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Until the early to mid 1990's the engine of choice for off road and recreational vehicles were mainly 2-stroke engines due to the superior power density and Wide Open Throttle (WOT) torque characteristics. With the introduction of increasingly more stringent emission control requirements there has been a large swing to 4-stroke engines. In recent years there has been significant development to improve the power density of the 4-stroke engine in an effort to match the performance 2-stroke engine, however at a compromise to the torque characteristics and the manufacturing and maintenance cost of the engine. This paper looks at a fresh approach to develop a new concept engine to deliver a better compromise between the 4-stroke and existing carburettor 2-stroke characteristics, and provides early experimental results from the development work for a preferred WOT torque characteristic.

Whilst Direct In-cylinder injection is now common in both the automotive and non-automotive markets, the very high performance 2-stroke engines pose specific challenges to the application of direct injection due to the increased fuelling levels, the high fuel turn down ratio requirements and the reduced fuel preparation times at high engine operating speeds. In addition, a high performance 2-stroke engine will usually have a highly tuned scavenging system, which puts further demands on the fuel and combustion systems to achieve the desired performance. The fuel and combustion systems must also retain the low emissions to meet the relevant emissions legislation with a minimum level of aftertreatment.

With the introduction of increasingly more stringent emission standards, Engine Management Systems (EMS), including port fuel injection, are now being introduced in the 4-stroke motorcycle marketplace. These systems have been generally derived from the automotive industry, albeit with some significant changes to satisfy the strict cost and packaging constraints of the motorcycle applications. Direct injection (DI) is positioned to become one of the next generation of technologies for the automotive engine, offering the potential for improved fuel economy, performance and emissions control. Direct injection can also provide similar benefits for motorcycle applications. However, direct transfer of the current production automotive systems is unlikely to suit the requirements of motorcycle applications, due to some of the specific challenges faced in the motorcycle market.

Orbital have been developing their stratified combustion process (Orbital Combustion Process OCP) for direct injection gasoline engines over the last 15 years, with successful production releases of the system in both the marine and automotive 2-stroke applications in 1996. This paper discusses how the same basic qualities of the air-assist fuel system and combustion process have been applied to automotive 4-stroke engines. The inherent qualities of the air- assist fuel system in combination with careful design of the combustion chamber has enabled high charge stratification with late injection timings and very stable combustion over a wide range of operating conditions. Experimental test data from a 4-cylinder, 16 valve 4-stroke development engine demonstrates the ability of this low pressure system to operate at very lean air/fuel ratios, with part load fuel economy improvements of up to 34 % at an operating condition equivalent to a vehicle speed of 40 km/hr.

It has been shown that an automotive, stratified charge, direct injected (DI) 2-stroke engine is capable of meeting the proposed Stage 3 European emissions standards and the Ultra Low Emissions Vehicle standards of California at low mileage. In order to meet these stringent standards and develop a high in-field mileage capability for this lean burn engine, new engine control strategies and catalyst formulations have been developed. Engine out emissions and catalyst temperatures over both the proposed Stage 3 New European Drive Cycle (NEDC) and Federal Test Procedure (FTP) are described. Also with catalyst light-off time being an important factor in determining the overall cycle emissions, engine calibration based Fast Light Off and light load exhaust temperature strategies have been developed, in conjunction with optimised catalyst system configurations.