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BASIC DESIGN PARAMETERS 3.1 Engine and Port Geometry

3.3 Stepped-Piston Engines

The stepped-piston concept overcomes some of the drawbacks of crankcase- compression engines. Increasing the delivery ratio, isolation of the fresh charge from the crankcase, and improving engine performance at high altitude have some advantages. The essential features of the stepped-piston two-stroke engine are shown in Figure 3.7.

The engine is constructed of a stepped piston and a stepped cylinder, thus forming three compartments: a power, a compression, and a crankcase compartment. With this arrangement, the fresh charge is compressed in the compression compartment, delivered to a receiver, and introduced to the cylinder through the scavenge ports. In some designs, the fresh charge enters the crankcase compartment prior to its admission to the cylinder.

Figure 3.5: Early stepped piston engine diagram

As the piston travels downward, the volume of the compression compartment increases, the pressure thus decreases, and fresh charge is admitted. Meanwhile, the exhaust port of the power cylinder is exposed first, and then the scavenging port and the burnt gases inside the power cylinder are scavenged by the fresh charge, which previously was compressed in the crankcase compartment. As the piston ascends, both the exhaust and scavenge the piston and, simultaneously, the intake reed valve of the compression compartment closes, the delivery reed valve opens, and fresh charge flows from the compression compartment to the crankcase cover ports of the power cylinder. Just before top center (TDC), the fresh charge in the power cylinder is ignited, combustion occurs, and the piston is pushed down for the power stroke.

Many designs incorporating stepped pistons were proposed in the early days of the internal combustion engine. Few of these apparently ever reached production, largely because of limitations of contemporary engine technology. The British Dimelt motorcycle, produced from 1919 to 1930, employed a simple version of the stepped-piston engine to improve performance by increasing the displacement of the crankcase pump. This resulted in an excellent torque curve, for which these machines became well known, at the expense, however, of high fuel consumption. The Elmore car, produced in the United States with three or four cylinders from 1909 to 1913, employed another version of the stepped- piston engine. In this engine, an ordinary trunk piston was provided, at its lower

end, with a circular flange carrying piston rings. This flange made a running fit in an enlarged bore concentric with the working cylinder. An annular space was thus formed between the small-diameter portion of the piston and its large bore, where the charge was compressed. A more effective pump than the crankcase resulted, as its clearance volume could be made as small as desired. This principle may be applied to both multi-cylinder and single-cylinder engines alike. For multi-cylinder engines, the charge compressed in the annular space in one

cylinder during the down stroke of the piston is transferred to another cylinder in which the piston has simultaneously performed an upstroke.

For single-cylinder operation, the fresh charge is compressed into a receiver, where it is stored during the next down stroke until the piston opens the scavenging ports. For applications at high altitudes, it has been found that a problem with the use of a crankcase-scavenged two-stroke engine is the sharp decrease in engine power with increase in altitude. This was attributed not only to the low density of the ambient air, but also to deterioration of the efficiency of the gas exchange process due to the decrease in the delivery ratio. The main reason for the decrease in the delivery ratio at high altitudes is the inability of the crankcase volume to admit enough air when the pressure difference between the ambient and the crankcase volume is small. Increasing the compression ratio of the crankcase is one possible solution; however, the

fresh charge supply to the scavenge ducts will most likely take place during only a small part of the scavenging period and the scavenging process would probably be less efficient. Increasing the delivery ratio can easily be achieved with a stepped-piston engine.

Compared with the conventional crankcase-scavenged engine, the stepped- piston engine offers better scavenging, but higher pumping work, because both the compressor and the crankcase are required. If the increase in engine volume and weight are ignored, an optimal aspect ratio that gives the highest thermodynamic efficiency exists. The engine's bulk, however, is an important parameter that directly affects specific power, usually an important feature of the two-stroke engine. Adding a compressor with a high compression ratio at the inlet of the crankcase appears to be an attractive solution. This modification facilitates breathing at high altitude (higher pressure difference between ambient and the compressor compartment), does not require a high aspect ratio (low engine bulk), and allows slower fresh charge delivery to the power cylinder during the scavenging period.

Chapter 4

THE ENGINE CONCEPT AND ITS DESIGN

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