In mid-2017, the new mild hybrid electric vehicles (MHEVs) began to enter the product line-up and are appearing in a broad range of models.
The new technology is well-suited for both diesel or gasoline engines and can reduce consumption in a V6 gasoline engine by up to 0.7 liters per 100 kilometers (0.2 US gal per 62.1 mi) in real-life driving conditions.
Audi offers two variants of MHEV drive systems. For the four-cylinder engines, they are based on the familiar 12-volt electrical system. The six- and eight-cylinder engines as well as the W12 in the Audi A8 are equipped with a new 48-volt system that serves as the main vehicle electric system. The lithium-ion battery with a capacity of 10 Ah that supplies this system is housed in the luggage compartment. A water-cooled belt alternator starter (BAS) is mounted to the combustion engine’s front end. A heavy-duty V-ribbed belt connects it to the crankshaft. The BAS generates a recuperation power of up to 12 kW and 60 Nm (44.3 lb-ft) of torque.
The 48-volt MHEV technology offers exceptional convenience and efficiency. When drivers take their foot off the accelerator pedal at a driving speed from 55 to 160 km/h (34.2 to 99.4 mph), the car can coast for up to 40 seconds with the engine shut off completely. When coasting at low speeds, the start-stop phase already begins at 22 km/h (13.7 mph). The drive management system utilizes information from the navigation map and on-board sensors to evaluate whether coasting is more fuel-efficient or recuperation. It selects recuperation, for instance, if the efficiency assistant detects a vehicle driving ahead. In the braking phases, the BAS assists in braking and can feed the recovered electrical energy into the 48-volt storage unit or use it directly for electrical consumers.
Compact powerhouse –
The belt-driven starter/generator is mounted on the front of the engine on the right-hand side.
When the driver presses the accelerator pedal again after a coasting phase or a stop, the BAS restarts the combustion engine. The conventional pinion starter is only used for the initial starting operation when the engine oil is still cold and a higher starting torque is required. The BAS start is controlled based on load demands by the driver. For example, if the air conditioner is activated and requires an engine start while the vehicle is stopped, the engine is restarted very comfortable. The same is true if the driver releases the brake pedal in a start-stop phase to rejoin other vehicles in traffic. If the driver presses the accelerator pedal further, signaling a wish for significant acceleration, the engine start is executed as quickly as possible. In addition, there is what is known as the predictive convenience start. Here, the engine is started as soon as the car ahead starts moving, even if the driver is still pressing the brake pedal.
The functional principal of MHEV technology that utilizes a 12-volt electrical system is the same as with 48 volts. However, the coasting phases, recuperation output and the CO2 savings are somewhat reduced.
Additional models will get mild hybridization features as early as next year. The new architectures enable even more power and torque, with innovative functions leading to greater fuel savings. Over the mid-term, the brand plans to convert ancillary units like compressors to 48 volts; they will then lend themselves to more precise need-based control as well as lighter and more compact construction. The same applies to large static convenience consumers such as window heating and sound systems. Small consumers such as control units and lights, on the other hand, will remain in the 12-volt system well into the future.
Three important elements –
The BAS converts deceleration energy into electricity and feeds it into the 48-volt battery. The DC/DC transformer connects the 48-volt main electricity system with the 12-volt subsystem.
Fuel consumption of the model named above:
Audi A8: Combined fuel consumption in l/100 km: 7.8 – 5.6 (30.2 – 42.0 US mpg); Combined CO2 emissions in g/km: 178 – 145 (286.5 – 233.4 g/mi)