Hybrid drive derives its name from Latin “hybrida” for crossbreed. In the motor industry, hybrid vehicles feature two energy converters. One of them is usually an internal combustion engine, the second one a complementary electric motor. However, a hybrid system may also consist of a single electric motor which is fed from two sources from energy (for instance battery and overhead wires).

Whether 100 years ago or 35 years ago – the aim of the engineers in developing hybrid drive systems always was to perfect existing configurations. The idea was to build upon existing drive systems instead of completely replacing them. So most hybrids combined an internal combustion engine with alternative types of drives, usually an electric motor.

However, the various designs varied greatly in their approach to optimization. In the case of early hybrid vehicles like the Mercedes Mixte, the main purpose was to reduce the mechanical stresses in an internal combustion engine with manual transmission, whereas the overriding objective pursued with the hybrid buses of the 1970s was to avoid local emissions of exhaust gases and noise, for example in sensitive downtown areas.

Current designs, by contrast, aim at reducing both the vehicles’ fuel consumption and environmental pollution. Advanced hybrid studies by Daimler AG have achieved this by expending large technical effort to combine two types of drive system to utilize the synergies of the two different units.

Basically, hybrid drives can be classified as serial or parallel. In the serial hybrid the individual drive systems are connected in series – the internal combustion engine drives a generator, for example, which in turn supplies energy to the electric traction motor. In the case of parallel hybrids, both drive units can be used either singly or together. In such solutions the electric motor provides a boost to the internal combustion engine, for example during acceleration.

The parallel hybrid has established itself as preferred solution for concept cars and prototypes mainly of passenger cars, SUVs and light-duty commercial vehicles. Compared to serial solutions, the parallel hybrid affords a bigger opportunity to utilize synergies between the two different types of drive system. Thus, as sole drive the electric motor can bring its stronger torque to bear for moving off; while on the go it then assists the internal combustion engine when needed so that the latter usually can operate in its optimum range.

Parallel hybrids are subdivided into “micro hybrids”, “mild hybrids” and “full hybrids”. The difference lies in the ratio of the two drive systems’ use. Full hybrids can run on the electric motor alone, for instance at low speed and when starting off from standstill. “Mild hybrids” by contrast use the electric motor only for boosting the internal combustion engine’s performance and efficiency under high loads, especially during acceleration. “Full hybrids” also offer this booster function, though with clearly higher additional output contributed by the electric motor. “Micro-hybrids” feature an automatic start/stop function as well as brake energy recuperation for charging the battery.

The so-called powersplit hybrid concept is an intelligent synthesis of serial and parallel hybrid. Unlike previous single-mode systems, the patented two-mode hybrid uses a significantly smaller electric motor. The system is thus more compact while on the other hand, the two-mode hybrid covers two operating ranges with maximum power output and reduced fuel consumption. The electric motor serves as a starter and as a start-off booster; it can also be used as a generator for converting kinetic into electric energy.