Search Results

The crankcase-scavenged two-stroke-cycle engine is preferred in cases where low weight and high power output are paramount requirements. These qualities are most important in small pilotless aircraft. It was found that the main problem in the use of two-stroke cycle engines for this purpose, is a sharp decrease in the engine power with the increase in altitude. This is attributed not only to the low density of the ambient air, but also to the deterioration of the efficiency of the gas exchange process. In order to improve the engine performance at high altitude, it is proposed here to employ a stepped-piston engine. The engine is constructed of a stepped piston and a single stepped cylinder thus forming three compartments; a power, a compression and a crankcase compartment. In this arrangement, the fresh charge is compressed in the compression compartment before it enters the crankcase compartment.

The relationship between the characteristics of a spray pattern that is generated by a simple external-mixing plain-jet atomizer, in pulse mode, and the control parameters of the injection system, has been investigated experimentally. The experimental conditions varied in the range of Re=1236 to 3540, We=65 to 595, and air/liquid velocity ratio from 15 to 110. The pulse duration varied between 3.4 and 18.5ms. It was found that, 1. the Sauter mean diameter is proportional to (Vf/Va3)0.5 over the entire range of operational conditions (within an error of ±7.5%), 2. a shorter pulse and a longer time interval between the pulses result in a smaller SMD, 3. the SMD may by decreased by some 25%, as compared to a continuous injection, when the pulse time duration reduces to 20% of the total cycle time, and 4. short time pulses and long interval between pulses, are always preferable to produce small droplets.

An improved mathematical model is presented here to simulate the gas exchange of a two-stroke, cross or loop-scavenged engine. In this model, the geometry of the cylinder and port assemblies, as well as the physical conditions of operation are fully represented. The basic model which was described by the present author in a former work, has been modified here to include the Woolley-Hatton model for turbulent flow. By comparing predictions from both models with experimental measurements, it seems that a significant improvement was achieved here in predicting the instantaneous interface profile between the incoming charge and the burnt gas.

A semi-empirical model is proposed to represent the scavenging process in cross, loop or uniflow scavenged engines. The model is based on the assumption that in most cases (if not in all) the time variation of the mass fraction of fresh air content in the gas passing through the exhaust port (β) exhibits an “S” type curve. An exponential function of the form of is suggested to fit this curve where the shape and the form factors may consider any combination of perfect displacement, perfect mixing and short-circuiting processes. The charging efficiency was then calculated and compared with other models - the isothermal mixing, the non-isothermal mixing, the Benson model and a detailed computer model. The new model has been found as a realistic model for modern engine design and an easy-to-use model for computer simulations.

A theoretical model to simulate the gas exchange process in a two-stroke, cross or loop-scavenged engine is presented. In this model, the geometry of the cylinder and port assemblies, as well as the physical conditions of operation are represented. The mathematical model consists of five conservation laws in quasi 3-dimensional form. These were transformed into a form which both allows the domain of solution always to be confined in the volume occupied by the gas and also fits the geometry of the cylinder ports. The transformed equations were solved numerically by a finite-difference method to yield the instantaneous and spatial distribution of the temperature, gas composition and velocity fields inside the cylinder from which the gas exchange process was characterized. The predictions were compared with some experimental observations obtained from a flow visualization rig and found in good agreement.

A new empirical model is presented to simulate the complete performance map of an automotive turbocharger which consists of a radial turbine and radial compressor. Each of these components is modelled by a particular system which includes basic elements such as orifices, diffusers and a head gain unit organized in series or in parallel. This model which is, in particular, practical for detailed computer program of engine simulation may be simply calibrated to reproduce a wide range of experimental performance maps of typical automotive turbochargers. The isentropic efficiency contours of each component have been approximated by a generalized mathematical expression. The relevant constants may be adjusted so as to fit a specific map.