Constantin Kiesling

Hydrogen is a promising option to decarbonize energy conversion in internal combustion engines (ICE). A key limiting factor of high-load hydrogen ICE operation is the occurrence of combustion anomalies, including pre-ignition, backfiring into the intake manifold and knocking combustion. The existing literature indicates that there currently exists no generally accepted conceptual model that is able to explain the occurrence of all combustion anomalies for hydrogen-fueled internal combustion engines. This publication is intended to help fill this research gap based on experimental investigations on a single-cylinder research engine (SCE) with hydrogen port fuel injection and spark ignition in the 3 dm ³ displacement per cylinder class and related 3D-CFD simulation work. To enable in-depth combustion analysis and particularly to identify the origin and root cause of pre-ignition, the SCE was instrumented with AVL List GmbH’s “Visiolution” system, an advanced optical instrumentation that relies on fiberoptic sensors in the combustion chamber. Different types of pre-ignition including backfire, early and late pre-ignition could be provoked during engine operation. Backfire and early pre-ignition were found to originate from the piston-liner edge at the “bottom” of the combustion chamber and tend to occur in specific areas at the piston circumference where comparatively rich local mixture prevails. Mixture homogeneity may be key to reduce the risk of backfire and early pre-ignition to occur. The locations of origin suggest that various ignition sources emerging from the piston top land crevice such as hot residual gases or reactive species (e.g. due to reverse blowby) are possible. Late pre-ignition events (i.e. shortly before ignition timing) occurred in large numbers with a random spatial distribution in the combustion chamber and appear to be mostly related to lubricant oil droplet combustion. Future research must study the impact of mixture homogeneity on pre-ignition and strive to better understand the root causes for the underlying processes that provide the required ignition energy.

Fully flexible large diesel-gas dual fuel engines are known for their ability to be operated in pure diesel mode as well as in dual fuel mode for applications such as power generation and transportation. Dual fuel operation with these engines is characterized by a premixed gas–air cylinder charge and compression ignition via a pilot diesel injection into the combustion chamber. Proven benefits such as the flexibility to adapt the type of fuel to the market, fail-safe operation and relatively low engine-out nitrogen oxides emissions come at the price of downsides such as comparatively low efficiency and combustion stability. To overcome these detriments, an in-depth understanding of the fundamental processes and effects must be obtained. The aim of this paper is therefore to study specific diesel-gas combustion phenomena in detail. The focus is on investigating the influence of the diesel fuel injection timing on mixture formation, ignition, and combustion of both diesel and natural gas–air mixture at nominal engine load and an energetic diesel fraction of only 1%. A combined approach consisting of single-cylinder research engine (SCE) testing and three-dimensional computational fluid dynamics (3D-CFD) simulation was followed. In the first step, experimental investigations were carried out on a large high-speed SCE with a displacement of 6.24 dm³ per cylinder and at a nominal indicated mean effective pressure (IMEP) of 24 bar. The combustion chamber of the SCE was optically instrumented with an endoscope and an illumination device. This provided detailed insight into the fuel injection process and combustion phenomena in the early combustion phase while being minimal invasive and withstanding the high mechanical and thermal loads at full load engine operation. To supplement the SCE measurement data and to enhance interpretation of the results, injection rate curves of the diesel injector were measured on a separate test rig. In the second step, the experimental data were used to calibrate and validate a corresponding 3D-CFD simulation model. Further insight into the entire combustion process was obtained from the results of subsequent simulations. The combined results comprehensively illustrate how the injection timing of the pilot diesel fuel affects mixture formation, ignition, and the combustion process. Besides basic SCE performance and emission results, the spatially and temporally resolved results from 3D-CFD simulation provide key information about the processes in the combustion chamber. While it is already known from the existing literature that at very early pilot fuel injections, combustion phasing is not directly linked to injection timing, the 3D-CFD results allow an analysis and detailed explanation of this behavior. It has been found that varying the injection timing has a considerable impact on ignition delay due to the different in-cylinder conditions at the start of injection. Ignition delay in turn affects local mixture quality at start of combustion (and vice versa), which results in significantly different combustion processes and related performance indicators. This detailed understanding serves as the basis for future research that must investigate the impact of additional influencing factors such as combustion chamber and injection nozzle geometries as well as pilot fuel injection strategies (e.g., multiple injections per cycle) to holistically improve the diesel-gas combustion concept in a target-oriented way.

Global issues such as climate change, environmental pollution, and conservation of resources require manufacturers of internal combustion engines (ICE) to achieve substantially reduced emissions of greenhouse gases and pollutants as well as increased engine efficiency and durability. Condition monitoring and predictive maintenance approaches for sliding bearings in ICEs are key tools for increasing engine durability and saving resources by exploiting more of the useful lifetime of a bearing while avoiding critical engine operation caused by bearing wear and failure. These approaches require appropriate measurement technology capable of acquiring informative parameters that reflect the current condition of the bearings while withstanding the high temperatures and mechanical loads encountered inside the engine and accommodating spatial constraints. This paper deals with research work related to advanced sensor technology that is highly integrated into sliding bearings so that information is obtained nearly directly from relevant areas such as the bearing running layer and the lubrication gap. An isolated, sputtered sensor layer with a thickness of a few micrometers is employed in combination with a laser structuring process to form the desired thin film sensor structure below the bearing running surface. While several measurement parameters and corresponding sensor types are conceivable, this paper focuses on temperature and strain measurements that rely on a change in the electrical resistance of the sensor layer material. Promising sensor layouts and positions targeted for use in condition monitoring applications in ICEs are elaborated in detail. Developments and challenges in implementing the sensor technology concept—in particular with regard to the process of manufacturing the sensor as well as the wire contacting—are outlined in depth. The paper concludes by presenting measurement results obtained with this sensor technology at lab scale as well as an outlook toward implementing the instrumented bearings in ICEs.

Condition monitoring of components in internal combustion engines is an essential tool for increasing engine durability and avoiding critical engine operation. If lubrication at the crankshaft main bearings is insufficient, metal-to-metal contacts become likely and thus wear can occur. Bearing temperature measurements with thermocouples serve as a reliable, fast responding, individual bearing-oriented method that is comparatively simple to apply. In combination with a corresponding reference model, such measurements could serve to monitor the bearing condition. Based on experimental data from an MAN D2676 LF51 heavy-duty diesel engine, the derivation of a data-driven model for the crankshaft main bearing temperatures under steady-state engine operation is discussed. A total of 313 temperature measurements per bearing are available for this task. Readily accessible engine operating data that represent the corresponding engine operating points serve as model inputs. Different machine learning methods are thoroughly tested in terms of their prediction error with the help of a repeated nested cross-validation. The methods include different linear regression approaches (i.e., with and without lasso regularization), gradient boosting regression and support vector regression. As the results show, support vector regression is best suited for the problem. In the final evaluation on unseen test data, this method yields a prediction error of less than 0.4 °C (root mean squared error). Considering the temperature range from approximately 76 °C to 112 °C, the results demonstrate that it is possible to reliably predict the bearing temperatures with the chosen approach. Therefore, the combination of a data-driven bearing temperature model and thermocouple-based temperature measurements forms a powerful tool for monitoring the condition of sliding bearings in internal combustion engines.

The aim of this paper is to identify and investigate the potential and limitations of diesel–gas combustion concepts for high speed large engines operated in gas mode with very small amounts of pilot fuel (<5% diesel fraction). Experimental tests were carried out on a flexible single cylinder research engine (displacement 6.24 dm3) equipped with a common rail system. Various engine configurations and operating parameters were varied and the effects on the combustion process were analyzed. The results presented in this paper include a comparison of the performance of the investigated dual fuel concept to those of a state-of-the-art monofuel gas engine and a state-of-the-art monofuel diesel engine. Evaluation reveals that certain limiting factors exist that prevent the dual fuel engine from performing as well as the superior gas engine. At the same NOx level of 1.3 g/kWh, the efficiency of the dual fuel engine is ≈3.5% pts. lower than that of the gas engine. This is caused by the weaker ignition performance of the injected pilot fuel compared to that of the gas scavenged prechamber of the gas engine. On the other hand, the dual fuel concept has the potential to compete with the diesel engine. The dual fuel engine can be operated at the efficiency level of the diesel engine yet with significantly lower NOx emissions (3.5 g/kWh and 6.3 g/kWh, respectively). Since the injection of pilot fuel is of major importance for flame initialization, and thus for the main combustion event of the dual fuel engine, optical investigations in a spray box, measurements of injection rates, and three-dimensional (3D) computational fluid dynamics (CFD) simulation were conducted to obtain even more detailed insight into these processes. A study on the influence of the diesel fraction shows that diminishing the diesel fraction from 3% to lower values has a significant impact on engine performance because of the effects of such a reduction on injection, ignition delay, and initial flame formation. The presented results illustrate which operating strategy is beneficial for engine performance in terms of low NOx emissions and high efficiency. Moreover, potential measures can be derived which allow for further optimization of the diesel–gas combustion process.