By Optimized E-Fuels Mix

Almost CO2-neutral Mobility

6. November 2018 | Engineering Service

Using Power-to-x, synthetic fuels can be produced from hydrogen and carbon. Ideally, the hydrogen is produced by means of electrolysis using renewable electricity. CO2 taken from the atmosphere or organic matter should be the source of carbon. Using these kinds of fuels in internal combustion engines would achieve virtually CO2-neutral mobility [1,2]. Because of their optimized molecular structure, they also offer a means of reducing exhaust emissions. However, to achieve a quick reduction in the amount of CO2 emitted by the transportation sector, these alternative fuels would need to be compatible with the existing vehicle fleet and infrastructure. Oxymethylene ethers are incompatible with some materials, making fuels, methane and dimethyl ether gases only partially suitable. Fischer-Tropsch (FT) products are very similar to fossil fuels and can be added to them without any problems [3].

Today’s FT processes are designed to yield as many long-chain, saturated hydrocarbons as possible. Alkanes and alkenes with medium-length chains could also be obtained using a high-temperature Fischer-Tropsch (HTFT) process. Adding a hydroformylation process to the end of production makes it possible to create new types of composite fuels (alkane/alcohol mixtures) using robust and easy-to-control technologies. Before a synthesizing plant begins operation, it cannot be predicted what the exact composition of the product will be because the alkanes and alcohols of differing chain lengths are formed only with the likelihood of polymerization. To cover the broad range of possible fuels, two extreme fuel mixtures consisting of C7 to C12 alkanes and C6 to C8 alcohols were chosen for engine testing.

Table 1: Properties of FTF1 fuels tested

The first fuel defined, hereafter referred to as “Fischer- Tropsch Fuel 1” (FTF1), represents a non-reactive fuel with a rather short chain and a very high concentration of alcohol at 40 percent m/m. By contrast, FTF2 creates a highly reactive fuel with a low concentration of alcohol at 10 percent m/m and longer molecules. In addition to these two FT alcohol fuels, we examined blends with fossil diesel fuel, with the concentration of FT alcohol fuels in each being 20 percent (with FTF1, referred to below as “Blend1” and referred to as “Blend2” when combined with FTF2).

Challenges for Engine Control

Fig. 1: Comparison between fuels for reference injection (Rotational speed: 1,500 1/min., PMI: 6.8 bar)

Despite the similarity of these fuels to fossil diesel fuel, there are some major differences in their combustion characteristics especially when an engine controller is used based on an engine map (Fig. 1). The combustibility of FTF2 is clearly seen in a very short ignition delay. By using EGR, the difference further increases to as much as 10° compared to diesel. Blend2 also shows this tendency.
Engine control systems based purely on engine mapping cannot exploit the full potential of e-fuels. Even customized calibration of the engine maps for a specific number of predefined fuels is not helpful because of their diversity, which results from the production process and mixture rate. Therefore, out of necessity, we use a control system that enables us to maintain the desired form of combustion regardless of the fuel being used.
A new type of control system based on an approach called “digital combustion rate shaping” (DiCoRS) may provide a solution. Especially in light of the unpredictable fuel compositions, this approach continues to guarantee reliable control of all target values without the need to know the fuel composition, neither during initial engine calibration or during subsequent vehicle operation.
The key difference between DiCoRS and conventional control systems built into engines is the control variable they use. Unlike a combustion-phased or PMI controller, no optimization of previously calibrated injection processes is performed in order to control individual parameters. Instead, DiCoRS controls combustion at a completely predefined crankshaft angle. This means that DiCoRS takes advantage of the maximum degree of freedom of combustion control.
From this operating condition-based standard of optimum combustion, a pre-control algorithm automatically calculates the proper injection rate necessary, including the number of injections, the electric trigger times, and the amounts to be injected. At this point, it should be noted that this approach causes a shift in conventional engine calibration from defining the process of injection to specification of an ideal process of combustion. At the same time, the time and effort required for calibration is reduced drastically and the desired thermodynamic optimum is achieved more reliably [4].

Experimental Results

The engine testing impressively demonstrates that the control algorithm developed can control acceptable combustion with a wide variety of fuel mixtures, even without calibration for varying fuel specifications. Through the use of DiCoRS, it is also possible for the first time to compare different fuels at a fixed, constant rate of combustion. This enables statements to be made about fuel emissions that are virtually independent of the form or phase of combustion.

Fig. 2: Comparison between fuels at three different EGR rates (rotational speed: 1,500 1/min., PMI: 6.8 bar)

Figure 2 shows an excerpt of the test results. At all EGR rates, the target progressions of the burn rate were easily achieved. There is a clearly noticeable shift in the injection rates in accordance with the fuels’ combustibility. At this load point, the reduction in soot emissions is quite pronounced thanks to use of the FT alcohol fuels. With FTF1, there is a reduction of up to 80 percent, and with FTF2, the amount was lowered by as much as 50 percent compared to fossil diesel fuel (Fig. 3).

Fig. 3: Comparison of fuel emissions under Euro 6 NOx levels (Rational speed: 1,500 l/min, PMI: 6.8 bar)

The particulate/NOx trade-off is illustrated in Fig. 4 for two load points. The alternative fuels shift the curve so that a more optimal solution can be reached. The strong advantage of soot can mainly be seen as higher EGR compatibility of the alternative fuels. In future calibration strategies, this could allow the EGR rate to be increased in order to reduce the NOx emissions, while still inside the engine.

Fig. 4: Lower particulate/NOx trade-off for two different engine loads and rotational speeds

Summary

In this examination, we assessed the potential for creating innovative e-fuels based on the Fischer-Tropsch process and by adding hydroformylation to the end of production. There is large variation in fuel composition possible because of the production process. The use of such FT alcohol fuels thus enable the CO2 footprint to be reduced considerably by increasing the concentration of regenerative energies in the transportation sector using existing engine technology and infrastructure. Thanks to their optimized physical and chemical properties, it is also possible to further lower pollutant emissions, especially particulates and nitrogen oxides, at the same time.
Based on the results, a statement regarding the emissions potential of the fuels with the same rates of combustion and, thus, the same combustion noise levels through the use of the controller can be acheived.
With respect to the effectiveness of the DiCoRS approach to control, we achieved excellent quality across all fuels, load ranges and EGR rates. In each case, the controller was able to make adjustments for the specified target combustion rate. It is conceivable that the system may be expanded to include an external control loop to complete the integrated e-fuel control system. Additional injection pressure and EGR controllers could take optimal advantage of the demonstrated lowering of the particulate/NOx trade-off. Depending on the fuel mixture and associated potential to produce particulates, the injection pressure or even the EGR rate can be subsequently adjusted online as needed to find an ideal compromise.

REFERENCES
[1] Leitner, W., Klankermayer, J., Pischinger, S., Pitsch, H., Kohse-Höinghaus, K., Advanced Biofuels and Beyond: Chemistry Solutions for Propulsion and Production; Angewandte Chemie International Edition 56 (20), 5412-5452, 2017.
[2] Artz, J., Müller, T. E., Thenert, K., Kleinekorte, J., Meys, R., Sternberg, A., Bardow, A., Leitner, W., Sustainable Conversion of Carbon Dioxide: An Integrated Review of Catalysis and Life Cycle Assessment; Chemical Reviews 118 (2), 434-504, 2018
[3] Gill, S. S., Tsolakis, A., Dearn, K. D., & Rodríguez-Fernández, J., Combustion characteristics and emissions of Fischer–Tropsch diesel fuels in IC engines. Progress in Energy and Combustion Science, 37(4), 503-523, 2011.
[4] Jörg, C., Zubel, M., Neumann, D., Heufer, A., Schaub, J., Weber, J., & Herrmann, O., Digital Combustion Rate Shaping Control as a Tool to Identify Modern Fuel Injection Strategies, 26th Aachen Colloquium Automobile and Engine Technology 2017, Aachen, 2017.

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