Design and Simulation of Two Stroke Engines

Chapter 2 - Gas Flow through Two-Stroke Engines
blunt truth is that he is "whistling in the wind." For those who are my contemporaries in this
technology, let them be assured that I readily admit to being as guilty of perfidy in my technical
publications as they are. It was this "guilt complex" which led to the design of the QUB SP
apparatus. The nomenclature "SP" stands for "single pulse."
The criteria for the design of the apparatus are straightforward:
(i) Base it on the assumption that a fast-response pressure transducer is the only accurate
experimental tool readily available,
(ii) The pipe(s) attached to the device must be sufficiently long as to permit visibility
of a pressure wave traveling left or right without undergoing superposition in the
plane of the transducer while recording some particular phenomenon of interest,
(iii) The cylinder of the device must be capable of having the mass and purity of its
contents recorded with absolute accuracy,
(iv) The cylinder of the device must be capable of containing any gas desired, and at a
wide variety of state conditions, prior to the commencement of the experiment
which could be the simulation of either an exhaust or an induction process,
(v) The pipes attached to the cylinder must be capable of containing any gas desired,
over a wide variety of state conditions, prior to the commencement of the experiment
which could be the simulation of either an exhaust or an induction process,
(vi) The pipes attached to the device must be capable of holding any of the discontinuities
known in engine technology, i.e., diffusers, cones, bends, branches, sudden expansions
and contractions and restrictions, throttles, carburetors, catalysts, silencers,
air filters, poppet valves, etc.
These design criteria translate into the single pulse device shown in Fig. 2.26. The cylinder
is a rigid cast iron container of 912 cm 3 volume and can contain gas up to a pressure of 10
bar and a temperature of 500°C. The gas can be heated electrically (H). The cylinder has
valves, V, which permit the charging of gas into the cylinder at sub-atmospheric or supraatmospheric
pressure conditions. The port, P, at the cylinder has a 25-mm-diameter hole that
mates with a 25-mm-diameter aluminum exhaust pipe. The valve mechanism, S, is a flat
polished nickel-steel plate with a 25-mm-diameter hole that mates perfectly with the port and
pipe at maximum opening. It is actuated by a pneumatic impact cylinder and its movement is
recorded by an attached 2-mm pitch comb sensed by an infrared source and integral photodetector.
Upon impact the valve slider opens the port from a "perfect" sealing of the cylinder,
gas flows from (or into in an induction process) the cylinder and seals it again upon the
conclusion of its passing. A damper, D, decelerates the valve to rest after the port has already
been closed. An exhaust pulse generated in this manner is typical in time and amplitude of,
say, a two-stroke engine at 3000 rpm. A typical port opening lasts for 0.008 second.
The cylinder gas properties of purity, pressure and temperature are known at commencement
and upon conclusion of an event; in the case of purity, by chemical analysis through a
valve, V, if necessary. The pressure is recorded by a fast-response pressure transducer. The
temperature is known at commencement and at conclusion, without concern about the time
response of that transducer, so that the absolute determination of cylinder mass can be conducted
accurately.
The coefficients of discharge, Cd, of the cylinder port, under wide variations of cylinderto-pipe
pressure ratio giving rise to inflow or outflow and valve opening as port-pipe area
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