In the period of shortage of gasoline after the Second World War, EV began to be listed in Japan as an alternative energy vehicle. In 1949, domestic EV production in Japan reached 3,299 vehicles, which accounted for 3% of the car ownership in Japan at that time. However, with the improvement of engine cars and the popularization of gas stations, the momentum of EV began to gradually decline in Japan.
After that, the automotive industry began to develop EVs in 1971 by positioning them as environmentally friendly vehicles. At the time, Japan ’s Ministry of International Trade and Industry ’s Industrial Technology Institute used a large-scale project system (participated by car, motor, and battery manufacturers) to start EV research and development. But after 1980, with the advancement of engine exhaust purification technology, EV disappeared again.
In 1990, 20 years later, California, the United States formulated the exhaust emission regulations "ZEV Act" (Zero Emission Vehicle Act). At that time, no vehicle other than EVs could meet this requirement, so EV development was started again.
The ZEV Act was implemented in 1998. Since it is necessary to sell a specified percentage of EVs, companies have begun to develop hard. However, the regulation was not implemented on schedule, and ended up with a limited production of several years.
HEV with EV element technology
As mentioned above, EVs have problems in travel distance, charging time, and cost, and so far, they have only been popularized in specific application fields such as forklifts.
To solve the above-mentioned problems of EVs, the cars with better fuel efficiency and lower emissions are the HEVs that were launched in the second half of 1990. Toyota launched the "Prius" in 1997, and Honda launched "Insight" in 1999.
These HEVs use EV element technology developed to comply with the ZEV Act. In particular, nickel-metal hydride rechargeable batteries were adopted in Toyota's "RAV4EV" and Honda "EV PLUS" which were put into practical use in 1996. Since it helps to extend the continuous driving distance of EVs, it is not an exaggeration to say that HEV cannot be realized without a nickel-metal hydride rechargeable battery. In addition to batteries, permanent magnet (PM) synchronous motors developed for EVs that use rare-earth magnets also contribute to the improvement of HEV performance.
Before introducing the HEV system, let's talk about the EV developed to comply with the ZEV Act. Figure 1 lists the system configuration of Toyota RAV4 EV. This system controls the inverter by the EV · ECU (Electronic Control Unit) based on the amount of depression detected by the accelerator sensor, and drives the travel motor. The motor uses a permanent magnet motor.
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| Figure 1: Toyota “RAV4 EV†system configuration An EV equipped with rechargeable nickel-metal hydride batteries was put into practical use in 1996. |
The battery that drives the motor uses a 288V nickel-metal hydride rechargeable battery. The battery ECU and EV · ECU monitor the charge and discharge status to calculate the remaining capacity at any time. When charging the battery, use the car charger to pass the AC 200V commercial power supply. Hereinafter, the high-voltage rechargeable battery driving the travel motor is referred to as a main battery, and the auxiliary driving rechargeable battery is referred to as a 12V battery.
In EV travel control, the required driving torque of the vehicle is determined using the driving torque map based on information such as accelerator opening, braking signal, gear position, and vehicle speed. The vehicle control unit of the EV · ECU issues a torque command to the motor control unit, and transmits a command to the inverter through a PWM (pulse width modulation) signal. Motor control adopts the method of using as an electric motor during acceleration or normal driving and as a generator during deceleration (Figure 2).
Figure 2: EV driving control determines the vehicle's driving torque based on the accelerator opening and braking signals.
Overview of the hybrid system
There are two classification methods for hybrid power systems. One is to classify according to different functions that can be realized, and the other is to classify by means of driving mechanism.
First, if you categorize it by function, it looks like Figure 3. Only those without idling function are called micro HEV or ISS (Idling Stop System). Adding acceleration assistance, energy regeneration, and efficient engine operation functions to this function is called weak HEV, and adding EV driving function is called strong HEV. The closer to a strong HEV, the less CO2 emissions and tail gas. The EV emissions are all zero. In addition, a plug-in HEV (PHEV) and a range extender-type EV that extends the driving distance by operating a power generation engine are positioned between the strong HEV and EV.
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| Figure 3: The CO2 emission reduction effect of HEV and EV classifies the hybrid system according to different functions. |
(1) Tandem HEV
The serial HEV is equipped with an engine and a generator that charge the main battery, and runs with the motor while always charging (Figure 4). It can also be considered that an engine and a generator are added to the EV. Compared with private passenger cars, this method is more commonly used on public buses. Toyota ’s “Coaster HEV†launched in 1997 and Mitsubishi Fuso Truck ’s “Aero Nostep HEV†launched in 2004. This way.
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| Figure 4: The structure of the tandem HEV is driven by the motor only. Equipped with high-power large motor. |
The characteristics of the tandem HEV are as follows.
· Only motors drive, so compared with other methods, motors and generators are high-power large products.
· All engine power is converted into electricity, so the energy efficiency is slightly lower.
· Drive force control and power output control are relatively simple.
· The engine runs in a stable state, so it is easier to purify the exhaust gas.
(2) Parallel HEV
The parallel HEV is equipped with an engine and a motor in parallel, and driving power can be supplied by both parties (Figure 5). In addition to Honda's practical use as "IMA (Intelligent Motor Assist)", it has also been adopted by Daimler "Mercedes-Benz S400 HYBRID" and BMW "AcTIveHybrid 7". In Honda's IMA, the engine and motor are connected directly and rotate at the same time. Unlike this, many manufacturers have developed a system that sandwiches a clutch between the engine and the motor and disconnects the clutch to achieve EV driving.
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| Figure 5: The parallel HEV (direct connection) motor only plays an auxiliary role and uses small products. |
The characteristics of the parallel HEV (direct connection) are as follows.
· It is only necessary to add a motor between the engine and gearbox of the previous model, so the structure is simple.
· The power output of the motor only plays an auxiliary role. The EV drive is hardly carried out, so the motor is a small product.
· The motor also acts as a generator, so the regenerative power can only be used for driving after being stored in the battery.
· By inserting a clutch between the engine and the motor, EV driving is possible, but this requires a large power-conveying motor.
(3) Series and parallel HEV
A representative example of the series-parallel HEV is "THSâ…¡ (Toyota Hybrid System II)" adopted by Toyota Prius and others. This method uses the planetary gear mechanism to integrate the three power sources of the engine, MG1, and MG2, and combines these power sources according to the driving state to drive them (Figure 6).
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| Figure 6: The configuration of series-parallel HEV has the advantages of both series and parallel systems. |
MG here refers to the acronym for motor and generator. Since it is necessary to frequently switch between the motor function and the generator function, the part originally called the motor or the generator is called MG.
The role of the engine is to drive the vehicle and drive MG1. In addition to charging the main battery, the role of MG1 also includes starting the engine as a motor and driving assistance for the vehicle. The role of MG2 is to achieve EV driving, acceleration assistance, and energy regeneration as a generator.
The characteristics of series-parallel HEV are as follows.
· It has the advantages of both series and parallel modes, taking into account both fuel efficiency and driving performance.
· The system efficiency is high, so the fuel efficiency improvement effect is remarkable.
· The system and control are more complicated.
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