Audi has introduced its first combustion engines with the new MHEV plus technology in the new A5 and Q5 series, which are based on the Premium Platform Combustion (PPC). The 48-volt mild hybrid system, with its unique interaction of the powertrain generator (PTG), belt alternator starter (BAS), and a lithium iron phosphate battery, supports the combustion engine while also lowering carbon emissions and increasing performance and agility. The PTG, which may be fully linked or decoupled, has integrated power electronics and an electric motor for partially electric drive. This decreases usage and improves the driving pleasure.
“With the new MHEV plus technology, we are driving forward electrification in our new combustion-engine vehicles based on the Premium Platform Combustion that is tailored to the needs of our customers. “This will strengthen our product portfolio of all-electric models, plug-in hybrids, and vehicles with efficient combustion engines.” said Geoffrey Bouquot, Member of the Board of Management for Technical Development at AUDI AG.
MHEV + has appealing features such as partially electric driving, electric boosting, and a considerable improvement in economy and comfort.
Situation-specific liquid cooling of the power electronics and electric motor allows the components to work in optimal circumstances to fulfill power and torque needs throughout all operational states. The new MHEV + technology permits entirely electric operation while simultaneously supporting the combustion engine. The technology improves performance and agility while lowering fuel consumption and CO2 emissions.
For example, in the A5 2.0 TDI (150 kW front/quattro) (combined fuel consumption in l/100 km: 5.7-4.8; combined CO2-emissions in g/km: 150-125; CO2 class: E-D), up to 10 g/km or 0.38 l/100 km can be saved, and in a 3.0 TFSI with V6 engine (270 kW quattro) (fuel consumption combined in l/100 km: 8. 0-7.4; combined fuel consumption; 182-169 (g/km) combined CO2 emissions; CO2 class: G-F), up to 17 g/km or 0.74 l/100 km (according to the WLTP) savings are possible.
Powertrain generator as an extra powerful driving module.
Another significant advantage is that the MHEV plus technology increases both performance and driving comfort. The new MHEV + system’s tiny yet powerful electric drive module is the powertrain generator (PTG). This component also marks the most significant departure from Audi’s earlier MHEV technology, which relied solely on a belt alternator starter. The PTG, mounted in a small device with integrated power electronics right on the transmission’s output shaft, may provide up to 18 kW (24 PS) of electrical power to the drive. The module allows for a maximum torque of 230 Nm at the gearbox output, which is immediately accessible as driving torque when the car starts.
The PTG’s small gearbox runs at a ratio of 3.6:1. To maximize efficiency, MHEV plus uses the PTG up to a maximum speed of 140 km/h. At higher speeds, the PTG disengages from the drivetrain via an inbuilt dog clutch.
The PTG weighs around 21 kilos and can rotate the output shaft at a maximum speed of 5,550 revolutions per minute. Depending on the vehicle and driving mode, this translates to a speed of 130 to 140 km/h.
With minimum changes to the surrounding parts and components, the necessary room was produced inside the existing vehicle tunnel boundaries to accommodate an electric motor at the transmission output. The arrangement right behind the gearbox has multiple advantages: the PTG’s 18 kW of driving power or up to 25 kW of regenerative braking power is accessible directly at the axle output with no further losses. This arrangement allows the PTG to be utilized in both front-wheel and all-wheel drive cars without requiring any changes and in a modular approach.
To achieve the system’s high comfort criteria, the electric motor’s torque, current, and speed must be precisely controlled. The working temperature ranges from -40 to +75 degrees Celsius. A water jacket protects the electric motor while simultaneously cooling the small and highly integrated power electronics in the common coolant circuit, which is installed directly on the electric motor to save space. The high-performance power modules in the power electronics are grouped around the heat sink. The intermediate circuit capacitors are enclosed by the heat sink in a space-saving and thermally efficient manner.
Development concentrated on client requirements.
Audi created the MHEV + technology with a focus on expected consumer needs. The traditional drivetrains available with first-generation start-stop or mild hybridization rely on important efficiency components such as engine stop when the vehicle comes to a halt, coasting, freewheeling with the engine turned off, and 12-volt or 48-volt energy recovery. The main benefits of increased electrification with new technology include the convenience of start-stop operation, emission-free coasting, energy recovery, partially electric driving, such as for electric parking and maneuvering, and increased performance due to electric support for the combustion engine.
This allows the car to be operated entirely electrically, with the combustion engine turned off for extended periods of time, such as while traveling slowly in the city, when traffic is moving slowly on routes outside of the city, or when nearing the next town. Furthermore, the vehicle’s start-up reaction is greatly enhanced and more spontaneous thanks to the PTG, which offers driving torque of up to 230 Nm even at slow speeds. This results in considerably improved response, which translates to a clearly noticeable increase in agility, particularly in the initial few meters after taking off.
The PTG can sustain the combustion engine at speeds ranging from zero to 140 kilometers per hour. This implies that MHEV Plus provides an extra electrical output of up to 18 kW, allowing the combustion engine to run as effectively as possible. In this speed range, the PTG can recover up to 25 kW of energy via regenerative braking until the vehicle comes to a halt. The integrated blending-capable brake control system allows pressure-free braking and effective regenerative braking, often without the need of friction brakes. The electric air-conditioning compressor in the MHEV plus allows the air-conditioning system to run continuously even when the combustion engine is turned off, such as when waiting at a red light.
BAS, lithium-ion battery, and iBRS: a perfect combo
The belt alternator starter (BAS) is a component of the MHEV plus technology that starts the engine and delivers electrical energy to the battery. The belt drive offers acoustic benefits over a pinion starter and allows the combustion engine to start more quickly. This leads in increased consumption and beginning comfort. When the engine is turned off, the belt alternator starter recovers its energy and positions the cylinders optimally for restarting.
The lithium-ion battery built from lithium iron phosphate (LFP) has a storage capacity of 37 ampere hours, or little about 1.7 kWh (gross). It has a maximum discharge power of 24 kW. Because of the requirements for availability, power, and torque, the battery is incorporated into a low-temperature water-cooling circuit that maintains ideal conditions between 25 and 60 degrees Celsius. This is Audi’s first use of an LFP battery in its mild hybrid systems.
The integrated brake control system (iBRS) contributes significantly to energy recovery. In vehicles equipped with MHEV + technology, iBRS offers pressure-free braking and deceleration using regenerative braking, eliminating the need for a mechanical wheel brake. Mechanical brakes are only activated when the brake pedal is pushed more firmly. This has no influence on the braking experience.
Sophisticated MHEV with operational strategy
In a hybrid system, a battery charged to 50-60% capacity works well since it can send strong currents to the electric motor while also storing high charging currents during energy recovery. The hybrid system focuses on draining and recharging the battery in quick cycles rather than on electric range. This allows as much energy as possible to be collected and immediately repurposed for the drive.
With MHEV plus technology, a control program examines the vehicle’s operational status to ensure optimal interaction between the combustion engine, PTG, and BAS. Characteristic values for optimal utilization of the two electric motors, as well as required torque levels for propulsion or energy recovery, are saved for this purpose. The battery’s charge level is also taken into account. The aim is steady functioning, and the control system produces varying results based on the scenario. This is because the operating strategy of the extra electric drives is tailored to each combustion engine. The end result is the lowest feasible usage without sacrificing driving performance.
The operating strategy takes into consideration the specified transmission mode as well as the accelerator pedal modulation. For example, in driving mode D, the powertrain generator applies the whole extra electric power of up to 18 kW only when the accelerator pedal or kickdown is at about 80 percent. In driving mode S, the additional 18 kW of power is already accessible at lower accelerator pedal positions. In D, the PTG can be disconnected at 85 km/h to reduce electrical losses in the PTG’s electric motor when traveling at a constant pace with the combustion engine on highways and roads beyond city boundaries.
The operating strategy differs between driving modes D and S, notably in terms of the goal SoC (state of charge) of the 48-volt battery. In D, an average SoC of 50 to 55 percent offers the best balance for having adequate energy available for electric assistance of the combustion engine up to partially electric drive. This SoC is also enough for storing the large amounts of recovered energy that result from mild and prolonged braking periods at traffic signals or while approaching towns.
The usage of the powertrain generator also has advantages in terms of driving dynamics, as the increased and instantly accessible torque allows the vehicle to respond more quickly to load changes and accelerate more agilely out of bends. The kind of load change is also adjusted differently in D and S modes, resulting in more pleasant handling in D and more responsive, dynamic handling in S.
Models with MHEV + may also operate entirely on electricity, such as when entering a town, and then maintain speed using the PTG. If the power required by the driver or the Adaptive Cruise Control (ACC) surpasses a particular threshold, the combustion engine kicks in and takes over propulsion.
If the present SoC is lower than the goal SoC, the combustion engine turns on sooner. On the one hand, this is to prevent using more energy for electric driving, lowering the SoC even further. On the other side, the combustion engine may raise the SoC as needed by increasing power in combination with the BAS and PTG, essentially recharging the battery. This does not apply to electrically powered maneuvering, crawling along in slow-moving traffic, or parking, which may be done at a much lesser charge.
If the present SoC exceeds the intended SoC, the combustion engine activates later, when there is a somewhat larger need for power. As a result, the 48-volt battery discharges towards the target SoC, allowing it to absorb adequate energy during subsequent energy-recovery phases. As vehicle speed increases, the threshold for seeking power from the combustion engine decreases. Simply said, the faster the speed, the more power the combustion engine provides to the vehicle.
When combined with whatever is in the tank, the powertrain’s efficiency gains significantly boost the vehicle’s total range. This makes vehicles equipped with MHEV plus technology far more suited for long-distance travel and much more pleasant.
