INTRODUCTION The carbon dioxide emitted by Japan’s transportation sector accounts for 20% of the country’s total emissions. Automobile manufacturers are currently engaged in research and development related to fuel-efficient and hybrid vehicles, with a goal of improving engine efficiencies and thus reducing carbon dioxide emissions [ 1]. Trucks account for a high percentage of these carbon dioxide emissions, so considerable effort is being made toward applying hybrid technology to small- and medium-size trucks. The authors have been conducting research related to the simultaneous improvement of the fuel efficiency and emission characteristics of hybrid trucks through the optimization of the hybrid control [ 2], [3], [4], [5]. Large trucks operate mainly at constant high speeds and so present fewer opportunities for energy regeneration. Therefore, if the fuel efficiency of these vehicles is to be improved through hybridization, it would be necessary to increase the sizes of the motors and batteries [ 6], [7], [8], [9], [10], [11 ], [12], [13], [14]. Since this would give rise to issues such as reduced load space and a higher purchase price, there has been little progress towards the commercialization of large hybrid trucks. The authors, therefore, are proposing a new-type of hybrid system [15]-[16], specifically for large hybrid trucks, which differs from a conventional hybrid system where the aim is to improve fuel efficiency for high-speed operation by reducing the weight of the hybrid system. This new hybrid system is based on the concept of improving fuel efficiency by operating the engine in the high thermal efficiency range by increasing the air flow into the combustion chamber regardless of the engine’s rotational speed. The regenerative energy produced by the electric motor during high-speed operation is used mainly in the electric supercharger, this being an auxiliary device with a low power consumption. This report describes the verification of the improvement in fuel efficiency, etc. It describes the prototyping of the electric supercharger, which is central to the application of this new-type hybrid technology to diesel engines, as well as the testing of a lar ge virtual hybrid truck equipped with the new hybrid system, using a hybrid power train bench-testing system [ 17]. The virtual truck was subjected to the operating patterns of the Japanese heavy-vehicle fuel-efficiency and emissions tests. CONCEPT OF NEW-TYPE HYBRID SYSTEM The conventional hybrid system applied to passenger cars as a means of improving fuel efficiency is shown in the upper half of Figure 1. Regenerative energy is used to power the electric motor linked to the driving axle, with large amounts of energy consumed by every assist, thus necessitating a large battery. On the other hand, as shown in the lower half of Figure 1, the “new-type hybrid system” proposed previously utilizes an electric supercharger (eSC) with a high-voltage driving battery acting as the power supply. This system compensates for the lack of engine torque encountered in the transient state when a small-displacement engine is employed. The axle-driving electric motor is mainly responsible for regeneration/generation while providing torque assist to compensate for the torque shortfall that occurs during acceleration with the small-displacement engine. Since A Study on Improvement of Fuel Economy of Heavy Duty Hybrid Trucks with New Type of Hybrid Electric Assist Engine System Nobunori Okui National Traffic Safety & Enviro Lab. ABSTRACT In order to improve the fuel economy of the heavy duty trucks at a highway driving condition, the heavy duty hybrid trucks with new type of hybrid electric assist engine system were proposed at the previous report. The new system consists of a downsizing diesel engine with a two-stage charging structure, which has an electric supercharger with bypass circuit and a conventional turbocharger, the hybrid electric motor and the small-cap