Development of a Compression Pressurized Direct Fuel Injection System for Retrofit to Two-Stroke Engines

Small displacement carbureted, crankcase-scavenged, two-stroke engines are commonly used for transportation needs in developing countries (Iyer 2000). The high emissions characteristic of this engine design creates localized acute air pollution problems and contributes significantly to global air pollution (Gorham 2002). A compression pressurized direct fuel injection system (CPDI) has been proposed as a lower cost alternative to the common air blast direct injection system (ABDI). This paper describes the application of the CPDI system as a retrofit to a two-stroke engine typical of those used in “tricycle” transports in the Philippines. Power production, emissions, and fuel consumption for the original carbureted engine, the ABDI system and the CPDI system are also compared. The DI systems show a significant reduction in both hydrocarbons (88% reduction for the ABDI system and 86% reduction for the CPDI system) and carbon monoxide (49% for the air blast system, 83% for the compression pressurized system), and similar power compared to the carbureted engine. INTRODUCTION: ABDI AND CPDI The air blast direct injection system, pioneered by Orbital Engine Company (Huston et al. 1998), has successfully been applied as a retrofit to small twostroke engines for the purpose of improving emissions via reduction of fuel short-circuiting (Willson 2002). In the ABDI system (figure 1), fuel is injected into a cavity in an air rail, which is separated from the combustion chamber by an outwardly opening, solenoid actuated, poppet valve called the “blast valve”. Pressurized air is also provided to the air rail by a mechanical pump, which is driven by the crankshaft. When the blast valve is opened the air “blasts” the fuel into the combustion chamber, finely atomizing it as it enters the combustion chamber. While this system is successful at reducing the hydrocarbon (HC) and carbon monoxide (CO) emissions of the engine, the intrusiveness of the air pump presents a challenge for retrofit applications. The rest of the components can be bolted on, either replacing an existing part (such as the head), or in an arbitrary location (such as the fuel pump). The air pump, however, requires durable mounting to the engine case and coupling to the spinning crankshaft. FIGURE 1 Schematic of an air blast direct injection system on a two-stroke engine. Note the air pump, which is driven by the crankshaft. One way to eliminate the air pump is to modify the ABDI system so that pressure for the blasting of the fuel is extracted from the combustion chamber during the compression stroke. This technique was first proposed by AVL List GMBH of Austria (Fraidl 1996), and later refined by Orbital Engine Corporation of Australia (Leighton 1994). The compression pressurized direct injection system (figure 2) uses most of the same components as the ABDI system, however, now the blast valve must do a double duty as it is responsible for the blasting of the fuel, and re-charging of the mixing cavity which replaces the air rail. FIGURE 2 Schematic of a compression pressurized direct injection system on a two-stroke engine. The mixing cavity is pressurized from the combustion chamber during the compression stroke. There are three phases to CPDI operation: cavity pressurization (recharge), fuel introduction, and cavity discharge (blast) to the combustion chamber. The pressurization phase takes place during the compression stroke and is completed before ignition. The blast valve is held open, and the compressed gases from the combustion chamber are admitted into the mixing cavity (figure 3). Once the mixing cavity has attained the appropriate pressure, 500 kPa for example, the blast valve is closed, sealing the mixing cavity. FIGURE 3 Blast valve is held open during compression stroke to pressurize the mixing cavity. Once the blast valve is closed fuel may be injected into the mixing cavity while the engine is in the combustion, expansion or exhaust portion of the cycle. The fuel is injected directly at the back of the blast valve (figure 4). As the gases in the mixing cavity are still hot from compression, some fuel may vaporize, however much of may remain as liquid droplets, or a liquid film on the back of the blast valve. FIGURE 4 Fuel is injected into the mixing cavity, partially evaporating, while the blast valve is held closed. Once scavenging is complete and the exhaust port is closed, the blast valve is again opened. As the pressure of the mixing cavity (approximately 500 kPa) is greater then the pressure of the combustion chamber (approximately 120 kPa) the contents of the mixing cavity are “blast” into the combustion chamber. Any remaining fuel droplets are finely atomized in the strong shear between the injected gases and the combustion chamber gases (figure 5). FIGURE 5 The fuel is blast into the combustion chamber at the beginning of the subsequent compression stroke. CPDI: SINGLE VS. DUAL VALVE ACTUATION In the CPDI design the blast valve may either be actuated twice per revolution (CPDI-2) of the engine, with separate “blast” and “recharge” events, or the two steps may be combined in a single actuation which holds the valve open continuously from the beginning of the blast event to the end of the recharge event (CPDI1). The CPDI-1 mode of operation has the advantage of requiring no additional electronic hardware, as the existing ABDI system’s electronic control unit (ECU) can be used to actuate the blast valve once per cycle. Inherent in the CPDI-1 technique, however, is the possibility of injecting fuel into the combustion chamber at relatively low differential pressures, thereby potentially degrading the atomization quality. The CPDI-2 technique has the ability to avoid this low-pressure injection by closing the blast valve while the pressure differential is still relatively high, and only reopening it for recharge after the pressure of the combustion chamber has exceeded that of the mixing cavity. The CPDI-2 technique, however, requires additional circuitry to operate the valve twice per revolution. We therefore used a separate computer system and injector driver circuit for operation of the blast valve in the CPDI-2 mode. In CPDI-2 mode, all of the other functions, such as ignition, liquid fuel injector actuation, fuel and oil pump operation, and throttle sensing, were performed by the ABDI ECU. ENGINE MODIFICATIONS: KAWASAKI HD-III As the focus of our project was the reduction of vehicular emissions in developing countries, we selected the Kawasaki HD-III 125cc two-stroke motorcycle for our modifications. This is the most common motorcycle in use in the Philippines, where it is often fitted with a sidecar, and used as a taxi, called a “tricycle” (figure 6). FIGURE 6 Kawasaki HD-III “tricycle” with sidecar at CSU’s Engines and Energy Conversion Laboratory. The majority of the fuel injection components (fuel pump, fuel rail, fuel injector, blast valve, oil pump, ECU, air pump, generator, index sensor) came from an Aprilia Scarabeo motor scooter, which uses an Orbital ABDI system. The side case of the engine was modified to accept the air pump, and an air pump driving cam for ABDI system testing (figure 7). An aluminum head was machined which accepted the ABDI hardware. The combustion chamber shape was designed based on a shape used by Orbital in similar displacement engines. FIGURE 7 Kawasaki HD-III with ABDI modification. FIGURE 8 The CPDI mixing cavity replaces the fuel rail of the ABDI system. The skew of the top flange allows easier access to the lower flange’s head bolts. FIGURE 9 Schematic of the CPDI mixing cavity. Cavity is formed in the space between the two injectors. Fuel Rail Head Crank Sensor Stop Cock Air Pump Cam Air Pump