Evo

IjO TDC2 Ej/C

BDC2 IVC a angle

Angle overlap

Figure 1.17 (a) Gas exchange four-stroke (Benson and Whitehouse)

when the piston is at or near the TDC position, is cleared, as far as possible, of the remaining products of combustion, by displacement of these 'residuals' by the incoming air. This period varies from very small values (< 40° CA) for road transport engines to substantial values (=t 120° CA) in medium speed highly turbocharged engines.

4-5: Induction (EVC to BDC2)

In naturally aspirated engines most of this period during which the fresh air charge is drawn into the cylinder, takes place under conditions of subatmospheric pressure, due to the pressure drop across the inlet valve. This pressure drop tends to reach a maximum value near mid stroke, with pressure returning to near atmospheric towards (BDC)2.

During this final period which is usually quite short, a small degree of pre-compression of the air charge takes place with some resultant increase in cylinder pressure before the inlet valve finally closes.

The sequence of events described above constitutes the 'pumping loop', as is obvious with reference to the pressure volume diagram, Figure 1.6b, which was briefly discussed in section 1.2.3 in connection with the evaluation of indicated work. In naturally aspirated engines, due to the excess of cylinder pressure during the exhaust stroke over that during the induction stroke, this pumping loop is invariably associated with negative work, while under favourable conditions in turbocharged engines it may make a positive contribution to cycle work.

1.5.2.2 Two-stroke engines (Figure 1.17b)

The two-stroke open period process has been briefly discussed already in section 1.2.3. Only two-stroke engines with asymmetrical timing will be considered; to avoid confusion an exhaust valve-in-head, inlet ported engine (section 1.2.1.2) is chosen. Unlike the four-stroke engine which depends largely on piston displacement action to effect the gas exchange process, the two-stroke engine receives no such assistance from the motion of the piston and instead has to rely on a positive pressure difference between inlet and exhaust manifold to force fresh air into the cylinder through the inlet valve or ports while simultaneously products of combustion escape through the exhaust valve or ports. The time available for the gas exchange process is greatly

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