Manufacturers offer multiple hybrid systems based on different philosophies. Here we provide an impartial overview of some of the technological options available on the market
by Gian Piero Repetti
The trend as regards vessels under 50 m is the parallel hybrid, a setup with the diesel engine connected directly to the drive shaft. The engineers have cut their imaginations loose, ranging from systems with power take-off (PTI/PTO) on the reduction gearbox to systems with in-line electric motors between the engine and gearbox, coaxially or on parallel axles. Then there are different types of electric motors, coupling systems, not to mention energy storage with batteries of different types and characteristics. We will try to give an overview of some of the technologies available on the market, without claiming to list all the manufacturers nor to express a preference for anyone in particular. Every manufacturer applies its logic, mainly to satisfy specific installation requirements. We will start from the most traditional of all, and also the least invasive: PTI/PTO on the gearbox. It is no surprise that all the gearbox manufacturers offer their variant. (LAYOUT A)
The figure shows a hybrid drive layout with the electric motor fitted on the gearbox power take-off.
The great advantage is an installation that remains just like any traditional diesel engine, with almost zero interference on the diesel drive. ZF, Reintjes, Twin Disc, and others less common in Italy all offer their systems. (FIGURES 1 AND 2)
We then consider coaxial in-line systems between the engine and gearbox, like Auxilia, E-motion, MAN, Volvo, MTU, FNM, etc. (FIGURES 3 AND 4)
Choosing a hybrid system requires great skill and accurate assessment to identify the system most suited to your needs.
The concept is the same, but these systems are distinguished by the type of electric motor and form of coupling between electrical and diesel. Permanent magnet motors, with axial or radial flux, are the most widespread, even though other valid and innovative alternatives have been adopted by a few manufacturers, like for example variable reluctance synchronous motors. The electric/diesel coupling/decoupling is achieved using single disc electromagnetic clutches, toothed clutches, or centrifugal clutches, according to the conditions of use or the constructional philosophy of the manufacturer. The final variant considered is when the electric motor is inserted between the engine and the gearbox, coupled to an axle parallel to the drive shaft. Examples include Esco Power and Transfluid. (FIGURE 5)
These systems are by nature much more complicated than traditional drives and need to offer added value that satisfies real needs, above and beyond mere technical thrills, fashion, and boating convenience.
The advantages and disadvantages of the various systems are as follows. A PTI on the gearbox is the conceptually simplest and least invasive since in cases of electrical faults it is located completely outside of the drivetrain. An adequate support structure must be provided for the electric motor, together with the necessary space in the stern. A PTI is a substantial accessory for the gearbox and as such is not cheap. Drive inversion must be implemented by the electric motor because it is not possible to use the gearbox reverse mechanism. There are no problems with the maintenance of the diesel engine since the electric motor is on a secondary branch of the system. Next, we consider a coaxial system between the engine and gearbox. The design is compact and the system could be defined as modular. The drivetrain is slightly elongated, but without bulk in the transverse directions. It is heavier but extremely practical, and in the case of the Auxilia even has an unpowered clutch (centrifugal clutch). The reverse drive is managed by the gearbox.
There are some concerns about faults in the electric motor since it is in-line with the diesel engine and this excludes the use of the diesel during maintenance. The third option is an electric motor between engine and gearbox, with the transmission on a parallel axle. This has many of the advantages of the systems already described, but the lateral bulk is increased and the electric motor has to be supported in some way. (LAYOUT C)
Different operating combinations are possible with all the systems described above: pure electric, booster, generating current, options that complicate the system to a lesser or greater degree. However, the question we need to pose ourselves is: what do we want to obtain from a hybrid drive boat? Apart from the dream of silent boating, which is possible with pure electric drive, the hybrid can offer a series of combinations: producing electrical energy when travelling with diesel, or a power reserve for use as a booster, for example. At the end of the day, we have a drive system available that combines a diesel engine and an electric motor, which together produce more power, making it possible to reduce fuel consumption (and therefore also exhaust emissions) dividing the power of the diesel and transforming the electric motor into a current generator. The overall bulk, in the end, is much the same as the diesel drive alone but offers considerably greater versatility. However, be careful not to overlook one critical factor: energy storage.
To run under electric power the system needs a supply, and so the batteries play a critical role. Once again there is a plethora of choice. Lithium-ion batteries are among the most common and efficient, but there are other types and each has characteristics that might make it preferable to the others. (LAYOUT B)
When thinking about energy storage, the first question is how much we need to drive power and how much autonomy this gives us. The other considerations include ease of management, safety, weight and bulk, and power consumption not from the drive but instead required to cool the batteries themselves when operating in electric mode. Users tend to only weigh up overall system performance while neglecting to assess how well the technical solution suits their specific requirements for use.
Lithium-ion batteries are among the most common and efficient, but there are other types and each has characteristics that might make them preferable to the others.
At the end of the day, these systems are by nature much more complicated than traditional drives and need to offer added value that satisfies real needs, above and beyond mere technical thrills, fashion, and boating convenience. Choosing a hybrid system is an issue for design specialists, requiring great skill and accurate assessment to identify the system most suited to your needs. This is because there is no single ideal option, but one that best fulfils our expectations. In the next issue, I will recount something of my own experience and the importance of choosing one system rather than another in order not to just have a hybrid on board, but to lend your boat added value in terms of comfort, which technology like the hybrid drive can certainly provide.
(Sailing in Silence 2, Hybrid systems – Barchemagazine.com – January 2021)