Electronically Controlled Batteryless Injection System for Small Two-Stroke SI Engines 2012-32-0118
The two-stroke SI-engine is the preferred drive for handheld power tools because of its high power density. Advantages of the two-stroke concept are its excellent reliability and the simple mechanical construction, which results in a good price-performance ratio. Usually these engines are equipped with carburators to meter the fuel. The tuning of carburetors is complex and compromises between idle, part load and full load settings are necessary.
This article describes the development of an electronically controlled fuel injection system for hand-held power tools that allows optimum performance in all operation points and an automatic engine adjustment for different ambient conditions and fuel qualities. The main requirements were a good reliability under adverse conditions (such as dirt, wet conditions and vibrations), low weight and a maintenance free system. Therefore, each component of the injection system was especially designed for the use in handheld power tools.
The central component of the injection system is the electronic control unit, which controls ignition and injection without requiring a battery. The energy for the fuel injection system is supplied by a generator connected to the flywheel.
A pressure-temperature sensor which is fitted to the crank case delivers the load signal. Engine speed is calculated by evaluating the generator signals. Injection takes place once per revolution in synchronism with the combustion cycle.
Characteristic maps are used for the basic stoichiometric setting. The data on the engine speed and throttle/load status of the engine enables the optimal operating parameters - such as injected fuel quantity, injection angle and ignition timing - to be selected for each stationary operating point.
Changing operating conditions, fuel qualities or wear can make it necessary to adjust the mixture preparation. This task is performed by intelligent map-based pilot controls and closed-loop controllers that determine the required corrective values and adjustments and apply them to the default operating variables.
The STIHL Injection system was introduced in a cut off machine. Benefits for the customer are the extremely simple starting procedure, a high power output and optimal fuel efficiency under all conditions.