Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
UNIFLOW scavenging where the brake specific hydrocarbon emission is still very high at
92.3 g/kWh. By comparison with a small industrial four-stroke cycle engine this remains
inadequately excessive, for such an engine will typically have a bsHC emission of between
15 and 30 g/kWh; however, in some mitigation. Many small four-stroke industrial engines
have bsCO emission levels exceeding 200 g/kWh to help reduce the NOx emission levels by
running rich, whereas this chainsaw simulation shows its bsCO level to be possible at 25
g/kWh by operating close to the stoichiometric air-to-fuel ratio.
7.3.2 The effect of air-fuel ratio
It can be seen from the measured or calculated data in Figs. 7.3-7.18 that optimizing the
air-to-fuel ratio means that it should be at one of two levels. If peak power and torque is the
design aim then an equivalence ratio, A,, of 0.85 will provide that requirement. If the minimum
emissions and fuel consumption are needed then optimization at, or close to, an equivalence
ratio, X, of unity is essential. For the simple two-stroke engine of conventional design that
will almost certainly not be good enough to satisfy current or envisaged legislation.
The most important message to the designer is the vital importance of having the fuel
metered to the engine in the correct proportions with the air at every speed and load. There are
at least as large variations of bsfc and bmep with inaccurate fuel metering as there is in
allowing the engine to be designed and manufactured with bad scavenging.
There is a tendency in the industry for management to insist that a cheap carburetor be
installed on a simple two-stroke engine, simply because it is a cheap engine to manufacture. It
is quite ironic that the same management will often take an opposite view for a four-stroke
model within their product range, and for the reverse reason!
7.3.3 The effect of optimization at a reduced delivery ratio
It is clearly seen from Figs. 7.3-7.18 that a reduction of delivery ratio naturally reduces
the power and torque output, but also very significantly reduces the fuel consumption and
hydrocarbon emissions of the engine. The reason is obvious from Chapter 3—a reduction of
scavenge ratio for any scavenging system raises the trapping efficiency.
Hence, at the design stage, serious consideration can be given to the option of using an
engine with a larger swept volume and optimizing the entire porting and inlet system to operate
with a lower delivery ratio to attain a more modest bmep at the design speed. The target
power is then attained by employing a larger engine swept volume. In this manner, with an
optimized scavenging and air-flow characteristic, the lowest fuel consumption and exhaust
emissions will be attained at the design point.
The selection of the scavenging characteristic for such an approach is absolutely critical.
The design aim is to approach a trapping efficiency of unity over the operational range of
scavenge ratios. The candidate systems which could accomplish this are illustrated in Fig.
3.13. There are three scavenging systems which have a trapping efficiency of unity up to a
scavenge ratio of 0.5. They are UNIFLOW, QUBCR and GPBDEF. The uniflow system can
be rejected on the grounds that it is unlikely to be accommodated into a simple two-stroke
engine. The remaining two are cross-scavenged engines, and the GPBDEF design is the better
of these in that it has a trapping efficiency of unity up to a scavenge ratio (by volume) of 0.6.
The physical arrangement of this porting is shown in Fig. 3.32(b).
486