Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
The front of all pressure waves travels at the local speed of sound, ao, which from Eq.
2.1.1 is:
a o = ^YR(Texc + 273) m/s (2.1.1)
For engines below 5 bar bmep the value of Texc will be in the region of 350°-450°C, and
R and yean be estimated to be as for air at atmospheric conditions, i.e., 287 J/kgK and 1.4,
respectively.
Although the exhaust pulse peak will propagate at supersonic velocity, the suction reflection
will travel at subsonic speed and the average can be approximated to the local speed of
sound (see Sec. 2.1.3). If you assume that the exhaust pulse peak occurs at 15° after exhaust
port opening, you would wish the peak of any suction reflection to return at or about the bde
piston position. This gives a reflection period in degrees crankshaft of 0^,:
6rp=^ L -15 (6.2.1)
The time, t^, taken for this double length travel along the pipe, Lj, must equal the reflection
period, where:
6rp 60 2Lj
T ~ 71^ X ~ T^Ti (6.2.2)
360 rpm 1000a0
Incorporating Eq. 6.2.1 and rearranging Eq. 6.2.2 to obtain the pipe length, Li, which is in
mm units:
=
41^.(6,-30)
If the appropriate numbers are inserted into this equation for a chainsaw at 8000 rpm,
with an assumed exhaust gas mean temperature of 400°C, and an exhaust period of 160°
crankshaft, then Li is derived as 352 mm. As there is clearly no possibility of installing a 352mm-long
downpipe on a compact chainsaw, the designer is left to dimension a silencer box
which will merely have the least possible restrictive effect on gas flow from the engine, while
still silencing it to the legislative requirement. This effect is abundantly evident from the
discussion in Chapter 5. Fortunately the two aims are not mutually incompatible, as the largest
possible box can be ingeniously designed into the available space so as to give the best
power output and the best silencing effect. See Chapter 8 for an expanded discussion on noise
and silencing.
The basic rule of thumb for simple exhaust silencer boxes, be they a single box, or the
double box design as seen in Fig. 5.6, is to set the total box volume to at least ten swept
volumes and the final outlet pipe to about 50% of the area of the exhaust port.
436
rpm