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Recently even for medium or heavy duty commercial vehicles (MD/HD CVs), hybrid electric vehicles (HEVs) or electric vehicles (EVs) have been launching in the world. In the current status, parallel type of HEV shows the most practicality in market. In that case, the optimal engine adaptation and the highly efficient brake energy recovery are the keys for efficient HEV. The EV application for MD/HD CVs is also the possible solution in the near future if some issues are cleared. Therefore it is hard to decide the most possible case for future CVs only from the existing point of view. The conceptual new name of “XEV” will be suitable to express those possible cases generically. The improvement of energy storage system (ESS) will have been the most prioritized issue for “XEV” for a while.

We consider the modeling and control design of the regenerative braking system for heavy duty hybrid electric vehicles (HEVs) which have an isolated air-over-hydraulic (AOH) brake system and a generator. A nonlinear model is set up to characterize the behavior of the brake system. Then, the brake control is formulated as a torque tracking problem according to the driver's operations. The AOH brake system is appointed to track a constant brake torque; meanwhile, the generator is designed to track the torque error between the desired braking torque and the torque output of the AOH brake system. Finally, numerical experiments are carried out to verify the proposed model and control algorithms.

Ultra-low energy consumption and ultra-low emission vehicle technologies have been developed by combining petroleum-alternative clean energy with a hybrid electric vehicle (HEV) system. Their component technologies cover a wide range of vehicle types, such as passenger cars, delivery trucks, and city buses, adsorbed natural gas (ANG), compressed natural gas (CNG), and dimethyl ether (DME) as fuels, series (S-HEV) and series/parallel (SP-HEV) for hybrid types, and as energy storage systems (ESSs), flywheel batteries (FWBs), capacitors, and lithium-ion (Li-ion) batteries. Evaluation tests confirmed that the energy consumption of the developed vehicles is 1/2 of that of conventional diesel vehicles, and the exhaust emission levels are comparable to Japan's ultra-low emission vehicle (J-ULEV) level.

Recently natural gas has attracted public attention as clean fuel for motor vehicles. We first developed a heavy-duty compressed natural gas (CNG) engine for city busses and manufactured many CNG-fueled engines. Both medium- and heavy-duty CNG engines achieved very low exhaust emissions. However, the fuel consumption of these engines for example the city-bus application are higher than that of a diesel engine. For this reason, these CNG engines always operate under the part-load conditions. Therefore, we developed a direct-injected CNG engine. Under a part-load condition, the engine is operated on the stratified-charged natural gas that is directly injected into the combustion chamber. It is the most important that the air/fuel ratio of the mixture stratified near the spark plug must be controlled to achieve the stable mixture condition.