. Fig.21: Schematic illustration of exhaust emissions reduction of Isuzu 2L DI proto-type diesel engine Fig 22: Fuel economy of Isuzu 2L DI proto-type diesel engine c). 3L DIESEL ENGINE This is in fact a future engine prototype which incorporates overall engine engineering as well as individual techniques to deal with fuel economy emissions, especially focusing on weight and size of the engine. Some of the new technologies incorporated into this engine are cylinder head and cylinder block both 125

8 made of aluminum alloy, which can achieve a significant weight saving compared with current diesel engines. The engine weight is about 20% lower than the current Isuzu 3.1L diesel engine. Target specific weight is 1.5 kg/kw and well below the current passenger car diesel engine level. As indicated, its overall size is much smaller than that of Isuzu L L diesel engines now in production. When this type of next generation diesel engine becomes available, light weight and compactness significantly expand freedom in installing diesel engines on future vehicles. V. NEW APPROACHES Efforts have been under way in Japan and the U.S. to research into a new compression ignition combustion concept to drastically reduce NOx and smoke emission. One example of such research is "premixed lean diesel combustion" (PREDIC) pursued by New ACE Institute in Japan, which uses diesel fuel and high compression ratio like that of a diesel engine[9][10]. A conventional diesel engine emits 400 ppm of NOx when the fuel is injected immediately after the top dead center (TDC) of the compression stroke. When the fuel is injected much earlier than the conventional engine and forms an air/fuel mixture in the cylinder by means of "side injector nozzles" arranged opposite to each other on the bore circumference, NOx dropped drastically. As shown in the figure, when the fuel is injected at 64 degree BTDC, NOx emission reduced to 250 ppm, and when 78 degree BTDC is selected, NOx dropped down to 20 ppm. Observation of combustion showed no luminous flame, which means lean combustion is taking place. This combustion, however, can be realized in only very limited operating conditions. Therefore, further research activities are required and feasibility for its practical application is not yet clear. Even so, this is quite interesting approach that may achieve very low NOx combustion. In addition, in the case of 64 degree BTDC injection, very high rate of heat release right after TDC is observed without knocking and this should provide significant improvement in thermal efficiency. These new approaches may develop into a new diesel engine technology in the future. VI. CONCLUSION 1) Trends of diesel engines and future diesel engine technologies are reviewed and evaluated with the focus on the simultaneous reduction of fuel consumption and exhaust emissions. Judging from the result of these studies, if a number of future technological challenges are made, the diesel engine is likely to be further improved in terms of fuel economy (CO2) and exhaust emissions meeting social demand in the future. 2) Various engines with some of such new technologies incorporated were built and evaluated. The resultant improvements are significant enough to support the prospects stated above. 3) As the power plant for passenger cars, diesel engine s advantage over gasoline engine in fuel economy is likely to remain in the future as well, and with further improvement in specific output and weight, more passenger cars are likely to be equipped with diesel engines. 4) Though not discussed in this paper, there are some other important fields of technology, some of which are improving fuel properties, reducing friction and heat losses, and reducing noise. VII. REFERENCES [1] J.Fairbanks "Development of LE-55 Diesel Engine Concepts" U.S. DOE, Customer s Coordination Meeting, Oct [2] T.Minami, K.Takeuchi, N.Shimazaki "Reduction of Diesel Engine NOx. Using Pilot Injection" SAE [3] L.Zhang, H.Kurihara, T.Ueda, T.Takatsuki, K.Yokota "The Combustion Improvement of Reentrant Chamber in DI Engines; The Effect on In-Cylinder Flow" IPC-8, No.218(JSAE) Nov [4] L.Zhang, T.Tsurushima, T.Ueda, Y.Ishii, T.Itou, T.Minami, K.Yokota "Measurement of Liquid Phase Penetration of Evaporating Spray in a DI Diesel Engine SAE [5] M.Kita, M.Wakabayashi, K.Honjo "New 12L 6WAITC Turbocharged Diesel Engine" SAE [6] T.Hasegawa, T.Iwasaki, T.Kobayashi, Y.Matsumoto "Charactristics of the Variable-Orifice Nozzle for Direct Injection Diesel Engines" JSAE Oct. 1996, No [7] K.Kishishita, K.Miyajima, K.Hirai "A Study of an Electrical Turbo-compound System" JSAE Review 16(1995) [8] S.Ishida, K.Sato "Japanese Approach to Small DI Diesel Engines" ATA 96A6011 [9] Y.Takada, K.Nakagome, K.Niimura "Emission Characteristics of Premixed Lean Diesel Combustion with Extremely Early Staged Fuel Injection" SAE [10] K.Nakagome, N.Shimazaki, K.Niimura, S.Kobayashi "Combustion and Emission Characteristics of Premixed Lean Diesel Combustion Engine SAE