What is the function of the deflector crown on the two stroke engine?

Answer 1

As there is a positive mechanical device, in the shape of a deflector on the piston, to promote the fresh air and fuel charge flow from the scavenge ports towards the cylinder head and away from the exhaust ports; such an engine tends to have good scavenging characteristics and such engines have, in general, excellent low speed performance behaviour. The scavenging process of the deflector piston engine is known as cross-scavenging and these engines are often described as cross-scavenged engines as well as deflector piston engines. A conventional deflector piston engine is shown in FIG. 1 with the piston at top-dead-centre position with its deflector of height H1 above the timing edges of the piston, T E1 for the exhaust port or ports and T S1 for the scavenge port or ports SC1. The piston rings P1 are located as close to these timing edges as is practical to ensure a good cylinder gas seal particularly when the piston rings P1 have uncovered the exhaust port and before the timing edge T E1 does so releasing the burned air/fuel mixture or exhaust gas into the exhaust duct via the port or ports E1. Consequently the convenion design has the timing edges T E1 and TS 1 located at the same vertical physical position on the piston by design and as close to the piston rings P1 as practical considerations permit. It is known that good scavenging is related, among other considerations, to deflector height H1 and in general, the taller deflector the better is the scavenging process. This produces undesirable side effects for the design of the combustion chamber. Whatever the height of deflector, because the timing edges T E1 and T S1 are at the same vertical position on the piston and separated by the deflector then, almost by definition, the combustion chamber becomes a divided chamber and cannot be described as a compact combustion chamber. In practice this produces a slow combustion process which results in inferior performance characteristics for the engine. Compounding this problem is the intrusion of the deflector into the combustion space with the consequence that it is difficult to effectively cool or transfer the heat away from the deflector to the body of the piston. Thus the deflector tends to run hot, to promote detonation in the best case and pre-ignition in worse circumstances. This again tends to limit the use of high compression ratios for the conventional engine which limits the theoretical, and experimental, efficiency of the power unit by comparison with engines where such effects do not occur. Often the deflector is so hot during the combustion process that more fuel than is normal must be introduced so as to cool the deflector and reduce the possibility of detonation or pre-ignition; this further reduces the power output and engine efficiency. Another important aim of the designers of combustion chambers is the generation of squish between the piston and the cylinder head to promote gas turbulence. This leads to faster flame propagation speeds in the combustion chamber, more rapid rates of pressure rise and shorter combustion times hence both theoretically, and experimentally, producing higher output and increased engine efficiency. To obtain this effect the designer must ensure that a considerable area of the piston surface mates closely with the cylinder head at the top-dead centre position and produces a high speed jet of turbulent fresh charge, designated S1 in FIG. 1, directed towards the flame and combustion created by the ignition I 1 at the spark-plug X 1 . In the case of the conventional deflector piston-engine this is quite difficult to achieve in practice because of the often complex shape of deflector required for good scavenging characteristics and because of the manufacturing consideration of often having cast but not machined piston crowns and cylinder heads. While this close mating can be obtained, albeit with difficulty, in single cylinder experimental engines by hand finishing; it is very difficult to achieve in the mass production of multi-cylinder power units. Consequently, in the case of the conventional deflector piston, the combustion process is characterised by poor squish, slow combustion rates, hot piston crowns, incipient detonation or pre-ignition and a necessity for fuel cooling all of which results in inferior power and thermal efficiency characteristics by comparison with two-stroke cycle engines with compact combustion chambers and good squish characteristics. This has led to a decrease in popularity of the deflector piston engine design.