The Charge Cycle In Two-Stroke Engines

Scavenging. The characteristic feature of different two-stroke engine designs is the respective type of cylinder scavenging and the related type of scavenging air supply. The selected scavenging approach greatly influences the complexity of the design, the component load, operating behavior, air/gas mixing conditions, fuel consumption, and the emissions of the engine.

When the cylinder is scavenged, the combusted mixture is displaced from the cylinder by fresh gas, without mutual mixing in the ideal exception of displacement scavenging. In contrast, when a cylinder is scavenged in a real engine, a mixture of fresh gas and exhaust occurs in addition to the displacement of the exhaust. As schematically illustrated in Fig. 10-55 (especially when there is a great deal of scavenging air such as at high load map points), a part of the scavenging gas mixed with the exhaust is expelled from the cylinder (loss of fresh gas). To evaluate the results or the efficiency of the scavenging procedure in two-stroke engines, the retention rate or air expenditure is used as an index in addition to volumetric efficiency.

Fig. 10-55. Mass balance of the two-cycle scavenging process according

Figure 10-56 shows an overview of the most important two-cycle scavenging procedures with methodically related advantages and disadvantages.

Fig. 10-56. Comparison of different scavenging approaches

Loop scavenging: With loop scavenging (according to Schnurle), the fresh gas passes into the cylinder generally through two to six scavenging channels (overflow channels) that symmetrically mirror each other across the midaxis of the exhaust port and run in the opposite direction of the exiting exhaust. The scavenging streams align with each other and form an increasing stream of fresh gas on the side of the cylinder opposite the exhaust port. The fresh gas stream reverses direction at the cylinder head and expels the exhaust from the cylinder.

This type of scavenging that is particularly widespread in small engines and is suitable for high rpms is easy to design and results in compact engine dimensions. With direct injection (DI) diesel engines, the combustion chamber recess can be located in the cylinder head where it is well cooled. Disadvantages are the asymmetrical thermal load on the piston, the endangerment of the piston rings from scavenging and exhaust ports, and the fact that the oil consumption is difficult to control when pressure circulation lubrication is used. In addition, other technical measures are required to create the conventional charge turbulence for DI diesel engines and to create an asymmetrical control diagram.

Uniflow scavenging: With uniflow scavenging, the fresh gas passes into the cylinder through intake ports in the perimeter of the cylinder and displaces the exhaust through several exhaust valves in the cylinder head that are controlled with the crankshaft speed. A tangential arrangement of the scavenging channels makes it comparatively easy to generate and influence the turbulence that supports the mixture. This turbulence generally lasts the entire work cycle while attenuating, and it does not have to be completely regenerated in the following scavenging cycle.

The advantages of uniflow scavenging are that it is comparatively effective (low air expenditure), asymmetrical timing can be achieved without additional constructive measures, and tried-and-true DI diesel combustion methods for four-stroke engines can be transferred largely unchanged to two-stroke engines. In contrast to loop scavenging, it is comparatively easy for the piston rings to freely rotate given a corresponding scavenging port design that increases their life.

The overall height of a cylinder head with valves yields a taller engine compared to similar four-stroke engines, especially with oversquare stroke-to-bore ratios, since the scavenging ports are covered by the piston shaft and a collision of the connecting rod with the piston shaft must be excluded in the design. In addition, there are substantial demands on the design of the exhaust valve drive because of the double valve actuation frequency and the limited valve opening (crank) angle with the simultaneous requirement for large opening time cross sections.

Opposed piston uniflow scavenging: With opposed piston uniflow scavenging, two pistons move in the opposite direction in one cylinder, and their inner end position encloses the combustion chamber (TDC position).

In their outer end position (BDC position), one of the pistons opens the intake ports, and the other piston opens the exhaust ports so that the inflowing fresh gas expels the exhaust from the cylinder with the main direction of flow along the cylinder axis. The advantages are effective scavenging, minimization of the combustion chamber surface heated in the high pressure phase, and easily realizable asymmetrical timing. Serious disadvantages of this approach result from the complex construction, bulky engine dimensions, extreme thermal load on the exhaust-side piston, and the limited transferability of the combustion process to modem four-stroke engines.

Reversed head scavenging: With reversed head scavenging, the fresh gas generally flows through at least two or three valves actuated at crankshaft speed at BDC into the cylinder and displaces the exhaust from the cylinder through the simultaneously opened exhaust valves supported by a reversal of direction at the piston floor. The advantage of this type of scavenging is the design of the engine-transmission unit that largely corresponds to that of a comparable four-stroke engine. Furthermore, the absence of scavenging in the exhaust ports reduces the hazard to the piston rings.

These advantages contrast with the great disadvantage that intake and exhaust valves must be located on the limited combustion chamber surface of the cylinder head. In contrast, for a comparable two-stroke engine with uniflow scavenging and, for example, with four exhaust valves, a basic approximation indicates that the available opening time cross sections are cut in half. At the same time, a great deal more scavenging air is required to introduce the same amount of fresh gas in the cylinder because of the less effective scavenging (mixture of fresh gas and exhaust from turbulence and contact of a large surface area of the gas stream) in reversed head scavenging. For this reason, the required charge cycle work and the resulting specific fuel consumption lies only within an acceptable range at low engine speeds.

These restrictions of the nominal speed and consumption run counter to the requirements of designs of drives for future passenger cars. Apart from that, short-stroke, reversed head two-cycle diesel engines and possibly two-cycle spark ignition engines hold promise for low-speed airplanes (no intermediate transmission, high propeller efficiency).

We refrain from discussing other types of scavenging such as cross scavenging, fountain scavenging, reverse MAN scavenging, and the various dual-piston scavenging approaches because of their limited efficiency, complicated design, or other disadvantages.