
How do you reduce the fuel consumption of a vehicle that does not use fuel? That is a question currently exercising the minds of Europe’s trailer manufacturers who are dealing with the curve ball thrown at them by European Union (EU) legislators when they decided that trailers should be included in carbon reduction targets using VECTO (Vehicle Energy Consumption Tool).
VECTO sets out the fuel consumption of various vehicle types, including key factors such as aerodynamic drag and the rolling resistance of tyres as well as the output of the vehicle’s powertrain. For trailers, the EU has suggested a reduction of 7.5% by 2030 from a 2024 baseline, and failure to conform will lead to fines being imposed on manufacturers.
While it is by no means clear if or when VECTO standards will be adopted in the UK, there can be no doubt that they will have some impact both on British trailer manufacturers who export to the EU, and the products offered to UK operators by EU manufacturers.
There are potentially positive and negative impacts on UK operators. Improved aerodynamics are an obvious route to fuel saving, and such savings will of course be welcome, but experience shows that aerodynamic aids such as trailer sideskirts are damage-prone and may not survive long enough to save sufficient fuel to cover their front-end cost.
It only takes one careless forklift driver to smash a sideskirt to pieces, and the operator is left with all the hassles of making the trailer safe to drive on the road before it can leave the location where the damage occurred, then arranging a repair, and fighting to recover the cost from an insurance company.
Savings from aerodynamics may not be real-world consistent anyway. Setting aside the difference between a headwind and a tailwind, there is the far more common situation of a crosswind (prevailing winds in the UK are west to east, while more miles are covered on a roughly north-south axis).
It is hard to see how increasing the side area of a trailer will help its fuel economy when it is being dragged through a crosswind). And even in the best conditions, improved aerodynamics are only really effective at higher speeds.
Rolling resistance is another matter. The less there is of it, the better.
CONCERNS
The most obvious place to start is with the tyres. Michelin’s director of product marketing François Le Hen suggests that while switching a tractor unit’s tyres up one grade on the EU tyre labelling scale can improve fuel economy by one litre every 100km, switching both the tractor and trailer tyres will double that improvement.
Operators may have legitimate concerns over upgrading tractor unit tyres (in general the tyres that have the lowest rolling resistance may achieve this at the expense of steering grip and traction, and durability) but these factors are less significant on trailer applications. When trailer axles are changed from dual to single-wheel designs (as most are nowadays) rolling resistance is reduced by 10% according to research carried out at MIRA in 2021.
A 2022 study, using otherwise identical DAF CF 440 6x2 tractors coupled to FFB cement tankers revealed that, compared to a truck fitted with new standard tyres all round, new low rolling resistance tyres returned a fuel saving of around 8% on real-life work in the south-east of England.
It was noted, however, that the low rolling resistance tyres wore faster than their standard counterparts thanks to their shallower treads and stiffer materials.
This would increase the fuel savings from reduced rolling resistance as the tyres wore, but there was an obvious penalty in the more frequent replacements that would be required.
But there is even less rolling resistance, and indeed tyre wear, when the tyres themselves are not in contact with the ground.
Axle weight legislation is to be respected, but at the same time not all trucks run fully laden all the time. Raising an axle (or even two) on a lightly loaded trailer will reduce both rolling resistance and tyre wear.
ZF makes trailer lift axle systems that automatically raise and lower one or two axles on a three-axle trailer bogie, while respecting weights limits on the remaining axles as the trailer’s ABS measures the loading on each axle.
ZF has also developed a suspension system that adjusts the trailer’s height while driving. A distance sensor connected to the trailer axle recognises dynamic movements of the chassis caused by uneven roads, curves and braking, and automatically controls chassis height while driving to yield improved stability and fuel economy.
The tail of the trailer is lowered by partially collapsing the suspension when travelling at high speeds in a straight line, improving its aerodynamic profile.
The manufacturer claims that this system can save 150 litres of fuel in a typical year’s operation and will also reduce wear and tear on the suspension components.
UNDER DEVELOPMENT
Nevertheless, it is hard to see how these innovations alone can return the fuel savings demanded by VECTO. There is, however, an escape clause in the legislation: trailers which are “equipped with devices that actually support their propulsion and have no internal combustion engine” are exempt from the legislation altogether.
Such technology has been under development for while – usually utilising an electric generator/motor which functions in the manner of the old Telma electromagnetic retarder under braking, but stores energy in an ultracapacitor or battery pack to provide additional power under acceleration.
This technology was used in the EU-Transformers project of 2013-17, alongside aerodynamic features including a trailer with a roof that could be lowered to the rear when partly laden and rear-end diffusers. Fuel savings of up to 9% were recorded during trials.
The drawback here is that powering a trailer axle directly could induce jack-knifing.
EU-Transformers partner, trailer-maker Schmitz CargoBull subsequently leveraged the axle technology to launch the successful S.KOe Cool fridge trailer, with the recovered energy cooling the load via an all-electric fridge unit rather than powering the vehicle.
More recently, BMW’s logistics division has claimed substantial fuel savings when a powered axle system developed by Trailer Dynamics was used on a curtainside trailer.
The 360kW (constant output) powered centre axle was used in conjunction with trailer-mounted batteries and a fifth-wheel incorporating sensors to ensure the rig’s stability.
Energy recovered under braking was used to aid the tractor’s driveline when pulling away and hill-climbing. The trailer batteries were also recharged from the grid when the truck was not in use.
On short runs over a hilly route incorporating motorways and main roads for a total distance of up to 250km a day, diesel savings of almost 47% were recorded. On a longer 450km route, the fuel savings rose to more than 48% with loads of over 16 tonnes on board.
The technology also has a significant contribution to make to extending the range of electric trucks. BMW said it achieved a range of over 600km without recharging when the trailer was coupled to an electric Volvo tractor unit.
BMW maintains the higher cost and unladen weight of the electric trailer is counterbalanced by lower operational costs when the system is in use.