Scania’s plug-in hybrid powertrain could be the answer to the current limitations of electric coaches

Electric propulsion has fundamentally reshaped the urban bus market in recent years, but electrifying long-distance buses and, in particular, coaches is a considerably more complex task. While all-electric propulsion has now become a widely applicable technology for urban buses operating on fixed routes with predictable daily mileage, vehicles covering long distances on variable routes still involve far more technical and operational compromises.

The greatest difference lies in the operating profile. The energy requirements and daily mileage of an electric urban bus can be planned relatively precisely, while charging can primarily rely on depot infrastructure and, where necessary, on-route fast charging. A coach, by contrast, may need to cover journeys of several hundred or even thousand kilometres, often on routes that change from day to day. Europe currently lacks a high-power charging network with adequate geographical coverage whose deployment and accessibility would generally meet the needs of large commercial vehicles. Moreover, a significant proportion of charging stations built for passenger cars cannot be used without difficulty by a 13–15-metre coach simply because of their physical layout.

Providing a long range with batteries also involves compromises. Increasing the required energy storage capacity raises the vehicle’s unladen weight, which may reduce the available payload and, depending on the installation method, may also affect luggage-compartment capacity. In addition, the vehicle carries the large battery pack even in operating situations where its full capacity is not required. Charging requirements must also be coordinated with driving and rest periods, the passenger itinerary and other route constraints, potentially limiting the traditionally high degree of operational flexibility offered by coaches.

None of this means that all-electric coaches are not viable. They can already be used on well-planned routes with suitable charging provision, and their field of application is expected to expand further as the technology develops. However, they are currently only suitable to a limited extent for fully replacing the exceptionally broad range of duties performed by diesel-powered coaches.

For this reason, a plug-in hybrid powertrain that can also be charged from the grid could represent one of the most viable transitional solutions in this vehicle category over the coming years. A smaller battery pack enables urban sections requiring low- or zero-emission operation to be completed in all-electric mode, while over longer distances the internal-combustion engine provides the range and operational flexibility expected of coaches. Moreover, the battery can be charged not only from an external charger: alongside regenerative braking, its state of charge can also be increased or maintained with the aid of the internal-combustion engine where necessary, allowing the vehicle to reserve electrical energy for a later urban section even if no suitable charging opportunity is available en route.

This operating philosophy also underpins the pairing of the Irizar i6S Efficient and Scania’s new-generation plug-in hybrid powertrain, which won the coach category of the Sustainable Bus Award 2027. The significance of the design lies not simply in its electrifiable powertrain, but in making the benefits of electric operation usable even within the current infrastructure conditions of long-distance transport, without fundamentally sacrificing one of the most important operational advantages of coaches: their high degree of flexibility.

The central elements of Scania’s new plug-in hybrid powertrain are the manufacturer’s 13-litre Super diesel engine and a new electrified transmission unit. The system is available with 420 hp and 460 hp diesel engines; the award-winning Irizar i6S Efficient uses the more powerful 338 kW, 460 hp version, with maximum torque of 2500 Nm. The engine can also run on renewable HVO fuel, meaning that alongside partial electrification, the powertrain also offers a means of reducing the use of fossil fuels in this respect.

Alongside improving efficiency and reducing emissions, considerable emphasis was also placed on increasing service life during the development of the 13-litre Super engine. The engine features a maximum cylinder pressure of 250 bar, upgraded XPI injection, an optimised high-pressure fuel pump, reduced internal friction and an improved combustion process. Scania specifies a design life of up to two million kilometres, representing a 25% increase over the previous generation. The engine was developed with increasingly stringent future emissions requirements in mind; this includes Scania’s Twin-SCR exhaust aftertreatment system, which reduces nitrogen oxide emissions through dual AdBlue dosing.

Scania offers the plug-in hybrid powertrain in both two-axle, 4×2 and three-axle, 6×2*4 configurations. The former can be built with a 3280 mm wheelbase, while the latter is available with 4385 or 4780 mm wheelbases. The PHEV powertrain uses the R660 rear axle, whose gear ratios have also been adapted for high-speed operation and whose design has been dimensioned to transmit the increased torque.

The braking system has also been further developed alongside the powertrain. The new 13-litre engine comes as standard with an exhaust brake positioned after the turbocharger, capable of delivering up to 200 kW of braking power. The Scania CRB compression-release engine brake, integrated into the cylinder head, is available as an option; it engages all six cylinders in deceleration, has maximum braking power of 350 kW and, according to the manufacturer, can in many bus applications also eliminate the need for a separate retarder.

However, the technically most interesting element of the design is Scania’s new six-speed powershift transmission, into which two electric machines, each rated at 145 kW, have been integrated. The electric drive therefore has a combined peak output of 290 kW, which, together with the 338 kW, 460 hp Scania Super diesel engine, means that the combined output of the power sources integrated into the powertrain exceeds 600 kW. The electric machines not only enable all-electric operation, but also supplement the diesel engine’s drive with additional power and torque in hybrid mode, even at higher travelling speeds. During deceleration and braking, their operation is reversed: acting as generators, they convert part of the vehicle’s kinetic energy into electrical energy and feed it back into the battery.

The electric drive is supplied by an NMC-chemistry lithium-ion battery with a nominal capacity of 89 kWh. Scania uses battery technology also employed by MAN: the battery pack, built from cells supplied by CATL, can operate at up to 90% depth of discharge, meaning that 80 kWh of its nominal 89 kWh is actually available for propulsion. This is a deliberately small capacity compared with the energy storage systems of several hundred kilowatt-hours used in all-electric coaches. Its purpose is not to provide battery power for an entire day’s operation, but to make available an electric range that can already be put to meaningful use on urban or emissions-sensitive sections, for example. Scania specifies an all-electric range of up to 70 kilometres in the three-axle version and up to 80 kilometres in the two-axle configuration.

The battery can be charged from an external power source via a CCS2 connector, at DC charging power between 20 and 130 kW and a current of up to 200 amps. According to Scania data, it can be charged from 20% to 80% state of charge in around 44 minutes. However, the system offers greater operational freedom than all-electric propulsion precisely because the battery’s state of charge does not depend exclusively on external infrastructure.

While driving, the energy storage system can also be charged through regenerative braking, and the diesel engine itself can additionally be used to increase the state of charge where necessary. Accordingly, the powertrain control system offers four basic operating strategies. In Electric, or all-electric, mode, the vehicle operates solely with electric drive, while in Hybrid mode the system manages the two energy sources according to prevailing conditions. Charge Sustain mode reserves the energy stored in the battery for a later electric section, while in Forced Charging mode the state of charge can be increased while driving with the aid of the diesel engine.

These latter two functions are particularly well suited to coach operation. On a longer motorway section, the system can preserve the energy stored in the battery or, in the absence of suitable external charging opportunities, can even generate the electrical energy required later, after which the vehicle can switch to all-electric operation when approaching its destination. Charging with the diesel engine does not, of course, replace grid charging in energy terms; its primary role is to ensure operational flexibility.

Scania Zone geofencing can also be linked to the powertrain control system. It enables geographical areas to be predefined in which the control system automatically modifies the vehicle’s operation. Thus, on entering a city centre or zero-emission zone, for example, the bus can automatically switch to electric mode, provided that sufficient battery charge is available. The system therefore does not leave it solely to the driver to decide where the available electrical energy is used.

Stopping the diesel engine does not impede the operation of auxiliary systems either. The PHEV platform has received an electric air compressor and electrohydraulic power steering, while the power electronics have their own liquid-cooling circuit. Scania itself carries out the battery-pack integration and uses its own battery management, charge-control and thermal-management systems.

A further advantage of the electrified powertrain is that regenerative deceleration also relieves the conventional service brake. The electric machines then convert part of the vehicle’s kinetic energy into electrical energy, which is returned to the battery. Naturally, the amount of recoverable energy depends on the route, so the system’s benefits are primarily realised on sections involving frequent deceleration and changes in elevation.

The award-winning vehicle combines the bodywork of the Irizar i6S Efficient with Scania’s new plug-in hybrid chassis – which is also available to other bodybuilders and has already been bodied by Castrosua. This is a design optimised from the outset to improve energy efficiency: during its development, Irizar achieved a weight reduction of up to 950 kilograms compared with the previous generation, while reducing aerodynamic drag by around 30%. As a combined result, the manufacturer specifies fuel consumption up to 13% lower than that of the previous i6S.

The advantages of the PHEV powertrain and the Efficient bodywork therefore build on one another. Scania had previously spoken of the potential for fuel savings of up to 40% under typical mixed operation; according to more recent data published in connection with the Sustainable Bus Award, regular external charging and appropriate use of electric operation can reduce fuel consumption by up to 47% under favourable conditions. However, these figures should not be regarded as universally guaranteed values, but as potential savings dependent on the operating profile, charging frequency and the proportion of route sections covered electrically.

Scania also communicates a far greater potential reduction in carbon dioxide emissions, of up to 94%, but this is no longer the result of the hybrid powertrain alone. The calculation is based on the combined use of the plug-in hybrid system, electricity from an external source, and HVO (Hydrotreated Vegetable Oil), a renewable diesel fuel based on hydrogenated vegetable oil that can be used in the diesel engine. The use of HVO alone can also deliver significant savings: according to Scania data, carbon dioxide emissions across the entire energy supply chain can typically be reduced by around 83% compared with fossil diesel, while reductions of 50–90% can be achieved depending on the feedstock and production method. Most 13-litre engines can also run on FAME (Fatty Acid Methyl Ester) biodiesel; for this fuel, Scania typically specifies a carbon dioxide emissions reduction of around 60%, or between 50% and 80% depending on the feedstock, compared with fossil diesel.

The essence of the design is therefore not to try to cover as many kilometres as possible electrically with a large battery. On the contrary, it uses a relatively small energy storage system to provide an electric range that can already be meaningfully used on urban and emissions-sensitive sections, while continuing to rely on high-energy-density liquid fuel to meet the energy requirements of long-distance travel. In this way, the powertrain makes coach operations partially electrifiable within the framework of current infrastructure while retaining their fundamental flexibility of use.