Functions of Electric Hybrid Vehicles

Some vehicle functions characterize electric hybrid powertrains and they are value added of electric hybrid systems for the customer. These functions pertain directly to the various operating modes presenting in Figure 4-3 and are explained briefly in this section. The main functions include:

 engine start & stop;

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 torque gap filling during gear shifts;

 all wheel driving;

 torque vectoring;

 external battery charging.

The listed functions are briefly discussed below.

Engine Start & Stop – The engine start-stop function is a basic function found in all hybrid vehicle concepts. As soon as the hybrid control system senses that the vehicle will come to a complete stop, for example at a traffic light, the engine will shut off and be prevented from idling. The engine is restarted by means of an electrical motor or starter-generator as soon as there is a power requirement that merits it to start again. In micro hybrid systems that do not offer electric driving, the automatic start stop feature is able to start the engine and have it available for acceleration in less than a second. The driver’s signal to start the engine is normally depressing the clutch for manual transmission cars or releasing the brake for automatic transmission cars. For architectures that have electric driving capability, the transition from rest to starting the engine can be delayed by using the electric driving mode as a first means of propulsion before starting the engine for additional power.

Sailing – The “sailing” function is somewhat trivial but never the less useful in optimizing a hybrid control strategy. The function refers to the decoupling of both the engine and the electric system from the wheels and using the force of gravity or the inertia to move the vehicle without friction losses of powertrain loads. Conventional vehicles can glide when placed in neutral during downhill operation.

Hybrids however, must have the ability to rapidly connect the appropriate powertrain that best suits the driving situation moving in and out of a gliding operating environment.

Regenerative Braking – The term regenerative braking refers to the capturing of braking energy that would normally be lost to friction and heat in conventional car systems. It is most relevant function to achieve higher fuel economy and lower CO2 emissions. Brake energy recuperation is achieved by setting the electric traction motor in a generative mode that serves as a counter force to the vehicle direction of movement. The energy obtained through regenerative braking can be directly stored in the traction battery and later used for boosting, electric driving or powering the electrical system components.

The use of electric motors as brakes could be sufficient for most braking situations. However, redundant friction braking systems are still required for safety purposes. Hybrids with enough power systems display a regenerative braking capability that can prolong the life of traditional friction brakes as an added benefit to the customer. As presented in Chapter #4.5.6, regenerative braking is limited by the battery system and motor ability to allow for impulse power storage in short time scales. Super capacitors have been proven to be well suited for regenerative braking in the case of micro and mild hybrid systems, where 2-3 seconds of high power inputs and outputs are used in charging and discharging from the capacitor device.

Power Boost – When the driver’s situation requires excess acceleration power beyond what the combustion engine can deliver the electric motors provide additional torque to the wheels known as

56 boosting. Power boosting situations also include driving on inclines or towing use cases. In this mode, the battery charge is depleted and delivered through the electric motors as an additional source power.

Boosting is particularly effective in improving car acceleration specifications (e.g. 0-100km). Figure 4-4 shows that the electric motor delivers the highest torque starting from rest and low RPM values (0-900 RPMs), whereas the typical otto-cycle combustion engine achieves maximum power at higher RPM values (2000-2500 RPMs). In a typical hybrid car the resulting system performance is enhanced when accelerating from rest by initially using the torque that the electric motor supplies to the drive train.

Figure 4-4 Example of Torque vs. Transmission Speed for an electric motor and combustion engine; Boosting function allows for additional torque for acceleration, especially when starting

Electric Driving – Electric driving is performed by using electric energy stored in the traction battery to power the propulsion motor that powers the wheels. During the electric driving mode, the combustion engine is decoupled from the powertrain. It is either shut-off or used to generate electric power. Electric driving is limited by the energy availability of the electrical storage system.

Battery Charging by Engine – Based on energy management logics the ICE is turned on to recharging the electric battery. In case of parallel hybrid powertrain the battery charging can be performed by the engine with the contemporary propulsion. In serial hybrid the engine is used only to recharge the battery with the aim to extend the electric traction operation. The battery energy management can be carried out in the folowing main modes: charge-depleting, charge-sustaining and blended operations.

Charge-depleting refers to a mode of vehicle operation that is dependent on the energy from the battery pack and is typical of pure electric vehicles. In charge-sustaining the battery state of charge (SOC) is maintened according to logics of energy management that alternate electric propulsion, engine proulsion and battery recharging modes, with the contemporary aim of minimizing fuel consumption. In case of plug-in hybrids blended charging modes are used. More details of battery energy management will be presented in Chapter #11.2.

Torque Gap Filling during Gear Shift - A typical issue of an Automated Manual Transmission (AMT) is the absence of traction during gear shifting, that is consequence of discomfort for the driver. The availability of an electric motor, positioned in the driveline downstream the gearbox (see P3 architecture in Chapter #5.2 ) or an electric driven axle (P4 architecture), allows to overcome the drawback. In this way the comfort of an AMT is comparable with other more expensive transmission systems (e.g. DCT and AT). The disadvantage of this function is the electric energy consumption, with negative impact on fuel consumption and electric autonomy.

57 All Wheel Drive (AWD) - By means of an axle electrification (see P4 architecture in in Chapter #5.2 ) is possible to implement the four wheels driving, without any changes on the conventional front/rear wheel drive powertrain of the vehicle. In this case the function is named e-AWD.

Torque Vectoring - Torque Vectoring (TV) is the ability to vary the torque delivered to each wheel. This function allows for the wheels to grip the road for better launch and handling. The TV function is typically performed by means of a mechanical differential on the same axle and a torque splitter between the front and rear axles, in case of AWD. These mechanical devices are electronic controlled in recent applications.

The use of an electric driven axle to implement an AWD vehicle makes the TV function available too.

External Battery Charging – External battery charging differentiates plug-in hybrid concepts from all other hybrid vehicle concepts. In addition to the typical hybrid components, a battery charging unit can be added to the car with the possibility to plug into an external electrical grid. The possibilities of connecting hybrid and electric cars to the electrical grid opens up possibilities for night time charging when electricity is cheapest and the electric load capacity of local power stations are at their lowest level. Vehicle to grid studies within electric mobility research are complementary areas of study that have garnered recent attention.

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