Emissions Testing of Gas-Powered Commercial Vehicles
The results of tests to measure the greenhouse gas and air pollutant emission performance of various gas-powered HGVs, on behalf of Department for Transport.
Prepared by Low Carbon Vehicle Partnership
BRIEF SUMMARY OF THE MAIN POINTS
- Freight transport is vital to economic growth, but also has significant environmental impacts. Heavy goods vehicles (HGVs) are currently estimated to account for around 16% of UK greenhouse gas (GHG) emissions from road transport and around 21% of road transport NOx emissions, while making up just 5% of vehicle miles.
- The £11.3 million Low Carbon Truck Trial (LCTT) 2, ran between 2012 and 2016.
- This programme of testing was designed to help the Department for Transport to develop its evidence base to inform future policy on gas vehicles, and gauge their proper GHG impacts.
- There has been an issue with emissions of unburnt methane (methane slip) from some of the participating vehicles, particularly the retrofit dual-fuel diesel/natural gas conversions.
- The tests covered the following vehicles/technologies:
Four dedicated OEM Euro VI natural gas vehicles, including a 40 tonne Scania, 40 tonne Iveco Stralis, one 18tonne Scania rigid and one 7tonne Iveco Daily.
a. One LCTT dual fuel (DF) diesel/natural gas retrofit by Hardstaff conversion to a Euro V 44t artic vehicle
b. One DF (diesel/natural gas) retrofit conversion by Prins to a Euro VI 44t Howard Tenens artic vehicle
c. One DF (diesel/LPG) retrofit conversion of a Euro VI 44t artic vehicle supplied by Mercury Fuel Systems Ltd.
- The baseline case for the dual fuel vehicles was provided by comparing emissions performance under dual fuel operating conditions with those when the same vehicle was operating in diesel-only mode.
- The test programme used the track-based test procedures and three drive cycles developed originally by LowCVP for its HGV retrofit (CO2 reducing) technology accreditation scheme (simulating long haul, regional delivery and urban delivery operations).
- GHG results summary for the dedicated natural gas vehicles show the GHG results are somewhat mixed. When comparing with a substantially higher-powered diesel vehicle (Dedi02), overall savings of 4-8% were measured, but in more like-for-like tests (Dedi01 and Dedi03), the savings were, at best, 5% and, at worst, the dedicated gas vehicle’s emissions were some 15% higher than the diesel comparator. These results suggest that there are quite high efficiency losses under some operating conditions in moving from a compression ignition, conventional diesel engine to a spark-ignition one of similar power output.
- The Draft report of the LCTT highlights the links between tailpipe CO2 emissions, energy substitution rates (ESR) and efficiency loss. The dedicated natural gas vehicles show efficiency losses of 20% - 45% The Natural gas/diesel dual fuel vehicles typically show efficiency losses of between 5% and 15%. The LPG/Diesel dual fuel vehicle while having lower substitution rates has lower efficiency losses, or indeed, small efficiency gains in the region of -6% - +4%.
- For the current-generation dual-fuel vehicles operating on diesel and natural gas, levels of methane slip were found to be substantial under all test cycles (9 – 18 g/km). When considering only tailpipe CO2 emissions, both these retrofit conversions (Dual01 to a Euro VI diesel and Dual02 to a Euro V) showed savings of between 4% and 11%, findings very much in line with those of the Low Carbon Truck Trial. When factoring in the measured methane slip, however, the overall GHG impacts of the dual-fuel vehicles rise by, on average, 26% for the Euro VI conversion and 37% for the older Euro V system, thus turning the CO2 “savings” into overall GHG increases over the diesel-only baselines of around 10 – 35%.
- The dual-fuel diesel and LPG retrofit conversion (of a Euro VI diesel tractor unit) also showed quite high levels of hydrocarbon (THC) emissions (1 – 2 g/km). These emissions are presumed un-burnt fuel, in this case LPG, not methane, and thus do not contribute to the overall GHG impacts. This system generally achieved modest, but measurable, average GHG savings of 2%.
- The Euro VI dedicated gas vehicles tested produced, on average, NOx emissions of about 135 mg/km, while the Euro VI diesel comparators produced, on average, about 230 mg/km. Testing for statistical significance, the results are sufficient to conclude that Euro VI dedicated gas vehicles emit lower levels of NOx than their diesel counterparts. The same is true for NO2 emissions with the gas vehicles producing about 20 mg/km on average, less than one-third of the 78 mg/km produced, on average, by the diesel comparators. Emissions of carbon monoxide, however, were typically higher for the dedicated gas vehicles than their diesel equivalents. Emissions of hydro-carbons (THC, un-burnt fuel) were also higher.
- For the dual fuel, diesel and natural gas conversion of a Euro VI vehicle (Dual01), the NOx emissions were, on average, higher in dual-fuel mode than with the same vehicle operating in diesel-only mode, but the CO levels were lower.
- For the diesel and LPG conversion of a Euro VI vehicle (Dual03), the NOx emissions were lower than when in diesel-only mode but the CO emissions were higher. These differences, as well as the THC (un-burnt gas) emissions were still within Euro 6 standards, suggest that current applications of retrofit dual-fuel technologies do involve some compromises with regard to the overall ability of the vehicles’ exhaust after-treatment systems to fully mitigate emissions of all the regulated pollutants.
(Note these are based on a limited programme of tests, on a limited number of vehicles, so care is needed if extrapolating the results to a UK-wide level. The data presented are based on actual tailpipe emissions and take no account of the GHG benefits of bio-fuel options)
Methane & CO2 emissions
- The Euro VI dedicated gas vehicles tested through this programme exhibit very low levels of methane slip, typically adding less than 0.5% to the overall GHG impacts of those vehicles compared with the CO2-only case. Current generation (Euro VI) dedicated gas vehicles, running on natural gas (rather than biomethane), are likely to have broadly similar GHG impacts compared to Euro VI diesel equivalents, to within +/- 10%.
- The only after-market dual fuel system currently available, converting a Euro VI diesel truck to diesel and natural gas operation, exhibited high levels of methane slip (sufficient to increase GHG emissions by c. 20%).
- An after-market dual fuel diesel and LPG system (conversion of Euro VI diesel) exhibited similarly modest GHG benefits to some of the dedicated gas vehicles tested (c. 5% savings), and although some slippage of hydro-carbons was evident, this is un-burnt LPG, not methane or any other GHG. The system tested has since undergone a software update that may well reduce the levels of hydro-carbon emissions.
- The after-market dual fuel (diesel/CNG) conversion of a Euro V vehicle exhibited high levels of methane slip (sufficient to increase GHG emissions by c. 20-30%).
- The testing indicates that the transition to Euro VI has, for diesel heavy goods vehicles, been effective in cutting overall NOx emissions by over 98% when compared to Euro V vehicles. A further move from Euro VI diesel vehicles to Euro VI dedicated gas increases that reduction in NOx emissions to at least 99%.
- The dual-fuel diesel and natural gas system retrofitted to a Euro VI diesel vehicle exhibited increases in average NOx emissions in dual-fuel mode compared its diesel-only mode. THC emissions also increased, but CO emissions were lower.
- The dual fuel diesel and LPG system retrofitted to a Euro VI diesel vehicle produced lower NOx emissions in its dual fuel mode compared to its diesel-only mode, but emissions of other pollutants (CO and THC) increased.
- The duel fuel (diesel and natural gas) system retrofitted to a Euro V vehicle consistently reduced NOx emissions but levels remain at least one order of magnitude higher than all the Euro VI vehicles tested (diesel, gas or duel fuel). Emissions of CO and THC increased.
- This study has shown that dedicated gas commercial vehicles have potential to deliver significant GHG savings when a non-fossil, bio- or synthetic methane blend is used. DfT should therefore continue to support the development of gas vehicle infrastructure and gas powered vehicles, particularly dedicated gas, while increasing the supply of low carbon/renewable methane as a sustainable transport fuel.
- This study has highlighted the potential for GHG savings from dual fuel diesel/LPG conversions, and the role of bio-LPG. DfT should also, therefore, consider enhancing its support mechanisms for this sustainable transport fuel.
- DfT should fund further research into N2O emissions from Euro VI diesel vehicles > 7.5t gvw.
- DfT should continue to develop its evidence on GHG and AQ performance of emerging commercial vehicle technologies.
With acknowledgement to the Low Carbon Vehicle Partnership, as well as to the Department for Transport and Transport for London