Design and Simulation of Two Stroke Engines

nilar,
is the
'S S^
stein
ifold
;ions
mm
*oc
Chapter 5 - Computer Modeling of Engines
breathing in most two-stroke engines, simply to acquire the performance enhancement from
exhaust tuning, with the more peculiar needs of high-speed racing engines being the exception
to this comment.
There comes a point in the design process for any multi-cylinder engines when consideration
should always be given to a four-cylinder engine, with firing intervals of 90°, and employing
lower exhaust port timings with a compact manifold, as shown in Fig. 5.8(c). Its
possible adoption depends on many factors, and while for many engines the three-cylinder
option remains the optimum, no designer can afford to blindly accept "custom and practice"
in one field as being universally applicable in all circumstances.
A four-cylinder engine
To illustrate the points made above regarding the effectiveness of a four-cylinder engine
optimally designed with the correct port timings, port widths and areas, together with a compact
manifold, a four-cylinder engine with the same bore and stroke is simulated at the same
engine speed point, 3500 rpm. The bore and stroke are 90 and 82 mm, respectively, but the
engine capacity is now reduced to 2100 cm 3 . The same Roots supercharger is employed
within the simulation, but downsized by 33%. The exhaust and inlet opening timings are at
108° and 132° atdc, respectively, and there are twelve ports around the cylinder, as in Fig.
3.41, four of which are exhaust ports and eight of which are scavenge ports. The piston rings
are unpegged. The four exhaust ports are each of 19.8 mm effective width and the eight
transfer ports are each of 16.2 mm effective width. The scavenging is simulated as being of
YAM1 quality (see Fig. 3.16) as in the three-cylinder engine simulation. The very considerable
increase in the effective width of the exhaust and scavenge ports, over that in the threecylinder
engine example, is clearly evident. The exhaust manifold corresponds to the sketch
in Fig. 5.8(c). The lengths of the various limbs are denoted as Li to L$ and they are 200, 100,
100,200,50 and 50 mm, respectively. The diameters of the various limbs are denoted as dj to
d-] and they are each set at 48 mm.
All other factors for this engine are as for the three-cylinder engine simulation, i.e., trapped
compression ratio, air-to-fuel ratio, intercooler or combustion characteristics, or engine and
duct surface temperatures, etc.
While the main point of the simulation is to illustrate the effectiveness of the trapping and
charging characteristics of a four-cylinder engine, a summary of the ensuing performance
characteristics is very relevant for direct comparison with the three-cylinder engine simulation.
The bmep is 7.9 bar, bsfc is 268 g/kWh, DR is 0.839, SE is 0.813, TE is 0.843, CE is
0.712, and T|m is 0.87. By comparison with the three-cylinder engine, it will be observed that
the attained bmep and supplied air flow rate are higher, and the trapping and charging efficiencies
are superior, while the specific fuel consumption and mechanical efficiency are somewhat
inferior. The latter effects are due to the greater pumping loss from a blower which is
operating at a higher pressure level to deliver the same air flow rate, as can be seen in Figs.
5.38 and 5.39.
The exhaust tuning of the four-cylinder engine
The cylinder firing order is 1-3-4-2. This means that the exhaust pulse from cylinder
number 3 is that which cross-charges cylinder number 1, and that from cylinder number 1
407