Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
1.4.2 Compression ratio
All compression ratio values are the ratio of the maximum volume in any chamber of an
engine to the minimum volume in that chamber. In the crankcase that ratio is known as the
crankcase compression ratio, CRCC, and is defined by:
V + V
CR = (1.4.3)
V,
cc
where Vcc is the crankcase clearance volume, or the crankcase volume at bdc.
While it is true that the higher this value becomes, the stronger is the crankcase pumping
action, the actual numerical value is greatly fixed by the engine geometry of bore, stroke, conrod
length and the interconnected value of flywheel diameter. In practical terms, it is rather
difficult to organize the CRCC value for a 50 cm 3 engine cylinder above 1.4 and almost physically
impossible to design a 500 cm 3 engine cylinder to have a value less than 1.55. Therefore,
for any given engine design the CRCC characteristic is more heavily influenced by the
choice of cylinder swept volume than by the designer. It then behooves the designer to tailor
the engine air-flow behavior around the crankcase pumping action, defined by the inherent
CRCC value emanating from the cylinder size in question. There is some freedom of design
action, and it is necessary for it to be taken in the correct direction.
In the cylinder shown in Fig. 4.2, if the clearance volume, Vcv, above the piston at tdc is
known, then the geometric compression ratio, CRg, is given by:
V + V
CR = sv cv (1.4.4)
V cv
Theoretically, the actual compression process occurs after the exhaust port is closed, and
the compression ratio after that point becomes the most important one in design terms. This is
called the trapped compression ratio. Because this is the case, in the literature for two-stroke
engines the words "compression ratio" are sometimes carelessly applied when the precise
term "trapped compression ratio" should be used. This is even more confusing because the
literature for four-stroke engines refers to the geometric compression ratio, but describes it
simply as the "compression ratio." The trapped compression ratio, CRt, is then calculated
from:
V, +V
CR = ts cv (1.4.5)
1.4.3 Piston position with respect to crankshaft angle
At any given crankshaft angle, 0, after tdc, the connecting rod centerline assumes an
angle, (|), to the cylinder centerline. This angle is often referred to in the literature as the
"angle of obliquity" of the connecting rod. This is illustrated in Fig. 1.10 and the piston
position of any point, X, on the piston from the tdc point is given by length H. The controlling
trigonometric equations are:
22
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