BOOST PORTS

CONFUSION EXISTS AS to the nature of boost ports and how they work, arising from the review of my 10 c.c. engine in the May, 1962, issue of AEROMODELLER.

There are two forms of port now in use, loosely termed "boost port". One is the form of port positioned diametrically opposite the exhaust port in Schnürle system engines (Fig. 1) whose function is to utilize the gases under the piston crown and so increase scavenging efficiency. The timing was originally rather "short" in relation to main transfer port timing, but subsequent development has enlarged this port, with increased understanding of its potential, until its chordal width is now governed solely by the room available before the positioning of the main transfer ports is detrimentally effected. In the same manner, the port height has increased so as to equal in some cases the height of the main transfer port. A second point with this form of port is that the thermal stresses on the piston are considerably reduced, due to the circulation of cool charge directly under the piston crown.

The apparent upper dimension of the port is not necessarily its true time-area, as for reasons of mechanical strength, it is impossible to cut a relating window of such chordal width in the upper regions of the piston. Therefore the piston port is normally narrow and deep, which naturally modifies the flow characteristics of the port. (Fig. 3).

In fact it is worth experimenting with the shape of this piston window to find the optimum performance of the port as a whole. The manner in which this is done depends on the concept of the designer.

It has been used with outstanding success by the M.Z. designer, Walter Kaaden, who obtained a 20 per cent. power increase by combining this port with the standard Schnürle system.

This form of port is not really new. Its use dates back to pre-War derivation from the Schnürle patent, which was used by Zundapp motorcycles of the period.

Description of my 10 c.c. engine in the May issue aroused considerable controversy and I ought to make it clear, that the above is the type of port used.

The second type of port in use is the Ehrlich boost port, for which patent No. 824,673 was granted in 1956. It is the invention of Dr. J. Ehrlich, the well-known two-stroke designer. The object of this port is identical to the type just described-i.e., to increase scavenge efficiency-but the principle is entirely different. These two ports are situated between the normal Schnürle ports and the exhaust port, at a similar angle to the main transfer ports. (Fig. 2.)

The operation of the port is as follows: near the maximum crankcase pressure for the desired operating r.p.m., two windows set high in the piston skirt uncover and charge the boost ports shown in Fig. 4. The piston continues its downward travel and the remaining charge in the crankcase is transferred at low velocity into the cylinder via the main transfer ports. Shortly after this main action, the boost ports open and pass their charge at high velocity into the cylinder. This gives in effect a two-stage transfer system which, with the development work carried out by Dr. Ehrlich, has been made to yield an impressive power curve which is much "fuller" than that usually associated with two-stroke engines of that power output.

As can be seen, the idea is capable of many variations. As regards dimensions, their limit of chordal width is dictated by the need to avoid upsetting the main transfer port positioning, and the excessive proximity to the exhaust port which, apart from raising the temperature of the stored charge, will lead to undue charge loss or pollution.

No information is available on the volume of these ports, but I would "guesstimate" that they represent a potential 15-20 per cent. of the total transferred charge.

As to height of the ports in relation to the main transfer ports, I should imagine their value to be approximately 70-80 per cent. of the latter.

Comparing the two systems just described, we see that, in terms of positioning as regards directional gas flow, the Ehrlich system would seem to have a better potential. The other system can tend, if not very carefully designed and timed, to reduce effective filling by pushing the normal Schnürle loop nearer the exhaust port.

In conclusion it is essential to realize that both forms of port are, as their name implies, secondary in importance to the normal porting used in modem racing two-strokes. Correct application can yield great improvements in power and power characteristics, and I feel boost ports have great potential in the future development of the two-stroke engine.