Mild hybrids shouldn’t exist.
The automotive engineer who decided to call their new idea “mild” probably didn’t get brownie points for hard-hitting promotion, but the term stuck and we now have mild hybrid electric vehicles (MHEVs).
Maybe the “m” in “mild” is a typo and was meant to be a “w”? Some say MHEVs shouldn’t exist as a concept, with their expensive add-on components to a conventional internal combustion engine (ICE) for marginal performance benefit.
However, short of wholesale and immediate conversion of every car offered on the market to full battery power, it was the only practical industry response to the International Council on Clean Transportation requirements for an urgent reduction in vehicle emissions. In the EU for example, by 2021, a manufacturer’s passenger-car fleet of models had to average less than 95gm CO2/km. That’s equivalent to about 58.8 mpg for a gas engine. Try getting that from your regular SUV.
So, offering smaller, high-selling vehicles with an add-on that pushes the average fleet emissions below the threshold can avoid big fines on manufacturers while they develop all-battery cars. Some cost can be passed on to the customer as a green sell. Because profits come from bigger, premium models, keeping them in production with their higher emissions offset by MHEVs is still important to manufacturers.
Therefore, MHEVs are here to stay for the medium term and automotive engineers have a new playground with a variety of possible implementation approaches.
What exactly is a mild hybrid EV? A defining feature is that it doesn’t plug in, so the electric motor energy comes from a battery charged from a generator connected to the drivetrain.
It sounds like perpetual motion, but charging energy occurs during braking or over-running and is released only during bursts of demand such as acceleration or as the car just begins to roll. The motor is typically around 15kW rating and the net effect is 13-22% savings in CO2 emissions, enough to dip below that 95 gm/km target for a small car. Driving experience can be enhanced as well though, with the electric motor and its instant torque response eliminating ICE turbo lag, for example.
MHEV categories are numerous
MHEVs fall into categories. The simplest implementation is the BiSG or Belt-integrated Starter Generator, where the belt-driven alternator in a conventional ICE is simply replaced with a 48V motor-generator. This is an example of a P0 architecture (see table).
Unfortunately, mechanical slip limits the belt connection and the torque required for cold starting is generally too high, so a separate 12V battery and starter motor is required. 12V is typically generated by a DC-DC converter off the 48V rail for legacy equipment that requires the lower voltage. If the DC-DC is bidirectional, it’s possible to use some 12V battery capacity for traction, adding back in to the 48V rail. This would be for exceptional reasons though, as the 12V battery would normally be relatively low capacity and being typically lead-acid, should not be deep-discharged and must keep plenty in reserve for cold cranking.
|PO||The electric machine connects with the internal combustion engine through a belt on the front-end accessory drive (FEAD).|
|P1||The electric machine connects directly with the crankshaft of the internal combustion engine (ICE)|
|P2||The electric machine is side-attached (though a belt) or integrated between the ICE and the transmission; the electric machine is decoupled from the internal combustion engine and has the same speed of the ICE (or multiple of it).|
|P3||The electric machine connects through a gear mesh with the transmission; the electric machine is decoupled from the ICE and its speed is a multiple of the wheel speed.|
|P4||The electric machine connects through a gear mesh on the rear axle of the vehicle; the electric machine is decoupled from the ICE and is located in the rear axle drive or in the wheels hub.|
MHEVs fall into different categories.
More complex MHEV arrangements are TiMG (Transmission-integrated Motor Generator) or CiSG, (Crankshaf- integrated Starter Generator). Both can provide cold starting from the traction electric motor. TiMG is most flexible, with the ability to provide minimal creep electric driving as well. CiSG has high efficiency, but one of its disadvantages is that it can’t provide regenerative energy if the ICE stops during coasting for economy reasons. Both schemes have relatively high integration costs into a conventional ICE platform. TiMG is an example of a P3 or P4 architecture and CiSG is P1 architecture.
MHEV batteries will all be 48V
The battery voltage for the electric motor has settled on 48V, although some CiSG arrangements have been higher due to torque demands requiring a more substantial motor. 48V will be the standard though, as more auxiliary equipment such as pumps, air conditioning and heating operates at this voltage and 48V battery modules achieve manufacturing economy of scale. The value also makes sense technically; motors and their drive inverters operate much more efficiently at 48V than at 12V and for the same power, current is one-fourth. This means less power loss in cables (reduced by a factor of 16 because of the I2R effect), or smaller, lighter and cheaper cables for the same loss.
For 15kW output from the motor at 48V, current is a substantial 312A, without factoring in efficiency losses, so resistive drops in cables, connectors and in any semiconductors in line need to be carefully minimized. At 12V, the peak current would have been over 1000A, unmanageable in conductor and inverter size and economics for what is already an add-on to the ICE costs. The 12V battery is still typically retained though, charged from a DC-DC converter off the 48V rail, as there is still a large stock of 12V accessories on shelves and full EVs typically still have a 12V auxiliary rail, bolstering the market.
Control, safety, communications and infotainment electronics will generally require lower voltage still, perhaps even sub-1V for a CPU or FPGA, requiring further DC-DC buck converters. Although these could be powered from the 12V rail, it is also possible to down-convert directly from 48V — a bus arrangement now favored in data centers — where it provides an efficiency edge, so dedicated control ICs and buck modules are common.
Safety sets a maximum battery voltage
Safety is also a consideration. 60VDC is set by international standards as a maximum before it becomes an electric shock hazard, so 48V is the practical nominal limit with the overhead of the charging voltage, typically 54.6V, tolerance and surges. Full EVs operate at up to 800V and potentially higher to keep currents within bounds, so techniques implemented to ensure safety are well practiced. This is a major cost overhead with heavier insulation, mandatory spacings and the need for galvanic isolation in the sometimes-bidirectional DC-DC conversion stages.