Being capable of driving for hundreds of kilometres at zero emissions, several thousands of metres or hardly any at all. This is the crucial difference between the three families of hybrid systems on the market, known as plug-in hybrid, full hybrid and mild hybrid, respectively. The ability to be fully independent of the use of a traditional combustion engine, slightly independent or not at all is the only objective parameter to find your bearings in the manufacturers' offerings, with an eye to the abbreviations conventionally used in price lists, i.e.PHEV standing for plug-in hybrid electric vehicle, HEV for hybrid electric (also known as full hybrid) vehicle and MHEV for mild hybrid electric vehicle. Let's recap the main differences from a technical point of view.
MILD HYBRID ELECTRIC VEHICLES
Several mild hybrid solutions have been added to the line-ups over the past years. These vehicles are electrified by adding an electric motor to the conventional engine to act as a starter and improve starting power by delivering extra torque. Technically, a mild hybrid solution does not offer zero-emission driving but it does have a favourable impact on fuel consumption and emissions even on lower range models that also the most affordable. The main advantage of a mild-hybrid electric solution is that it can be easily adapted to use with a conventional petrol, diesel and soon even LPG/natural gas engine.
The principle is to add thrust for starting off and recover energy when decelerating or braking. The vehicle is complemented by a 12V electrical system or, in some cases, better still, by a second 48V system that offers four times the voltage. It means that the current intensity can be reduced at the same power but also that the electric motors can be four times more powerful. Furthermore, the cable size can be reduced and more rational use can be made of the lithium batteries dedicated to the hybrid system, which are in any case separate from the conventional 12V battery.
FULL HYBRID ELECTRIC VEHICLES
There are currently five different full hybrid platforms with different abbreviations indicating different interactions between the components. A common mistake is to define a hybrid electric as a “dual drive” vehicle. In actual fact, these vehicles pack a real energy-generating system on-board capable of using the kinetic force developed by the internal combustion engine for traction and also convert it into electrical energy and then use it to power an induction motor drive system, which in turn is used to move the wheels, exclusively in some cases. In short, instead of putting the convention engine at centre stage, in this energy system, it is part of a complex assembly that has been heading towards more prevalent electric traction solutions for several years. While the first full hybrid electric systems were based on a mainly parallel layout, the next-generation evolution favours traction coming exclusively from the electric motor, which is the only one connected to the wheels, the petrol engine serving only as an energy generator supplying the current needed to regulate the energy flow to the batteries. The so-called series/parallel full hybrid electric vehicles, with an internal combustion engine connected to the wheels that can also be used exclusively to produce electrical current, fall somewhere between these two extremes. In any case, full hybrid electric vehicles are going strong in every market and in some cases have already overtaken sales of their diesel counterparts.
PLUG-IN HYBRID ELECTRIC VEHICLES
How can you extend the zero-emission range of a hybrid vehicle? That's easy. Simply by upgrading its electrical part, that is by increasing the battery capacity and charging them through the mains. The emergence of plug-in hybrid electric vehicles may appear to be propelled by a linear reasoning but this is not the case at all, if only because many automakers have deliberately skipped the full hybrid step and focused solely on PHEVs. This could mean that they are on technically different paths compared to manufacturers who have chosen to work on both formulas, sharing part of the mechanics across the two layouts.
Plug-in hybrid electric vehicles are expressly designed to extend the electric driving range even beyond the 50-kilometre mark, achieved with battery packs that go from the average 1.5 kWh capacity of a full hybrid to a maximum of 18.1 kWh. That is thirteen times larger but also bigger and heavier. As a consequence, plug-in hybrid electric solutions are restricted to medium and large-sized vehicles and - all importantly - has a significant impact on the retail price.
Of key relevance is also the management software logic, which must allow switching from hybrid to electric driving when entering restricted traffic zones from outside the city. To do this, the battery charge must be managed with surgical precision for use only where it is needed. Finally, the truly more advanced solutions allow the internal combustion engine to charge the battery while it also takes care of traction , effectively freeing the plug-in hybrid electric vehicle from the plug-in concept itself, avoiding the need to stop at the charging stations or connect the car to the power mains at home.
