Air Flow in a Naturally Aspirated Two-stroke Engine

Abstract

The problem of scavenging of naturally aspirated, two-stroke cycle engines by utilization of wave effects in the exhaust and induction system is treated theoretically and investigated experimentally by systematic variation of the exhaust and induction system, as well as engine speed. In the theoretical treatment accuracy is achieved with a minimum of numerical computation by the adoption of the theory of waves of finite amplitude and the use of charts, wherever possible. These two features constitute the most significant departure from List's very thorough analysis (List 1949/50)‡, which is based throughout on the small-wave theory and in which little attention appears to have been paid to the simplification of numerical work. The theoretical section is divided into two parts dealing with (1) the analysis of cyclical pressure fluctuations in the engine cylinder and pipe system, and (2) the treatment of the gas-exchange process in the cylinder on the basis of ‘perfect mixing’ and the derivation of the conditions for dynamic similarity. A novel experimental technique is used, air from an external compressor being admitted to the cylinder of the motored engine through a rotary valve. In this manner release pressures comparable with those obtaining in the firing engine are attained, while the effects of high temperatures, which are of secondary importance in relation to wave phenomena, are eliminated. The experimental work is divided into two parts: (1) tests corroborating the wave theory; and (2) systematic air-consumption trials with variable induction-pipe lengths and exhaust systems, incorporating ( a) plain pipes, ( b) an exhaust ejector similar to that described in the Kadenacy patent specifications, ( c) an expansion chamber, and ( d) a diffuser. The general conclusions are: (1) good agreement between observations and analytical results is obtained, (2) high air-consumption is possible with natural aspiration provided that effective use is made of wave action by correctly designed exhaust and inlet systems; and (3) satisfactory scavenging is only achieved if not only the external pipe system but also the engine speed and geometry conform to certain requirements conveniently expressed in the form of dimensionless groups.