Analysis of a hydrostatic transmission driveline for its use in off-road multiple axle vehicles

There is a vast range of off-road multi-axle wheeled vehicle configurations. Some of the most common are the three axle rigid vehicles or the four axle articulated vehicles. However, these types of vehicles have the problem of using very complex transmission configurations. In addition, the requirements in terms of torque in each of the wheels are quite variable and non uniform. This work aims to model and study, from the standpoint of performance and energy efficiency, the driveline of such vehicles. The modelling process for the design and analysis of a hydrostatic transmission aimed at off-road multiple axle vehicles has been conceptually described. Mathematical models for the main components of the transmission and a global model of the driveline have been defined. A specific example study is presented, applying the described procedure. Results show that the overall performance of the transmission is highly dependent on the operating conditions, on the selected configuration and on the used components. The results also show that the actual instantaneous efficiency of each of the components is usually far below their maximum catalogue value. In the case study efficiencies up to 64% have been reached for the overall transmission.     

基于智能驾驶电动汽车的多车?桥梁耦合系统动力学特性研究

迫于能源和环保问题的压力, 电动汽车及智能驾驶受到了各国高度重视. 轮毂电机驱动电动汽车车轮振动剧烈, 与桥梁路面动力学相互作用更加突出, 现有研究主要针对传统汽车, 关于电动车轮与公路桥梁接触动力学相互作用及智能驾驶车队的多车?桥梁耦合作用研究尚不多见. 本文以轮毂电机驱动电动汽车为研究对象, 考虑车轮和桥面多点接触关系, 研究了两个智能驾驶汽车过桥时的车桥耦合动力学特性. 分析了电机质量、电机激励、轮胎悬架刚度非线性、车距、车速对系统振动特性的影响, 以及桥面不平顺激励、三重耦合激励对电动汽车平顺性的影响. 研究表明: 车距和车速是影响车?桥系统振动特性的重要因素, 在车?桥耦合动态设计中, 车距和车速的影响应重点关注; 桥面越平坦, 电机激励及桥面二次激励对车辆平顺性和道路友好性影响越加显著, 当汽车行驶在平坦桥面时两种激励对轮毂电机驱动电动汽车的影响不容忽视. 所建模型有望为智能驾驶电动汽车与桥梁的耦合作用研究提供理论参考.      

Torque control strategy for a parallel hydraulic hybrid vehicle

Hydraulic hybrid system is an important branch of hybrid technology, which has the advantage of high power density and the ability to accept the high rates/high frequencies of charging and discharging, therefore hydraulic hybrid technology is well suited for off-road vehicles and heavy-duty trucks. Relatively lower energy density and complicated coordinating operation between two power sources require a special energy control strategy to maximize the fuel saving potential. This paper presents a new configuration of parallel hydraulic hybrid vehicle (PHHV) to improve the braking energy regenerated potential and engine work efficiency. Based on the analysis of optimal energy distribution for the proposed PHHV over a representative urban driving cycle, a fuzzy torque control strategy based on the vehicle load changes is developed to real-time control the energy distribution for the proposed PHHV. Simulation results demonstrate that the proposed PHHV with torque control strategy takes advantage of the high power density and efficiency characteristics of the hydraulic hybrid system, minimizes the disadvantages of low energy density and effectively improves the fuel economy of PHHV.     

Prediction of impacts of wheeled vehicles on terrain

Traffic of off-road vehicles can disturb soil, decrease vegetation development, and increase soil erosion. Terrain impacts caused by wheeled off-road vehicles were studied in this paper. Models were developed to predict terrain impacts caused by wheeled vehicles in terms of disturbed width and impact severity. Disturbed width and impact severity are not only controlled by vehicle types and vehicle dimensions, but also influenced by soil conditions and vehicle dynamic properties (turning radius, velocity). Field tests of an eight-wheeled vehicle and a four-wheeled vehicle were conducted to test these models. Field data of terrain–vehicle interactions in different vehicle dynamic conditions were collected. Vehicle dynamic properties were derived from a global position system (GPS) based tracking system. The average prediction percentage error of the theoretical disturbed width model is less than 20%. The average absolute error between the predicted impact severity and the measured value is less than an impact severity value of 12%. These models can be used to predict terrain impacts caused by off-road wheeled vehicles.     

Mobility analysis of off-road vehicles: Benefits for development, procurement and operation

Nowadays the requirements on off-road vehicles are rising steadily. The ideal vehicle has to provide excellent off-road capability with low fuel consumption, offer a high customizability for each specific mission and, last but not least, it has to be easy to operate. To meet these demands, on the development side a lot of parameter studies have to be carried out. The customer has to compare offers from a multiplicity of suppliers to decide which vehicle fulfills the designated mission task best. And finally, the operator needs the best training on the vehicle to cope with all possible situations in off-road mobility. In response to these needs, the presented simulation program WinMaku was developed to offer a tool to facilitate development, procurement and operator training. Exemplary simulation results show, on the one hand, the influence of specific design parameters, e.g. tire size, engine power, torque characteristics, gear shifting, and engine working conditions, and on the other hand the (beneficial or adverse) effects of operational parameters like driving with maximum/partial engine load, gear selection, engine speed, tire inflation pressure or track tension, on mobility performance. Furthermore results of vehicle comparison analysis are presented. These types of analysis show comparisons of mobility performance of different vehicle types or vehicle concepts (e.g. wheel vs. track) in fulfilling a certain mission profile, characterized by passing a sequence of different soils with various inclines. Endowed with such capability, the presented simulation tool serves as a training tool for operators, provides a cost effective method to assess possible development steps, allows customers to run a pre-selection process prior to expensive and time-consuming field tests, and finally supports mission planning by providing data like expected fuel consumption or time needed to pass a certain mission profile.     

Improvement of traction performance and off-road mobility for a vehicle with four individual electric motors: Driving over icy road

To control speed and wheel slip for severe conditions of tire-surface interaction is a challenging task in the design of traction control system for electric vehicles with off-road capability. In this regard, the present paper focuses on a specific traction control for an electric vehicle with four individual in-wheel motors over icy road. The study demonstrates that a proper integration of the speed controller and wheel slip controller can essentially improve the mobility of the vehicle in the cases of acceleration and slope climbing. The paper discusses relevant case studies with particular attention given to the system architecture (sliding mode and PID control methods), extremum-seeking algorithm for maximum tire-road friction and corresponding slip value, and experimental validation of the tire model used in the controller with the help of the Terramechanics Rig in the Advanced Vehicle Dynamics Laboratory (AVDL) at Virginia Polytechnic Institute and State University.     

Vehicle movement patterns and vegetative impacts during military training exercises

The operation of off-road vehicles during military training exercises can affect the environmental conditions of training lands by removing or disturbing vegetation. The use of global positioning systems (GPS)-based vehicle tracking systems can help to characterize the movement of vehicles during training exercises for the purpose of quantifying vegetative impacts. The combination of GPS positions of vehicles in the field during a training exercise, and geographic information system (GIS) maps of the training installation can provide information about vehicle-specific vegetation impacts of a training exercise, as related to vehicle locations, turning radius and velocity. Such relationships can be used to estimate off-road vegetation impacts. Twenty GPS-based vehicle tracking systems were installed on vehicles of the US Army 3rd Brigade 1/14 Cavalry to evaluate vegetation impacts during a 10 day reconnaissance training exercise at Yakima Training Center in Yakima, WA. The vehicle tracking systems were programmed to record the position of the vehicles every second. The resulting vehicle tracking data were analyzed for quantity of travel per day of the training activity, quantity of travel on and off roads, off-road vehicle dynamic properties turning radius and velocity, and off-road vegetation removed. The vehicles were in motion an average of 8.4% (approximately 2 h per day) of the training exercise time. The average distance traveled per day on roads was 33.5 km, and the average distance traveled per day off-roads was 7.7 km. On average, the vehicles spent 16% of their off-road traveling time at turning radii less than 20 m. Vegetation impacts were compared for different missions. The zone reconnaissance mission produced the highest vegetation impact per distance traveled.     

Diversification and sophistication

In this report, progress and state-of-the-art of research on vehicle and machinery designs are described for vehicles such as boat tractors, wheel type vehicles, tracked vehicles and a new locomotion system. The development stages and aspects of off-road vehicles are different in countries/districts, showing many kinds of running devices. Reflecting a high-technology in some countries, the mechanism and the control system of off-road vehicles are becoming more and more sophisticated.     

A comparative study of hybrid electric vehicle fuel consumption over diverse driving cycles

Environmental pollution and declining resources of fossil fuels in recent years, have increased demand for better fuel economy and less pollution for ground transportation. Among the alternative solutions provided by researchers in recent decades, hybrid electric vehicles consisted of an internal combustion engine and an electric motor have been considered as a promising solution in the short-term. In the present study, fuel economy characteristics of a parallel hybrid electric vehicle are investigated by using numerical simulation. The simulation methodology is based on a fast forward facing simulation model of a parallel hybrid and an internal combustion engine powertrains. The objective of this study is to present the main parameters which result in an optimum combination of hybrid powertrain components in order to obtain a better fuel economy of hybrid powertrains regarding different driven cycles and hybridization factors. Then, the fuel consumption of the parallel hybrid electric vehicles are compared considering various driven cycles and hybridization factors. The results showed that the better fuel economy of hybrid powertrains increases by decreasing average load of the test cycle and the point of the best fuel economy for a particular average load of the cycle moves towards higher hybridization factors when the average load of the test cycle is reduced.     

A method of torque control for independent wheel drive vehicles on rough terrain

Our previous research has revealed that, for vehicles with independently driven wheels, a torque distribution based on the ratio of the vertical load of each wheel to the total vehicle load is efficient for driving on flat ground. In this research, this method of torque distribution was extended to electric off-road vehicles driving on rough ground. In order to examine the driving efficiency of these vehicles, a numerical vehicle model was constructed in the pitch plane. Simulations using the numerical vehicle model on rough ground were conducted with a proposed torque distribution and control method. The numerical results from these simulations were compared with those of a conventional vehicle to evaluate the driving efficiency and trafficability on ground with various profiles. A comparison between the simulations demonstrated that the proposed method of torque distribution to the front and rear wheels based on the ratio of the vertical load is efficient for driving on rough ground.     

A roll stability performance measure for off-road vehicles

The roll stability is significant for both road and off-road commercial vehicles, while the majority of reported studies focus on road vehicles neglecting the contributions of uneven off-road terrains. The limited studies on roll stability of off-road vehicles have assessed the stability limits using performance measures derived for road vehicles. This study proposes an alternative performance measure for assessing roll stability limits of off-road vehicles. The roll dynamics of an off-road mining vehicle operating on random rough terrains are investigated, where the two terrain-track profiles are synthesized considering coherency between them. It is shown that a measure based on steady-turning root-mean-square lateral acceleration corresponding to the sustained period of unity lateral-load-transfer-ratio prior to the absolute-rollover, could serve as a reliable measure of roll stability of the vehicle operating on random rough terrains. The robustness of proposed performance measure is demonstrated considering sprung mass center height variations and different terrain excitations. The simulation results revealed adverse effects of terrain elevation magnitude on the roll stability, while a relatively higher coherency resulted in lower terrain roll-excitation and thereby enhanced vehicle roll stability. Terrains with relatively higher waviness increased the magnitude of lower spatial frequency components, which resulted in reduced roll stability limits.     

Non-conventional land transport systems

Locomotion in saturated terrain and some special vehicle solutions for saturated agricultural terrain were discussed, as well as arrangements that increase the mobility of off-road vehicles for military purposes, component and motor alternatives for off-road vehicles, and the new Maglev and Eurotren Monoviga guided transport systems .     

A skid steering model using the Magic Formula

The paper describes a computer model for predicting the steering performance and power flows of a notional skid steered tracked vehicle. The force/slip characteristics of the rubber track pads are calculated by means of the so-called Magic Formula. Relevant parameters for the Magic Formula are derived from the limited amount of data available from traction tests with a tracked vehicle on a hard surface. The computer model considers the vehicle in steady state motion on curves of various radii and allows for lateral and longitudinal weight transfer, roll and pitch motions and the effects of track tension forces. Vehicle dimensions, Magic Formula parameters and the equations of motion are set up in a Microsoft Excel spreadsheet and solutions obtained using the Solver routine. Model outputs are described in terms of driver control input and various power flows against lateral acceleration. Maximum lateral acceleration is generally limited by the available engine power. In some conditions the outer track sprocket could be transmitting almost twice the maximum net engine power. For vehicles with a single electric motor/inverter driving each sprocket, these units would need to be able to transmit these high intermittent powers.     

Overland transport without roads

The need for off-road vehicles as applied to commerce, warfare, and recreation is examined. Most of the developments in the mobility of off-road vehicles have had a primarily military background. The main impediments to off-road mobility are defined as major obstacles, soft ground, and rough ground. Optimization of track design for accommodating these impediments in military vehicles reveals the importance of the ratio of Mean Maximum Pressure to Mean Pressure of a traced vehicle. It appears that contemporary commercial cross-country vehicles contain limitations in their designs which culd be overcome with available knowledge. These limitations include soft ground mobility, and speed over rough ground.The role of hovercraft in providing off-road mobility is reviewed, and special attention is given to the most recent requirements for avoiding environmental disturbance when using off-road vehicles in the Canadian North.     

Analysis of vehicle platoon movement and speed-spacing relationships during military exercises

In this study a method that identifies off-road vehicle column movement was developed and evaluated. Previous studies have revealed that multiple vehicle passes produce detrimental soil and terrain impacts. Identifying the frequency and location of this type of multi-pass impact during military maneuvers is difficult. This method will aid in the assessment of environmental impacts of off-road military vehicles by allowing land managers to characterize vehicle movement patterns, especially column movement, at military training installations during maneuvers. GPS units mounted on military vehicles collected on and off-road tracking data during a reconnaissance maneuver at Fort Lewis Military Installation, Washington. A set of data utilizing a Stryker platoon of four vehicles was used to evaluate this method. The GPS coordinates, speed, and direction of travel of each vehicle was collected at each second. A criteria to identify platoon column movement was developed based on vehicle proximity, speed and direction of travel. The results of this study show that the method can correctly identify off-road column movement for the purpose of evaluating the multi-pass impacts on the terrain. In addition, using this approach the vehicle movement patterns associated with on- and off-road platoon movement (i.e. vehicle speeds and spacing) were evaluated.     

“Wheels vs. tracks” – A fundamental evaluation from the traction perspective

The issue of wheeled vehicles vs. tracked vehicles for off-road operations has been a subject of debate for a long period of time. Recent interest in the development of vehicles for the rapid deployment of armed forces has given a new impetus to this debate. While a number of experimental studies in comparing the performances of specific wheeled vehicles with those of tracked vehicles under selected operating environments have been performed, it appears that relatively little fundamental analysis on this subject has been published in the open literature, including the Journal of Terramechanics. This paper is aimed at evaluating the tractive performance of wheeled and tracked vehicles from the standpoint of the mechanics of vehicle–terrain interaction. The differences between a tire and a track in generating thrust are elucidated. The basic factors that affect the gross traction of wheeled and tracked vehicles are identified. A general comparison of the thrust developed by a multi-axle wheeled vehicle with that of a tracked vehicle is made, based on certain simplifying assumptions. As the interaction between an off-road vehicle and unprepared terrain is very complex, to compare the performance of a wheeled vehicle with that of a tracked vehicle realistically, comprehensive computer simulation models are required. Two computer simulation models, one for wheeled vehicles, known as NWVPM, and the other for tracked vehicles, known as NTVPM, are described. As an example of the applications of these two computer simulation models, the mobility of an 8 × 8 wheeled vehicle, similar to a light armoured vehicle (LAV), is compared with that of a tracked vehicle, similar to an armoured personnel carrier (APC). It is hoped that this study will illustrate the fundamental factors that limit the traction of wheeled vehicles in comparison with that of tracked vehicles, hence contributing to a better understanding of the issue of wheels vs. tracks.     

A comparison of ground vehicle mobility analysis based on soil moisture time series datasets from WindSat,LIS, and in situ sensors

Soil moisture is a key terrain variable in ground vehicle off-road mobility. Historically, models of the land water balance have been used to estimate soil moisture. Recently, satellites have provided another source of soil moisture estimates that can be used to estimate soil-limited vehicle mobility. In this study, we compared the off-road vehicle mobility estimates based on three soil moisture sources: WindSat (a satellite source), LIS (a computer model source), and in situ ground sensors (to represent ground truth). Mobility of six vehicles, each with different ranges of sensitivity to soil moisture, was examined in three test sites. The results demonstrated that the effect of the soil moisture error on mobility predictions is complex and may produce very significant errors in off-road mobility analysis for certain combinations of vehicles, seasons, and climates. This is because soil moisture biases vary in both direction and magnitude with season and location. Furthermore, vehicles are sensitive to different ranges of soil moistures. Modeled vehicle speeds in the dry time periods were limited by the interaction between soil traction and the vehicles’ powertrain characteristics. In the wet season, differences in soil strength resulted in more significant differences in mobility predictions.     

Magnetogasdynamic generation of electrical energy in models of aircraft with a high-speed jet engine

The possibility of generating electric power in a plane model of an integral high-speed hydrogen-burning jet engine by mounting a magnetogasdynamic (MHD) generator at the combustion chamber exit is discussed. Attention is concentrated on clarifying the effect of MHD energy extraction from the stream on the aircraft’s thrust characteristics. The internal and external flows are simulated numerically. The two-dimensional supersonic gasdynamic flow inside the engine (in the air-intake, combustion chamber, MHD generator, and nozzle) and the supersonic flow past the aircraft are described on the basis of the complete averaged system of Navier-Stokes equations (in the presence of turbulence), which includes MHD force and heat sources, a one-parameter turbulence model, the electrodynamic equations for an ideal segmented MHD generator, and the equations of the detailed chemical kinetics of hydrogen burning in air. The numerical solution is obtained by means of a computer program that uses a relaxation scheme and an implicit higher-order version of the Godunov method. It is shown that MHD electric power generation can be realized without disturbing the positive balance in the relation between the thrust and the drag of the aircraft with the engine operating with allowance for the MHD drag, but with some loss of effective thrust.     

The ride comfort vs. handling compromise for off-road vehicles

When designing vehicle suspension systems, it is well-known that spring and damper characteristics required for good handling on a vehicle are not the same as those required for good ride comfort. Any choice of spring and damper characteristic is therefore necessarily a compromise between ride comfort and handling. The compromise is more pronounced on off-road vehicles, as they require good ride comfort over rough off-road terrain, as well as acceptable on-road handling. In this paper, the ride comfort vs. handling compromise for off-road vehicles is investigated by means of three case studies. All three case studies indicate that the spring and damper charcteristics required for ride comfort and handling lie on opposite extremes of the design space. Design criteria for a semi-active suspension system, that could significantly reduce, or even eliminate the ride comfort vs. handling compromise, are proposed. The system should be capable of switching safely and predictably between a stiff spring and high damping mode (for handling) as well as a soft spring and low damping mode (for ride comfort). A possible solution to the compromise, in the form of a four state, semi-active hydropneumatic spring-damper system, is proposed.