2. Fachtagung TestRig - September 2024 83 DME plus X as a potential Fuel Hichame Ait El Mallali, M. Sc. Tec4Fuels GmbH, Aachen, Germany Vishalkumar Patel, M. Sc. Tec4Fuels GmbH, Aachen, Germany Ashrith Arun, M.Sc. Tec4Fuels GmbH, Aachen, Germany Dr. Simon Eiden Tec4Fuels GmbH, Aachen, Germany Abstract Given the growing urgency to reduce carbon emissions, the development of alternative fuels to mitigate CO 2 emission are of utmost importance. Within the DMEplusX project, DME (Dimethyl Ether) has been investigated as a fuel in HiL (Hardware-in-the-Loop) tests by TEC4FUELS. DME exhibits promising potential as an alternative fuel, particularly due to its possible production through solar and wind energy in conjunction with CO 2 . Naturally, DME is in gaseous form, necessitating careful adjustment of test benches to evaluate it as a liquid fuel. In recent years, TEC4FUELS has successfully developed Hardware-in-the-Loop test benches for liquid fuels. To test DME in liquid phase at room temperature it must be compressed to 5 bar during testing. Due to this requirement, the HIL test bench in TEC4FUELS has been accordingly modified. This enables the examination of the effects of DME and DME blends on various automotive components such as injectors, high-pressure pumps, and rails, etc. Additionally, special reactors have been developed to investigate the impact of DME and DME blends on corrosion of steel materials. This article elaborates extensively on the operation of the HiL-DME test bench and its application in different scenarios. 1. Introduction Hardware-in-the-loop (HiL) is a method that allows the most important components of fuel supply system to be tested without having to carry out a combustion test. HiL test benches investigate the effects and compatibility of various fuels and their constituent parts. The automotive sector uses these test recently developed benches. [1]. Fuel tanks, in-tank fuel pumps, filters, electric motors, high-pressure pumps, common rails, and injectors make up the HiL test bench as shown in figure 1. These components are coordinated by an adjustable control system to maintain continuous fuel circulation [2]. Fig. 1: General diagram [1]. At low flow rates, the in-tank fuel pump is triggered and fuel flows to the high-pressure pump. A filter then allows clean fuel to flow through the system, providing a barrier against small particles making it from the tank to the engine. The High-pressure pump is coupled to an electric motor [2]. This method allowed to test the compatibility of different fuels with state-of-the-art system components as well as prototype parts like rails, injectors and pumps. TEC4FUELS has successfully tested not just fossil fuels but also synthetic fuels, such as Polyoxymethylene dimethyl ethers (OME), Methanol to Gasoline (MtG) and also blends of alcohols with fossil fuels. In this work, we will focus on one of the challenging fuels, DME (Dimethyl ether), because this fuel is gaseous at room temperature, whereas all successful HiL tests at TEC4FUELS have been carried out with liquid fuels at room temperature. In ambient conditions, DME is a colorless, non-toxic, slightly narcotic, and highly combustible gas. However, under mild pressure, it can be used as a liquid fuel. DME is a clean fuel among numerous alternative fuels; it emits comparatively little pollution to other conventional fuels. It performs similar to diesel engines in terms of output and thermal efficiency, and has a high cetane number and distinct autoignition properties [3]. 84 2. Fachtagung TestRig - September 2024 DME plus X as a potential Fuel 2. Method Fig.2: Schematic diagram of the DMEplusX hardwarein-the-loop test bench Tab.1. Tested fuels and used parameters. Tested Fuels Run time (h) Pressure (bar) Injector Temperature °C Diesel EN590 200 700 230 100% DME 80%Diesel + 20 % DME 80 % Diesel + 20 % DME +Additive At room temperature, DME (dimethyl ether) requires at least 5 bar to convert from a gaseous to a liquid state. Due to higher temperature expected in the return flow of the high-pressure pump, DME was pressurized to 12 bar inside a stainless-steel tank. The pressure tank was specifically designed for a maximum pressure of 45 bar. As an additional safety measure, the test bench was operated with nitrogen. Nitrogen also served to adjust the system pressure. Due to the applied pressure, all fuel lines in the system were made of stainless steel. In this test setup, no in tank pump was required to deliver the fuel to the high-pressure pump because the pressure in the tank was sufficient. Therefore, the fuel goes directly to the high-pressure pump, which is coupled to an electric motor. The fuel is then distributed at 700 bar rail pressure to the corresponding injector. The other outlets of the rail are blocked. The injector is activated by a current profile provided by the injector control unit. The injector sprays the fuel into a reactor. To simulate the influence of combustion heat on the injector, the injection nozzle is additionally heated to the chosen temperature. The sprayed fuel condenses, leaves the reactor, and is fed back into the tank. The return line of the high-pressure pump is cooled to regulate the temperature of the fuel tank. The complete testing cycle is divided into three main phases: continuous operation (120 min), pause (45 min), long pause (120 min), and cycled operation. In the continuous operation phase, the fuel circulates repeatedly through the components. For this test, a runtime of 200 hours has been selected, meaning the effective runtime is 100 hours. All parameters are controlled by software, ensuring that data is saved every second. On the other hand, a corrosion test was conducted on steel samples to investigate their compatibility with DME and DME blends as a liquid fuel. As shown in Figure 3, the reactor was filled with DME in its liquid state. This setup allowed for the observation development of corrosion over time. Fig.3: a) Reactor used for corrosion test, b) Close-up of the reactor showing the samples in contact with the fuels. 3. Results HiL enables to determine the compatibility of the fuel with the components (high-pressure pump HPP, rail, injector, and tank) in short test times. The results of DME 100 % and DME blends (as listed in table 1) are compared with those diesel EN 590 as reference fuel. For each type of fuel, a new component was used to assess the comparison between the fuels: 3.1 High-pressure pump After the 200-hour test period, the system components were visually evaluated. The following figures (Figure 4) provide photo documentation of the high-pressure pump parts. No significant abnormalities were observed after the diesel test as shown in Figure 4a. However, in the case of DME, signs of corrosion were evident in the high-pressure pump (HPP) and observed more clearly in the spring (Figure 4b). A reduction in corrosion was observed for the blended fuel compared to 100 % DME, particularly in the spring, with no difference being observed between the additive and non-additive blends (Figure- 4c & 4d). Despite the corrosive attacks during the 200-hour runtime, there was no component failure or pressure loss. An 2. Fachtagung TestRig - September 2024 85 DME plus X as a potential Fuel explanation for the corrosion observations in the highpressure pump when using DME has not yet been fully clarified. The difference between DME and diesel is that DME has an oxygen content of 35-% [4], whereas diesel contains no oxygen. This could lead to the formation of corrosive and oxidizing intermediates. Fig.4. HPP after 200 h with different fuels, a) Diesel EN590, b) 100 % DME, c) 80 % Diesel + 20 % DME, d) 80 % Diesel + 20 % DME + Additiv 3.2 Injector Fig.5. Injector peak status after 200 h test with Diesel EN590 , DME 100 %, 80 % Diesel + 20 % DME and 80 % Diesel + 20 % DME + Additive Due to the thermal stress on the injector tip, slight discoloration can be seen after the test with diesel. As can be seen in Figure 5, the injector tip shows severe coking after the test with 100 % DME compared to diesel. In contrast, a reduction in coking was observed in tests with blended fuel compared to 100 % DME. A significant improvement and reduction in coking at the injector tip was observed, particularly with additive blended fuels. 3.3 Rail The tests were carried out on the rail fuel distributor with both diesel and DME (dimethyl ether) fuels Figure 6. No anomalies were found during these tests, indicating the versatility and reliability of the system. the rail fuel distributor can operate smoothly with all types of fuel without any adverse effects or loss of performance. Fig.6. Rail after the test run with Diesel EN590 3.4 Tank No abnormalities were observed after the test with diesel. In contrast, the tests with dimethyl ether (DME) showed the formation of deposits, which are shown in Figure 7. These deposits appear as fine metallic particles when viewed visually. This could indicate a lack of material compatibility of the components with DME, which leads to the formation of such particles. Deposits with the blend (80 % diesel + 20 % DME) were observed similar to those with 100 % DME. However, after dosing the additive into this blend, the deposits were almost completely removed as shown in figure 7 (two rights pictures). Further investigations are required to clarify the exact mechanisms of deposit formation and their effects on system compatibility. Fig.7. Tank status after DME test for 200 h. 3.5 Corrosion test The corrosion test of the tested fuels was carried out by filling each reactor with 25 % water, 25 % fuel and 50 % air, so that at the end there was 50 % liquid phase and 50 % atmospheric phase. The first test with diesel showed that the phase in contact with water had a corrosion spot, as can be seen in Figure 8 (left photo). The test with 100 % DME and water showed a different behavior, where the corrosion spot was observed in the gas phase (Figure 8b). The blend of 80 % diesel and 20 % DME showed several corrosion spots in both phases (Figure 8c). With the additivation of the blend, not a single corrosion spot was found after 26 days of storage at room temperature, showing that additives are very important for DME blends. 86 2. Fachtagung TestRig - September 2024 DME plus X as a potential Fuel Fig.8. a) Steel sample after 100 % Diesel, b) Steel sample after the storage in DME 100 %, c) Steel sample after Diesel + 20 % DME, d) Steel sample after Diesel + 20 % DME + Additive Conclusion The HiL testing evaluated the compatibility of DME and its blends with diesel fuel in key fuel system components over 200 hours. Key findings include: High-Pressure Pump (HPP): Diesel showed no issues, while DME caused corrosion, especially on the spring. Blends reduced but did not eliminate corrosion. Injector: Diesel caused slight discoloration, while DME led to severe coking, significantly reduced in blends, particularly with additive. Rail: No anomalies were found, indicating reliable operation with all fuels. Tank: Diesel caused no issues, but DME resulted in metallic deposits. Blends had similar deposits, which were significantly reduced with additives. Corrosion Test: Diesel showed water-phase corrosion; DME showed gas-phase corrosion; blends had corrosion in both phases, eliminated by additives. The reactors for the storage of DME as a liquid gas and its compatibility with steel samples enabled efficient observation of corrosion over time, showing traces in the atmospheric phase. This method also allows testing other materials with liquid gas, such as elastomers and thermoplastics. Additives are crucial for improving DME blend performance, reducing corrosion, and minimizing injector coking. Further research is needed to understand and mitigate these issues for better compatibility. Bibliography [1] D. C. Sanchez, “Data analysis in Hardware-in-theloop applied in a complete common rail system for testing of fuel-component compatibil-ity.,” Technische Akademie Esslingen (Fachtagung TestRig), p. 121, 2022. [2] H. Hoffmann, „A Contribution to the Investigation of Internal Diesel Injector Deposits“, Aachen: Shaker Verlag, 2018. [3] Lee, „Fuel Properties and Emission Characteristics of Dimethyl Ether in a Diesel Engine,“ p. 113, 2017. [4] Zoha Azizi, “Dimethyl ether: A review of technologies and production challenges,” Chemical Engineering and Processing: Process Intensification, pp. 150-172, 2014.