Pole Arrangements that Introduce Prismatic Joints into the Design Space of Four- and Five-Position Rigid-Body Synthesis

Although a general five-position, rigid-body guidance problem admits a discrete number of revolute–revolute (RR) dyads, this paper identifies arrangements of five task positions that result in a center-point curve. For these special arrangements, a one-dimensional set of revolute-prismatic (RP) dyads exist to achieve the task positions. Other five-position arrangements are identified where a one-dimensional set of prismatic-revolute (PR) dyads exist to achieve the task positions. For a general case of five task positions, neither PR nor RP dyads are possible. In a general case of four-position rigid-body guidance problems, a unique PR dyad and RP dyad exist. Four-position arrangements are identified where the associated center-point curve includes the line at infinity and admits a PR dyad with a line of slide in any direction. Likewise, arrangements of the four positions are identified where the circle-point curve includes the line at infinity, permitting a one-dimensional set of RP dyads. These special four and five positions lead to dyads that can be coupled to solve a rigid-body guidance synthesis problem with a PRRP or RPPR device which is generally not possible. These solutions are particularly useful in design situations where actuation through a prismatic joint is desired

Integrating Numerical Models for Efficient Simulation of Compressor Valves

The motion of automatic, self-acting valves is a primary aspect in achieving superior compressor reliability and performance. Simulation of compressor valve mechanics involves a complex set of interactions that include characteristics of the compression chamber, thermodynamics, gas flow, valve motion, and pressure pulsations through the valve passages. One-dimensional (1D) lumped simulation models that encompass these interactions have been formulated and refined over the past several decades. During that same time period, finite element theory for fluid-structural-interaction (FSI) has been developed. Â FSI provides three-dimensional (3D) results across the entire fluid and solid domain and is well-suited for compressor simulation. However, the usage of FSI on real problems has not been adopted until much more recently, as sufficient computational resources became available. Still, the high computational expense and significant run times create a practical barrier to using FSI as a routine design tool. This paper presents techniques for integrating 1D-lumped models with 3D-FSI models. Methods to properly formulate the 1D-models are discussed. Once formulated, these 1D-models provide quick and accurate results that are used to narrow various design alternatives. Additionally, the 1D-models provide initial conditions for the higher resolution 3D-FSI models in critical regimes of operation. Lastly, experimental data is shown to confirm the techniques

A Semi-Empirical Prediction Model for the Discharge Line Temperature of Hermetic Compressors

Predicting the discharge line temperature (DLT) of air conditioning and refrigeration compressors is important to ensure sufficient lubricant properties and proper performance of components that are positioned in the exhaust stream. Understanding the DLT is also necessary for the design of waste heat recovery systems, which are of increasing interest. However, compressor performance information published by manufacturers does not typically include DLT values. Assumptions made in established thermodynamic models result in only modest correlation between predicted and observed DLT values. Numerous comprehensive DLT prediction models have been developed with excellent accuracy, but require many details of a particular compressor. This paper presents an assessment of various thermodynamic DLT prediction methods that do not rely on compressor-specific parameters. Prediction methods considered include an entropy-based model, a polytropic model and an energy model. The energy model was expanded to include an empirical component to account for high-side heat transfer and exhibits an accuracy that significantly exceeds the other established methods. The model was evaluated for both traditional refrigeration and air-conditioning operating conditions and translates well to vapor-injected, economizer cycles. Lastly, a study was conducted to determine the robustness of the empirical component by analyzing the relationship between the accuracy of the model and the number of experimental points used to produce the model

Attainable Moment Set and Actuation Time of a Bio-Inspired Rotating Empennage

Future tactical aircraft will likely demonstrate improvements in efficiency, weight, and control by implementing bio-inspired control systems. This work analyzes a novel control system for a fighter aircraft inspired by the function of – and the degrees of freedom available in – a bird’s tail. The control system is introduced to an existing fighter aircraft design by removing the vertical tail and allowing the horizontal tail surfaces to rotate about the roll axis. Using a low-fidelity aerodynamic model, an analysis on the available controlling moments and actuation speeds of the baseline aircraft is compared to that of the bio-inspired rotating empennage design. The results of this analysis at a takeoff and approach flight condition indicate that the bio-inspired tail design is able to improve upon the baseline in terms of control power available for yaw by up to 170%, while also improving the actuation speed by about 450 milliseconds for moments about the pitch axis. The bio-inspired design is shown to have actuation times that are up to 600 milliseconds slower for generating yawing moments and a reduced roll control contribution from the tail in certain moment combinations. The impacts of these issues on control will need to be determined with analysis at additional flight conditions and a flight dynamics analysis

Kinematic Synthesis of Planar, Shape-Changing, Rigid Body Mechanisms for Design Profiles with Significant Differences in Arc Length

This paper presents a kinematic procedure to synthesize planar mechanisms capable of approximating a shape change defined by a general set of curves. These “morphing curves,” referred to as design profiles, differ from each other by a combination of displacement in the plane, shape variation, and notable differences in arc length. Where previous rigid-body shape-change work focused on mechanisms composed of rigid links and revolute joints to approximate curves of roughly equal arc length, this work introduces prismatic joints into the mechanisms in order to produce the different desired arc lengths. A method is presented to iteratively search along the profiles for locations that are best suited for prismatic joints. The result of this methodology is the creation of a chain of rigid bodies connected by revolute and prismatic joints that can approximate a set of design profiles

Kinematic synthesis of planar, shape-changing, rigid body mechanisms for design profiles with significant differences in arc length

This paper presents a kinematic procedure to synthesize planar mechanisms capable of approximating a shape change defined by a general set of curves. These “morphing curves,” referred to as design profiles, differ from each other by a combination of displacement in the plane, shape variation, and notable differences in arc length. Where previous rigid-body shape-change work focused on mechanisms composed of rigid links and revolute joints to approximate curves of roughly equal arc length, this work introduces prismatic joints into the mechanisms in order to produce the different desired arc lengths. A method is presented to iteratively search along the profiles for locations that are best suited for prismatic joints. The result of this methodology is the creation of a chain of rigid bodies connected by revolute and prismatic joints that can approximate a set of design profiles

Development of a Spring-Based Automotive Starter

Automotive starting systems require substantial amounts of mechanical energy in a short period of time. Lead-acid batteries have historically provided that energy through a starter motor. Springs have been identified as an alternative energy storage medium and are well suited to engine-starting applications due to their ability to rapidly deliver substantial mechanical power and their long service life. This paper presents the development of a conceptual, spring-based starter. The focus of the study was to determine whether a spring of acceptable size could provide the required torque and rotational speed to start an automotive engine. Engine testing was performed on a representative 600 cc, inline 4-cylinder internal combustion engine to determine the required torque and engine speed during the starting cycle. An optimization was performed to identify an appropriate spring design, minimizing its size. Results predict that the test engine could be started by a torsional steel spring with a diameter and length of approximately 150 mm, similar in size, but lower weight than an electrical starting system of the engine. A proof-of-concept prototype has been constructed and evaluated

A Mechanical Regenerative Brake and Launch Assist Using an Open Differential and Elastic Energy Storage

Regenerative brake and launch assist (RBLA) systems are used to capture kinetic energy while a vehicle decelerates and subsequently use that stored energy to assist propulsion. Commercially available hybrid vehicles use generators, batteries and motors to electrically implement RBLA systems. Substantial increases in vehicle efficiency have been widely cited. This paper presents the development of a mechanical RBLA that stores energy in an elastic medium. An open differential is coupled with a variable transmission to store and release energy to an axle that principally rotates in a single direction. The concept applies regenerative braking technology to conventional automobiles equipped with only an internal combustion engine where the electrical systems of hybrid vehicles are not available. Governing performance equations are formulated and design parameters are selected based on an optimization of the vehicle operation over a simulated urban driving cycle. The functionality of this elastically-based regenerative brake device has been demonstrated on a physical prototype

Simulation Model of an Automatic Commercial Ice Machine

Automatic commercial ice making machines that produce a batch of cube ice at regular intervals are known as “cubersâ€. Such machines are commonly used in food service, food preservation, hotel, and health service industries.  Machines are typically rated for the weight of ice produced over a 24 hour period at ambient air temperatures of 90°F and water inlet temperature of 70°F. These cubers typically utilize an air-cooled, vapor-compression cycle to freeze circulating water flowing over an evaporator grid. Once a sufficient amount ice is formed, a valve switches to enable a harvest mode, where the compressor’s discharge gas is routed into the evaporator, thereby releasing ice into a storage bin. The U.S. Department of Energy has set a target of reducing energy usage by 10 – 15% by 2018.  Engineering models are not publicly available to assist designers in achieving the new energy regulations. This paper presents an engineering simulation model that addresses this need. This model simulates the transient operation of a cuber ice machine based on fundamental principles and generalized correlations. The model calculates time-varying changes in the system properties and aggregates performance results as a function of machine capacity and environmental conditons.  Rapid “what if†analyses can be readily completed, enabling engineers to quickly evaluate the impact of a variety of system design options, including the size of the air-cooled heat exchanger, finned surfaces, air / water flow rate, ambient air and inlet water temperature, compressor capacity and/or efficiency for freeze and harvest cycles, refrigerants, suction/liquid line heat exchanger and thermal expansion valve properties. Simulation results from the model were compared with the experimental data of a fully instrumented, standard 500 lb capacity ice machine, operating under various ambient air and water inlet temperatures. Key aggregate measures of the ice machine’s performance are: (1) cycle time (duration of freeze plus harvest cycles), (2) Energy input per 100 lb of ice, and (3) Energy usage during 24 hours. For these measures, the model’s accuarcy is within 5% for a variety of operating conditions.

Steady-State Modeling of Condensing Units with an Economizer Loop

This paper presents an engineering model that simulates the steady-state operation of air-cooled condensing units. Packaged, air-cooled, condensing units includes a compressor, condensing coil, tubing, and fans, fastened to a base or installed within an enclosure. To increase capacity, modern condensing units are being equipped with a brazed-plate heat exchanger for an economizer loop, configured in either upstream or downstream extraction schemes