According to incomplete statistics, there are more than 0.8 billion motor vehicles around the world, all of which consume more than 1 billion tons of petroleum per year. Furthermore, these figures are increasing with year. The exhaust greenhouse gas CO2 and various poisonous substances bring about increasingly serious environmental pollution and damage. On the other hand, petroleum resources all over the world can only last for the next few decades. Thus, intensive attention has been paid to energy saving automobiles to address the problems of pollution and energy shortage due to automobile booms.
According to statistics from DOE of United States, only 15% of the energy of the gasoline in an automobile tank is used to drive the automobile to travel or to power other useful accessories such as an air conditioner. The remainder energy is wasted. Part of the load and energy consumption of an automobile is to overcome the rolling friction between the tires and the ground, which is proportional to the overall weight of the automobile and the rolling friction coefficient, with the consequential required power slightly increasing with increases in automobile velocity. Another part of it is to overcome the wind drag due to the movement of the automobile, which is proportional to the square of the velocity, with the consequential required power being proportional to the cube of the velocity. A third part of it is used to start, accelerate and climb so as to provide power necessary to increase its kinetic or potential energy. This portion of kinetic or potential energy is transited into friction heat in braking processes. In city road conditions, the energy consumption at low velocities and low loads and at standby accounts for 17.2%, the energy consumed by the generator and the compressor of the air conditioner accounts for 2.2%, the loss in transmission accounts for 5.6%, braking consumption accounts for 5.8% and loss due to rolling friction and aerodynamic drag accounts for 6.8%. Only 12.6% of heat energy in gasoline is consumed during traveling. If the energy consumption at low velocities and low load, at standby, and at braking, which totally accounts for 23%, is reduced by one half, then 50% energy could be saved. Therefore, there is a great potential to save energy through advanced techniques.
The predominant power device of motor vehicles is an internal combustion engine, especially, a gasoline engine, whose dynamic characteristics presents a rotation speed of 600-6000 rpm, a substantially constant (slightly convex) torque profile vs. rotation speed, and an approximately linear proportional torque profile vs. throttle open degree. Universal characteristics of an internal combustion engine demonstrate that it presents a high efficiency only when the rotation speed and load is in a certain range, and that its efficiency may be significantly decreased in case of low rotation speed and low load. Moreover, characteristics of a gasoline engine and those of a diesel engine are slightly different, e.g., the higher efficiency ranger for a diesel engine is relatively broader. In rating conditions, the average effective efficiency of the engine itself is only 37.6% (218 g/kwh). In city traffic conditions, most working periods of the engine are in low load state, e.g., in a state that the load is about 10-30% of the rating load and the average efficiency of the engine is only 16-18% (480 g/kwh).
Approaches for saving energy and decreasing emissions mainly include improving the heat efficiency of the engine itself (such as the technique of a diesel engine with a high compression ratio), decreasing the power consumption due to drag (for example, lowering the vehicle weight, decreasing the wind drag and the rolling drag), employing new power and new energy (such as electricity, hydrogen fuel cells and methanol), improving the energy efficiency of the whole vehicle through changing the transmission manner (such as taking use of transmissions of higher efficiency, adopting continuously variable transmission, employing hybrid power, and regenerating braking energy). However, no excellent energy-saving approaches have been developed for hundreds of million vehicles that have been put into use nowadays.
The present invention fulfills the purposes of saving energy and decreasing emissions mainly through improving the energy efficiency of the whole vehicle, which is realized by changing the transmission manner, and through employing electric technologies.
In an automobile power system, a transmission plays a role only second to that of the engine. According to their operating and transmission manners, automobile transmissions may be classified into two categories, that is, manual/automatic stepped transmissions and continuously variable transmissions. A manual stepped transmission itself is of a higher efficiency. However, it is difficult to optimize its cooperation with the internal combustion engine, leading to a low overall efficiency; and frequent shifting is a labor-intensive burden. An automated mechanical transmission (AMT), which may be employed to replace manual manipulations, imposes a higher requirement on intelligentized control of the system. Even an automatic transmission (AT), which is composed by a hydraulic torque converter and a planetary gear train, may increase the engine efficiency, its efficiency is relatively low, its control system is complicate, and its manufacture is difficult. A continuously variable transmission is an ideal transmission that is pursued for a long time. In some continuously variable transmissions, such as those of a chain or steel belt type, continuously variable transmission is realized via friction force, such that the efficiency is greatly increased. However, their ranges of speed shifting and torque converting are limited due to the limitations on material properties and minimal curvature of steel belts, thus they still need to function in cooperation with hydraulic torque converters or starting clutches. In some of them, a hydraulic variable displacement pump/motor is employed to constitute a fully hydraulic continuously variable transmission system, whereas the overall efficiency of the system is less than 80% even if an individual pump or motor has efficiency up to 90%. As an improvement, U.S. Pat. No. 6,145,409 (entitled by Hybrid Gearbox), Komatsu Ltd. in Japan and some universities in China propose to carry out multiple step continuously variable transmission in a mechanical-hydraulic power split manner, that is, they propose a power split transmission solution which combines hydraulic continuously variable shifting and mechanical transmission. Even though this approach achieves an excellent energy-saving effect, it can only be applied to coaches, school buses, tractors, and mobile machinery shops for special purposes due to its large volume and high weight.
There are three challenges associated with currently widely-used motor vehicles which employ internal combustion engines for power, that is, the low efficiency (no matter what transmissions are employed) in low load conditions (such as the conditions of uniform motions at low velocities), kinetic energy regeneration during braking and the contradiction between the system dynamics and economy, which inspire a hybrid mechanical-electrical power mode and a hybrid mechanical-hydraulic power mode.
Hybrid mechanical-electrical driving systems can be classified into three categories according to the cooperation manner of the mechanical system and the electrical system, that is, a serial mode, a parallel mode and a hybrid mode. According to the relative extent of electrical power, the systems may be classified into strong hybrid ones and weak hybrid ones. In weak hybrid systems, the released electrical power is only used to control the starting and shutdown of the engine and to aid in accelerating, thus the energy-saving effect is limited. By contrast, strong hybrid systems may independently drive vehicles to travel via electrical power. For general cars in urban traffic conditions, this brings about a fuel consumption of 3.5-5.5 L per one hundred kilometers (decreased by 30-50%) and a 90% reduction in emission. Electrical power is mainly used in low speed and low load conditions, and the engine is started once beyond a certain velocity. Since it makes use of a motor characteristic that the torque is relatively large in low speed conditions so as to directly drive the automobile to start and to accelerate, the driving current is large and the efficiency is low at this time. Its energy transfer chain of the electrical power is very long, that is, from engine to generator, to convertor, then charging and discharging the cell pile, then to convertor, to motor, and finally to automobile, thus the efficiency of utilizing electrical power can only amount to 70%. In high speed conditions, the motor must participate in operation, leading to a decreased overall efficiency in such conditions. During automobile braking, in order for kinetic energy to be efficiently regenerated, motors of large power and the cell pile capable of being repeatedly charged and discharged with large currents should be employed, thus resulting in a very high cost.
Besides the hybrid mechanical-electrical mode, a hybrid mechanical-hydraulic driving method is also applicable. Since the loss of the hydraulic energy storage is low, its power density is large, the regeneration rate of a hydraulic system is higher than that of an electrical system, hydraulic techniques are fully developed, and at the same time hydraulic systems have long service life and low cost, researchers in Europe and Japan conducted numerous experiments to explore this technique in 1970s during which period a global energy crisis prevailed. However, the developments were below expectations due to the limitations from the state of arts at that time. Nevertheless, some researchers in American have been keeping researching since then. Hybrid mechanical-hydraulic systems can also be classified into a serial mode and a parallel mode. In the serial mode, such as Hydraulic Launch Assist (HLA) which has been tested by U.S. Environmental Protection Agency, FORD and EATON, a hydraulic pump is serially provided in the original transmission system to regenerate kinetic energy during braking, which is to be released during restarting of automobiles. It is reported that this may regenerate 60% kinetic energy during braking, the energy consumption in urban traffic conditions is reduced by 25-35%, and emission is decreased by 50%. Similar systems are also disclosed in CN patent 200420040920.8, entitled by “hydraulic hybrid power transmission device of a serial mode for buses”. However, energy-saving effect of serial systems in high velocity conditions is not significant. In a parallel mode, for example, as disclosed in U.S. Pat. No. 6,719,080 (entitled by “Hydraulic Hybrid Vehicle”) and CN patent 01803550 (entitled by “Vehicles Driven in a Hydraulic Hybrid Mode”), both granted to U.S. Environmental Protection Agency, and CN patent 200420040925.0 (entitled by “Hydraulic Hybrid Power Transmission Device of a Serial Mode for Bus”), there are provided at least two hydraulic pump/motor and two storages of higher and lower pressure. In this mode, shifting and transmission are performed completely via hydraulic means. Even though the efficiency of the engine is greatly improved, the efficiency of hydraulic transmission is low, which counteracts part of energy-saving effect. As improvements to these systems, U.S. Pat. Nos. 4,441,573, 5,088,041 and 5,495,912 proposed various mechanical-hydraulic hybrid driving mechanisms and a variety of energy management methods, such as a dual-engine mode and a multi-stage pressure mode, etc. Among others, a more advanced technique is a dual-mode mechanical-hydraulic hybrid driving mode, that is, continuously variable transmission is performed completely via hydraulic means in low velocity conditions, and purely mechanical transmission is employed in high velocity conditions, with switching therebetween being carried out by two clutches. This approach may improve the energy-saving effect, nevertheless to a limited extent in that the efficiency of hydraulic transmission is low in low velocity conditions and the contradiction between system dynamics and economy remains unsettled due to purely mechanical transmission in high velocity conditions. In China, School of Mechanical and Vehicular Engineering, Beijing Institute of Technology conducted numerous theoretical researches and explorations on hybrid power and continuously variable transmission via a mechanical-hydraulic split power manner, and achieved some simulation results.
In summary, the three key challenges confronting energy-saving could not be solved through employing a pure automobile transmission, whatever it is a manual transmission, an automatic transmission or a continuously variable transmission. In a mechanical-electrical hybrid driving manner, even though the energy-saving effect is significantly improved, the cell property and system cost become key restrictions to its development. There are many problems to be solved, such as realizing quick charging and discharging cells, improving the energy storage efficiency and extending the service life of cells; in addition, the driving system is complicated, and required mechanical-electrical switching control systems, cell piles and high capacity motors are of high costs; furthermore, it is impractical to retrofit about 0.8 billion vehicles, which are in use throughout the world, in this manner for the purpose of saving energy. The prior art mechanical-hydraulic hybrid driving mode still bears the disadvantages of low specific energy and low transmission efficiency of hydraulic systems. Therefore, the accordingly designed structures are complicated, heavy and bulky, and thus can only be applied to large-scale vehicles that frequently travel in urban areas, for example, such buses, mail cars and garbage trucks, etc. The overall energy-saving effect of the system is not as significant as expected. International automobile industry is currently unanimously optimistic about plug-in mechanical-electric hybrid automobiles in that this solution may enhance energy-saving effect, lower emission and conserve environment. However, a challenge is how to reduce its cost. The object of the present invention is to solve all these problems.