Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
levels at full load published by Batoni [7.1, Fig. 8] where the value at 4 bar and 3000 rpm is
shown as 7%. In other words, at that point it is almost certain that some stratified combustion
is occurring.
This engine provides an excellent example of the benefits of stratified charging. It also
provides a good example of the mechanical disadvantages which may accrue from its implementation.
This design, shown in Fig. 7.34, is obviously somewhat bulky, indeed it would be
much bulkier than an equivalent displacement four-stroke cycle engine. Hence, one of the
basic advantages of the two-stroke engine is lost by this particular mechanical layout. An
advantage of this mechanical configuration, particularly in a single-cylinder format, is the
improved primary vibration balancing of the engine due to the opposed piston layout.
Nevertheless, a fundamental thermodynamic and gas-dynamic postulation is verified from
these experimental data, namely that stratified charging of a two-stroke engine is a viable and
sound approach to the elimination of much of the excessive fuel consumption and raw hydrocarbon
emission from a two-stroke engine.
The Ishihara option for stratified charging
The fundamental principle of stratified charging has been described above, but other
researchers have striven to emulate the process with either less physical bulk or less mechanical
complication than that exhibited by the Piaggio device.
One such engine is the double piston device, an extension of the original split-single Puch
engines of the 1950s. Such an engine has been investigated by Ishihara [7.7]. Most of these
engines are designed in the same fashion as shown in Fig. 7.38. Instead of the cylinders being
placed in opposition as in the Piaggio design, they are configured in parallel. This has the
advantage of having the same bulk as a conventional twin-cylinder engine, but the disadvantage
of having the same (probably worse!) vibration characteristics as a single-cylinder engine
of the same total swept volume. The stratified charging is at least as effective as in the
Piaggio design, but the combustion chamber being split over two cylinder bores lends itself
more to stratified burning than homogeneous burning. This is not necessarily a criticism.
However, it is clear that it is essential to have the cylinders as close together as possible,
and this introduces the weak point of all similar designs or devices. The thermal loading
between the cylinder bores becomes somewhat excessive if a reasonably high specific power
output is to be attained.
Another design worthy of mention and study, which has considerable applicability for
such designs where the cost and complexity increase cannot be excessive due to marketing
and packaging requirements, is that published in the technical paper by Kuntscher [7.23].
This design for a stratified charging system has the ability to reduce the raw hydrocarbon
emission and fuel consumption from such engines as those fitted in chainsaws, mopeds, and
small motorcycles.
The stratified charging engine from the Institut Francais du Petrole
This approach to stratified charging emanates from IFP and is probably the most significant
yet proposed. The performance results are superior in most regards to four-stroke cycle
engines, as is evident from the technical paper presented by Duret et al. [7.18]. The fundamental
principle of operation is described in detail in that publication, a sketch of the engine
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