What is the MHEV plus mild hybrid technology that Audi offers in its new A5 and Q5, and how does it work?

A few years ago, Audi was one of the manufacturers most committed to mild hybrid MHEV technology. The brand with the four rings began offering this technology back in 2017, in both TDI and TFSI engines, across practically its entire range of combustion-engine models. Two MHEV systems were even developed: a simpler one for 4-cylinder engines, based on the traditional 12-volt electrical system, and another for six- and eight-cylinder engines, in this case at 48 volts.

This is how the conventional MHEV system works

Basically, this mild hybrid system, which many other manufacturers also used and which Audi still uses in many models of its current range, works as follows. A belt-driven starter-alternator, known by its acronym BAS, is connected to the engine’s crankshaft by a belt. This alternator, which is also water-cooled, can provide additional torque to assist the combustion engine—in the case of Audi, up to about 60 Nm with the 48-volt system—or recover energy during deceleration and braking phases—up to 12 kW—which is stored in the system’s battery.

Thanks to MHEV, the vehicle can coast in certain phases in sailing mode with the engine disconnected, with the consequent fuel savings. In Audi models with this first-generation system, this can last a maximum of 40 seconds when the driver lifts off the accelerator at speeds between 55 and 160 km/h. The system makes it possible for all electrical consumers to keep working even with the engine off, since it draws on the energy stored in the battery. For this same reason, another advantage of MHEV is that the start-stop system can be activated and the engine switched off before the car has come to a complete stop. In this case, Audi does so from about 22 km/h.

The BAS not only works as an alternator, but also takes care of restarting the engine after a coasting phase or a stop. It does so more smoothly than the conventional pinion starter system, which is retained but only for the initial start-up, when the engine oil is cold and a higher starting torque is required. In addition, under hard acceleration the BAS also provides extra thrust to the combustion engine, which improves the driving experience. That said, with conventional MHEV, this electric motor that acts as an alternator can never move the wheels on its own; in other words, the car never moves in electric-only mode.

In the case of Audi, MHEV technology works the same way with 12 and 48 volts, the difference being that the coasting phases, the power output and the fuel savings are somewhat smaller in the 12-volt system.

The MHEV plus of the Audi A5 and Q5: almost like a “real” hybrid

Audi has now evolved the MHEV system and, for the new engines based on the so-called Premium Combustion Platform, which for now are the new Audi A5 and Audi Q5, it uses what it calls MHEV Plus, which offers a higher degree of electrification without using high-voltage systems like conventional hybrids do.

In practical terms, the most important difference is that with MHEV plus, a small electric motor can propel the vehicle on its own in very specific situations and for a very short time, such as low-speed manoeuvres—parking—or to maintain the vehicle’s speed when approaching a junction or roundabout until it is time to brake, which helps reduce consumption. There are also other advantages: the additional torque provided by MHEV plus is greater and available sooner, which improves acceleration and recovery. And the capacity to recover energy during braking is also higher. We could say it is “almost” like a “real” hybrid—or at least like those first hybrids of Toyota, but with a far less complex and sophisticated system.

Compared with the simpler MHEV system, MHEV plus improves performance and agility to a greater extent, and reduces consumption and emissions further. To be clear, it is not a full hybrid, which are more sophisticated and technically complex; but it is an increasingly valid solution for more and more manufacturers. In fact, what is now new for Audi has been used by other brands for some time, even in popular models. This is the case with Stellantis, which has adopted a very similar technology and offers it from a Opel Corsa which, incidentally, the brand has no qualms about selling as a Hybrid, although it really is not one.

Audi claims fuel savings of up to 0.4 l/100 km for the A5 with the 150 kW 2.0 TDI, and up to 0.7 l/100 km for the 3.0 TFSI. These figures are very similar to those achieved—according to official data—with the simpler MHEV systems, although it is true that the new A5 is a larger, better-equipped and therefore heavier car.

This is how MHEV plus works: a powerful additional generator

How does MHEV plus work? By adding an electric motor in addition to the belt-driven starter-alternator (BAS), which Audi calls the PTG, which integrates the power electronics and is installed directly at the output of the transmission shaft. In this case, the PTG can deliver up to 18 kW (24 CV) of power, produces a maximum torque of 230 Nm and has the capacity to regenerate energy during braking with a power of up to 25 kW.

The PTG weighs about 21 kg, has liquid cooling, and above 140 km/h it disengages from the drivetrain via a clutch. In the case of the Audi A5 and Q5, a new integrated brake control system combines regenerative braking with the hydraulic brakes so that regenerative braking is used to the maximum, with no noticeable change for the driver in the transition between the two. An additional advantage is that MHEV plus allows the air conditioning to run continuously even when the combustion engine is switched off, by powering the electric air-conditioning compressor directly.

Here, the belt-driven BAS alternator also takes care of starting the engine and supplying energy to the battery, which, instead of lithium-ion, is of the LFP type, with a capacity of 1.7 kWh. A sophisticated control software evaluates how the vehicle is operating to decide how the combustion engine, the PTG and the BAS should interact, always with the main goal of achieving the lowest possible consumption. Among the parameters taken into account are the position of the accelerator pedal or the mode selected in the transmission, changing in each case the speed at which the PTG disengages, as well as the target state of charge (SoC) of the 48-volt battery.

One point to note is that Audi does not use the MHEV plus system in all of its engines in the A5 and Q5 ranges. For example, in the A5 it is fitted to the 150 kW TDI engines and the S5’s V6 TFSI, but not to the 2.0 TFSI, which does, however, have this hybridisation in the Q5 range.

Audi MHEV plus technology – Image gallery