For hybrid engine two things can be understood: a thermal engine capable of exploiting two different fuels, for example petrol + LPG, or a dual engine system, for example petrol + electric. The most centered definition is the second, also because i engines petrol + LPG are correctly defined as bifuel more than hybrids.
Once they agree that in the language of transport for hybrid engine we mean a system equipped with a double engine, a thermal (usually petrol or diesel) and an electric one (let's call the others bifuel) you need to understand what kind of hybrid engine you are talking. There are in fact three variants of these systems.
The first type is the hybrid engine series which imitates the electric traction of trolley buses and trains, and which also finds application on large ships. This system includes a heat engine which, via an alternator, supplies energy to the batteries that power an electric motor.
The second type is the parallel hybrid engine, considered more suitable for automotive and road transport vehicles. In this case the two motors, the thermal one and the electric one, are powered independently and transmit energy to the same shaft engine. A peculiarity lies in the fact that starting, accelerating and braking (i.e. the most demanding 'ideas' from the point of view of consumption) insist on the electrical part and this helps to reduce the consumption of the hybrid engine.
The third type is the so-called idle hybrid engine, in which the electrical component intervenes only in the starting phase and in running at minimum speed. Hybrid engines of this type, however, represent a small percentage compared to the other two, in particular theparallel hybrid which is the most common type.
The advantage of the hybrid engine is that it allows for good performance by reducing fuel consumption and with lower overall exhaust emissions. This is also possible because the electrical system allows the energy of braking to be recovered (especially in the city) with consumption of 10-15% lower.