Direct Injection as a Retrofit Strategy for Reducing Emissions from 2-Stroke Cycle Engines in Asia

Direct Injection as a Retrofit Strategy for Reducing Emissions from 2-Stroke Cycle Engines in Asia Dr. Bryan Willson Professor & Research Director, Engines & Energy Conversion Laboratory Department of Mechanical Engineering, Colorado State University Fort Collins, CO 80523-1374 USA E-mail: Bryan.Willson@colostate.edu ABSTRACT There are an estimated 70-100 million 2-stroke cycle engines in Asia, powering motorbikes, mopeds, threewheelers, tuk-tuks, and tricycles. These 2-stroke engines are characterized by very high levels of hydrocarbon (HC), carbon monoxide (CO), and particulate (PM) emissions. These high emissions levels are primarily caused by scavenging losses produced when the fresh air/fuel mixture is used to flush the exhaust gases from the previous stroke out of the engine; over 35% of the fuel is typically lost in the scavenging process. The application of direct in-cylinder fuel injection (or direct injection DI) can be used to reduce HC and CO emissions by over 70%. This initial reduction allows the use of an oxidation catalyst to further reduce emissions, for a potential emissions reduction of well over 90%. The DI technology reduces fuel consumption by approximately 35% and dramatically reduces particulate emissions. The DI technology is being used by manufacturers in Europe for scooters, and in the U.S. for outboard motors and personal watercraft. This paper describes the fundamentals of the DI technology. Applications to alternative fuels such as CNG and LPG are discussed. The application of DI in retrofit applications is discussed, with a case study of a retrofit application that reduced CO and HC emissions by over 99%. Finally, a demonstration of DI retrofit technology in Asia is proposed. Keywords: two-stroke, direct injection, 2-wheeler, 3-wheeler, air pollution 1.0 INTRODUCTION Air pollution in many Asian cities is increasing due to the proliferation of vehicles powered by simple two-stroke cycle engines. These engines produce high levels of carbon monoxide, unburned hydrocarbons, and particulates. The principal cause of high emissions is the simple scavenging process (called crankcase scavenging ) that allows 35%+ of the engine s fuel to escape unburned from the engine. Two-stroke cycle engines ( 2-strokes ) are utilized due to their rugged construction, low cost, and high power / weight ratio. Two-stroke-powered vehicles include scooters and mopeds (aka two-wheelers ) and three-wheeled motorized tricycles (aka three-wheelers, tuk-tuks, tricycles ) that are used for taxi service and as utility vehicles. Two-stroke cycle engines are widely used throughout Asia for personal transportation. A recent estimate in India calculates that 80% of the 2-wheelers are powered by two-stroke engines, 1 although recent sales data for new 2-wheelers indicate a shift toward four-stroke engines. In 1992-93, twoand three-wheelers with two-stroke cycle engines were reported to account for 70% of the total unburned hydrocarbons (HC) and 46% of the carbon monoxide (CO) emissions in India 2 and were a significant contributor to particulate emissions. This is due to the large number of vehicles with two-stroke engines and the particularly high emissions from these engines. Emissions values from small and medium 2-stroke engines are presented in Table 1, showing hydrocarbon emissions which range from 134 236, and CO emissions which range from 185-522. Brake specific emissions data (i.e. ) are presented instead of driving cycle data (typically grams/km) to focus on the engine only and avoid the contributions of vehicle weight and drivetrain efficiency. Some existing engine emissions regulations are presented in Table 2. Note that NOx emissions from small two-stroke cycle engines are generally low due to their use of relatively rich air/fuel mixtures. The environmental impact from these two-stroke engines is pervasive. Carbon monoxide is a powerful respiratory irritant. Particulate emissions contribute to respiratory disease. Hydrocarbon emissions contribute to ozone formation. A few Asian countries still use leaded gasoline. Research has shown that the lead emitted in unburned hydrocarbons (i.e. scavenged fuel from carburetted 2 strokes) is much more damaging to the nervous system of children than the lead which is emitted as products of combustion. PS-8 1

CO HC NOx CO HC NOx 55 kw 2-stroke 185 134 2.74 Indian 2-stroke 603 166 - outboard engine 3 genset regulation 4 7.4 kw 2-stroke Johnson OMC- J10RCSE engine 5 519 236 0.73 US EPA 2-stroke emission factor, 1999 522 206 0.67 Cooper GMVC- 10C natural gas engine with direct fuel injection 6 2.0 CARBURETTED vs. DIRECT-INJECTED TWO-STROKE ENGINES The high emissions illustrated in the first two rows of Table 1a are not inherent to the two-stroke engine cycle. Instead, they result from of the use of simple carburettion combined with crankcase scavenging. The crankcase-scavenged two-stroke cycle engine was developed by Sir. Dugald Clerk in the 1890s and is still widely used due to its simplicity, durability, power density and low cost. Technology now exists to produce much cleaner two-stroke cycle engines. This technology relies on direct in-cylinder fuel injection (DI), which can reduce hydrocarbon emissions by 90%, and carbon monoxide emissions by 70%. Particulates and visible smoke are dramatically reduced through the use of direct injection technology. A comparison of carburetted and DI two-stroke engine operation is illustrated in Figure 1. When an oxidation catalyst is added, CO and HC reductions of over 99% have been demonstrated. It should be noted that the use of an oxidation catalyst without direct injection results in excessive catalyst temperatures that can reduce catalyst life, lead to catalyst meltdown and cause safety problems. The same direct-injection technology that reduces emissions also reduces fuel consumption. By eliminating the 35%+ of the fuel which escapes unburned, engine efficiency is increased. By reducing carbon monoxide production, combustion efficiency is further increased. The result is a 40%+ reduction in fuel consumption, producing a corresponding reduction in the production of greenhouse gases. The unfavorable emissions from carburetted two-stroke cycle engines are primarily due to: 1) shortcircuiting of fuel during scavenging, 2) incomplete scavenging, 3) the use of waste lubrication, and 4) poor control of air/fuel ratio. A review of these factors follows: 2.1 Short Circuiting 1.88 9.1 0.67 California 4- stroke emission factor < 19 kw (25 hp) Table 1a. Typical 2-Stroke Emissions 322 5.4 2.4 Table 1b. Current 2-Stroke Regulations In a carburetted crankcase-scavenged two-stroke engine, the combustion products from the previous cycle are forced from the cylinder with a new air/fuel charge. This charge is compressed in the crankcase by the underside of the piston and then enters the cylinder when the piston uncovers the transfer port. Unfortunately, the exhaust port is open during the entire time that the transfer port is open allowing part of the air/fuel mixture to short circuit through the cylinder during the scavenging process. This is the major source of the high hydrocarbon emissions from crankcase-scavenged engines, allowing 35%-40% of the fuel to be lost directly out of the exhaust ports during the scavenging process. 7 The scavenging losses at idle can be as high as 70%. 2.1 Incomplete Scavenging The maximum volume change in the crankcase of a crankcase-scavenged engine is equal to the swept volume of the engine. Due to pumping losses, the volume of air/fuel mixture used for scavenging is significantly less than the swept volume of the cylinder (i.e., the delivery ratio is less than unity). There is not enough mixture to completely force the old exhaust products from the cylinder, resulting in high levels of residual exhaust, which remain in the cylinder. This is a major contributor to combustion instability that in turn contributes to high carbon monoxide and hydrocarbon emissions. In order to stabilize the combustion process, richer mixtures are used, again leading to high CO emissions. The efficiency of the scavenging process can be improved by increasing the scavenging volume, but this will increase the hydrocarbon emissions if the engine is scavenged with an air/fuel mixture. PS-8 2

2.2 Waste Lubrication Crankcase-scavenged two-stroke cycle engines use a waste lubrication system. In small two-stroke engines, the oil is mixed with the fuel during refuelling, or the driver purchases premix. Some of the oil is deposited on the appropriate components (crank bearings, rod bearings, cylinder walls) while the mixture is in the crankcase. The remaining oil then travels with the air/fuel mixture into the cylinder where it is either FUEL & OIL INTAKE CARBURETTOR CONVENTIONAL ENGINE WITH CARBURETTOR UP TO 70% FUEL CHARGE LOST: POOR FUEL ECONOMY & EXH EMISSIONS STRATIFIED FUEL INJECTION AFTER PISTON COVERS PORT INTAKE AIR & METERED OIL DIRECT INJECTED ENGINE ONLY AIR LOST Figure 1 Differences in Operation Between Carbureted and Direct-Injected 2-Stroke Engines short-circuited or trapped in the cylinder. The short-circuited oil contributes to the hydrocarbon emissions. The trapped oil does not burn readily and becomes a major source of the visible smoke produced by small two-stroke engines. Direct-injected two-stroke engines still use a waste lubrication system. However, since the oil is not dissolved in the fuel, it deposits more effectively on the walls and bearings where it is needed. This reduces the oil migration into the combustion chamber, which dramatically minimizes the smoke caused by combustion of the lubricating oil. 2.3 Air/Fuel Control Although high hydrocarbon emissions are inevitable in carburetted two-stroke engines, high carbon monoxide emissions are not. They arise from rich carburettor tuning. In the range at which maximum power occurs, CO concentration levels are typically 1.5% - 2.5%. 8 If a carburettor is set for leaner operation, misfire may occur during acceleration transients. Due to the time delays involved with crankcase scavenging, acceleration enrichment is generally ineffective, so the engine is typically tuned rich at all times. Carburettors are also tuned rich to provide fuel cooling of the cylinder to help prevent overheating of the piston. 3.0 DIRECT INJECTION TECHNOLOGY In a carburetted engine, there is ample time for the fuel to vaporize to form a homogeneous air/fuel mixture. In a direct-injected two-stroke engine, the fuel is injected into the cylinder late in the cycle, as the piston is returning. The time available for vaporization and mixing is short, so the fuel must be atomized into very fine droplets to allow the fuel to vaporize for combustion. The fuel is injected directly into the cylinder, so the injector must withstand combustion pressure and temperature. These factors provide challenges to designers of DI fuel injection equipment. 3.1 Types of DI Technology There are several competing DI technologies produced by different manufacturers, although they can be grouped by atomising technology. High-pressure DI systems used high fuel pressure to atomize the fuel, in a manner similar to diesel fuel injection. High pressure DI systems have been developed by Bombardier (who PS-8 3

Figure 2 Fuel Spray from an Orbital Air- Figure 3 Cutaway of an Orbital Air- Assisted Fuel Injector Assisted Fuel Injector holds the license to the Ficht DI system), Yamaha, BKM, and others. There several manufacturers of highpressure direct injection equipment for four-stroke automotive engines, a technology known as Gasoline Direct Injection, or GDI. A second alternative uses compressed air to break up the fuel droplets. This is known as air atomisation or air blast fuel injection. This technology was developed by the Orbital Engine Company in Perth, Australia and is referred to as the Orbital Combustion Process (OCP). The fine atomisation produced by the Orbital air-assisted fuel injection system is illustrated in Figure 2 above. The results of the Orbital OCP system on a 150 cc engine in a typical 3-wheeler application are documented in Table 2. These results show that the use of direct injection reduces CO by 37%, HC by 75%, and improved fuel economy by 36%. Results later in the paper will show that even greater improvements can be achieved through the use of an oxidation catalyst. Further improvements without a catalyst may be achievable by optimising the injector location, port geometry, and shape of the piston crown. The system is in commercial use in two-stroke engines sold by Mercury Marine (the Optimax engine), Bombardier-Rotax, Tohatsu, Aprilia, Piaggio, and Peugeot Motocycles. Other two-stroke and directinjection four-stroke applications are in various stages of commercialisation. The durability of the Orbital OCP system has been demonstrated by durability tests 9 and by the successful conclusion of a large fleet trial. In this trial, 100 Ford Festiva vehicles were equipped with two-stroke Orbital engines. These vehicles accumulated over five million kilometers of operation and reported excellent reliability and durability. 10 Finally, the Orbital OCP system has now been in use for over fives years on Mercury Optimax outboards with generally excellent reliability. 3.2 Fuel Injection System Components The key piece of technology for the Orbital OCP system is the air/fuel injector. There are many variations of the injector, one of which is shown in Figure 3 above. The injector consists of two parts. The upper part of is the fuel injector, which meters fuel into the lower component, the air injector. The timing and quantity of the air and fuel injection are controlled separately. At light loads, fuel is injected relatively late to create a stratified charge with a richer mixture around the spark plug than in the rest of the cylinder. At higher loads, the fuel is injected earlier to allow greater penetration and a more homogeneous mixture in the cylinder. The air injector, fuel injector, and spark plug are controlled by an engine control unit (ECU). Figure 4 shows the results of computational fluid dynamic (CFD) modelling by Orbital. Using CFD, the instantaneous air/fuel distribution in the engine can be predicted. Computational tools such as these assist in the optimisation of the engine and fuel injector. 4.0 RETROFIT POTENTIAL Currently, direct injection equipment is being supplied to manufacturers of new two-stroke engines by Orbital, Bombardier and others. The author and his research group have been engaged in development and application of direct-injection technology for retrofit applications on small gasoline engines and on large industrial engines. The results to date have been very successful and suggest the potential for widespread retrofit application of direct injection to two- and three-wheelers in Asia. PS-8 4

62º Before Top Dead Center 56º Before Top Dead Center 43º Before Top Dead Center 28º Before Top Dead Center Figure 4 Fuel Distribution in Cylinder during Compression. Piston shape has been altered to minimize hydrocarbon losses. 4.1 Gasoline Retrofit The results in Table 2 suggest the effectiveness of converting existing engines to direct injection. Components that must be added include the fuel injector, fuel pump, air compressor, crank sensor, throttle sensor, and engine control unit (ECU). Estimates of conversion cost range from $150-$300 for a singlecylinder conversion. The improvements presented in Table 2 can be improved through the addition of an oxidation catalyst. A student team at Colorado State University used the Orbital OCP system and an oxidation catalyst to convert a 600cc 3-cylinder Arctic Cat snowmobile to direct injection. The results of this retrofit conversion are illustrated in Figure 5. These results show that the use of direct injection alone reduced CO emissions by 70% and the HC emissions by 99%. By adding an oxidation catalyst, emissions were further reduced to a final control level of 99.7% for HC and 99.9% for CO. To restate these results, more than 300 modified snowmobiles could be driven to produce the same environmental impact as one standard snowmobile. 4.2 Alternative Fuels Retrofit Carburetted Engine DI Engine HC + NOx 5.3 g/km 1.3 g/km CO 2.7 g/km 1.7 g/km Fuel Consumption 29.6 km/litre 40.4 km/litre Table 2 Comparison of Carburetted and DI Fuel-Injected Operation. Results from a 150 cc engine in a typical 3-wheeler application tested on the Indian Driving Cycle PS-8 5

Figure 5 Results from Conversion of a Carburetted 2-Stroke Cycle Snowmobile Engine to Direct-Injection using an Oxidation Catalyst. The Orbital system is well suited for use with gaseous fuels such as compressed natural gas (CNG), liquid petroleum gas (LPG), or even more exotic fuels such as hydrogen. This capability results from the ability to use the air injection stage of the OCP injector as an injector for gaseous fuels. The use of DI for alternative fuels helps to solve part of the range problem which plagues most two-stroke engines that operate on alternative fuels. Loss of fuel during the scavenging process is largely eliminated. This is expected to result in an estimated improvement in vehicle range of 35%. Since a DI system injects part of its fuel after port closure, it can help to offset the displacement of air that typically reduces power in a CNG engine by 10%-15%. 11 Gaseous fuels are stored under pressure. A fuel pressure regulator is required to maintain a consistent fuel pressure to the injection system. However, the air compressor and fuel pump required for a gasoline-based system are not required, which is expected to significantly simplify the conversion process. Direct injection of natural gas into two-stroke engines may appear exotic in automotive applications, but has a 60+ year history in industrial engines. Figure 6 is a picture of a Cooper-Bessemer GMV engine. This is a direct-injected natural gas engine used to provide power to compress natural gas so that it can be moved through the pipeline system in the United States. Variations of engines similar to this have been in use in the United States since the 1940s, and are still in use due to their high reliability and overall efficiency. Emissions from a similar engine are presented in the bottom row of Table 1a, showing generally excellent emissions levels. Figure 7 shows comparison between the results of computational fluid dynamic modelling of natural gas injection (left half of picture) and experimental planar laser induced fluorescence (PLIF) imaging of the same fuel injection event (right half of picture). Both parts of the image were produced in the author s laboratory during studies of direct fuel injection into large two-stroke cycle engines. 5.0 POTENTIAL FOR A RETROFIT PROGRAM IN ASIA The potential of direct injection to reduce emissions and fuel consumptions suggests that it is a cost effective improvement that manufacturers can utilize on new two-stroke cycle engines. Various estimates of the existing vehicle population suggest that there may be 70-100 million two-stroke vehicles in use in Asia. The useful life of a two-wheeler is generally estimated to be less than 10 years, but the life of a three-wheeler can be significantly longer than 10 years. Three-wheelers are also characterized by lower fuel economy (due to their heavier weight) and higher mileage than two-wheelers. The population of three-wheelers in India ( three-wheelers ), Indonesia, Thailand ( tuk-tuks ), and the Philippines ( tricycles ) is estimated at over two million vehicles. Three-wheelers are almost all commercial vehicles, and their owner/drivers are more likely to be motivated by the fuel savings than a commuter with a two-wheeler. These commercial vehicles are suggested for further evaluation to assess the cost/benefit of a retrofit program. This evaluation should include the evaluation of high-population models, locations for pilot projects, and the potential for inclusion of alternative fuels. Stakeholders from affected groups (drivers, manufacturers, environmental officials, funding agencies) should be included in the evaluation. 6.0 CONCLUSIONS This paper illustrates that the use of direct in-cylinder fuel injection can dramatically reduce the emissions from small two-stroke cycle engines and can improve their fuel economy. ACKNOWLEDGEMENTS The author would like to express his gratitude to the Orbital Engine Company for their permission to use the Figure 6 Cooper-Bessemer GMV Direct-Injected Two-Stroke Cycle Natural Gas Engine Figure 7 In-Cylinder Natural Gas Injection: CFD Modeling (left) and PLIF Visualization (right) PS-8 6

illustrations in Figures 1-4, and for the data in Table 2. REFERENCES 1 N. V. Iver, Technology and policy options to control emissions from in-use two- and three-wheelers. In Clearing the Air, Better Vehicles, Better Fuels, Ranjan Bose, editor. Tata Energy Research Institute, 2000. 2 B P Pundir, Amar K Jan, Dinesh K Gogia, 1994, Vehicle Emission and Control Perspectives in India: A State of the Art Report, Indian Institute of Petroleum. 3 S W Coates, G G Lassanske, Measurement and Analysis of Gaseous Exhaust Emissions from Recreational and Small Commercial Marine Craft. SAE #901597. 4 Notification, Ministry of Environment & Forests, No. Q-15017123/98-CPW, November 5, 1999. 5 John B Heywood, Eran Sher, The Two-Stroke Cycle Engine: Its Development, Operation, and Design. SAE / Taylor & Francis, 1999. 6 14 bore x 14 stroke. 300 rpm. 1342 kw (1,800 hp). Highly turbocharged. 7,550 Btu/bhp-hr 7 Marco Nuti. Emissions from Two-Stroke Engines. Society of Automotive Engineers. Page 85. 8 Marco Nuti. Emissions from Two-Stroke Engines. Society of Automotive Engineers. Pp. 99-101. 9 K Eisenhauer. Durability Development of an Automotive Two-Stroke Engine. SAE paper #956006. 1995. 10 David Shawcross, C Pumphrey, D Arnall. A Five-Million Kilometre, 100-Vehicle Fleet Trial of an Air- Assist Direct Fuel Injected, Automotive 2-Stroke Engine. SAE paper #2000-01-0898. 11 B. Willson, "Evaluation of Aftermarket Fuel Delivery Systems for Natural Gas and LPG Vehicles," published by National Renewable Energy Laboratory, NREL/TP-420-4892. September 1992. Uncontrolled CO Level of 108,150 ppm Uncontrolled HC Emission of 19,343 ppm 75177 ppm removed with DI 32,908 ppm removed with catalyst 17,293 ppm removed with DI 1986 ppm removed with catalyst 65 ppm remaining Remaining HC emissions of 64 ppm PS-8 7