Scavenging the two-stroke engine

Modeling the Scavenging Process in the Two-Stroke Engine-An Overview

Prediction of the Gas Exchange Performance in a Two-Stroke Cycle Enginep

The primary concern associated with a two-stroke engine performance is its gas exchange
process (scavenging process). The success of the scavenging process greatly affects the
thermodynamic properties of cylinder content at the trapping conditions and hence the
combustion and power output. The unsteady gas flow in engine pipes has a marked influence
on the scavenging process.
In the current study, the two-stroke engine simulation model has been developed to predict
the steady state performance characteristics of a crankcase compressed, piston port-timed,
two-stroke engine equipped with expansion chamber. The characteristics include parameters
such as engine torque and power, BSFC, scavenging efficiency and charging efficiency. The
model also has the ability to predict the unsteady gas dynamic behaviour in various engine
pipes. The instantaneous pressure fluctuation and mass flow rates at inlet port, transfer port
and exhaust port were calculated and analysed.
The model has several advantages compared with other one-dimensional isentropic model.
It includes an improved procedure to account for the variation in geometry of pipe and to
determine the thermodynamic states in cylinder/crankcase. The model also considered the
temperature discontinuity at port/pipe interface. All these efforts increase the accuracy and
numerical stability of the prediction.
A single cylinder two-stroke engine dynamometer rig and dedicated fast data acquisition
hardware and software have been developed in the project. Engine torque, speed and fuel
consumption can be measured on the dynamometer rig. The dynamic pressure signals in
engine cylinder, transfer port and exhaust port, together with crankshaft position signal, can
be acquired at a speed of 50,000 sample/second per channel, sufficient for accurate
acquisition of experimental data.
Substantial simulations and experiments were performed and the computer model was
validated. A variable exhaust system, as described in this study, was used to improve a twostroke
engine's performance under off-design engine speeds. This was realised by making the
mid-parallel section of a conventional expansion chamber extendable, while the other
dimensions of that chamber remained unchanged. In the study it was found that with the
adjustable expansion chamber, the pressure wave timing at the exhaust port was under control
within the test speed range (2200 - 4400 RPM) so that such wave timing matched the engine
port timing, leading to a relative optimum in performance. The numerical approach proposed
could satisfactorily handle the complicated pipe flow calculation and predict both engine
performance and dynamic wave variations in the test engine. Thus the program can be used
for improving the design and development of naturally aspired, port-controlled crankcase
compressed two-stroke engines.

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Modelling the scavenging process in a two-stroke i.c. engine

Analysis of Loop Scavenging Process in a Small Power SI Two-Stroke Engine

There has been few investigations of the relation between scavenging flow of a two-stroke cycle engine and its efficiency. To investigate the relation, the process in which the water in a transparent cylinder was scavenged by injection of a colored salt solution was photographed. By measuring the specific gravity of contents in the cylinder after scavenging, scavenging efficiency was calculated, and the results were compared with theoretical scavenging characteristic curves. These experiments were carried out on a two-dimensional scavenging flow in a model with a fixed piston and on a three-dimensional scavenging flow in a model with a movable piston. It is concluded that, (1) scavenging efficiency varies continuously with delivery ratio, (2) vortex motion of scavenging does not appear within the delivery ratios of engines which are in practical use, and (3) the deterioration of scavenging efficiency is caused mainly by the pure short-circuiting loss.

Takeshi Oka

Papers

Relation between Scavenging Flow and Its Efficiency of a Two-Stroke Cycle Engine