The absence of the separate induction and exhaust strokes in the two stroke is the fundamental difference from four stroke engines. In two-stroke engines, the gas exchange or scavenging is the simultaneous emptying of the burned gases and the filling with the fresh air or air/fuel mixture.

Ideally, the fresh charge would solely displace the burned gases, but in practice there is some mixing. There are many different scavenge arrangements, and some of the more common systems are shown in fig 1.4.

fig 1.4: the different scavenging arrangements and the associated geometry for two stroke engines.

(a)Cross-scavenging; (b) loop scavenging; (c) Schnurle loop scavenging; (d) uniflow scavenging with poppet exhaust values; (e) uniflow scavenging with opposed pistons

With the cross scavenge arrangement fig 1.4a the charge could flow directly into the exhaust system, but this tendency was reduced by the deflector on the piston.

The troublesome deflector on the piston was avoided by the loop scavenging system (fig 1.4b) with the exhaust ports above the inlet ports. The incoming air or mixture is directed towards the unported cylinder wall, where it is deflected upwards by the cylinder wall and piston.

A modiefied form of loop scavenging was devised by Schnurle in 1920(fig 1.4 c), in which two pairs of inlet ports are located symmetrically around the exhaust ports. With this system, the flow forms a ‘U’ shaped loop.

The final two types of scavening system both employ a uniflow system, either by means of exhaust calves (fig 1.4d), or with an opposed piston arrangement (fig 1.4e). Both these systems are particularly sited to Diesel engines, since the inlet ports can be arranged to generate swirl. Swirl is very important in promoting effective Diesel engine combustion, yet if swirl was used with either cross or loop scavenging, then there would be significant mixing of the inlet flow with the burned gases.