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A simple yet reliable method to evaluate the overall friction power of an internal combustion engine is proposed. The method is based on the measurement of the angular deceleration of the engine speed. The tested engine is coupled to a flywheel having a known high moment of inertia, fired and accelerated to a high engine speed. The system is kept running until steady conditions are attained. Then, the ignition system is shut-off, and the engine speed deceleration vs. time, α, is recorded. The total friction-mean-effective-pressure, tfmep vs. time, may then be evaluated from the fundamental relationship between the two (tfmep=2πIα/Vd, where, Vd is the engine displacement volume, and I is the moment of inertia of the integrated system). The proposed method, is a modification of the direct motoring method, in which the engine is motored by an external means under conditions as close as possible to firing.

The effect of atmospheric pressure on the performance characteristics of a crankcase-scavenged spark-ignition two-stroke cycle engine has been investigated. An air-borne, 350 cc opposed 2-piston engine was tested on an experimental test bench where both the pressure at the inlet manifold and the pressure at the exhaust pipe were controlled separately. The experimental results were analyzed by using a computer program, the MICE Program, which simulates in detail the various processes occurring inside the cylinder of an internal combustion engine. The computer program has been calibrated by using the measured results at sea-level. An empirical correlation has been proposed for the correction factor of the engine power in the range of 100 to 44 kPa, where the low pressure corresponds to a standard altitude of 21,000 ft (7 km).

An effective and simple method to improve the performance of a crankcase-scavenged two-stroke engine under off-design conditions is to reduce the fuel loss, due to short-circuiting, by controlling the pressure at the exhaust port. The present work presents a systematic study of the effect of throttling the exhaust pipe as a means of improving the torque, fuel consumption and emission in a two-stroke-cycle engine. The experimental observations were analyzed and extrapolated with the aid of a detailed computer program which simulates the engine cycle. It was concluded that: 1. The hydrocarbon emissions are significantly reduced when an exhaust contraction is applied. A reduction of 28% may be achieved at a wide range of engine speeds and loads, without a noticeable deterioration in die operation stability. 2. An optimum contraction ratio for HC emission has been observed and was found to be a complex function of the engine speed and load, but mainly dependent on the engine load. 3.