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Since 2013, Euro VI heavy-duty on-road vehicles have been on the market in the Europe. Regulated exhaust emissions, including nitrogen oxides and particulate matter, have been cut down to a very low level, independent of fuel (diesel or natural gas). Multiple research papers have shown that the regulated emissions from the Euro VI and US 2010 heavy-duty on-road vehicles tested on chassis dynamometers really deliver emission levels which correspond the type approval requirements, independent of the test cycle used. In-service conformity (ISC), which is included in the Euro VI legislation, requires heavy-duty on-road engine manufacturers to test and prove their engines to comply with the emission legislation during the engine in-use period. The measurements are carried out in the field using PEMS (Portable Emission Measurement System) equipment. This kind of testing, depicting real-world emissions is the final stage to confirm low real-life emissions. Although there is evidence that the exhaust aftertreatment systems of Euro VI heavy-duty on-road engines work well on an average, there is only a limited data of city buses covering their functionality over longer operation periods, especially not at lower ambient temperatures. Especially reduction of nitrogen oxides in the SCR-system (Selective Catalytic Reduction) is sensitive to SCR catalyst operation temperature, and also to impurities transferred from engine oil or fuel. This paper present results of a unique combination of chassis dynamometer measurements, on-road PEMS measurements and continuous NOx-emission monitoring during operation in winter condition from four Euro VI diesel city buses operating in the Helsinki metropolitan area. Based on the research done, four Euro VI city buses monitored and measured during the project showed decreased NOx conversion rate and increased tailpipe NOx emissions when operating in ambient temperatures below 0 °C. Especially in ambient temperatures below -10 °C, the continuous monitoring showed dramatically increased tailpipe NOx concentrations.

Over the years, natural gas has been promoted as a clean-burning fuel, especially for transit buses. A decade ago one could claim that natural gas buses deliver significant emission benefits over diesel buses, especially regarding particulate emissions. The spread in nitrogen oxide emissions has always been significant for natural gas engines, high for lean-burn engines and low for three-way catalyst equipped stoichiometric engines. With the introduction of US 2010 and Euro VI (effective as of 2014) exhaust emission regulations, independent of the fuel, the regulated emissions of all engines have been brought close to zero level. This means that the advantage of natural gas as a clean fuel is diminishing, especially in a situation in which electric transit buses are also entering the market. The motivation to use natural gas could still be diesel fuel substitution and to some extent, also reduction of greenhouse gas emissions. However, looking at the full fuel cycle and also taking into account methane leakages in various part of the fuel cycle, natural gas is, at its best, comparable to diesel regarding greenhouse gas emissions, not better. One of the reasons for this is the low fuel efficiency of spark-ignited gas engines compared to diesel engines. To deliver an advantage regarding greenhouse gas emissions, the efficiency of the gas engines should be significantly improved. Alternatively natural gas should be replaced with biogas. The paper presents a comparison of diesel and natural gas buses for regulated emissions, selected unregulated emission components, greenhouse gas emissions and energy efficiency.

Helsinki Region Transport, Neste Oil, Proventia Emission Control and VTT Technical Research Centre of Finland carried out a 3.5 year PPP venture "OPTIBIO" to demonstrate the use of paraffinic renewable diesel (hydrotreated vegetable oil HVO) in city buses. The fleet test in Metropolitan Helsinki involving some 300 buses is the largest one in the world to demonstrate this new fuel. The fuels were a 30 % blend of renewable diesel and 100 % renewable diesel. This paper describes the overall set-up of the project, gives an overview of the emission results as well as presents experience from the field.

Over the past decade significant research and development activities have been invested in alternative fuels in order to reduce our dependency on fossil fuel sources and reduce CO2 and local emissions from traffic. One result of these R&D efforts is paraffinic diesel fuels, which can be used with existing vehicle fleets and infrastructures. Paraffinic diesels also have other benefits compared to conventional diesels, for example a very high cetane number and the lack of sulfur and aromatic compounds. These characteristics are beneficial in terms of exhaust gas emissions, something which has been demonstrated in numerous studies. The objective of this study was to develop low-emission combustion technologies for paraffinic renewable diesel in a compression ignition engine, and to study the possible benefits of oxygenated paraffinic diesel. Hydrotreated vegetable oil (HVO), which is a commercial example of paraffinic, renewable diesel, was used with and without oxygenate in comparison with conventional diesel. Exhaust emissions were measured in three steady state conditions. The adjusted engine parameters, such as inlet valve closure and injection timing, injection pressure and amount of exhaust gas recirculation (EGR) were optimized for HVO. The results demonstrate that significant reductions of particulate matter (48-61%), polyaromatic hydrocarbon (75-87%) and NOx (31-54%) emissions can be achieved simultaneously by using HVO with adjusted engine parameters.

Hydrotreated vegetable oil (HVO) named NExBTL is a 2nd generation renewable diesel fuel made by a refinery-based process converting vegetable oils to paraffins. Also animal fats are suitable for feedstocks. Properties of this non-ester type biobased fuel are very similar to GTL. It contains no sulfur, oxygen, nitrogen or aromatics. Cetane number is very high (∼90). Cloud point can be adjusted by severity of the process from -5 to -30°C, heating value is similar to diesel fuel, storage stability is good, and water solubility is low. Emissions of two heavy duty engines and two city buses are presented with HVO and sulfur free EN 590 diesel fuel. The effect of HVO on regulated emissions compared to EN 590 fuel was: NOx -7 % ⋯ -14 % PM -28 % ⋯ -46 % CO -5 % ⋯ -78 % HC 0 % ⋯ -48 % Aldehydes, PAHs, mutagenicity and particulate size were also measured.

In a diesel engine, both physical and chemical properties of the fuel affect exhaust emissions. This study focused on the separation of some physical and chemical effects of the fuel on the NOx emission. The tests were conducted with a bus engine using four diesel fuels with different density and aromatics levels. The measurements were performed using five injection timing settings to screen the effects of changes in actual injection timing. When conventional diesel fuel was replaced by reformulated diesel, NOx was reduced 7-13 %. Changes in the injection timing due to differences in the physical properties accounted only for a minor part (10-25 % relative) of the total reduction of NOx, whereas the greater part of the reduction (75-90 % relative) was due to other reasons, mainly fuel chemical properties.

Unregulated emissions of reformulated diesel fuels (sulfur < 50 ppm, aromatics < 20 vol-%) were compared to the European EN590 specification fuel (sulfur < 500 ppm, aromatics < 35 vol-%) in three IDI passenger cars and one DI van using FTP and/or ECE/EUDC emission test procedures. The effect of reformulated diesel fuels on the mutagenicity of particulate soluble organic fraction (SOF) was studied. Fuel reformulation reduced particulate emissions in IDI cars. Reformulating fuel by decreasing heavier aromatics-without decreasing final boiling point-reduced particulate mutagenicity on emission basis. At low ambient temperature (-7°C) particulate PAH and mutagenic emissions increased compared to the standard ambient temperature (+22°C) with all fuels.