Comparability of the routes
The results were achieved in the years 2020–2023. In total, distances of more than 1 million kilometers were driven with filling station diesel B7 and with HVO diesel. As the journeys in 2020 were still significantly influenced by the restrictions in connection with the COVID-19 pandemic, Fig. 9 shows the journeys from 2021 with > 750,000 km for the period 2021 - April 2023.
When averaging the values recorded for the individual vehicle pairs, it is noticeable, that the average consumption of the vehicles fueled with HVO is even slightly lower than the consumption of the vehicles fueled with diesel B7. This means that the additional volumetric consumption expected due to the lower density cannot be measured. The project team attributes the lower consumption to the drivers' awareness that they are using an ecological fuel and that they are driving more consciously.
In order to investigate the above thesis, the consumption data stored in the fleet management system of the vehicles was evaluated. The vehicles are evaluated anonymously as pairs, each of them drove the same route profile and comparable load. Alfons" refers to the HVO-fueled variant and "Anna" to the B7 diesel-fueled variant. The quality of the comparability of the data can be seen, for example, in the monthly presentation of the routes in Fig. 10.
a) Influences on fuel consumption
Using the test pair "Alfons" and "Anna" as an example, the slight reduction in consumption is constant across all months. This means that one-off, strongly deviating events can be ruled out, as can temperature and climatic influences on consumption are the same.
The analysis of driving behavior on fuel consumption is only possible to a limited extent (on a monthly base see Fig. 11) for data protection reasons. As a strong indicator of a more or less restrained driving profile, the fleet management system counts the events referred to as so called "strong acceleration". These events describe the use of the vehicle in the characteristic map ranges that correspond to significantly stronger acceleration. However, these events are themselves strongly influenced by traffic. Journeys with a lot of roadworks traffic (April-December 2021) causes an accumulation of these events, as shown in Fig. 12 can be seen
Figure 12 shows a restrained driving profile with reduced fuel consumption for the HVO fueld trucks over several months.
b) Oil analysis as a long-term sensor
The piston rings do not completely seal off the combustion chamber from the crankcase and the oil circuit in a gas-tight manner. As a result, combustion gases enter the crankcase as so-called blow-by gases and come into contact with the engine oil. Some of the exhaust gases are returned to the combustion chamber via the crankcase ventilation and exhaust gas recirculation, while another part condenses and mixes with the engine oil as a liquid. This effect is stronger when the engine is cold and running at partial load than at full load. The liquid mixed to the oil consists mainly of water, which is produced when the hydrocarbons react with the oxygen in the air. Acidic combustion residues produce a weak acid that has a corrosive effect on metals, among other things. Unburned fuel components deposit on the cylinder wall and thus also enter the engine oil. This effect is also relevant during cold starts, under partial load and especially during post-injection to regenerate the particulate filter. Fine soot particles produced during combustion can also get into the engine oil and lead to oil thickening.
The engine oil is therefore a kind of permanent sensor and reacts to any combustion changes caused by the fuel. Two representative vehicle pairs were selected with the aim of representing as many different vehicle manufacturers, operating conditions and mileages as possible. The boundary conditions for the two vehicles to be compared with conventional diesel and HVO diesel should be as similar as possible in order to obtain reliable comparisons (see also Fig. 10).
To analyze the corresponding effects, samples were taken at three intervals (s. Figure 13) and analyzed in Neste's certified oil laboratory. The oil change intervals were reduced, compared to the manufacturer's specifications in order to be able to analyze any temporal influences that may occur. Over the period between two oil change intervals, exhaust gas components, condensation water from combustion and soot can accumulate in the engine oil and cause interactions with the engine oil. The amount of foreign substances introduced by the blow-by gases is a good indicator of the quality of the fuel used, especially when assessing it as a 1:1 replacement for the existing B7 petrol station diesel.
In addition, there is the so-called oil dilution caused by unburned fuel, which enters the engine oil during particulate filter regeneration, for example. Elements of the fuel can accumulate there, e.g. if the fuel has an unfavorable boiling behavior. If fuel jets hit the liner, possible mixing of the fuel with the oil during continuous operation is crucial for maintaining viscosity and therefore lubricity.(s. Figure 14)
The dilution of the engine oil with water reacts less sensitively to part-load operation, as shown in Fig. 15. Neither the water content nor the fuel dilution are outside the usual ranges; there are no fuel-related abnormalities.
Another important element that can be observed with the oil analysis is soot deposition. Its values are also at a very low level and indicate normal combustion and soot emissions. Fuel-related deviations are not detectable.
Further influences on the wear behavior can be caused by acid formation, e.g. in connection with nitrogen oxides NOx and SO2 in the raw emissions. The combustion of paraffinic diesel HVO is accompanied by reduced soot formation, as several analyses have shown (12). Increased nitrogen oxide emissions were also not observed in the "reFuels - Rethinking Fuels" project and without sulphur as a fuel component, SO2 emissions are also below the measurement limit (3).
Effects in a changed wear behavior can be detected very well by the metal content in the oil, here exemplified by the iron content in the oil samples taken (see Fig. 16).
The iron content increases as expected over the duration of the test, but at a maximum of 30 ppm is a very low, good level and does not indicate increased wear. The curves between the reference vehicles with conventional diesel B7 and the test vehicles with HVO diesel run parallel at the same level. The same applies to the aluminum values at an even lower level.
Two pairs of vehicles were tested over a period of one year, one vehicle with conventional diesel and one vehicle with HVO diesel. The Scania with conventional diesel covered 116,939 km during the test period, the one with HVO diesel 123,104 km. Both vehicles were in long-distance operation. All oil parameters tested were within the expected permissible range and no indication of fuel-related differences in operating behavior can be observed.
The MAN vehicles tested in short-haul traffic covered 26,001 km (vehicle with conventional diesel B7) and 26,726 km (vehicle with HVO diesel), and here too there was no indication of fuel-related operating differences.
Over an operating period of one year and mileages of around 26,000 km (MAN in short-haul traffic) and 120,000 km (Scania on long-haul routes), HVO diesel demonstrated its unrestricted suitability for this application. In particular, the problems of oil dilution and soot thickening observed with some alternative fuels could not be observed.