DESIGN OF A DIRECT INJECTION RETROFIT KIT FOR SMALL TWO-STROKE ENGINES
By Nathan Lorenz, Tim Bauer, Bryan Willson
EnviroFit International, Colorado State University
2005
Carbureted 2-stroke engines are a worldwide pandemic. There are over 50 million 2-stroke cycle engines in Asia alone, powering motorbikes, mopeds, "three-wheelers", "auto-rickshaws", "tuk-tuks", and "tricycles". These carbureted 2-stroke engines are characterized by high levels of hydrocarbon (HC), carbon monoxide (CO), and particulate matter (PM) emissions. Direct injection is a technology that has shown a great ability to reduce these emissions while at the same time improve fuel economy. A prototype kit has been designed for use in retrofitting existing carbureted two-stroke engines to direct injection. The kit was designed for use on a Kawasaki HDIII; a motorcycle from the Philippines that is commonly used as a taxi. It is however, a relatively common engine design and Kawasaki manufactures similar models for sale all over the world. The retrofit kit incorporates the Orbital air blast direct injection system. This injection system has been implemented in Original Equipment Manufacturer (OEM) applications for over six years. The design involved replacing the existing cylinder head with one designed to incorporate the direct injection valves as well as a modified combustion chamber. An external compressor was added to supply compressed air to the system. The carburetor was replaced with a throttle body outfitted with a position indicator, and an encoder system was added to provide speed and position feedback to the engine control unit (ECU). Once design and manufacture of the system was complete, it was installed on the motorcycle. The motorcycle was then mounted in a low inertia eddy current dynamometer test cell for calibration. Calibration was done on the dynamometer for power and engine performance. The system was also tuned in real world road tests for drivability. When calibrations were complete emissions and fuel consumption measurements were taken for the vehicle. The results showed an 88% reduction in hydrocarbon emissions and a 72% reduction in carbon monoxide emissions versus the baseline engine, while at the same time virtually eliminating visible smoke. The retrofitted system also showed a 32% increase in fuel economy, and had similar to better performance than the carbureted engine. The retrofitted system also showed improved cranking and idling characteristics over the carbureted engine.
INTRODUCTION
Air pollution is on the increase in many Asian cities due
in part to the widespread use of carbureted two-stroke cycle
engines. These engines are typically used as the power source
for "two-wheelers" (i.e. motorcycles, mopeds, etc.) and
"three-wheelers" such as tricycles and tuk-tuks. The main
reasons for this are: 1). their rugged yet simple construction,
and 2.) their low cost and 3.) their high power-to-weight ratio.
Unfortunately, two-stroke cycle engines are also characterized
by high levels of unburned hydrocarbons, carbon monoxide, and
particulate emissions.
The high hydrocarbon emissions from carbureted two-stroke engines result from the scavenging process used. Scavenging refers to the process by which the burned exhaust gasses are flushed from the engine. In a conventional "carbureted" two-stroke engine the fuel is entrained in the intake air stream before the combustion air enters the crankcase. The charge is compressed in the crankcase by the underside of the piston, and enters the cylinder when the piston uncovers the transfer ports. Combustion products from the previous cycle are forced or "scavenged" from the cylinder with this new air/fuel charge. Unfortunately, the exhaust ports are also open at this time, allowing 30%-40% of the fuel to be lost directly into the exhaust stream.1 At idle conditions the losses can be as high as 70%.
The high carbon monoxide emissions result from rich air to fuel ratio typically seen in these engines. High residual gas fractions within the cylinder lead to an environment in which consistent ignition is difficult. In order to improve combustion stability rich air fuel mixtures are typically used. This excess of fuel leads to incomplete combustion and high carbon monoxide levels.
Finally, the high particulate emissions result from the unstable combustion, excessive lubrication (typical in small two stroke engines), and a lubrication system which allows lubricating oil to be dissolved in the fuel.4 In a typical 2-stroke, the oil mixes with the fuel at the carburetor. As the air/fuel/oil mixture transfers into the crankcase, the fuel dissolves the oil. This action reduces the amount of oil deposited on the cylinder wall (or other critical components) as it is essentially ‘washed’ out of the engine by the fuel.
