Hybrid Drives: This Is How They Work

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Tags: Emobility

Hybrid electric vehicles can make an important contribution to reducing fuel consumption and emissions. For this, they combine combustion engines and electric drives. However, hybrid concepts vary considerably.

Martin Westerhoff, November 07, 2018

Martin Westerhoff studied technology journalism and writes about vehicles and technologies since then. He has a soft spot for motorsports and racing cars.

One of the most important vehicle development targets of today is to reduce greenhouse gas emissions. Ultimately, the aim is to decelerate the progressing climate change. According to a resolution of the EU states, for instance, by 2030, new vehicles shall emit 35 percent less CO2 than in 2020. This target can neither be achieved by purely battery-electric vehicles due to their low number nor by only optimizing current gasoline or diesel engines. The solution is to combine combustion engines and electric motors to form a hybrid drive. The larger the range that can be covered purely electrically, the better for the environment.

Different hybrid concepts

But hybrid drives are not all the same. Depending on the arrangement and task distribution of combustion engine, electric motor, generator and transmission, experts divide the hybrid concepts into series, parallel and power-split hybrid drives. Within these categories, different hybrid concepts have been developed over the years. When you take a look at the hybrid architectures currently used in volume production, however, you see some main development trends.

Series hybrid: two motors, one drive

A series hybrid drive consists of a combustion engine which is coupled to a generator. This is supplemented by an electric motor to drive the wheels. The combustion engine's only task is to drive the generator for power generation. This electric power is used to drive the vehicle, either by one electric motor with differential or by two motors per axle without differential.

This concept's main advantage: The combustion engine can run independently from driving conditions and the battery can also be charged with the vehicle standing still. The energy stored in the battery of the serial hybrid also enables purely electric driving with the combustion engine switched off. In current battery-powered electric vehicles, the combustion engine is also used as a so-called range extender. It ensures that the vehicle does not stop moving because of an empty battery.

Combustion engine and drive wheels are not linked mechanically. The combustion engine drives a generator whose generated energy is stored in a battery. This energy is used for the electric motor that drives the wheels.

Parallel hybrid: two motors, two drives

Unlike the series hybrid, a parallel hybrid features a mechanical link between combustion engine and wheel. The power transmission in the drivelines powered by the combustion engine or electric motor respectively can be varied flexibly – the vehicle is then either driven purely electrically, purely by the combustion engine or by both drives simultaneously. In parallel hybrids, the power output of electric motor and combustion engine is added up to form the total output. Apart from the combustion engine and electric motor, such hybrid drives also need one or several transmissions, clutches or freewheels.

Depending upon the power output and function scope, experts distinguish between mild and full hybrids. The mild hybrid is the simplest and most cost-efficient hybrid. It is integrated into the combustion engine's belt drive as a starter generator or is directly connected to the crankshaft. With a comparably low system voltage of 48 volts, a mild hybrid already enables the fuel-saving recuperation of otherwise unused braking energy. Furthermore, it enables the fuel-saving automatic engine stop and coasting functions which always switch off the combustion engine when it is not needed. Last but not least, a mild hybrid also supports the combustion engine when accelerating.

Mild hybrid that is integrated into the combustion engine's belt drive as a starter generator or is directly connected to the crankshaft. With a system voltage of only 48 volts, a mild hybrid enables the fuel-saving recuperation of otherwise unused braking energy. Also possible: The fuel-saving automatic engine stop and coasting functions that always switch off the combustion engine when it is not needed.

In the significantly more powerful full hybrid system, the electric motor for the drive is integrated into the vehicle transmission. A considerably higher system voltage of up to 600 volts also enables purely electric driving for heavy vehicles such as SUVs with adequate performance for daily driving tasks as well as for longer distances.

To save installation space, the electric motor is integrated into the transmission housing. This hybrid version enables the vehicle to be driven purely electrically or with the combustion engine only. During boosting, the electric motor supports the combustion engine when accelerating. During braking, the electric motor turns into a generator converting braking energy into electric energy that is stored in the battery.

All-wheel drive thanks to hybridization

In contrast to series hybrids, parallel full hybrid systems only need one electric motor that works either as a drive motor or as a generator, depending upon the operating state. The power output of combustion engine and electric motor can be scaled independently, meaning system components can be better tailored to the driving profile.

Current vehicle concepts are generally based on cost-saving modular concepts. Here, passenger cars use a common platform. Different modules would enable the realization of various drive concepts. The parallel hybrid concept offers great flexibility here. In the transmission-side full hybrid, the complete hybrid drive is integrated into the transmission. This makes it relatively easy to integrate the new technology into the driveline of rear-wheel drive vehicles, since only a few changes are needed.

However, other parallel hybrid architecture as well as system configurations are also possible depending on the vehicle platform, available installation space, costs and the desired function scope. Instead of the conventional, non-driven rear axle, an assembly consisting of axle, input shaft, electric motor and control electronics is installed. As added value, this concept also offers an all-wheel drive function. In this case, the traction control activates the electric rear-wheel drive in addition to the front-wheel drive powered by the combustion engine on demand.

Instead of the conventional, non-driven rear axle, an assembly consisting of axle, input shaft, electric motor and control electronics is installed. As added value, this concept also offers an all-wheel drive function.

Plug-in hybrid: parallel hybrid with power socket connection

All the hybrid variants described can be configured as plug-in hybrids. The plug-in hybrid is characterized by a battery with higher capacity. It enables purely electric driving over longer distances. The special feature of a plug-in hybrid: The battery is not only recharged during recuperation - when braking, for instance - but can also be charged by plugging the vehicle into an external power supply system. The larger the battery, the longer the range that can be covered purely electrically. The plug-in hybrid is the bridge to the pure battery-electric vehicle on the one hand, but on the other hand it also offers unlimited long-distance mobility thanks to the combustion engine.

Power-split hybrid: three engines on board

The power-split hybrid is a combination of serial and parallel hybrid. In this type of hybrid, the power of the combustion engine is divided into a mechanical and an electrical component. At the heart of today's standard systems is a planetary gear - in addition to the combustion engine and to two electric motors. One of the electric machines works as a motor, the other one as a generator. In this system, the combustion engine transmits part of its power directly to the wheels, while the generator simultaneously converts the rest into electrical energy. A battery stores this energy and, if necessary, transfers it to the electric motor, which is integrated into the power flow of the wheels. On the one hand, this makes purely electric driving possible, and on the other hand the combustion engine can be operated in consumption-optimized speed and load ranges, as energy that is temporarily not required for the drive is temporarily stored. Due to the specific properties of a planetary gear, however, the power output of the combustion engine and electric motor must be precisely matched in this hybrid configuration.

Driveline electrification will be standard soon

Electrification of the combustion engine driveline will start with the 48-volt mild hybrid for vehicles up to the mid-size segment while full hybrids will be mainly used for larger vehicles. The plug-in hybrid best combines the advantages of electric drive and combustion engine and offers the greatest degree of flexibility to its users.