A mild-hybrid is a car whose conventional powertrain is supplemented by an electric motor and an additional battery. This modification enables the mild-hybrid to make better use of the recuperated energy.
The MHEV belongs to the category of hybrid vehicles. The basis of each mild-hybrid is an electric motor and a car of conventional construction, i.e. with an internal combustion engine. Instead of a conventional alternator, the MHEV uses an electric motor (called a reversible alternator) to help drive the internal combustion engine in certain driving modes. For example, when starting, sailing or accelerating. By replacing the conventional alternator with an electric motor, the mild-hybrid has a much wider range of possibilities in recuperating and using the recuperated energy. The system stores the recovered energy in an additional 48-volt battery.
A car like the MHEV cannot run purely on electricity, it always needs an internal combustion engine! The electric motor is only connected to the drive chain via a belt drive and acts as a so-called start-generator. It can also switch off and restart the combustion engine while driving, which would be technically unthinkable with a conventional starter motor. Most MHEVs also use the Boost function (full acceleration) and can add a few horsepower to the system via the electric motor whenever needed. This improves not only the economy of the ride, but also its dynamics and ride comfort. MHEVs can also make better use of the sailing function, which, in conjunction with the predictive adaptive cruise control, also helps to reduce fuel consumption.
As already mentioned, an integral part of MHEVs is the 48 V lithium-ion battery, which is usually located under the passenger seat and is used to store recuperated energy. This additional battery does not have as large a capacity as, for example, the battery in PHEVs. For example, the capacity of the Audi A8 MHEV battery is only 0.48 kWh. MHEV batteries cannot be recharged from a plug-in battery, as in PHEVs.
MHEVs are based on a conventional car and therefore retain a conventional 12 V on-board network with a standard battery. In order to recharge this battery while driving, a DC/DC converter takes care of the power transfer between the two batteries.
Advantages of MHEVs:
The benefit of MHEVs is that they allow a conventional car to be equipped with a smaller and more economical combustion engine, which can be assisted by an electric motor if more power is suddenly needed. Thanks to such a combination, real fuel consumption can be reduced, practically without loss of dynamics. Thus, the principles of downsizing are used.
If the driver applies the brakes, the system can recover up to 40% of the braking energy stored in the 48 V battery. The reported average saving is 0.4 litres per 100 kilometres.
With increasing pressure to reduce CO2 emissions and emission penalties, it is likely that MHEV hybrid technology will become standard as it contributes to lower fuel consumption, but is also relatively simple and affordable. In other words, the hybrid PHEV variant is a full-size conventional car with an internal combustion engine and a full-size battery car, which is the most expensive solution.