1. Development background

Improving the mileage and emissions performance of public service buses that ply the streets of our cities today is an urgent task. Reductions in NOx, particulate matter (PM) and other tail-pipe emissions, together with improvements in fuel economy - which in turn reduce CO2 emissions - help to reduce atmospheric pollution and global warming respectively.

The rapid aging of the Japanese population is another factor to be taken into consideration, and means that buses with stepless floors are likely to become the leading type of public service vehicle in the near future. The layout of the propulsion system in today's buses, however, restricts the area of floor that can be made flat and makes it difficult to satisfy requirements for improved passenger movement, or for wheelchair access, through the bus.

The Mitsubishi HEV system provides the ideal propulsion system for large public service buses and is readily applicable. Using a diesel engine to drive the electricity generator, this system does not require the establishment of any special infrastructure to go into widespread service as a low-emission vehicle throughout Japan. In addition, realizing major reductions in emissions, fuel consumption and noise, it offers greatly enhanced environmentally acceptability, while its rational layout enables a more extensive stepless floor area.

Mitsubishi FUSO Aero No-step HEV
(Hybrid electric large public service bus)

2. HEV drive system profile

Type:	Series-configuration hybrid system
Generator engine:	6M6 8.2-liter diesel
Generator:	100kW
Traction motors:	2 x 150kW max.
Control system:	VVVF inverter with brake energy regeneration
Storage batteries:	High-density 648V lithium-ion

3. HEV system features

The Mitsubishi HEV system employs series-configuration hybrid propulsion, in which the diesel engine is used only to generate electrical power. The benefits of this configuration include the fact that the engine drives the generator at its most efficient speed, and that the bus is in the EV (battery power) mode with the engine off when stationary or moving off. As well as enabling tuning for optimal emissions and mileage performance, this configuration will enable compliance with the next regulated NOx and PM levels expected to follow the current New Short-term Regulations.

1. System configuration
   
   

   : Separating traction motors and generator engine brings greater freedom in layout design. : Generator engine runs at fixed speed, reducing emissions and fuel consumption



2. System operation
   
   

   A : Moving off and low-speed operation : Bus moves off or operates at low speed using electrical energy from the storage batteries. : Bus uses twin auxiliary power system to operate brakes and power steering when engine generator is not running. : Realizes major reductions in noise when operating in strictly regulated areas or at night.

   B : Hard acceleration, uphill driving, charging : Engine-generator produces electrical energy that is stored in batteries. Power from generator and from batteries provides gutsy traction for hard acceleration, climbing gradients, etc. : Twin motors deliver dependable, gutsy power.

   C : Braking : System motors use inertial energy from braking to generate electricity. Boasting input density on par with a capacitor, the lithium-ion batteries and twin motors enable high-efficiency regeneration. : Latest CAN (Controller Area Network) technology enables the regenerative braking and Electrically-controlled Braking Systems to convert braking energy even at the slowest vehicle speeds.



3. Diesel engine speed band
   
   

   In the series-configuration HEV system, the generator engine operates at a fixed speed independent of vehicle speed or running loads. This makes it possible to configure the system so that electricity generation occurs at a point midway between the high-efficiency combustion and emission zones, and thereby achieve low consumption and emissions performance.

   
   
4. Emission levels
   
   In the series-configuration HEV system, the generator engine operates at a fixed speed independent of vehicle speed or running loads. This enables major reductions in emissions compared with conventional systems which use the engine over its full speed range.
   
   
   
   The Mitsubishi HEV system uses the engine solely for generating purposes, making possible the use of gasoline, liquid petroleum gas, compressed natural gas and other fuels. MMC is also looking at the use fuel cell technology in its on-going development of the Mitsubishi HEV system.
   
   The Mitsubishi HEV Bus uses a new type of lithium-ion battery developed for HEV use that features high input/output and energy density properties, and uses the traction motor as a generator to convert inertial energy into electrical energy when the brakes are applied. In concert with the Electronically Controlled Braking system, this enables over 80% of braking energy to be used in power regeneration and realizes a 70% improvement in mileage over a diesel-powered bus with hydromechanical automatic transmission (as measured by MMC).
   
   
   
   
5. New lithium-ion storage cell
   
   The new lithium-ion storage cell for HEV use improves on the properties of the lithium-ion battery for EV use.
   
   
   1. Components of lithium-ion cell
      
      

      Lithium-ion batteries for large bus Cell voltage / weight (V/kg) 3.6 / 1.7 No. of cells (per module) 30 No. of modules 6 Total battery voltage (V) 648

      
      

      Note:(

      A Mitsubishi FTO-EV prototype powered by lithium-ion batteries for EV use covered a total distance of 2,142 km between 8am 19 December and 8am 20 December at the MMC Car Research & Development Center Proving Ground to rewrite the world 24 hour record for an EV.)

   
   
   2. Battery performance comparison
      
      The chargeability (input density) of the new HEV lithium-ion storage cell has been improved to a level similar to that of a capacitor. Its energy density is 10 times that of a capacitor, and as well as enabling the bus to run under battery power, it charges much faster because of its higher output density.
      
      

      	Lithium-ion		Capacitor(reference)
      	For HEV use	For EV use *
      Input density (W/kg)	425	165	450
      Output density (W/kg)	1000	1050	450
      Energy density (Wh/kg)	60	92	6.1

   
   
6. Super-single tires (rear only)
   
   The compact dimensions of the traction motor unit and the use of Super-single tire rear wheels realize an aisle that is 70% wider over the rear wheels than its diesel-powered counterpart, without requiring any suspension modifications.
   
   

   Size: 435/54R22.5 Rolling resistance: 15 % less than double tires on diesel-powered bus Weight: 18% less than double tires on diesel-powered bus

4. HEV large stepless bus profile

The stepless floor stems from the transverse mounting of the engine and generator unit at the rear of the vehicle and of the battery modules in the roof. This arrangement has also realized a more spacious floor area over the wheels. And moving the rearmost seats 500mm rearward has increased the floor area and enables a greater degree of freedom in locating the doors.

Delivering generously ample performance for urban operation, the Mitsubishi HEV Bus also realizes the smooth ride and shift jerk-free acceleration only possible with electric motor propulsion.

The Mitsubishi HEV Bus is also much easier to operate, with the electric traction motor eliminating the need for a clutch and realizing fully automatic drive.