Electrified trailers: generator axles, driven e-axles and electric reefers

An electrified trailer carries an electric machine on one of its axles. Three makers describe three arrangements: a generator axle that turns the vehicle’s kinetic energy into electricity during braking or overrun and feeds equipment on the trailer, such as a cooling unit or the pumps of a tanker or silo (SAF-HOLLAND’s SAF TRAKr); a driven e-axle with its own battery that recuperates braking energy and also adds traction to the combination (ZF’s TrailTrax with the AxTrax 2 axle); and a fully electric refrigerated semi-trailer with a generator axle (Schmitz Cargobull’s S.KOe COOL). According to the maker’s release of July 2023, the overall vehicle type approval granted for that semi-trailer by the German Federal Motor Transport Authority includes the generator axle.

Status as of 13 November 2026. Read on that day: SAF-HOLLAND’s SAF TRAKr product page; ZF’s TrailTrax campaign page and its AxTrax 2 product page; Schmitz Cargobull’s press release of July 2023 on the S.KOe COOL type approval and its S.KO COOL product page. The SAF and ZF pages carry no publication date.

Three arrangements

SAF-HOLLAND and Schmitz Cargobull call their axles generator axles; ZF describes TrailTrax as also propelling the combination. ZF’s page lists “Trailer EBS Integration” as one of three main components and says TrailTrax “provides holistic control of braking including recuperation, stability control, and electric traction”. What UN Regulation No. 13 requires of the braking of an O3 or O4 trailer, and how the electric control line and the ISO 7638 connector work, is set out in trailer brakes and EBS; recuperation on trucks and buses themselves is covered in regenerative braking.

Generator axle: SAF TRAKr

SAF-HOLLAND files the SAF TRAKr under “Electrified Suspensions”. Its product page says the axle “uses recuperation to recover energy, for example to operate a cooling unit or pumps in tankers or silos”, and that “during braking or in overrun phases, the centrally located high-voltage generator unit of the SAF TRAKr converts the kinetic energy of the vehicle into electrical energy”. The axle can be combined with SAF’s INTRA or MODUL air suspension.

Among its main features SAF lists a standardized system “identical in construction to standard axles”, that retrofitting is possible, and that the axle is independent from the trailer manufacturer. Its list of uses runs: cooler, silo, moving floor, lifting platform and transportable e-forklift.

The page gives no date for the start of production and no energy yield per kilometre or per year.

Driven e-axle with battery: ZF TrailTrax

ZF calls TrailTrax its “Electrified Trailer System”. The page describes it as “integrating ZF´s AxTrax 2 electric axle plus a battery system box with battery modules and battery management in the trailer”, which “allows braking energy to be recuperated, and the recovered power can support traction and ease the workload of the primary engine in the truck”. ZF names three components:

• Battery system box: it “integrates components such as high-voltage battery modules, cooling system and DC/DC converter”.
• Trailer EBS integration: listed by ZF for safety, energy recuperation and propulsion.
• e-Axle: ZF’s in-house AxTrax 2, with up to 210 kW continuous power for traction support.

ZF describes the system as working with any truck powertrain: diesel trucks “can be transformed into hybrid vehicles”, and trucks with a battery-electric or fuel-cell drive “can use the additional electric drive on the trailer as a zero-emissions range extender”. It also lists support for electric power take-off systems “to supply reefers and other external electric auxiliaries”, among the benefits it places after road approval (see below). The mechanical kind on trucks is covered in power take-off.

ZF’s figures and their conditions

ZF lists Drive-Off-Support, Traction Boost, Up-Hill-Support, the up to 60% recuperation and the support for electric power take-off under the sentence “There are even more benefits once the system is approved for road use”. The same page says the first electric trailers are expected to operate on EU roads once regulatory classification has been received. It gives no date for either. It also says traction assistance “effectively turns the truck into a 4x4 axle configuration in slippery conditions”.

On the market, ZF’s page states that Kässbohrer and Krone, and the axle supplier BPW, “are adopting ZF’s TrailTrax technologies”, and that ZF contributes TrailTrax and associated components such as the e-axle, batteries and control unit to ZEFES, an EU project on zero-emission long-haul freight transport.

AxTrax 2 on the truck side

ZF’s trailer page describes AxTrax 2 as series technology “from the new e-Drive platform for trailers, buses and trucks”. ZF’s separate AxTrax 2 product page covers the truck application: an “integrated and modular electric axle for medium- and 6x4 heavy-duty truck applications”, designed “to replace engine, transmission, and conventional axle”, with a hairpin electric motor of 210 kW continuous power and a maximum output torque of up to 34,400 Nm, an integrated 800 V silicon carbide inverter and a 3-speed transmission. That page does not describe the trailer version; the 210 kW continuous figure is the one both pages share.

Fully electric reefer semi-trailer: Schmitz Cargobull S.KOe COOL

In July 2023 Schmitz Cargobull announced that it had included “the electric generator axle of the S.KOe COOL semi-trailer in its overall vehicle type approval”, granted by the German Federal Motor Transport Authority (Kraftfahrt-Bundesamt). According to the release, the S.KOe COOL “meets all the relevant statutory safety, environmental and production requirements of the European Union”; proof was provided by prescribed type tests on a vehicle or its components, the systems were approved with the support of TÜV Rheinland, and the type test for the complete vehicle approval of the “three-axle, fully-electric semi-trailer” was carried out by the technical service of SGS TÜV. The release says Schmitz Cargobull had worked with TÜV Rheinland on all its electrification projects since 2014.

The conventional reefer semi-trailer of the same family is described on the Schmitz Cargobull S.KO COOL page. On its current S.KO COOL product page, Schmitz Cargobull lists an optional roof-mounted solar panel that supplies the refrigeration unit’s battery “when the vehicle is stationary or disconnected”, a Body Control Unit that “increases the refrigeration unit’s operating range through intelligent battery and system management”, and, in its TrailerConnect portal, “a battery monitor for electric refrigeration machines” alongside a tank sensor for diesel-powered ones. The temperature classes and test rules for refrigerated bodies themselves are covered in ATP refrigerated transport.

Related rules on other pages

The CO2 emission targets for trailers and semi-trailers in Regulation (EU) 2019/1242, as amended by Regulation (EU) 2024/1610, and the trailer vehicle groups behind them, are set out in trailer types explained and EU CO2 standards for trucks and buses. ZF’s page describes TrailTrax as “a key technology for trailer builders and fleets to meet upcoming local decarbonization legislations”.