How Hybrid Vehicles Work: An Easy Guide for your Service Team

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By Ted Ings, Executive Director

In the past, shops located in the middle of the country could go years without seeing a Toyota Prius or Honda Insight. 

But things have changed, and hybrid vehicles are no longer relegated to the liberal, coastal states – they’re everywhere. For service advisors, this means you need to know about components like motors and inverters. Otherwise, you’ll struggle in an industry that’s quickly being taken over by hybrid and electric cars.

Types of hybrid cars

Not all hybrids are the same. There are three basic types: 

Micro and belt alternator starter (BAS): Both of these layouts use a lower-voltage, belt-driven motor for engine start/stop. Also, the electric unit provides auxiliary power and regenerative braking. 


Full: Full hybrids use one or more high-voltage motors to power the drive wheels. Depending on the vehicle, the motors may either propel the vehicle on their own or in combination with the gasoline engine.

Plug-in: Plug-in hybrid electric vehicles (PHEVs) add a charging port to the full hybrid design. 

Hybrid powertrain designs

Full and plug-in hybrids can be classified by their powertrain layout:

Series: This design uses the gasoline engine as a generator.
•    The gasoline engine never directly drives the wheels. Instead, it either powers the electric motor or charges the battery.
•    The electric motor is attached to the transmission and turns the drive wheels. 
•    An example is the Chevy Volt. 


Parallel: With this layout, a direct mechanical connection links the gasoline engine to the high-voltage electric motor. 
•    The electric motor never propels the vehicle on its own. Instead, it either provides torque assist to the gasoline engine, or acts as a generator. 
•    Honda Civics built before 2014 are an example.

Series-parallel: This design uses a split device or planetary gear set to disconnect the electric motor(s) from the gasoline engine. 
•    The electric motors can either power the vehicle on their own, or in conjunction with the gasoline engine.
•    The Toyota Prius is an example of this design. 

How hybrid vehicles work

A basic understanding of hybrid sub-systems will help you recognize how these sophisticated vehicles work. The powertrains found in full and plug-in hybrids can be broken down into four main sections: 


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High-Voltage battery: Hybrids have two batteries. The first is the traditional 12-volt unit, which is used for the vehicle electronics. Then there's the high-voltage battery used in the hybrid powertrain. The HV unit can churn out as much as several hundred volts. Output depends on vehicle design and electric motor demand.

Gasoline engine: Hybrid engines aren’t much different from those found in traditional vehicles. Some use an atypical combustion cycle, called the Atkinson-Cycle, but for the most part, that doesn’t affect service or repair. 

Hybrid drive system: All hybrids have one or more electric motor/generator (MG). These units run off alternating current (AC) voltage and are much like oversized alternators. The voltage from the hybrid battery powers the motor when it’s used for vehicle propulsion. Under deceleration or braking, the motor acts as a generator and produces the voltage to charge the battery.
Control electronics: The control electronics manage the hybrid powertrain. The following components are included: 

Inverter: The hybrid electric motors require AC voltage, but the hybrid battery produces DC voltage. It’s the job of the inverter to convert energy from the battery into what the motor can use. When the vehicle is in regenerative mode, the unit also converts AC voltage from the motor into DC voltage for the battery. Plus, the inverter usually houses high-voltage capacitors. These electronic storage devices supply the motor with extra voltage under heavy acceleration.

DC-DC converter: The DC-DC converter changes high-voltage from the hybrid battery into lower voltage for the car’s 12-volt system. Basically, the DC-DC converter replaces the alternator found in traditional vehicles. The converter is often housed inside the inverter.


Hybrid ECU: The hybrid electronic control unit (ECU) monitors the operation of the hybrid powertrain. In some cases, the engine control module is integrated with the hybrid ECU.

Interlock circuit: High-voltage hybrid electronics are dangerous. The interlock circuit disconnects the HV battery from the vehicle when the hybrid system has been “opened up” for service. For example, when the inverter cover is removed, the hybrid ECU will isolate the high-voltage battery from the car. As a result, the vehicle will be prevented from going into “ready to drive” mode. 

It should be noted that not all vehicles contain an interlock circuit. Also, the hybrid system should be manually discharged before service or severe injury (or death) can result. Typically, this is accomplished by removing a service plug, then waiting for the system’s capacitors to discharge.