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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.

This paper investigates strategies intended for development towards Super Ultra Low emission levels (4 Star) from the existing 3 Star emission status for 2 stroke Direct Injection and 4 stroke outboard engines. The current 5 Mode ICOMIA outboard emission status is considered for Hydrocarbons, Nitrogen Oxides and Carbon Monoxide. From these a gap analysis is developed and possible strategies to close the gaps are considered. The potential of these strategies for 2 stroke air assisted Direct Injection outboards is reviewed together with supporting data where appropriate. Initial data show high potential for reduction of emission levels beyond the 3 Star standard, whilst further work will confirm the levels that may be feasible in a production outboard engine.

There is a growing requirement for lightweight high performance engines capable of operation with heavy fuels such as JP5 (F44), JP8 (F34) and diesel fuels as well as maintaining the capability of running on unleaded gasoline. Traditionally heavy fuels are associated with operation in compression ignition engines which exhibit poor power to weight characteristics. Today's engine applications call for much greater mobility and flexibility in use, especially in applications such as outboard engines, motorcycles, All Terrain Vehicles, Light Aircraft and portable generator sets. Compression ignition engines with their poor power to weight and vibration characteristics are less likely to succeed in these applications. These requirements are more likely to be met by the development of a spark ignition engine capable of operating on these heavy fuels.

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.

The Indian motorcycle and scooter market is faced with the combined challenge of exhaust emissions control from the environmental perspective and fuel economy demands from the consumer. Indian motorcycles are unique in their strong focus on reduced fuel consumption and this has resulted in the development of engines with quite different combustion characteristics in comparison to their European and ASEAN counterparts. Aprilia, Peugeot and Piaggio have recently released new scooter models utilizing air assist direct injection on their small 2-stroke engines to meet the ever increasing pressure on exhaust emissions, fuel economy and performance. These new scooter models are achieving good acceptance with the customer due to their combination of superior performance and improved fuel economy.