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無錫太湖學(xué)院 Designing and Modeling a Torque and Speed Control Transmission (TSCT) 1 Background The Partnership for a New Generation of Vehicles (PNGV) was formed between the Federal Government, Ford Motor Company, General Motors Corporation, and Chrysler Corporation. The goal of this partnership was to allow the major U.S. automotive manufactures to collaborate with each other and produce high fuel efficiency, low emissions vehicles for sale to the general public. The performance objective for these manufacturers was to create mid-sized passenger cars capable of attaining an 80 mpg (gasoline) composite fuel economy rating on the Environmental Protection Agency (EPA) city and highway cycles. Hybrid vehicle technology has shown great promise in attaining the goals set forth by the PNGV. Hybrid electric vehicles (HEV ) employ technology that helps bridge the gap between the future hope of an electric vehicle (EV) and today’s current vehicles. Within the past year hybrid electric vehicles have gained an important place in the vehicle market. American Honda Motor Company, Inc. is currently releasing their first generation HEV, the Insight. The Insight is a compact, two pass engey, parallel HEV which achieves more than 65 mpg (composite) on the EPA test cycles: the highest of any production vehicle ever tested. Toyota Motor Corporation has also released a hybrid vehicle for sale to the general public. The Toyota Prius is currently for sale in Japan and will come the United States in the beginning of the year 2000. The Prius is a four passenger combination hybrid employing an a gasoline engine, high power electric motor, and an electromechanical continuously variable transmission (CVT) comprised of a planetary gear train and a high power alternator/motor. It is through technology incorporated in vehicles such as the Prius that automotive transmission design and operation will make significant new advances. 1.1 Current Automotive Transmission Technologies With the advent of the automobile also came the creation of the automotive transmission. Early vehicles were simple with manual controls for all functions including the transmission. As advances have been made in vehicles over the past several decades, transmission technology has also advanced. The automatic transmission has nearly replaced the manual transmission in all but economy and performance cars. This trend can be attributed to ease of use, higher power engines becoming available, and congestion in urban areas. Another new transmission technology beginning to see application particularly in foreign markets is the continuously variable transmission that offers continuous operation without shifting between a high and low gear ratio. These three types of transmissions are all similar in function though their objectives are accomplished in different ways. The capabilities of these transmissions are limited to decoupling the engine speed from the speed of wheels and thereby providing one of several forward or reverse gear ratios. Each transmission is also a single input (engine) and single output (drive device). There are typically no provisions for attaching multiple power sources or for extracting power from more than one point. The exception to this is heavy-duty transmissions equipped with provisions for a power take off for driving auxiliary mechanical equipment. Single input, single output operation limits drive train flexibility for newer systems employing multiple power sources such as those used in the next generation of hybrid vehicles. 1.1.1 Manual Transmission Operation Manual transmissions are the least complex and oldest design of power transmission available. In simplest form, a manual transmission is a linear combination of a clutch and a directly geared connection. More sophisticated examples rely on this design but add the ability to select other gear ratios to allow different output speeds for the same input speed. Of these types of transmissions, there are two variations: synchronized and unsynchronized. Synchronized manual transmissions are typically used for light duty applications. Coupled to each gear is a synchronizer that allows the operator to disengage the clutch and select whatever gear necessary. The selection of a different gear engages the synchronizer, which then matches engine input speed and transmission output speed before the gears are engaged. Unsynchronized manual transmissions are more robust by nature. The operator must double-clutch between shifts to match engine and transmission speed manually. However, this allows a transmission of a given size to handle greater load as space previously occupied by the synchronizers can now be dedicated to the use of wider gears. Applications of these types of manual transmissions are for over-the-road trucks and up to larger equipment with total vehicle weights over 100 tons. [1] 1.1.2 Automatic Transmission Operation Automatic transmissions are a complex assembly of many components that allow for seamless power transmission. Those currently available in production vehicles use torque converters, clutches, and planetary gear sets for the selection of different output ratios. The engine is connected to the torque converter that acts very much like a clutch under some conditions while more like a direct connection in others. The torque converter is a hydraulic coupling that will slip under light load (idle), but engage progressively under higher load. While the torque converter transmits power to the transmission there is a speed reduction across the unit during low speed operation. This reduction is typically between 2.5:1 to 3.5:1 .Once higher vehicle speeds are attained, the torque converter input and output may be locked together to achieve a direct drive though the unit. The output of the torque converter is typically connected to a hydraulic pump that provides the necessary pressure to engage different clutches within the transmission and the planetary drive. Different gear ratios are created through the use of two or more planetary gear sets. These gear sets are combined with clutches on different elements. By clutching and declutching different elements, multiple gear ratios are possible. Basic automatic transmissions are equipped with a single control input that is throttle position. The combination of this with the hydraulic pressure created within the transmission allows for mechanical open loop control of all gear selections. Newer variations of the automatic transmission are equipped with electronic feedback controls. Shift logic is dependent on many more variables such as engine speed, temperature, current driving trend, throttle position, vehicle accelerations, etc. This allows the transmission controller to monitor vehicle operation and using a rule-based control strategy decide , which gear is best suited to the current driving conditions. Newer systems are also integrated with the engine controller such that a vehicle control computer has authority over engine and transmission operation simultaneously. This allows for such features as increasing engine speed during high-speed downshifts to match engine and transmission speed for smoother shifting and retarding fueling and ignition timing during high power up shifts to reduce ‘jerk’. Previously, transmission control was much simpler because overrunning clutches were employed in higher gears that only allowed for coasting to conserve fuel. [1] 1.1.3 Continuously Variable Transmission Operation Continuously variable transmissions are one of the emerging transmission technologies of the last twenty years. This type of transmission allows power transmission over a given range of operation with infinitely variable gear ratios between a high and low extreme. These transmissions are constructed using two variable diameter pulleys with a belt connecting the two. As one pulley increases in size, the other decreases. This is accomplished by locating on one shaft a stationary sheave and a movable sheave. For automotive applications, a hydraulic actuator controls movement of the sheave. However, centrifugal systems along with high power electronic solenoids may be used. A second shaft in the CVT contains the other stationary sheave and movable sheave also controlled hydraulically. A flexible metal belt is fitted around these two pulleys and the movable sheaves are located on opposite sides of the belt. There are two variations of this type of transmission: push belt and pull belt Pull belt CVT were the first type to be manufactured due to simplicity. A clutch is attached between the first pulley and the engine while the output of the second pulley was connected to a differential and thus the wheels. A hydraulic pump is used to control the diameter of the two different pulleys. As power is applied the first pulley creates a torque that is transmitted through the belt (under tension) to the second pulley. Control of the transmission ratio is usually a direct relationship dependent upon throttle position. Push belt CVT, similar in design to the Van , are much the same as pull belt CVT , except that power is transmitted through the belt while under compression. This provides a higher overall efficiency due to the belt being pushed out of the second pulley and lowering frictional losses. Current work with these transmissions is being focused on creating larger units capable of handling more torque. Efficiency of the CVT is directly related to how much tension is in the belt between the two pulleys. CVT torque handling capacity increases as tension in the belt increases. However, this increased tension lowers power transmission efficiency. The belt must slide across the faces of each pulley as it enters and exits upon each half rotation. This sliding of the belt creates frictional losses within the system. In addition, there may be significant parasitic losses associated with raising the hydraulic pressure required to move or maintain the position of the sheaves in each pulley. [2] 1.1.4 Automatically Shifted Manual Transmission Operation Automatically shifted manual transmissions are a fairly recent innovation. The benefit of the manual transmission is that (due to the direct mechanical connection through fixed gears) efficiency is very high. The drawback is that there must be some interaction with the user in the selection and changing of gears. Automatically shifted manuals were created to address this issue. These types of transmissions are traditionally synchronized manual transmissions with the addition of automation of the gear selection and control of the clutch. A logic controller is also employed to decide when and how to shift. Automatic shifting is usually accomplished through the use of electro-hydraulics. A high-pressure electric pump supplies pressure to hydraulic solenoids that are used to shift the transmission. A hydraulic ram is also used to engage and disengage the clutch. Current versions of these transmissions also employ unsynchronized gears. This allows for overall smaller packaging to accomplish the same task. Input speed of the engine is monitored along with lays haft speed. When a gear change is initiated, the controller opens the clutch, shifts to the desired gear while matching engine and lay shaft speed, and then closes the clutch again. This shifting operation can all be achieved in less than one third of a second. Automatically shifted manual transmissions shift gears faster than humanly possible. [3] 1.1.5 Manually Shifted Automatic Transmission Operation Manually shifted automatic transmissions are a variation on control of the transmission. The user is allowed to select either automatic or manual shifting modes. During automatic mode, the transmission functions identically to an automatic transmission. While in manual shift mode however, the transmission controller allows the user full authority over gear changes as long as the gear change will not over speed the engine. This mode of operation traditionally offers the user tighter, more positive shift feel. The only requirement of an automatic transmission for manual shifting is that shifts must be accomplished rapidly enough to allow the user a feeling of fluidity. The act of shifting must provide the immediate desired response. [3] 1.1.6 Planetary Gear Drive Transmission Operation Planetary gear sets are unique in that the combination of gears creates a two degree-of-freedom system. The gear sets are comprised of a ring gear, a sun gear in the center, and planetary gears that contact both the ring and the sun gears. Motion of the planetary gears is controlled by the carrier on which each of the planetary gears rotate. The carrier maintains the position of the planets in relation to each other but allows rotation of all planets freely. Inputs (or outputs) to the gear train are the ring gear, sun gear, and planetary carrier. By prescribing the motion of any two of these parameters, the third is fixed in relation to the other two. By employing one planetary gear train, a fixed ratio between input and output is created. Increasing or decreasing the number of teeth on the sun and ring gears can change this ratio. This in turn changes the number of teeth on the planetary gears, which has no other effect as these gears act as idlers. When combining more than one planetary gear train at one time, braking or allowing the movement of different elements can create a wide range of effective operation in terms of relative speeds, torque transfer, and direction of rotation. This is the type of system that is used in automatic transmissions described above. These systems are also employed in large stationary power transmission applications. [1] 1.2 Current Hybrid Electric Vehicle Transmission Design Hybrid vehicles are vehicles that utilize more than one power source. Current propulsion technologies being favored are compression ignition (CI) engines, spark ignition(SI) engines, hydrogen-fueled engines, fuel cells, gas turbines, and high power electric drives. Energy storage devices include batteries, ultra-capacitors, and flywheels. Hybrid power trains can be any combinations of these technologies. The aim of these vehicles is to use cutting edge technology combined with current mass-produced components to achieve much higher fuel economy combined with lower emissions without raising consumer costs appreciably. These vehicles are targeted to bridge the gap between current technology and the future hope of a Zero Emission Vehicle (ZEV), presumably a hydrogen-fueled fuel cell vehicle. The operation of these systems must also be transparent to the user to enhance consumer acceptability and the vehicle must still maintain all required safety features with comparable dynamic performance all at an acceptable cost. By combining multiple power sources, overall vehicle efficiency can be improved by the ability to choose the most efficient power source during the given operating conditions. This is key in improving vehicle efficiency because current battery technology dictates that nearly all total energy used by the vehicle across a reasonable range of driving comes from the on-board fuel. Highly adaptive control strategies that may be employed in the next generation of HEV may monitor vehicle speed, desired torque, energy available, and recent operating history to choose which mode of operation is most beneficial. These advanced control schemes will maximize the usage of the fuel energy available by choosing the most efficient means of power delivery at any instant. The reduced usage of energy for a given amount of work may also result in lower exhaust emissions due to a reduction in fuel energy used. 1.2.1 The Advantages and Disadvantages of Series Hybrid Vehicles Series hybrid vehicles typically have an internal combustion engine (ICE) that is coupled directly to an electric alternator. The vehicle final drive is supplied entirely by an electric traction motor that is supplied energy by the battery pack or combination of engine and alternator. The benefit of this type of operation is the engine speed and torque are decoupled from the instantaneous vehicle load and the engine needs only to run when battery state of charge (SOC) has dropped below some lower level. This allows engine operation to be optimized for both fueling and ignition timing in the case of a spark ignited engine, or fueling and injection timing for a compression ignition engine. The engine is also operated in the most efficient speed and torque without encountering transient operation regardless of load. The result is excellent fuel economy and low emissions. Series HEV operation is exceptionally well suited to highly transient vehicle operation which is prevalent in highly urban areas and city driving. The disadvantage to series hybrid operation is the efficiency losses associated with converting mechanical to electrical and then electrical to mechanical energy. Further losses in system efficiency are realized when the energy is stored in the battery pack for later use. Only a fraction of the energy put into the batteries can be returned due to the internal resistance of the batteries. The mechanical energy of the engine is directly converted to electricity by an alternator that has losses both in internal resistance and eddy currents present. Further losses are incurred when this electrical energy is converted back to mechanical energy by the traction motor and controller. Dynamic performance is also limited, as the engine cannot supplement the traction motor in powering the vehicle. 1.2.2 The Advantages and Disadvantages of Parallel Hybrid Vehicles Parallel systems also employ two power sources, typically an engine and a traction motor with both directly coupled to the wheels typically through a multi-speed transmission. This requires that the engine see substantial transient operation. However, the motor can act as a load-leveling device allowing the engine to operate in a more efficient operating region. When the vehicle is operating in a low load state the engine can be decoupled from the drive train and shut off, or the motor can be used to charge while driving creating a greater power demand for the engine and storing energy in the battery pack. The disadvantage of parallel hybrids is that direct connection of the engine to the wheels requires transient engine operation. This operation lowers fuel economy and increases exhaust emissions especially when employing SI engines. Ignition timing and fueling cannot be optimized for a single region of operation either. However, dynamic performance of parallel hybrids is much better than that of series hybrids using the same components. Much more power is available as both the engine and motor can provide power to the wheels simultaneously. These characteristics lend parallel HEV to excel in higher load, less transient situations and when using high efficiency engines such as CI engines. 1.2.3 The Advantages and Disadvantages of Combination Hybrid Vehicles The third variation of hybrid vehicle drive trains is the combination, which is a system that can function both as a series and parallel hybrid. Complex combinations of engines, alternators, and motors can accomplish this with geared connections and multiple clutches. By clutching and declutching different elements, a combination can be designed to function as a series hybrid under low speed transient conditions and then as a parallel hybrid under higher speed and load. This allows for increased efficiency as each mode of operation is employed under the ideal operating conditions. Drawbacks to these systems are increased mechanical and drive train control complexity along with higher weight associated with more components. Controlling these types of systems is much more difficult than either a series or parallel HEV. The system must first be capable of operating as a