Design and Simulation of Two Stroke Engines

INFLOW
mass dm|
enthalpy dH|
airdlli
Chapter 2 - Gas Flow through Two-Stroke Engines
CYLINDER
mass mc
energy Uc
purity nc
pressure Pc
temperature Tc
volume VQ
dVC
mass dm£
enthalpy dHg
airdllE
Fig. 2.25 The thermodynamics of open cycle flow through a cylinder.
toward the cylinder. All ports and valves experience backflow at some point during the open
cycle. The words "inflow" and "outflow" are, in the case of an engine cylinder, a convenient
method of denoting ports and valves whose nominal job is to supply air into the cylinder of an
engine. When dealing with an intake plenum, or an exhaust silencer box, "inflow" and "outflow"
become a directionality denoted at the whim of the modeler; but having made a decision
on the matter, the ensuing sign convention must be adhered to rigidly. The sign convention,
common in engineering thermodynamics, is that inflow is "positive" and that outflow is
also "positive"; this sign convention is employed not only in the theory below, but throughout
this text. Backflow, by definition, is opposite to that which is decreed as positive and is then
a negative quantity. The mesh computation must then reorient in sign terms the numerical
values for the right- and left-hand ends of pipes during inflow and outflow boundary calculations
as appropriate to those junctions defined as "inflow" or "outflow" at the cylinders or
plenums. While the computer software logic for this is trivial, care must be taken not to
confuse the thermodynamic needs of the mesh spaces defined in Sec. 2.18.9 with those of the
cylinder or plenum being examined here.
Heat transfer is defined as positive for heat added to a system and work out is also a
positive action. Employing this convention, the First Law of Thermodynamics reads as:
heat transfer + energy in = change of system state + energy out + work done
The entire computation at this stage makes the assumption that the previous application
of the boundary conditions, using the theory of Sees. 2.16 and 2.17, has produced the correct
values of the terms dmi, dHi, dPi, diriE, dHg and dP£. To reinforce an important point which
has been made before, during the application of the boundary conditions in the case of a
163