Dead Center Position Research Articles

Shape Optimization Analysis of Connecting Rod Using Growth-Strain Method

Because of space restrictions and the necessity of minimum mass, the connecting rod is one of the more heavily stressed parts of the car engine. The heaviest load occurs by gas pressure during firing at the top dead center position of the piston. The second is tensile load which occurs by the inertia force of the piston assembly mass and around the small end of the connecting rod mass. The third stress due to a lateral inertia, or “whip” of the connecting rod is usually negligibly small. In this paper we investigated the optimization of the shape of the normal connecting rod for the car engine, applying the growth-strain method. The analyses were done in case of compressive and tensile loads separately. The growth criterion parameters of the principal stress on the connecting rod were determined from the fatigue strength of the original design data and the actual safety factor. We thought that it is most compatible to use the fatigue strength which is the safety side, smallest value of the material constants, because the connecting rod receives the reciprocating compressive and tensile hard loads. After analysis, the optimized configurations of the connecting rod were compared with that of the original one. Then the reduction rate of the volume of the optimized connecting rod was approximately 22.8%.

Dynamic Modelling and Control of Semifree-Piston Motion in a Rotary Diesel Generator Concept

A new semifree-piston rotary generator concept is modelled dynamically and reduced to a single equation for piston stroke motion. This new concept comprises a toroidal-segment piston and cylinder, which orbit on separate generator disks, coupled by a pair of torsion springs to form a balanced mass-elastic system capable of spin. Conventional cyclic combustion takes place in the cylinder causing resonant motion of the disks. A two-part control strategy is proposed and tested by simulation to address the multi-objectives of maximum mechanical power transfer, minimum peak generator torque, and accurate piston top dead center (TDC) position control. A Part I strategy initially assumes that the combustion gas pressure is a function of time only. This produces torque control that follows a stroke velocity feedback law, which maximizes power transfer and implicitly minimizes generator torque, at the same time as power generation. When stroke-dependent gas pressure is introduced, however, the Part I strategy creates an unstable self-excited nonlinear system. The Part II strategy is designed to control piston TDC position and stabilize the response. This uses proportional control of gas pressure rise, assumed possible through fuel injection control and in-cylinder pressure sensing. An ideal-air-standard-dual-combustion two-stroke cycle is then adopted for nonstochastic simulation purposes, excluding the effect of delays and coupled system dynamics. A study is undertaken of a nominal 1.42 l, 200 mm orbit-radius, constant-pressure-scavenged diesel design with three different spring stiffness values. By focusing near the minimum compression ratio for diesel, to give a lower bound on the possible ideal output power, control gains are found that produce stable motion with piston TDC position errors of less than 1%. The power range is from 16 kW to 336 kW, depending mainly on spring stiffness. Since the concept can also store significant kinetic energy, it is potentially attractive as a range-extender for electric vehicles.

Relative centers of motion, implicit bars and dead-center positions for planar mechanisms

This paper characterizes the dead-center positions of a planar mechanism in terms of implicit bars, that can be described in their turn in terms of relative motion centers. Consequently, a graphical procedure for finding motion centers leads to a geometric description for dead-point positions. We give a survey of existing geometric constructions for motion centers, and we illustrate a new technique that makes use of the “Baracs construction”.

Full Rotatability and Singularity of Six-Bar and Geared Five-Bar Linkages

Full rotatability identification is a problem frequently encountered in linkage analysis and synthesis. The full rotatability of a linkage refers to a linkage, in which the input may complete a continuous rotation without the possibility of encountering a dead center position. In a complex linkage, the input rotatability of each branch may be different. This paper presents a unified and comprehensive treatment for the full rotatability identification of six-bar and geared five-bar linkages, regardless of the choice of input joints or reference link. A general way to identify all dead center positions and the associated branches is discussed. Special attention and detail discussion is given to the more difficult condition, in which the input is not given through a joint in the four-bar loop or to a gear link. A branch without a dead center position has full rotatability. Using the concept of joint rotation space, the branch of each dead center position, and hence, the branch without a dead center position can be identified. One may find the proposed method to be generally and conceptually straightforward. The treatment covers all linkage inversions.

Stretch rotation and complete mobility identification of Watt six-bar chains

Mobility of linkages refers to the problems concerning branch, full rotatability, singularities, and order of motion. This paper uses the stretch and rotation of a four-bar loop to convert a Watt six-bar linkage to an equivalent simple Stephenson linkage. It shows the mobility of a Watt six-bar linkage is affected by a hidden five-bar chain. The equivalency offers a simple and clear visual explanation on the formation of branches and sub-branches and how Watt and Stephenson linkages differ in mobility. The resulting mobility algorithm requires no stretch rotation. The dead center positions in the second four-bar loop are the branch points. It reveals that although a Watt six-bar linkage may have only up to four-branches, a branch may have up to six sub-branches. The results offer a simple algorithm suitable for automated identification of branch, sub-branch, and full rotatability. The algorithm is valid for Watt linkages with or without prismatic joints and is independent of linkage inversions. Examples are presented for illustration.

Experimental study of the effects of vegetable oil methyl ester on DI diesel engine performance characteristics and pollutant emissions

Vegetable oil methyl ester (VOME) is produced through the transesterification of vegetable oil and can be used as biodiesel in diesel engines as a renewable, nontoxic, and potentially environmentally friendly fossil fuel alternative in light of growing concerns regarding global warming and increasing oil prices. This study used VOME fuels produced from eight commonly seen oil bases to conduct a series of engine tests to investigate the effects of VOME on the engine performance, exhaust emissions, and combustion characteristics. The experimental results showed that using VOME in an unmodified direct injection (DI) diesel engine yielded a higher brake specific fuel consumption (BSFC) due to the VOME fuel’s lower calorific value. The high cetane number of VOME also imparted a better ignition quality and the high intrinsic oxygen content advanced the combustion process. The earlier start of combustion and the rapid combustion rate led to a drastic increase in the heat release rate (HRR) and the in-cylinder combustion pressure (ICCP) during the premixed combustion phase. A higher combustion rate resulted in higher peaks of HRR and ICCP as well as near the top dead center (TDC) position. Thus, it was found that a diesel engine fueled with VOME could potentially produce the same engine power as one fueled with petroleum diesel (PD), but with a reduction in the exhaust gas temperature (EGT), smoke and total hydrocarbon (THC) emissions, albeit with a slight increase in nitrogen oxides (NO x ) emissions. In addition, the VOME which possesses shorter carbon chains, more saturated bonds, and a higher oxygen content also yields a lower EGT as well as reduced smoke, NO x , and THC emissions. However, this is obtained at the detriment of an increased BSFC.

Branch identification and motion domain analysis of Stephenson type six-bar linkages

A general approach for branch identification and motion domain analysis of Stephenson type six-bar linkages is presented. By applying the Sturm theorem to the input-output polynomial equation, the dead-centre positions of the linkage are first evaluated and classified into two groups in order to discriminate the upper and lower bounds of the motion domains. The circuits of the linkage are then identified by matching the dead centres to the branches, which are attributed in accordance with the case where the input is assigned to a joint within the four-bar chain. Finally, the branches and motion domains of the more complicated case where the input is given through one of the uncoupled joints within the five-bar chain, are identified by mapping the circuits onto the domain of the specified input joint. This approach does not rely on the coupler curve of the constituent four-link mechanism. This is also suitable for computer implementation and can be systematically applied to all types of Stephenson linkages, regardless of the types of joints and the selection of input-output pair.

On the dead-centre position analysis of Stephenson six-link linkages

A new method for analysing the dead-centre positions of Stephenson type six-bar linkages is presented in this article. This method uses the input-output polynomial equation of the linkage and the corresponding Sturm functions to formulate the necessary condition of the dead-centre configurations as a polynomial equation in terms of the input parameter only. A simple analytical method for identifying the true dead-centre positions among the real solutions to the condition equation is also developed. The proposed method is conceptually straightforward and does not rely on any structure-related geometric conditions; therefore, it can be systematically applied to all types of Stephenson linkages and other multiple-loop, single degree-of-freedom linkages regardless of the selections of the input-output pair and the type of the joints.

Reconstruction and prediction of the in-cylinder pressure attractor of an internal combustion engine using locally constant models

In this research, the auto-mutual information function and correlation dimension are used for determination of lag and embedding dimension needed for reconstruction of the attractor of in-cylinder pressure of an internal combustion engine. Subsequently, a locally constant model is used for a 20-step prediction. The time series are acquired at three different test conditions and consisting of succeeding pressures at compression Top Dead Center (TDC) position of one of four cylinders. We have concluded that at least at relatively low engine speeds (below 2000 rpm) this method produces acceptable results.

Study of Dead-Centre Positions of Single-Degree-of-Freedom Planar Linkages Using Assur Kinematic Chains

The article presents an original technique, using the concept of Assur kinematic chains (AKCs), to determine whether a single-degree-of-freedom planar linkage is in a dead-centre position, i.e. a position where the input link is instantaneously stationary. An AKC is a special structure with mobility zero from which it is not possible to obtain a simpler substructure of the same mobility by removing one or more links. The article presents the concept of modularization of planar linkages into AKC based on the choice of the input link. Then, the article presents the constraints on the locations of the instantaneous centres of zero velocity (or instant centres) for a single-degree-of-freedom planar linkage to be in a stationary configuration, i.e. a configuration where one, or more, of the links is instantaneously stationary. The article shows that constraints on the locations of the instant centres for a stationary configuration are satisfied if an AKC, as part of the linkage, gains a degree of freedom. As the modularization of a planar linkage is based on the choice of the input link, the stationary configurations, determined by this method, are in fact dead-centre positions. Finally, this method is applied to indeterminate linkages, i.e. a class of single-degree-of-freedom planar linkages for which it is not possible to locate all the secondary (or unknown) instant centres by the direct application of the Aronhold—Kennedy theorem.

A Study of the Duality Between Planar Kinematics and Statics

This paper provides geometric insight into the correlation between basic concepts underlying the kinematics of planar mechanisms and the statics of simple trusses. The implication of this correlation, referred to here as duality, is that the science of kinematics can be utilized in a systematic manner to yield insight into statics, and vice versa. The paper begins by introducing a unique line, referred to as the equimomental line, which exists for two arbitrary coplanar forces. This line, where the moments caused by the two forces at each point on the line are equal, is used to define the direction of a face force which is a force variable acting in a face of a truss. The dual concept of an equimomental line in kinematics is the instantaneous center of zero velocity (or instant center) and the paper presents two theorems based on the duality between equimomental lines and instant centers. The first theorem, referred to as the equimomental line theorem, states that the three equimomental lines defined by three coplanar forces must intersect at a unique point. The second theorem states that the equimomental line for two coplanar forces acting on a truss, with two degrees of indeterminacy, must pass through a unique point. The paper then presents the dual Kennedy theorem for statics which is analogous to the well-known Aronhold-Kennedy theorem in kinematics. This theorem is believed to be an original contribution and provides a general perspective of the importance of the duality between the kinematics of mechanisms and the statics of trusses. Finally, the paper presents examples to demonstrate how this duality provides geometric insight into a simple truss and a planar linkage. The concepts are used to identify special configurations where the truss is not stable and where the linkage loses mobility (i.e., dead-center positions).

Poster 147: The effect of wheelchair seat position on peak shoulder joint moments during propulsion up an incline.

Objective: To assess the effect of changes in wheelchair seat position on peak shoulder joint internal moments during propulsion up an incline. Design: Randomized interventional study. Setting: Motion analysis laboratory. Participants: 11 subjects (9 men, 2 women) with paraplegia who used a wheelchair for mobility. Interventions: Subjects propelled up an 8% incline in a wheelchair with an instrumented pushrim and an adjustable axle that produced 4 different elbow angles by raising or lowering the seat. Main Outcome Measures: Kinetic information from the instrumented pushrim and kinematic information from a 10-camera motion analysis system were used to calculate seated elbow angle and mean peak sagittal, frontal, and transverse plane internal shoulder joint moments during the propulsive phase. Results: The elbow angle with the participant’s hand at the top dead center position of the pushrim ranged from an average of 55.1°±6.4° of flexion in the highest seat position to 76.9°±4.5° in the lowest. The shoulder flexors, adductors, and external rotators generated the largest moments in the sagittal, frontal, and transverse planes, respectively, during the propulsive phase of the cycle. Repeated-measures analysis of variance revealed that changes in seat position did not significantly affect the peak moment values in the sagittal and transverse planes. In contrast, the frontal plane moment values were significantly higher in the low seat position conditions ( P=.004). Conclusions: It has been suggested in the literature that low seat positions result in greater propulsion efficiency. This study showed that, during a challenging activity such as ramp ascension, lower seat positions resulted in a significantly higher shoulder adductor moment. This may be because greater shoulder abduction is present during the stroke cycle with the seat low. Therefore, this position may lead to increased fatigue of the shoulder adductors.

Synthesis of a planar seven-link mechanism with variable topology for motion between two dead-center positions

An analytical method of synthesis of a planar seven-link mechanism with variable topology for motion between two dead-center positions is suggested. The method is useful to reduce the solution space. The proposed method is non-iterative. A complex number, dyadic approach of synthesis of mechanism for motion, path (with prescribed timing) and function generation is considered. An alternative choice for Phase-II is also suggested.

Classification and branch identification of Stephenson six-bar chains

A Stephenson six-bar chain contains a four-bar loop and a five-bar loop, and its branch condition may be affected by the rotatability of both loops and the interaction between them. It may have up to six branches and its branch condition is far more complicated than that of a four-bar chain. In this article, the method to identify the effects of both loops on the rotatability of any Stephenson six-bar linkage and the algorithms to identify its branch condition are presented. The results resolve one of the most complicated and troublesome problems encountered in the finite position synthesis of Stephenson linkages. The proposed method is based on the rotatability of the common joints between the two loops and no coupler curve is used. The method is directly applicable to any type of Stephenson linkages regardless of whichever link is used as the input or fixed link. It is also valid for linkages containing prism joints. The algorithms are equally effective for path, motion, and function generation and also for any number of precision positions. Once the branch problem is resolved, other mobility problems, such as the full rotation, dead center positions, and the motion order, can be identified easily as in planar four-bar linkages.

Acoustic detonation suppression in internal combustion engine

First Page

On the Dead-Center Positions of Planar Linkage Mechanisms

In dead-center positions, a mechanism loses its mobility and has zero mechanical advantage. Based on the concept of instant centers, we develop a methodology to generate planar linkage mechanisms in dead-center positions. This methodology also can be applied to generate planar toggle linkage mechanisms. All possible configurations of six-bar planar linkage mechanisms, with one degree of freedom, in dead-center positions are listed.

Internal combustion piston engine using air chamber in piston driven in resonance with combustion wave frequency

First Page

Friction and wear processes in piston rings

It has been recognised that a large part of the top piston ring wear of an ic engine takes place in boundary lubrication around top dead centre (tdc) position. A quantitative assessment of the friction behaviour using actual piston ring and cylinder liner under conditions close to tdc has been made. The factors responsible for wear under these conditions have been identified as surface temperature, peak combustion pressure, total energy on the wearing surfaces and other physical properties of the material under sliding

Synthesis of double-rocker mechanism with optimum transmission characteristics

A simple analytical procedure is presented for the synthesis of the planar 4-bar double-rocker mechanism for coordinating the prescribed extreme positions. For optimum transmission characteristics, the design criterion used is the equal deviation of the transmission angles from 90° at the two dead-centre positions. Numerical examples are provided to illustrate the method.

Optimizing 4-bar crank-rocker mechanism

Closed form equations are developed for the synthesis of the 4-bar crank-rocker mechanism in which the angle between dead-centre positions of the rocker and the corresponding angle turned by the crank are prescribed. All the four different situations are considered and the minimum transmission angle, for the complete rotation of the crank, is obtained for each case. The upper and lower bounds on the crank angle, corresponding to a dead centre position of the rocker, are obtained by using Grashof's criterion. The design is optimized by maximizing the minimum transmission angle. Each case is illustrated by providing a numerical example.