Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
The deflector chamber, type (d)
The calculation for the squish flow area, Asq, is as follows. The flow will be along the
axis of the cylinder and across the width of the deflector, and if the flow is considered to take
place midway through the incremental step, then:
— Y .I Y J.
A„„ = xd xs + ' - I (4.5.11)
"sq
where xj is the deflector width.
It is clear that even for equal values of csq, higher squish velocities will be given for those
chambers with more offset.
It is also possible to determine the turbulence energy induced by this flow, on the assumption
that it is related to the kinetic energy created. The incremental kinetic energy value at
each time step is dKESq, where:
2
dKEsq=dmsq^- (4.5.12)
The total value of turbulence kinetic energy squished, KEsq, is then summed for all of the
calculation increments over the compression process from trapping to tdc.
4.5.2 Evaluation of squish velocity by computer
The equations in Sec. 4.5.1 are programmed in Prog.4.1, SQUISH VELOCITY, available
from SAE. All of the combustion chamber types shown in Fig. 4.13 can be handled by this
program. You are prompted to type in the chamber type by name, either "central," "offset,"
"total offset," or "deflector." Of course, there will appear a design for a combustion chamber
which is not central, yet is not sufficiently offset as to be described by category (b) in Fig.
4.13. Such a chamber would be one where there is still a significant radial clearance from the
bore edge so that the squish flow can proceed in a radial fashion around the periphery of the
bowl. In that case, and it is one requiring user judgment, the "chamber type?" prompt from the
computer program should be answered with "central."
A typical output from the program is shown in Fig. 4.14, where all of the relevant data
input values are observed, with the output values for maximum squish velocity and maximum
squish pressure ratio at the crank angle position determined for these maxima, and the total
kinetic energy which has been squished. The screen picture is dynamic, i.e., the piston moves
from trapping to tdc, and the squish velocity graph is also dynamically created. By such
means the operator obtains an enhanced design feel for the effect of squish action, for it is
upon such insight that real design experience is built.
The sketch of the engine, cylinder, and cylinder head which appears on the screen is
drawn to scale, although the head is drawn as a "bathtub" type purely for ease of presentation.
Thus, the physical changes incurred by altering bore-stroke ratio, squish area ratio, squish
clearance, exhaust port timing and trapped compression ratio are immediately obvious to the
designer at the same time as the squish velocity characteristics.
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