Types of Hybrid Systems in Vehicles



Automotive engineering

Release date:2022/8/24         

 ・In Japanese
Prerequisite knowledge
 ・AC current
 ・DC-AC conversion, DC-DC conversion, AC-DC conversion
 ・Battery capacity


■Basic system of hybrid cars

A hybrid car is a car that is powered by an engine and a motor, but there are various types of hybrid systems, so we will explain them. First, we will explain the parts and concepts that are common to all hybrid systems. Below, we will use the series-type hybrid system described below as an example.



① Motor
To operate the motor, several hundred volts of AC current (three-phase AC) is required. The power source is a lithium-ion battery or a generator powered by the engine. However, in order to match the operating voltage of the motor, the voltage is boosted by an inverter before being supplied to the motor.

② Generator
It generates electricity using the engine as power. It is called a generator because it is mainly used for generating electricity, but the structure is the same as a motor. Torque is generated when the engine starts.

③ Inverter
Generally, inverters are DC-AC converters, but in this world, inverters are modules that also perform DC-DC conversion and AC-DC conversion. They convert the direct current from the battery to alternating current (DC-AC) to supply it to the motor, convert the alternating current from the generator to direct current (AC-DC) to supply it to the battery, and step down the direct current from the lithium-ion battery to direct current (DC-DC) to supply it to a lead battery.

④ Battery
You might think that hybrid and electric vehicles only use lithium-ion batteries, but 12V lead batteries are also used. This is because electrical equipment (meters, sensors, etc.) used in automobiles may use parts from conventional engine vehicles, so a power source is required to supply 12V. Lithium-ion batteries are several hundred volts, so the voltage is stepped down before being supplied to the lead battery.

⑤ Engine
The purpose of the system varies, such as for driving or generating electricity. Recently, it is called ICE (Internal Combustion Engine).

■Classification of Hybrid Vehicles

Hybrid systems can be classified in several ways.

<Aspect: Positional relationship of the motor and engine to the drive shaft>
When considering the positional relationship of the motor and engine to the drive shaft, there are three main hybrid types.

<Series type>
The series type is a system in which the vehicle is driven only by the motor, and the engine is used to generate electricity to run the motor. The only interface that transmits power is the motor, and the engine and motor are in series when viewed from the drive shaft, so it is called a series type.

If the engine generates power to drive the motor, it would be more efficient for the engine to directly transmit power to the drive shaft, but the engine is inefficient in transient states such as when starting or accelerating, so it is more efficient to drive it with the motor, even if you take into account the loss of conversion to electricity. However, when the vehicle speed is high and the vehicle is in steady operation, the engine is more efficient, so that is a disadvantage of the series type.



Note that the range extender is also a series type. A range extender is a system that extends the driving range that is insufficient with battery energy alone by generating power from the engine. According to the definition of the US authorities, it is as follows, and if it meets this requirement, it is treated as part of an electric vehicle and can receive tax benefits.

① The driving distance on a single charge is 75 miles (121 km) or more.
② The driving distance on electricity generated by the engine dedicated to generating electricity is less than the driving distance on a single charge.
③ The engine must not operate until the battery power is low.

<Series-parallel type (split type)>
The series-parallel type is a method of driving a vehicle using both the motor and the engine, and the motor and the engine are used at their most efficient points. It is called this because it has the characteristics of both the series type and the parallel type. This is the type mainly adopted by Toyota, and although the structure is complicated, it has an advantage in terms of efficiency.



<Parallel type>
The parallel type allows both the motor and the engine to transmit power to the drive shaft at the same time. That is why it is called the parallel type. However, the motor output is not that high, and the engine is the main driver of the vehicle. The motor is positioned to assist the engine in areas where it is inefficient. However, some vehicles can run on the motor alone in certain areas, such as steady driving. Also, the motor cannot be driven when it is generating electricity (The reverse is also true).

The biggest advantage is that it can be realized at a lower cost than the other two methods.



<Aspect 2: Position of the motor relative to the drive shaft>
Aspect 1 mainly showed the positional relationship with the engine, but this does not only show the positional relationship with the engine, but also the position of the motor on the drive shaft, and they are called P0-P5 hybrid systems.



<Aspect 3: Motor's contribution as power>
When considering how much the motor contributes to driving the vehicle compared to the engine (in simple terms, the size of the motor), we can classify them into Strong hybrid, mild hybrid, and micro hybrid. Strong hybrids can run on their own using the motor, while mild hybrids only assist the engine while driving, and micro hybrids only assist when starting from idle stop.



<What is a plug-in hybrid>
A plug-in hybrid is one that can be charged from a household outlet (plug) or the like. Electric vehicles must be able to be charged from an outlet, so we don't call them plug-in electric vehicles. Not all systems implement the plug-in function, and it is not implemented in systems with small battery capacity. This is because the battery quickly becomes full even if it is charged, so there is little benefit to adding the function. Therefore, it is not implemented in mild hybrids or micro hybrids.

■Summary of Hybrid Systems

To summarize, it is as follows.











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