Mitsubishi Heavy Industries Technical Review Vol. 47 No. 4 (December 2010)17

Development of Highly Efficient Power Train forLarge Electric Buses on Regular Routes

Tr a n s p o r ta t i o n S y s t e m s & A d v a n c e d
Te c h n o l o g y D i v i s i o n
Advanced Mechanical Systems
D e pa r t m e n t

To achieve a low-carbon society, Mitsubishi Heavy Industries, Ltd. (MHI) is currently
developing an electric power train consisting of batteries, a motor, and an inverter for use inlarge electric buses of a highly public nature that run on regular routes. The widespread use oflarge electric vehicles is thought to be a development of the distant future, because the capacityof the batteries required for electric vehicles usually increases with the vehicle weight. However,large electric buses on regular routes can be used in practical operations without mounting alarge battery because approximately 80% of all routes in urban Japan are shorter than 15 km.

|1. Vehicle Concept
To enable an early market launch, MHI selected the low-floor, 65-passenger series hybrid
bus made by Mitsubishi Fuso Truck and Bus Corporation (MFTBC) as the base vehicle anddecided to collaborate with MFTBC in the joint development of large electric buses on regularroutes. As shown in Figure 1, the low-floor bus runs on a motor that is powered by electricity
generated by an engine and then stored in a battery. By replacing the engine with a combination ofrechargeable, replaceable batteries and developing an appropriate way to manage the batteries,
large electric buses that do not need large-capacity batteries can be used on regular routes. MHI
mounted its Li-ion batteries, drive motor inverter, and an electric air conditioner on the bus toprovide a highly efficient electric power train, with the result that the total energy management
improved the rate of electric consumption (travel distance per unit of power consumed), thereby
further reducing CO2 emissions and improving the operability with a small capacity battery.

|2. Solutions to Improve the Rate of Electric Consumption by Total
Energy Management
The following outlines some of the solutions discussed to improve the rate of electric
consumption through total energy management.

Figure 1

Series hybrid bus

Figure 2 Distribution of the workload
during operation on practicalroutes

Mitsubishi Heavy Industries Technical Review Vol. 47 No. 4 (December 2010)18

2.1

Drive Motor Inverter
For the drive motor inverter, a solution can be designed specifically for regular-routebuses, and for enhancing the performance of the device itself. Although the routes ofregular-route buses vary from one operation route to another, the buses do not move randomly
like general vehicles do, and their running and stopping patterns are similar. Consequently, thedrive motor work does not deviate markedly from the distribution shown in Figure 2.
Therefore, the rate of electric consumption can be improved, for example, by setting the best
efficiency of the motor in the region where the work is greatest.

2.2

Reducing Power Consumption for Air Conditioning
Table 1 shows conventional solutions for reducing the air conditioning power
consumption and the effects of the solutions. When compared with measures for altering theheat conduction and heat radiation of the vehicle itself, the following solutions were far more
effective in reducing the air conditioning power consumption: (1) a “door-air-curtain” can beused to prevent heat from entering or leaving the bus when the door is opened at stops, which isa situation peculiar to regular-route buses; and (2) exhaust heat can be recovered from the drive
motor inverter to secure a heat source for heating air.
Figure 3 shows an example of an analysis of the effect of the door-air-curtain mentioned
in solution (1). The air-curtain blown from the upper portion of the door significantly reduces
the outflow of heat during heating, but has little effect during cooling.
Regarding solution (2), the greatest weakness of electric vehicles is the lack of a heatsource for heating; this said, vehicles equipped with an internal combustion engine use exhaust
heat from the engine. Therefore, recovering the exhaust heat from the drive motor inverter is aneffective countermeasure.
Table 1 Solutions to reduce the air conditioning
power consumption and effects thereof
Solution(a) Without the air-curtain

Improved heat insulation for(%)
the vehicle bodyHigh-barrier(%)
double-insulating glass
High solar reflective coating (%)
Door-air-curtain(%)
Exhaust heat recovery(%)

Reduction in the air
conditioning power
consumption
Cooling Heating-3-9-9-4-2-1--15-16(b) With the air-curtain

Figure 3

Effect of the door-air-curtain

Figure 4 Comparison of the simulation results
and measured values|3. Driving Simulator and Validation Thereof
MHI developed a driving simulator as a tool for evaluating highly efficient power trains. To
validate its accuracy, MHI used an existing motor inverter and batteries, and applied a loadequivalent to that of legal driving mode on a test bench. The simulation outcomes and measured
values of the remaining battery power are compared in Figure 4. The difference between thesimulated and measured values was less than 1%, showing excellent agreement and confirming thesimulator as an evaluation tool for highly efficient power trains.