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9.1 Introduction

Since air breathing capacity and utilization determine the output of the diesel engine, the flow characteristics of the intake and exhaust systems are crucial in the achievement of good performance. In addition, the amplitude of the air pressure pulsations of the reciprocating engine must be attenuated in order to achieve reasonable levels of intake and exhaust noise.

The design and development of effective intake and exhaust systems remains an engineering challenge. Routinely intake and exhaust external flow systems are now designed and optimized using various design analysis software systems. These are based on various mathematical approaches, including complete one-dimensional solution of the wave equation for engine simulation. Complete three-dimensional calculations (computational fluid dynamics solutions) are used locally for optimization of elements such as exhaust downpipes, catalyst connections and also to determine optimum designs for internal components such as ports.

The intake ports of a diesel engine may have to generate air motion ('swirl') in the cylinder so that the air charge retains significant angular motion when the fuel is later injected, improving air-fuel mixing. Most engines rely on some swirl but modern high-speed direct injection engines require only low levels of swirl, much of the air-fuel mixing energy coming from the fuel spray.

Volumetric efficiency is determined by the complete flow system calling for care in selection and application of all components including intake and exhaust silencers.

Four-stroke engines have employed sleeve valves and rotary valves but it is the poppet valve that is most common. Two-stroke engines use a variety of airflow controls mostly piston-covered ports for scavenging, with valve-actuated exhaust systems in higher ratings.

This chapter covers mainly the design of poppet valve systems and piston-covered ports for two-stroke engines. Many of the design calculations covered here are often computed today, but the fundamentals remain unchanged. A section on factors affecting silencer design and selection is included.

9.2 Gas flow

Assuming an incompressible fluid the fundamental equation for fluid flow, derived from Bernoulli's equation is v = cJWi (9.1)

where v = Velocity of flow in m/sec; c = Coefficient of discharge; g = Gravitation acceleration = 9.80665 m/sec2; h = Head of fluid causing the flow in metres.

Further if

W = Mass of fluid flowing kg/sec; A = Area through which the fluid flows (m2); p = Density of the flowing fluid (kg/m3).

For the present discussion the fluids involved in measurements are air, water, mercury. For these the densities at room temperature taken to be 20°C (293 K) are:

Water

Mercury

1.205 kg/m3 998.2 kg/m3 13554 kg/m3

the pressure, or in some cases the pressure drop across an orifice, against a head of water, sometimes mercury for larger pressures, contained in a manometer. Since pressure = ph h ■ = h "air 'l\

0 0