Audi has been offering models as plug-in hybrids since as far back as 2014. Plug-in hybrid is the colloquial term for a plug-in hybrid electric vehicle, abbreviated PHEV. Traditionally, a powertrain is called a hybrid whenever two propulsion technologies – such as an IC engine and a motor and the respective energy storage systems – are combined in one vehicle.
In automotive engineering, the combination of an IC engine with an electric motor has become established for PHEV models. A typical characteristic of so-called parallel hybrids combining an IC engine and an electric motor in one powertrain unit is that the electric motor can either be used as the sole, locally emission-free traction motor or assist the IC engine in both part load operation and with extra boost capacity during stages of higher acceleration. The electric motor also serves as a generator for recuperation, converting kinetic energy into electric energy and storing it in a lithium-ion battery. On a plug-in hybrid, the battery can be charged externally at a charging station or power outlet.
In designing plug-in hybrid models, Audi focuses on three objectives: long and frequent electric driving segments plus low overall energy consumption, easy charging management and high suitability for everyday use.
The electric driving experience, for one, is ensured by powerful electric motors enabling all-electric driving across a wide speed range in various driving situations. Smart powertrain management – designed for maximum efficiency and customer comfort – provides the basis for frequent and sustained electric driving. In addition to the automatic selection of hybrid mode, drivers have the choice between “EV” and “Hold” modes. In “EV” mode, the car, subject to the battery’s relevant state of charge, is propelled strictly by electric power as long as the driver does not exceed a variably perceptible pressure point in the accelerator pedal. “EV” mode is the default setting anytime the car is started. In “Hold” mode, the powertrain management system controls the powertrain so that the existing charge status of the battery is maintained, for instance to enable subsequent all-electric driving in urban areas. Even so, this mode also includes fully electric driving segments.
In addition, by using the commonly known switch of the Audi drive select driving dynamics system, the driver can choose between “comfort,” “efficiency,” “auto” and “dynamic” modes and thus influence the setup of the powertrain, suspension and steering systems. During acceleration, depending on the setting, the thresholds change at which both propulsion systems start working together or the electric motor provides boost and thus maximum torque. In “dynamic” mode, the electric motor increasingly assists the IC engine with its electric boost capacity for maximum driving dynamics.
The sophisticated powertrain management substantially contributes to Audi’s PHEV models enabling long electric driving segments combined with moderate battery sizes. The Predictive Efficiency Assistant (PEA) and the Predictive Operating Strategy (POS) are the two core technologies of the powertrain management system. The Predictive Efficiency Assistant – known from Audi models with conventional powertrains – situationally controls the propulsion and recuperation behavior to suit immediate and obvious vehicle information such as predictive route data supplied by the navigation system or the front camera. The Predictive Operating Strategy controls the propulsion and recuperation behavior along the entire route. During active route guidance the POS analyzes route information along the travel distance such as traffic jams and types of roads like backroads and freeways. The area surrounding the destination is included in planning powertrain management as well. The objective pursued by route planning is to maximize electric driving in urban areas and to arrive at the planned destination with a near-empty battery. The reason is that as much electric energy as possible should be used in order to recharge the vehicle after arrival at the destination.
The active accelerator pedal that assists drivers in maximizing the efficiency of their driving style by providing haptic feedback is linked to powertrain management as well. It indicates when drivers are leaving the all-electric driving stage via a pressure point in the accelerator pedal. Based on the information from the Predictive Energy Assistant, it also prompts drivers to lift for coasting or recuperation.
The electric motor handles all minor decelerations – in other words, the majority of braking events in everyday driving. In moderate braking events, it shares deceleration duties with the hydraulic wheel brakes, which are used alone only beyond a deceleration level of 0.4 g. A complex vehicle setup results in a nearly imperceptible transition between the generator brake of the electric motor and the conventional wheel brake (“blending”), combined with a carefully defined and constantly identical pressure point in the brake pedal.
The PHEV models are charged at 400-volt three-phase AC chargers with a capacity of up to 7.4 kW, which takes around 2.5 hours. This charging time is ideal for the typical user patterns of PHEV customers. It enables easy and fast charging of the car once or twice a day, either at home or additionally at work. Fully charging the battery on a 230-volt household outlet takes about 6.5 hours.
Audi ensures high suitability for everyday use of its PHEV models by providing ample space as well as high variability and usability. The batteries are installed in a compact way underneath the trunk floor – without sacrificing a lot of space and without including a step. For all PHEV models, except the Audi A8, operation with a trailer hitch continues to be possible.
Audi offers plug-in hybrid models both in the compact and full-size car class. On the mid-size and full-size models A6, A7, A8, Q5 and Q7, power is transferred to the road via all four wheels – resulting in maximum traction even in poor road and whether conditions as well as high agility for a sporty driving style.
The battery system consists of liquid-cooled lithium-ion cells. The power electronics system, which is integrated in the cooling circuit as well, converts the direct current of the high-voltage battery into three-phase alternating current for the electric motor. During recuperation the process is reversed. Audi’s mid-size and full-size PHEV models, except for the Q7, use a complex thermal management system controlling the high-temperature circuit for the IC engine, the transmission and accessories as well as the low-temperature circuit for the battery, the charger, the electric motor and the power electronics. Integrated in the thermal management system is a heat pump in which electrical energy is converted into thermal output. The heat pump is coupled with the coolant circuit of the air conditioning system and uses the waste heat of the high-voltage components for fast and efficient temperature control of the car’s interior.