Saturday 27 February 2021

Low-carbon options on trial

With the deadline for the phase-out of conventional diesel-engined trucks now set for 2040, the UK’s Low Carbon Vehicle Partnership has published the results of £20 million trials it has run assessing various low and zero-carbon freight transport technologies in real world and laboratory situations.

The results of the Low Emission Freight Trials (LEFT) reveal, as might be expected, that battery-electric vehicles (BEVs) of all sizes achieve the greatest and most consistent reductions in CO2 and other pollutants, but some other technologies can also produce worthwhile savings without some of the practical constraints of electric vehicles.

Operators taking part in the trials included John Lewis Partnership, Asda, Cenex, Howard Tenens, Kuehne + Nagel, Gnewt Cargo/Menzies Distribution, Howdens, Aberdeen City Council, London Fire Brigade, Ocado, Veolia, Westminster City Council, Yorkshire Ambulance Service, Tesco, and UPS.

One note of caution is that winter conditions can have a significant impact on the reallife operational range of BEVs, and operators trying these vehicles would do well to bear this in mind when scheduling demonstrators.

BEV trials covered 17 heavy vehicles and 26 light vans on urban cycles, and revealed average cost per km savings of 64 per cent across the piece. Energy consumption was higher in the winter: increasing by 24 per cent on the heavy trucks, which was blamed largely on the need to re-heat the cab every time the doors were opened.

On a positive note, concerns about range anxiety proved unfounded in use, with some vehicles arriving back at the depot with as much as 62 per cent charge remaining. A separate trial took place of 7.5-tonne trucks converted from conventional Euro V diesels to electric power with small diesel Euro 5-equivalent engines acting as rangeextenders to charge the batteries.

Energy savings were in the order of 43 per cent, but again reduced with the onset of winter. The retrofitted vehicles could be used as straight replacements for conventional diesels, there were no range considerations, and the drivers enjoyed improved comfort and lower workload, with the number of gearchanges reduced from over 1,000 a day to none.

Geofencing reduced urban pollution by only running the diesel engine in non-sensitive areas, and toxic emissions were in any case reduced by converting the trucks from Euro V diesels. Further improvements could be made by fitting more sophisticated range-extender engines to future models.

Gas-powered trucks took part in separate trials, and their performance was inconsistent. There are two technologies commercially available: spark ignition (Iveco and Scania) and compression ignition using a small amount of diesel to trigger combustion (Volvo).

Trials of spark-ignition trucks indicated that the use of biomethane made for 69 –81 per cent reductions in CO2 output, but using ‘fossil’ gas reduced well-to-wheel CO2 output only at higher speeds. Lower speeds and urban driving bared the inefficiencies of the spark-ignition gas engines, increasing both tailpipe and well-to-wheel CO2 output on fossil gas compared to diesel.

In terms of toxic emissions, their performance was mixed, with no consistent benefit or disadvantage when compared to Euro VI diesel. Although maintenance and capital costs were higher for gas trucks, participating fleets reported positive operational performance and driver feedback across the trials: drivers found the trucks noticeably quieter than diesels, and the refuelling process was cleaner.

The reduced cost of gas compared to diesel meant that the increased capital and workshop costs of gas trucks was absorbed in two years at 160,000 km pa. Reliability, however, was no different to diesel trucks.

Methane slip, either from the fuelling process or through the tailpipe, was insignificant. The use of certain feedstocks for biogas can actually reduce emissions of methane to the atmosphere from waste if it is used as a fuel, meaning that operating gas truck operation can reduce atmospheric greenhouse gas overall.

An expanding network of biogas filling stations made the technology increasingly attractive to operators, and participants in the trials have since ordered over 200 more spark-ignition gas trucks for their fleets.

The study concluded that spark-ignition gas trucks were 23-27 per cent less energyefficient than diesels (thanks to their lower compression ratios), and were best suited to long-haul tasks where these inefficiencies were better masked.

Biomethane had to be used if significant savings in greenhouse gas emissions were to be achieved. Two compression-ignition trucks participated in the trial: unlike the spark-ignition trucks, these retained the efficiency of conventional diesel, and recorded savings in greenhouse gas of between eight and 14 per cent using fossil methane and standard diesel.

However, particularly when used at high speed, emissions of NOx (another greenhouse gas) were up, not only on those recorded by the sparkignition gas trucks but also conventional diesels. Particulate and other toxic emissions were also worse than for conventional diesel trucks, although all emissions remained within Euro VI limits.

One project trialled the use of diesels modified to use hydrogen to displace some 15 to 60 per cent of the conventional fuel. Here CO2 reductions were in line with the amount of hydrogen used (the vehicles could run on 100 per cent diesel, but not 100 per cent hydrogen), and there was no major power penalty for using increased amounts of hydrogen.

However, using grey hydrogen (produced from fossil methane) actually increased well-to-wheel greenhouse emissions by 10 – 30 per cent over pure diesel. If green hydrogen (produced by electrolysis of water using renewable electricity) was substituted, then well-to-wheel greenhouse gas savings of 10 – 35 per cent over diesel were made.

Unlike methane, there were valuable savings in NOx at city speeds, but this was often accompanied by increased emissions of carbon monoxide and particulate (although Euro VI limits were not breached). There were no significant changes in toxic emissions on other test cycles.

Participant operators had difficulty in obtaining hydrogen from public sources, and turned instead to producing their own by electrolysis using green electricity. Operationally, there was little difference in vehicle performance, and drivers felt comfortable using hydrogen.

Data from the trials has since been used to recalibrate vehicle systems to address the rich burn that caused excessive carbon monoxide emissions. The study also examined interventions that might reduce the greenhouse gas emissions of trucks with standard diesel drivelines.

Two double-deck trailers with aerodynamic features were tested at 44 tonnes. At speeds of over 55 km/h, fuel consumption reductions of over two per cent were recorded. Conversely a lightweight trailer simulation (loaded to two tonnes less than standard) yielded consumption reductions of three to six per cent, decreasing as road speeds increased.

If the two-tonne saving could be replicated by refining the trailer’s design, then this would alternatively allow extra payload to be carried. Results from a kinetic energy recovery system (KERS) fitted to ‘urban’ semitrailers operating at 37.5 tonnes were judged inconclusive.

Each trailer had an axle equipped with a regenerative braking motor-generator. When the vehicle slowed, the generator harvested braking energy which was stored in capacitors, which then drove the axle when the truck accelerated again.

Technical challenges delayed the trials, and this, combined with the relatively consistent high speeds that the trucks operated at in real life, meant little conclusive data could be gathered, but lab-based tests showed small savings in fuel could be made, particularly in city-centre driving, while if driving style was adapted to exploit the technology, savings as high as 10 to 15 per cent might be possible.

In conclusion, it was found that retrofitting an electric axle to a trailer was more complex than expected, and driveaxle pattern tyres would be required. There was additional parasitic drag from the system at high speed, which in its current form made it only suitable for city applications. An attempt to fit it to an 18-tonne truck revealed it was not suitable for rigids.

Potential weak points were identified in the safety of the system, and it was suggested that training may be required for those coming into contact with the vehicle, but the KERS system was positively received by drivers.

The report is available at: www.lowcvp.org.uk

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