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Not All Hybrids Are Created Equal

Understanding the Different Systems

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KEYWORDS hybrid cars
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A Hybrid Electric Vehicle (HEV) vehicle relies on two different sources of power for vehicle motion: the fuel in the tank (gasoline/diesel) and an electric motor. But there are different levels of vehicle hybridization that are in production and showing up in our shops for service and repairs. 

There are currently five types of hybrid vehicles and three hybrid configurations being used and produced almost always combined with an Atkinson cycle engine. Recognizing the differences and characteristics of each of these hybrid types and configurations is crucial for techs for repairs and diagnostics. And for shop owners, it helps to understand the service opportunities each will present.

Hybrid Types

Micro Hybrid Vehicle (mHEV): This is the most basic form of hybridization but is one of the most common: the start-stop vehicle.

Because the Internal Combustion Engine (ICE) is the only drive system responsible for vehicle motion, the start-stop vehicle is rarely thought of as a hybrid. During start-stop operation when the ICE is not running or stopped, it is saving fuel and tailpipe emissions. But the vehicle is still in run mode and operating off the vehicle’s 12V battery, keeping the comfort, entertainment and driver systems functional and waiting to immediately restart the ICE when needed. 

Most start-stop vehicles will modify the charging profile of the 12V battery by increasing the charge rate during vehicle deceleration and will use an AGM battery. The fuel savings per vehicle are low on this type of hybrid (about 1%) but it’s large enough for most manufacturers to use this form of hybridization due to its simplicity and lower initial cost. 

Mild Hybrid Vehicle (MHEV): The mild hybrid will have an electric motor involved in the vehicle’s drive system, but it will not propel the vehicle on its own and is only used to support the power output of ICE. The electric motor can boost the engine performance during acceleration by providing torque, eliminating the need for a traditional stator motor during start-stop operation and recuperate more energy during deceleration and braking than the mHEV can. The MHEV will have two electrical systems: a traditional 12V low voltage system and a high voltage system that can range from 36V to 160V depending on the make and model. Each MHEV will have a DC-to-DC convertor to exchange energy between these systems. 

There are two familiar types of MHEV: The Belt-Alternator-Starter (BAS) system and the Integrated Motor Assist (IMA). The BAS is considered a bolt-on hybrid solution that can be applied to almost all vehicle platforms with few modifications. The amount of torque that a BAS system can transmit is limited by the slippage of the drive belt. The IMA MHEV made popular by Honda does not suffer from this limitation because the electric motor is connected directly to the crankshaft. 

GM has used the BAS, and FCA (Fiat Chrysler Automobiles) is currently using the BAS on its 48V eTorque system that is an option available to Ram 1500 trucks and Jeeps.

Hybrid Vehicle (HEV) or Full Hybrid Vehicle (FHEV): The HEV will use both the electric motor and ICE to provide vehicle motion. The HEV permits full-electric driving but only for short trips and will take advantage of all the mild hybrid characteristics. The HEV will have a higher high voltage battery system typically 300V to 400V. 

The HEV will have a more powerful electric motor than that of the mild hybrid and a more capable high voltage battery for supplying and storing more energy. 

Toyota and Lexus have been the leading manufacturers of HEV vehicles, but the list of HEV vehicles is much bigger in 2021 with Hyundai, Ford, Audi, Nissan, Honda, Chevrolet and Kia and others all producing HEV vehicles.

Plug-in-Hybrid Vehicle (PHEV): The PHEV will share all the characteristics of the HEV but will now be equipped with a high voltage battery that can be charged at home or at other locations using a special cable and plug. 

The PHEV can be driven in full-electric mode more frequently and farther (about 45 miles) than the HEV because it has a larger more efficient battery and a more robust electric motor.

Range Extender Electric Vehicle (REEV): The REEV is regarded as a series hybrid because the ICE motor is only used to power the electric generator in most applications. The BMW i3 and the Chevy Volt are examples of current models that are REEV.

Hybrid Configurations

Series Hybrid: The ICE does not directly power the wheels on the vehicle. The ICE drives a generator that will either drive the electric motor or charge the battery pack.

Parallel Hybrid: This type of hybrid has two drive systems: an ICE and an electric motor. The vehicle can be driven by just the ICE or just the electric motor or a combination of both. With the parallel hybrid the power of the ICE and the electric motor are added together to form the total power of the vehicle. 

Series/Parallel Hybrid: This hybrid merges the advantages and complexity of both the series and parallel systems. The combinations of both designs allow for both the ICE and electrical motor to drive the wheels (parallel) or only the electrical motor (series). Allowing for only the ICE-only or electric-only means that the ICE can operate at maximum efficiency more frequently. This translates to more series hybrid operation at lower speeds and parallel operation at higher speeds. This system is more expensive to produce, requires a larger battery, a generator, and more complex computer controls for seamless operation. But it is more efficient than either the series or parallel systems when used alone. This design has been made popular by Toyota and was first used on the Prius. 

Atkinson Cycle Engine: Atkinson cycle engine uses the same four strokes as the Otto cycle engine does, but it leaves the intake valves open 20% to 30% longer during the intake stroke. By delaying the closing of each intake valve, the intake stroke becomes longer than the compression stroke. This increases thermal efficiency, reduces fuel consumption, and lowers exhaust emissions. But delaying the closing of the intake valves closing comes at a cost. The Atkinson engine suffers from poor low-speed torque and weak throttle response. To compensate for this deficiency the electric motor will provide the low-end torque and power to alleviate this problem. The combination of the efficient Atkinson cycle engine with an electric motor is ideal for hybrid vehicles and is an amazingly effective pairing that results in good fuel mileage and driveability.

Service Opportunities

There are many similar services and repairs on hybrids that are similar to non-hybrid vehicles. But some may require removing disconnecting the high voltage battery, such as an AC compressor replacement. This is when we need to talk about safety. 

Safety: Following the step-by-step instructions and using the proper approved and tested safety equipment is imperative if you are dealing with a high voltage system. Take the time to read any service precautions fully to familiarize yourself with what needs to be done to safely the service vehicle. The proper safety equipment and following the proper procedures cannot be stressed enough.

Servicing an AC compressor that is run by the high voltage system will require the high voltage battery to be disconnected using the service disconnect procedure. All the service information steps must be followed to ensure that all the voltage is removed from the system before starting the repair. 

All hybrids are going to have a 12V battery. Normally, a hybrid will use an AGM 12V battery, and they need to be treated differently from traditional lead-acid batteries. AGM batteries have a low internal resistance and are sensitive to charging voltages and charging rates. The proper battery charger and test equipment must be used for accurate test results and charging.

All current hybrids will have some form of ICE that will need to be serviced and repaired. They will need oil changes, filters, spark plugs and other repairs. The diagnostics and repairs of these engines are generally the same as a non-hybrid engine but getting the hybrid ICE to stay running during a diagnostic is a common issue that arises, even if it’s just to warm the ICE up for an oil change.

Keeping the ICE running typically requires entering a maintenance mode. Getting a Toyota or Lexus into maintenance mode can be done in one of two ways: using a scan tool and enabling maintenance mode or by doing the shifter pedal dance. 

The procedure is as follows. Perform the following steps within 60 seconds.

Turn the power switch to run without pressing the brake.

Fully depress the accelerator pedal twice with the shift lever in P (no brake pedal).

Press the brake and shift to N and fully depress the accelerator pedal twice. 

Press the brake and shift to P and fully depress the accelerator pedal twice.

Check that the MAINTENANCE MODE is displayed on the information display.

Start the engine by turning the power switch on (READY) while depressing the brake pedal. MAINTENANCE MODE should still be displayed on the information display.

After maintenance mode is entered, the idle speed will be approximately 1,000 RPM with the shift lever in park. The engine speed will increase to 1,500 RPM when the accelerator pedal is depressed halfway with the shift lever in park. If the accelerator pedal is depressed more than halfway, or when the accelerator pedal is fully depressed, the engine speed increases to approximately 2,500 RPM. Cycling the ignition switch will take the vehicle out of maintenance mode when done. 

Note: This procedure has worked for me as well on Honda and Hyundai hybrids, but if in doubt look up how to enter maintenance mode in your information system.

Cooling system repairs are another area that is often overlooked when it comes to hybrids. HEV and PHEV will often have a separate cooling system for the generator, invertor and electric motor and other parts that need thermal management. This cooling system has a service life and a service interval. Toyota/Lexus wants the invertor coolant changed at 150,000 miles and when doing so they recommend the use of a vacuum filler to ensure all the air is removed.

The BAS hybrid is dependent on the serpentine belt and tensioner for power transmission. The older GM BAS requires the use of a special tool to remove or install the belt that was under heavy tension. The current eTorque that FCA is using does not require any special tools other than a ½ inch breaker bar to release the tension. 

Thermal management of the hybrid battery is important. Most hybrid batteries are air-cooled using a blower motor that will force air over the high voltage battery pack to maintain the proper temperature. These motors must be kept clean and free of debris. In early Toyota/Lexus hybrids, only a grate was used to stop large debris from entering the battery’s forced-air cooling system and these could plug up and lead to the overheating of the battery or cooling fan failure. Toyota is now using a filter system like a cabin air filter to deliver clean filtered air to the battery pack and recommends that it is checked at every service interval or sooner (3,000 miles). They have even issued a TSB T-SB-0062-20 and a Tech Tip # T-TT-0432-17 that addresses the issue on most of their vehicles.

There are many valuable services that can be performed on hybrids without a lot of specialty equipment. Not all hybrid vehicles will have a high voltage battery, but they will all have a 12V battery and some form of ICE engine that will need to be serviced and diagnosed at some point over its lifetime. Hybrid cooling system services, air filtration, and specialized belts and tensioners on BAS hybrid will all need to be serviced at some point. 

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