The areas with the worst air quality include large urban agglomerations [1
]. On the one hand, there is the highest density of moving vehicles, and therefore a huge number of emission sources. On the other hand, vehicle traffic conditions in large cities are usually unfavourable (low average speed, frequent braking, stopping and accelerating) and favour high fuel consumption and high exhaust emissions. In urban agglomerations, city buses, along with trams, metro and sometimes trolleybuses, are usually the main element of public transport. Traffic conditions of city-buses are particularly unfavourable here due to the necessity of frequent stops at bus stops and traffic congestion. Additionally, in cities, the dense development of buildings hinders the dispersion of exhaust gases emitted by buses and other vehicles. At the same time, a large number of people are exposed to the pollution because of the high population density in cities. It is well known that in buses, the most common source of propulsion, also in hybrid solutions, is the diesel engine. The toxic effects of diesel engine exhaust on the human body are well known [4
], including carcinogenic effects [7
]. It is therefore understandable that reducing harmful exhaust emissions from city buses is particularly desirable. Over the last 20 years or so, exhaust emissions from diesel vehicles have been significantly reduced. First of all, thanks to the use of advanced injection systems and effective exhaust aftertreatment systems [9
]. This is perfectly illustrated by the lower and lower levels of exhaust emissions permitted by law [13
]. The introduction of reformulated diesel fuels, containing less than 10 ppm of sulphur, also played an important role.
From the legal point of view, reducing the exhaust emissions is forced by increasingly rigorous emission limits as well as stricter and more extensive measurement procedures [15
]. In this respect, note in particular that emission tests are carried out in real operating conditions [16
]. These studies provide more reliable results, which sometimes critically verify theoretical assumptions or the predictions made on their basis. For example, in studies by Liu et al. [22
] the exhaust emissions of buses were investigated in real conditions. A total of 234 city buses with different drive systems and emission standards were tested, including a large sample of Euro V buses (59 conventional and 26 hybrid). Based on this research, it was found that hybrid buses may emit more PN (particle number), NOx
and HC than buses with conventional drive. Other results presented by Keramydas et al. [23
] have shown that on some urban routes, hybrid buses may consume more fuel than conventional diesel buses. In turn, Dreier et al. in their research they found that plug-in hybrid bus provide reductions in greenhouse gas emissions of up to 72% [24
Due to the ongoing COVID-19 pandemic, forecasts regarding the development of transport and other branches of the economy are subject to considerable uncertainty [25
]. Nevertheless, the forecasts presented by the OECD in Transport Outlook 2021, even in the most pessimistic variant, predict an increase in the demand for passenger transport in cities [26
]. Total urban passenger transport demand is projected to grow by 59% to 2030 and 163% by 2050 from the base year 2015 under the Recover scenario. Characteristically, all scenarios are expected to increase the share of public transport at the expense of individual (private) transport.
Four main energy carriers are available for city buses: fossil fuels, biofuels, electricity and hydrogen. For all of these options, there are different application solutions, using one fuel or a combination of more than one energy carrier. The prospects for the development of modern city buses were studied under the European initiative CIVITAS (Cleaner and better transport in cities). The results obtained in this project show that buses powered by: compressed natural gas, electricity, second-generation biofuels and hybrid configurations combining electricity with hydrogen or a diesel engine are considered as the most promising in terms of technology and environment [27
]. There are many studies available in the literature describing the environmental benefits associated with the use of the above-mentioned technologies, see, e.g., in [28
Diesel engine is at present, and certainly will remain in the near future, the dominant drive in city buses. The service life of a city bus in Polish conditions is ~15 years. Therefore, it is justified to look for solutions reducing the toxicity of exhaust gases emitted from these vehicles. One possible solution is the use of oxygenated fuels. At the turn of the 20th and 21st centuries, many projects were implemented in this area, which confirmed the beneficial effect of oxygenated fuels on the composition of exhaust gases from diesel engines [34
]. Usually, reductions in the emissions of incomplete combustion products: PM, CO and HC were observed, while maintaining an acceptable NOx
emissions. A reduction in PM emissions is regarded as the main environmental benefit of oxygenated diesel fuel application. However, in the view of minor changes in NOx
emissions, an improvement is also achieved with regard to so-called PM/NOx
trade-off. The advantage of synthetic oxygenated compounds compared to the commonly used fatty acid methyl esters (FAME) is the high oxygen content—e.g., ~36% m
for glymes, and even 53% for dimethyl carbonate, compared to only ~10% for FAME. Thus, even a small addition of such synthetic oxygenates made it possible to achieve an oxygen concentration in the fuel that would reduce PM emissions, while not significantly affecting the fuel’s physical properties.
Similarly, the authors of this study conducted in the past extensive research on the influence of oxygenated fuels on the exhaust emissions from diesel vehicles [40
]. Among the 12 tested oxygenated compounds, PM emissions were reduced the most by carbonates; however, taking into account the PM/NOx
trade-off, glycol ethers were the most favourable oxygenates. Among the six tested compounds from the glycol ethers group, in the authors’ research the most favourable results were obtained for triethylene glycol dimethyl ether. With the content of 10% v
of TEGDME in diesel fuel, for a Euro 4 diesel passenger car over the NEDC cycle, PM emissions were reduced by 32%, HC by 34%, CO by 30%, with no changes in NOx
]. For this reason, the authors decided to use this oxygenated compound in the studies presented in this article.
The studies published by other authors also show favourable changes in exhaust emissions associated with application of glycol ethers as diesel fuel components. For example, mention can be made of the work of Delfort et al. [35
]. They tested passenger car emissions over the NEDC and achieved a reduction in PM emissions. The study by Hallgren and Heywood [36
] also reveals a reduction of PM emissions with the use of glycol ethers as diesel fuel components. Additionally, in this research it was also established that oxygenated compounds reduce mainly soot fraction, and only slightly soluble organic fraction. In another study (Yeh et al. [39
]), it was found that when glycol ethers are used, except of PM, also CO and HC emissions are reduced, however, NOx
emissions increase. Some further examples of studies showing the favourable effects of glycol ethers in diesel fuels on exhaust emissions can be given, including such as Porai et al. [45
], Dumitrescu et al. [46
], Serhan et al. [47
] and Pellegrini et al. [48
Despite the development of exhaust gas aftertreatment systems, oxygenated fuels are still of interest to researchers, as evidenced by the emerging scientific works in this field, see, e.g., in [49
]. The authors of this study also decided to investigate the effect of diesel fuel containing triethylene glycol dimethyl ether on road exhaust emissions of a hybrid bus. Unfavourable operating conditions of city bus engines may favour the benefits of using oxygenated diesel fuels. To the best of the authors’ knowledge, studies such as those undertaken in this article have not yet been published anywhere.
It should be emphasised that the driving conditions of a vehicle have a decisive influence on the operating conditions of its engine. These, in turn, determine the level of fuel consumption and exhaust emissions [53
]. Therefore, particularly valuable results are provided by measurements on real routes of city buses, possibly in test cycles mapping various road traffic conditions. This approach was used in this work.
The Solaris Urbino 18 Hybrid bus used in the research was equipped with a serial hybrid drive system. It was noticed that for this bus, the differences between exhaust emissions in each type of SORT cycle (1, 2 and 3) were small, while for the conventional bus, exhaust emissions recorded in individual variants of SORT cycles were more differential. The reasons for this can be found in the characteristics of the series hybrid drive system, where despite the change in vehicle speed and power on the wheels, the operating conditions of the internal combustion engine change slightly. The exhaust gas aftertreatment systems, including SCR and DPF, also play an important role here. Both of these factors are the reason why the use of oxygenated fuel did not have a significant effect on the emission of any of the exhaust emissions components in any of the SORT cycles.
The reduction in emissions of some exhaust components was found when the Solaris Urbino 18 Hybrid bus was fuelled with oxygenated fuel during its actual operation on the bus line 76 of the Municipal Transport Company in Poznań. There was a reduction of CO emissions by ~50% and NOx emissions by ~10%, almost identical levels of PM and HC emissions were observed and smoke opacity for both fuels. These are noticeable emission benefits, but rather not significant enough to justify the use of oxygenated fuel in the day-to-day operation of such a bus.
Analysing the obtained test results, it can be concluded that the influence of oxygenated diesel fuel on the exhaust emissions of modern city buses is quite favourable, although not as spectacular as in the case of older type vehicles. In modern vehicles, low exhaust emissions are due to the effective operation of advanced exhaust aftertreatment systems. Oxygenated fuels affect emissions by limiting the formation of toxic exhaust components during the combustion of fuel in the engine, thus their effect can be “masked” by a high-efficiency exhaust aftertreatment system. Nevertheless, the beneficial effect of oxygenated fuel may be, for example, less need to regenerate the diesel particulate filter, or to keep vehicle emissions low despite aging of catalytic converters and filters. Fuels also still have an important role to play in reducing greenhouse gas emissions. Renewable fuels, including biofuels and Power-to-Liquid technologies, have a potential in this respect. Thus, when deciding to apply oxygenated fuels, it is worth ensuring that oxygenates used are renewable components.