Operation behaviour
Figure 1 shows the share of work the tractors had to perform in the time period from 2015 to 2017. Depending on the structure of the farm (i.e. size, livestock, share of grassland and arable land), the tractors were used for a wide range of different types of work. However, the test tractors carried out work types (e.g. transporting, ploughing, cultivating, harrowing or mowing) that are comparable to the working profiles of conventional diesel fuelled tractors under Bavarian farming conditions.
Because of the different physical properties of R100 compared to diesel fuel (e.g. higher viscosity), cold start and idling are challenging operation points. Figure 2 shows the evaluation of the engine cold start by the tractor operators. The cold start behaviour of the tractors is predominantly rated as good or medium. Tractors 3, 4, 7, and 12, which feature a dual fuel management system, where the cold start is done with diesel fuel, usually performed a good engine start. Also, tractor 18 is equipped with a dual fuel management system, but because of irregular malfunction of the switchover valve in the low-pressure fuel system, no start was possible on four days during the three years of operation. The cold start behaviour of the tractors which operate solely with R100 is judged to be slightly less positive but still good or medium for more than 75% of the cold starts. For four of these tractors, only on one day during the three years a start was not possible. For tractor 5, this was caused by damage to the starter motor after 3500 h of operation. For the other tractors, exchanging the fuel filter solved the problem. The cold start behaviour is not directly related to the ambient temperature. This is because for plant oil compatible combustion engines suitable technical measures, such as heating devices, are applied to ensure good cold start behaviour. However, at low temperatures, technical deficiencies in the fuel system (such as weak aged fuel pumps, air in the pipes) or in the preheating systems (such as defective fuses) become more obvious.
In summary, the technical reliability of the tractors running on R100 seems to be more or less the same as that of diesel-fuelled tractors. However, manufacturers should pay special attention to optimising the cold start behaviour and fuel system.
Engine oil quality
The R100 content in the engine oil of the tractors is shown in Fig. 3. The tractors in engine category A show a much higher plant oil intake into the engine oil than the tractors in the other engine categories. After an operation time of 200 h, the engine oil was diluted, reaching an R100 content of more than 0.1 g g−1. The origins of the fuel intake can be leakages in the engine-oil-lubricated high-pressure unit pumps and injected fuel that was sprayed on the cylinder liner and washed into the engine oil. In contrast to diesel fuel and similarly to fatty acid methyl esters (FAME), R100 does not evaporate out of the engine oil under usual conditions because of its higher distillation curve but accumulates during the engine oil operation time [30, 31]. A high content of R100 in the engine oil increases the risk of oil polymerisation and subsequent engine damage [31]. Limit values for acceptable R100 content are not well defined and vary between 0.06 and 0.25 g g−1 [32]. Hence, the engine oil drain interval was reduced from 500 to 250 h for these tractors.
In contrast, with fuel content values under 0.05 g g−1, after 500 h of operation time, the tractors of engine category B are characterised by a low intake of R100. The tractors are equipped with CR injection systems, where the high pressure is generated by a fuel-lubricated high-pressure pump (tractors 5 and 6) or two engine-oil-lubricated high-pressure unit pumps (tractor 3). In comparison to the tractors of engine category A, which were equipped with four (tractor 1) and six (tractor 2) engine-oil-lubricated unit pumps, the number of possible leakages in the fuel system for the tractors of engine category B is less. Furthermore, tractor 3 is equipped with a dual fuel management system, where the cold start of the engine is performed with diesel fuel. Especially at cold start and during engine warm up, unfavourable conditions for mixture preparation in the combustion cylinder prevail and the risk of wetting the cylinder wall with fuel is higher, especially for plant oils that are characterised by high viscosity. For tractors 5 and 6, the cold start behaviour with R100 was optimised by adaptation of the injection timing and pressure within the engine control unit. The low fuel intake rates of these tractors, which are at the same level as for tractor 3 with the fuel management system, indicate high-quality optimisation for R100 operation.
For the tractors of engine categories C to E, low fuel intake into the engine oil far below 0.10 g g−1 of fuel after 500 h of operation time can be observed as well, except for tractor 18. Overall, the data basis for the tractor 18 is still too small to indicate a clear reason for the increasing R100 content in the engine oil (e.g. post-injections of fuel, incorrect operation during cold starts carried out with R100).
Further results of engine oil analysis focus on engines of the categories B to E with CR injection systems.
The results of total acid number (TAN) and total base number (TBN) of the engine oils are shown in Fig. 4. For the tractors of engine category B, the TAN of the engine oils increased by 0.3 mg KOH g−1 per 100 h of operation on average. About the same was observed for the tractors of engine categories C, D, and E, with an average increase of TAN between 0.2 and 0.4 mg KOH g−1 per 100 h. The TBN decreases with increasing operation time of the engine oil between 0.2 and 0.5 mg KOH g−1 per 100 h on average for the tractor engine oils. After 500 h of operation, which is the regular draining interval, the relative decrease did not exceed a level of 50%. An engine oil drain is recommended when the TBN is reduced by 50% from the initial value [33]. The decrease in TBN is at about the same level as the increase in TAN. After 500 h of operation time of the engine oil, the TBN is still higher than the TAN. Furthermore, the TAN does not show an accelerated increase. These factors indicate a sufficient alkaline reserve of the engine oil.
The resulting alkaline reserve of the engine oil after 500 h of operation time is dependent on the starting level of TBN. The ash-reduced engine oils of the John Deere tractors and tractor 18 are characterised by a lower starting TBN. The reason for the lower TBN might be that the additives for controlling the TBN often contain ash-forming elements and are therefore reduced.
Figure 5 shows the development of the iron content in the engine oil of the different tractors. Iron in the engine oil is a result of wear of different engine parts. The average increase of iron content varies between 4 and 8 mg kg−1 after 100 h of operation for the different tractors and can be assessed as uncritical. For example, the warning limit for iron defined by John Deere is 50 mg kg−1 per 100 h of operation time of the engine oil [20].
Also, for the other wear metals, no problematic concentrations in the engine oils were observed. The additive content and the analysed kinematic viscosity of the engine oils were also inconspicuous. The results of engine oil analysis do not indicate excessive wear or malfunction of engine parts.
Malfunction and repairs
No serious engine damage was observed for any tractor. The most cost-intensive repair concerning the engine was incurred for tractor 1 after 7000 h, to replace a broken control rack for the unit fuel pumps. This damage was not caused by R100 operation.
For tractors 1, 2, 5, and 6, after around 3000 to 4000 h, the low-pressure fuel pump had to be replaced. The cost of the pumps was below 500 € each. Since pure rapeseed oil is characterised by a higher viscosity compared to diesel fuel, it can be expected that the pumps are working at higher load, which may cause a shortening of the lifetime. Tractors 5, 6, 8, 9, and 10 are equipped with similar engines, and for this engine a malfunction of a relief valve at the CR was observed after around 1000 to 2000 h of operating time with R100. For two tractors, cleaning the valve was sufficient to solve the problem, and for the other tractors, this valve was replaced as it is not cost intensive. This malfunction was recognised once for each of these tractors but not again during ongoing operation time.
Tractors 3 and 4 had problems with air in the low-pressure rapeseed oil fuel system within the first year of operation. As a consequence, the fuel management system only allowed diesel fuel operation during most of the time. The reason was a tiny crack in an inspection glass on the fuel filter. Furthermore, the fuel switchover valve of tractor 3 had to be replaced twice. Additionally, a damaged switchover valve was also replaced on tractor 18.
Beside this, some other repairs that were not linked to R100 operation were necessary, such as damage to electronic or transmission parts. Small repairs, such as on the electronic or on screw connections were comparable to conventional diesel according to the information from the service workshops.
Considering that the usage of R100 in tractors is not common practice, the number and expenditure of necessary repairs of the 18 plant oil tractors were rather low. This shows that reliable long-term operation of plant-oil-fuelled tractors is possible.
Power performance
The power performance.of seven tractors was measured according to OECD Code 2 [29] several times at different tractor ages. It has to be considered that the maintenance of the tractors was carried out according to the manufacturer’s recommendations. Figure 6 shows the power at the power take-off (PTO power) at rated speed and full load. For all tractors, the variation of PTO power over operating time is within ± 5%. Simple linear regression analysis shows no significant influence of the operating time on the PTO power except for tractors 5 and 18. A significant increase of PTO power over operation time is calculated for tractor 5, while a decrease is calculated for tractor 18. Except for tractor 18, no trend of a significant power loss that might indicate engine malfunction is observable. Since there are some 100 operation hours in practice and some years between the PTO power measurements of the tractors, the range of variation is assessed as non-critical. The variations might result from different conditions of the engine, transmission, and auxiliary equipment but also minor differences in framework conditions during the measurements. For tractor 18, the detected power loss was 5 kW within 750 h of operation.
The injected mass of rapeseed oil per stroke at rated speed and full load is shown in Fig. 7. The injected mass of rapeseed oil varies by ± 4% during the operation time of the tractors and partially explains the differences in PTO power. Except for tractor 18, no trend of a significant decrease of the injected mass R100 per stroke that might indicate unusual deterioration of the fuel injection system is detectable.
For the most modern tractor, no. 18, the injection mass of R100 decreases significantly over time, which explains the power loss. One reason for the reduced injection mass may be the formation of injector deposits. Injector deposits can affect the hydraulic flow in the injector nozzle holes, resulting in a reduced fuel flow rate and injection quantity [34,35,36]. Especially modern engines with more sophisticated fuel injection equipment are more prone to form different types of deposits in connection with the fuel type and quality [37]. This trend can also be seen for the most modern tractor, no. 18, within the R100 tractor fleet using the PTO power and injection quantity as indicators, but further results are needed.
Emission performance
The emission performance of the tractors is evaluated on the basis of nitrogen oxides (NOX) and particulate matter (PM) emissions, since these are the most relevant exhaust components for diesel engines. Figure 8 shows the results of the emission measurements of the tractors fuelled with R100 and diesel fuel, except for tractor 17, where only measurements with R100 were performed. With increasing exhaust gas stage of the tractors, which in a way represent the technical evolution, the emissions of NOX and PM decrease for both fuels. For the tractors of engine categories A and B (tractors 1, 2, 4, 5, and 6), which were not equipped with exhaust gas aftertreatment systems, up to 25% higher NOX and up to 60% lower PM emissions were observed with R100 than with diesel fuel. Tractor 12 shows about the same NOX emissions with both fuels but higher PM with R100. This tractor features a DOC and SCR system for reduction of NOX but no DPF. In contrast, tractor 13 is equipped with a DPF, which explains the low but similar PM concentrations for both fuels. The same applies to the NOX emissions of tractor 13. Tractors 17 and 18, which comply with exhaust stage IV, are equipped with DOC, DPF, and SCR systems. While the NOX emissions of tractor 18 are equal for both fuels, PM emissions are lower during R100 operation than with diesel fuel. Although diesel fuel was not investigated for tractor 17, the results with R100 of both exhaust stage IV tractors correspond well with each other. The low levels of NOX and PM emissions demonstrate the high effectiveness of exhaust gas aftertreatment systems during R100 operation.
Higher NOX emissions of diesel engines without the SCR system fuelled with R100 compared to diesel fuel were also observed in previous research, especially at higher engine load [8, 17, 18, 38, 39]. This is probably a result of several overlaid effects, as discussed in detail in Emberger et al. [8], and is also proposed to explain the higher NOX emissions of FAME compared to diesel [40,41,42,43,44].
Summarising, it can be stated that the emission behaviour of the investigated tractors does not show any extensive deviation between R100 and diesel fuel operation. Furthermore, new tractor models with exhaust gas aftertreatment systems including SCR and DPF are characterised by low NOX and PM emissions in particular.
Six John Deere tractors (13–17) of exhaust stages IIIB and IV are equipped with a DPF and an upstream DOC, enabling continuous DPF regeneration (continuously regenerating trap). In diesel serial application, the DPF regeneration is usually done by actively dosing an additional amount of diesel fuel either in the cylinder at the end of combustion or in the exhaust stream before the oxidation catalyst. The resulting temperature increase of the exhaust gas through fuel oxidation in the catalyst leads to oxidation of the soot in the DPF. Because of the high viscosity and evaporation temperature of rapeseed oil, the dosing system does not work with R100. Thus, it had to be deactivated and the tractors were operated with passive regeneration instead. By solely passive regeneration it is intended to achieve a balance of the separated and oxidised plant oil soot quantity in the DPF, as shown by Düsseldorf [23]. To prove this theory, tractor 13 was equipped with a data-logging system to evaluate the pressure difference over the DPF during operation (logging frequency of 1 Hz). The resulting pressure difference of the DPF is an effect of not only the amount of soot and ash but also the mass flow of exhaust gas, which depends on the speed and load of the engine. For analysis of the pressure difference, only values where the engine speed was within the range of 1700 to 2100 rpm and the engine load was between 70 and 100% were considered. The measured pressure difference over a period of 2000 h is shown in Fig. 9. Although data were only used from a certain range of engine speeds and loads, a high variation of the pressure difference can still be noticed. Nevertheless, no excessive increase in differential pressure over a period of 2000 h was observed, and hence the passive regeneration of the DPF during R100 operation is suitable for long-term operation. There are multiple reasons for this. The oxidation reactivity of soot from vegetable oil combustion is higher than that of diesel [45], which leads to lower soot oxidation temperatures. Furthermore, the exhaust gas temperature in the DPF during many standard farm operations exceeds 300 °C for longer time periods, which allows long-term reliable and efficient passive DPF regeneration [46].
A linear regression analysis of the pressure data shows that the differential pressure increases by around 0.7 kPa in 1000 h. This increase can be explained by the continuous deposition of ashes in the DPF that originate mainly from additives in the engine lubricant [46, 47].
Besides tractor 13, the other five tractors with deactivated active regeneration systems were also operated without failures related to the DPF. This was favoured by tractor working profiles with recurrent high load operation. However, the possibility that passive regeneration will be insufficient for other DPF types and for tractors performing mainly low load operation cannot be excluded. Under these circumstances, active DPF regeneration with R100 will probably become necessary.
Fuel consumption and greenhouse gas emissions
In the years 2015 to 2017, the tractors consumed 233.777 m3 of R100 and 38.765 m3 of diesel fuel (Table 4). For 10 tractors, the share of R100 in the total fuel consumption was 0.95 m3 m−3 or higher. All these tractors were John Deere tractors that were optimised for R100 combustion, even at cold start. Tractors 1, 2, and 5 were also optimised for R100 combustion during cold start but show a lower share of R100 consumption of 0.69 to 0.76 m3 m−3. For these tractors, a higher share would have been possible. These tractors were all located at the same farm and because of difficulties in the supply of R100 the tractors were temporarily operated with diesel fuel.
Table 4 Fuel consumption of the monitored tractors in the years 2015 to 2017 Tractors 3, 4, 7, 12, and 18 were equipped with a dual fuel management system, where some diesel fuel is always needed for cold starts. Four of the tractors also showed a rather high R100 share of 0.74 to 0.82 m3 m−3; tractor 3, however, showed a lower share of 0.57 m3 m−3. The reason for this is that the operator drove the machine for about five months with only diesel fuel for organisational reasons on the farm.
Analysis of the R100 quality showed that the requirements of DIN 51605 are met for most of the delivered fuel batches. In the years 2015 to 2017, only one clear violation was recognised for one batch, where the content of calcium was 1.7 mg kg−1 instead of the maximum limit of 1.0 mg kg−1. The high fuel quality is the basis for reliable operation of the plant-oil-fuelled tractors.
The co-product from rapeseed oil is press cake, which is used as animal feed. For the calculation of GHG emissions, the life cycle analysis data of Dressler et al. [2] was used, since they assessed region-specific data. They reported specific GHG emissions of 11.9 g CO2eq MJ−1 from pure rapeseed oil from decentralised oil production in Bavaria, considering the substitution of soy meal by rapeseed cake and the land use change by soy cultivation in South America [2]. Taking into account the lower calorific value of R100 (34.5 MJ dm−3), the GHG emissions of the tractors amount to a total of 95.98 t CO2eq for the three years 2015 to 2017. Supplying the same energy content by diesel fuel would have resulted in 758.14 t CO2eq assuming specific GHG emissions of 94 g CO2eq MJ−1 as suggested by the European Council [5]. In total, 662.16 t CO2eq or 30.3 t CO2eq per 1000 operating hours were saved with all 18 tractors in the years 2015 to 2017.