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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.

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.

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 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.

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.

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.

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.

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 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.

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.