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Using a low-speed high-torque (LSHT) pump/motor to provide the speed range and torque for a hydrostatic automobile offers a number of advantages over using a high-speed low-torque pump/motor, combined with a gear reducer. However, there appear to be no LSHT units commercially available that have true variable displacement capability. Because of this void, a variable displacement pump/motor has been designed and built that could provide a direct drive for each wheel of a hydrostatic automobile. The unit uses some components such as the cylinder block, piston and modified rotating case from a commercially available radial piston pump/motor. Initial preliminary testing of the pump/motor indicates that it has good efficiency and performance characteristics, and, with further development should be very attractive for automotive use. This paper focuses on the design and kinematics of the device.

Utilization of hydrostatic drives for power transmission, along with hydropneumatic accumulator energy storage can provide significant opportunities for improvements in vehicle design. These advantages are particularly relevant when considering advanced concept vehicles such as flying automobiles. Research and development in this technology has primarily focused on the fuel economy improvements that can be achieved in automobiles using a hydrostatic transmission and hydropneumatic accumulator energy storage. The accumulator permits the engine power to be uncoupled from the road load, thus enabling the engine to be operated at a more efficient point. By using wheel drive units that can operate as either motors (when driving) or pumps (when braking), regenerative braking can also be achieved, with the energy stored in the accumulator. In addition to improved fuel economy, several other significant design opportunities can be exploited.

The use of a hydraulic drive system with accumulator energy storage has the potential of providing large gains in fuel economy of internal combustion engine passenger automobiles. The improvement occurs because of efficient regenerative braking and the practicality of decoupling the engine operation from the driving cycle demands. The concept under study uses an engine-driven pump supplying hydraulic power to individual wheel pump/motors (P/M's) and/or an accumulator. Available P/M's have high efficiencies (e.g., 95%) at the ideal point of operation, but the efficiency falls off considerably at combinations of pressure, speed, and displacement that are significantly away from ideal. In order to maximize the fuel economy of the automobile, it is necessary to provide the proper combination of components, system design, and control policies that operate the wheel P/M's as close as possible to their maximum efficiency under all types of driving and braking conditions.

Positive displacement flowmeters can be used to simply and accurately calibrate common flow transducers such as axial turbine and target flowmeters. Two means of utilizing positive displacement devices were studied for use as a laboratory flowmeter calibration. The first method employed a fixed displacement axial piston motor. This proved unsatisfactory due to the difficulty in quantifying flow losses. The second method used a large hydraulic cylinder. An optical encoder measured the position of the cylinder rod, permitting a direct calculation of the flow through the in-line flowmeter being calibrated. Because cylinder leakage is virtually zero at low pressure, flow can be readily calculated knowing the effective cylinder diameter and piston velocity. The method described in this paper permits flow rates to be measured with an accuracy of ±0.1% of the volumetric flow rate. This paper discusses details of the design of the flowmeter and calibration method.

Off-highway mining and construction equipment typically converts all the power output of the engine to hydraulic power, with this power then used to perform the earth-moving operations, and also to propel the vehicle. This equipment presents significant opportunities for a new type of powerplant designed to deliver hydraulic power directly. An alternative to the conventional engine driven pump is a free-piston engine-pump (FPEP). The FPEP incorporates the functions of both an internal combustion engine and a hydraulic pump into a single, less-complex unit. The design presented in this paper utilizes two double-ended, reciprocating, opposed pistons, with combustion at one end of each piston and pumping at the opposite end. The opposed piston layout provides balance and also facilitates uniflow scavenging through intake and exhaust ports in the combustion section of the engine. An important feature of this FPEP design is the rebound accumulator circuit.

The modified hypocycloid (MH) mechanism, which uses gears to produce straight line motion, has been proposed as an alternative to the slider-crank mechanism for internal combustion (IC) engines. Advantages of the MH mechanism over the slider-crank for an IC engine include the capability of perfect balancing with any number of cylinders and the absence of piston side loads. The elimination of piston side load has the potential for lower piston friction, reduced piston slap, and less susceptibility to cylinder liner cavitation. To evaluate the concept, an experimental single cylinder four-stroke engine which utilizes the MH mechanism is currently being built at the University of Wisconsin-Madison. The MH engine has an increased number of friction interfaces compared to a conventional slider-crank engine due to additional bearings and the gear meshes. Thus, the lubrication of these components is an important issue in total MH engine friction.

The modified hypocycloid engine incorporates a unique geared drive that imparts straight-line, sinusoidal motion to the one-piece piston and rod assembly. These kinematic characteristics provide a variety of potential benefits not possible with traditional slider-crank kinematics. Perfect engine balance is achieved through the use of two sets of counterweights. The absence of piston side thrust promises reductions in piston assembly friction and piston slap, even with smaller piston skirts. Additional potential benefits include improved combustion characteristics and reduced piston manufacturing costs. Although simpler hypocycloid designs provide the same motion, the modified hypocycloid engine reduces gear and crankshaft loading. A description and design details of a prototype engine currently under construction are presented. Patented design improvements over previous hypocycloid designs are described.

In conventional hydraulic systems, a fixed or variable displacement pump is used to supply a number of separate branches of a circuit, each with potentially different flow rate and pressure requirements which can vary widely with time. Conventional approaches to distributing the flow to the individual branches generally involve valves controlling the flow to each individual branch. This can lead to significant energy losses from valve throttling, depending upon the actual flow and pressure requirements of each part of the circuit. In the system discussed in this paper, the entire output of the pump is quickly and sequentially directed to each individual branch of the circuit. The average speed of the actuator is controlled by the proportion of time that the pump flow is being directed to that branch. A small accumulator is incorporated in each branch of the circuit to smooth the velocity of the actuator.

The geared hypocycloid mechanism, a kinematic arrangement that provides a straight-line motion, can be used as the basis for an internal combustion engine. Such an engine would have a number of advantages: Perfect balance can be achieved with any number of cylinders. The straight-line motion eliminates the need for a wrist pin bearing, further allowing a very short piston to be used without danger of cocking. Piston side load is virtually eliminated, and “piston slap” will not occur even with a large piston/cylinder clearance. These features make it particularly attractive for small single cylinder engine applications where vibration is undesirable, and also for the uncooled “adiabatic engines”, in which piston cylinder lubrication and friction are major concerns.

A direct acting free piston internal combustion engine/hydrostatic pump is analyzed. This device would take the place of a conventional engine-driven hydrostatic pump, and would be expected to offer significant advantages in cost, weight, and efficiency. The free piston configuration eliminates the need for a crankshaft-connecting rod system, and the comparable mechanism of the pump that converts rotary to reciprocating motion. Analysis of the design was done by computer simulation using a thermodynamic model of the combustion cylinder in combination with the system dynamics. A parametric study was performed to determine operating characteristics with a wide variety of mechanical parameters, and as a guide to developing a preliminary design. The results show that good performance is possible with reasonable mechanical dimensions and other parameters. Several different design configurations are presented.

Four alternative fluid power systems were analyzed to determine their efficiencies and standby power consumption while providing hydraulic power for power steering and a hitch for an agricultural tractor operating over a typical duty cycle. The investigation shows that a significant reduction in losses can be achieved with systems incorporating both split flow and variable delivery capabilities. These characteristics reduce losses by providing the required flow and pressure to each function without a large pressure drop for the function operating at the lower pressure. These systems can be used to improve efficiencies whenever there are multiple, simultaneous hydraulic power demands which require significantly different pressures.

Recent advances in hydrostatic transmission efficiency and accumulator technology make the hydropneumatic energy storage automobile appear quite attractive as a means of improving fuel economy. The system examined in this paper utilizes a conventional internal combustion engine, and two hydrostatic pump/motor units with an accumulator between them. The accumulator allows regenerative braking and permits the engine operation to be uncoupled from the road load. Detailed, second-by-second driving cycle simulations have been used to study the fuel economy possible with various combinations of component parameters. The design can provide excellent fuel economy with a moderate size accumulator.

The standard vehicle propulsion system and its controls are compared with a flywheel propulsion system. Different concepts of control and various system configurations are explored. Some considerations for the design of a general purpose automatic flywheel transmission vehicle are presented and discussed. Specifications required for a flywheel transmission system which can achieve substantial mileage improvements and provide high performance are presented. The resulting vehicle would have performance of 0–60 mph in less than 10 seconds and achieve 50 miles per gallon on the Federal Urban Driving Cycle (FUDC) at an inertia weight of 3,000 lb. Higher mileages are possible for lighter vehicles. Fuel economy is achieved by (1) engine operation only at minimum BSFC, (2) elimination of engine idle, (3) recovery of energy from braking and (4) minimizing transmission losses.

A flywheel to manage energy between a prime mover and a load has been used in many engineering applications. Automotive applications, however, pose a number of difficult problems which can be overcome only with proper design. Substantial mileage and performance improvements while meeting emission constraints can then be accomplished with the concept. An experimental flywheel car has been designed and built at the University of Wisconsin that has demonstrated a mileage improvement of about 50% over a corresponding production vehicle on the EPA/FUDC. With continued research and development gains of 100% appear feasible.

Hybrid vehicles, i.e., those containing two or more sources of power, have the potential of increased fuel economy under certain types of driving conditions. Systems currently being investigated include combinations of heat engines, electric drives, fly-wheels, and accumulators. In order to obtain fuel economy improvements over conventional vehicles, efficient components are required as well as a good system design. Hybrid powerplants appear more promising for heavier vehicles.

The electric car has many features that make it attractive for urban use. Currently, its principal shortcomings are its short range and poor efficiency for a realistic driving cycle. An electric hybrid car of advanced design, such as the University of Wisconsin model described here, can overcome the limitations of the all-electric car, while retaining most of its advantages, but only at the expense of greater complexity. More research and development is required before either version can be an adequate replacement for our present internal combustion engine cars.