DIRECT INJECTION AS A RETROFIT STRATEGY FOR REDUCING EMISSIONS FROM 2-STROKE CYCLE ENGINES IN ASIA
By Dr. Bryan Willson
Professor & Director, Engines & Energy Conversion Laboratory
Department of Mechanical Engineering, Colorado State University
2002
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.
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 India2 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 g/kW-h, and CO emissions which range from 185-522 g/kW-h. Brake specific emissions data (i.e. g/kW-h) 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.
