The world is facing a major transformation of its energy infrastructure shaped by industrialization. A shift from burning fossil fuels to more sustainable alternatives is now needed to stop climate change and ensure energy supply in the future. To achieve this, research into transition technologies that can convert fossil fuels with high efficiency while flexibly adapting to demand is particularly important. These technologies must be able to cope with the large fluctuations in the supply of solar and wind energy that cannot be compensated by existing storage capacities. By making fossil fuel energy conversion more flexible, the expansion of sustainable energy sources can continue without jeopardizing grid stability.

One way of increasing the efficiency of energy conversion processes is polygeneration. In such a process, the conversion of primary energy sources to mechanical energy or electricity, heat and the material conversion, e.g., to useful chemicals, are coupled in one process. In the concept investigated in this work, a fuel-rich internal combustion engine is used as a reactor to simultaneously generate electricity, heat and useful chemicals through the partial oxidation of methane or natural gas. This type of process allows flexible switching between maximum power generation (conventional engine operation) and reduced power and heat output with simultaneous chemical production (fuel-rich operation).

In this work, the influence of different operating strategies on the fuel-rich operation of an internal combustion engine is investigated. A modified BASF octane test engine is used as the test carrier. It is operated either in HCCI mode using the ignition-accelerating additives n-heptane, dimethyl ether, diethyl ether and ozone, or in spark ignition mode using pure oxygen as oxidizer. In addition to varying additive fractions and equivalence ratios (Φ = 0.5 to 12), the compression ratio (ε = 4.5 to 20) and inlet temperature (T_in = 50°C to 190°C) are also varied. Addition of CO₂ ranging from 0 to 33 mol% of the inlet mixture is also investigated. The influence on power and heat release, operating stability, product gas composition, soot formation and the combustion process will be investigated. The aim is to identify framework conditions and useful operating parameters for fuel-rich engine operation, which is still little researched.

The results show that HCCI operation can generally be realized with different additives and operating parameters. Compared to lean HCCI operation, fuel-rich operation is much less susceptible to high pressure rise rates and knocking because the high fuel content acts as a thermal buffer. It is shown that additive fraction, intake temperature, and compression ratio can be effectively used to shift the combustion phasing and induce stable engine operation. Reactivity and heat capacity are the primary factors in the choice of additives. The addition of small amounts of ozone as an additive is shown to be very effective. By adding only 75 ppm ozone to the total mixture, the DME content required for stable operation can be reduced in the experiment from 11% to 5.3% of the total fuel. High compression ratios result in significantly reduced additive fractions overall in HCCI operation and can even allow HCCI operation without additive while maintaining otherwise constant parameters. While HCCI operation is shown to be a reliable way for stable engine operation at equivalence ratios in the range of Φ > 1.5 when air is used as the oxidizer, for some studies with the addition of CO₂ to the feedstock, spark ignited operation with pure oxygen as the oxidizer is used. The high oxygen content enables stable operation even at high equivalence ratios in the range 1.9 < Φ < 2.5 by increasing the flame speed in the spark ignited mode. In this type of operation, up to 40% of the added CO₂ is converted at some operating points. Moreover, with this type of operation, the highest hydrogen yield of 62% can be achieved at a low compression ratio of ε = 4.5 at Φ = 2.3.