Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
2.18.12 The modeling of engines using the GPB finite system method
In the literature there has been reported correlation of measurement and this theory in
several international conferences and meetings. The engines modeled and compared with
experiments include two-stroke and four-stroke power units. One reference in particular [2.32]
describes the latest developments in the inclusion within the GPB modeling process of the
scavenging of a two-stroke engine. The present method employed for the closed cycle period
of the modeling process is as I describe [2.25, Chapter 4] using a rate of heat release approach
for the combustion period. The references list these publications [2.31, 2.32, 2.33, 2.34, 2.35,
2.40 and 2.41].
2.19 The correlation of the GPB finite system simulation with experiments
A theoretical simulation process in design engineering which has not been checked for
accuracy against relevant experiments is, depending on the purposes for which it is required,
at best potentially misleading and at worst potentially dangerous. In the technology allied to
the simulation of unsteady gas flow, many experiments have been carried out by the researchers
involved. Virtually every reference in the literature cited here carries evidence, relevant or
irrelevant, of experimentation designed to test the validity of the theories presented by their
author(s).
I am closely associated with a new series of experiments, reported by Kirkpatrick et al.
[2.41, 2.65, 2.66], designed specifically to test the validity of the theories of unsteady gas
flow, and in particular those presented here, and to compare and contrast the GPB finite
system simulation with both the experiments and with other simulation methods such as
Riemann characteristics, Lax-Wendroff, Harten-Lax-Leer, etc. The experimental apparatus
is quite unique and is detailed fully by Kirkpatrick [2.41]. It will be described here
sufficiently well so that the presentation of the experimental test results may be understood
fully. Although the main purpose is to determine the extent of the accuracy of the GPB
simulation method, the test method and the experimental results illustrate also many of the
contentions in the theory presented above.
2.19.1 The QUB SP (single pulse) unsteady gas flow experimental apparatus
Most experimenters and modelers in unsteady gas dynamics have correlated the measured
pressure-time diagrams in the ducts of engines, firing or motored, against their theoretical
contentions. As all unsteady gas flow within the ducts of engines is in a state of superposition,
this makes the process of correlation very difficult indeed. It is almost impossible to
tell which wave is traveling in which direction. While fast-response pressure transducers are
still the best experimental tool that the theoretician in this subject possesses, the simple truth
is that they are totally directionally insensitive. That much is manifestly clear in just about
every numerical example quoted up to this point in the text. Worse, the correlation of mass
flow is in an even more parlous state when working with engines, either motored or firing.
While the cylinder pressure may be recorded accurately, the density record in the same place
is non-existent since a temperature, or purity, or density transducer with a sufficiently fast
response has yet to be invented. Thus, while the experimenter may infer the mass of trapped
charge, or as a matter of even greater necessity the mass of trapped air charge, in the cylinder
from the overall engine air consumption and the cylinder pressure transducer record, the
170