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The distribution of spray droplet sizes plays a pivotal role in internal combustion engines, directly affecting fuel-air mixing, evaporation, and combustion. To gain a precise understanding of droplet size distribution in a two-dimensional space, non-intrusive optical diagnostics emerge as a highly effective method. In the current investigation, two-dimensional (2D) diesel spray droplet sizes mapping using a simultaneous combination of planar laser-induced fluorescence (PLIF) and Mie-scattering techniques is introduced. The assessment of droplet diameter relies on the interplay between fluorescent and scattered light intensities which correspond the light based on volumetric droplets and surface area of the droplets. This calculation is made possible through the LIF/Mie technique. However, traditional LIF/Mie methods are plagued by inaccuracies arising from multiple light scattering.

Chemical-kinetics-based multizone models (MZM) are effective tools for performance-oriented simulations of low-temperature combustion concepts. It demonstrates a better trade-off between simulation speed and predictivity than both high-fidelity computational fluid dynamics (CFD) and low-fidelity data-driven models. This study applies a newly developed MZM, referred to as UVATZ, to simulate reactivity-controlled compression ignition (RCCI) combustion, fueled by natural gas (NG) and diesel. In such a concept, in-cylinder conditions at intake valve closing (IVC) largely define the kinetically dominated combustion predictions. To secure IVC predictions accurately, UVATZ is for the first time coupled with a detailed air/fuel path dynamics model created in commercial engine modeling software (GT-Suite), forming a 1D simulation framework.

The paper presents novel studies on the impact of different ignition control parameters on combustion stability and spark plug wear. First, experimental results from a 32.4-liter biogas fueled large bore single cylinder spark ignition engine are discussed. Two different ignition systems were considered in the experiment: a DC inductive and an AC capacitive. The spark plugs used in the experiment were of dual-iridium standard J-gap design of different electrode gaps. Test results show the importance of different degrees of freedom to control a spark. A robust ignition is found to be achieved by using a very short spark duration, which in turn reduces total energy discharge at the gap. Further observations reveal that once a stable and self-propagating flame kernel is developed, it becomes independent of the spark energy further added to the gap. Finally, results from the spark plug wear tests using a pressurized rig chamber are discussed.

Spray evolution in diesel engines plays a crucial role in fuel-air mixing, ignition behavior, combustion characteristics, and emissions. There is a variety of phenomenological spray models and computational fluid dynamics (CFD) simulations have been applied to characterize the spray evolution and fuel-air mixing. However, most studies were focused on the spray phenomenon under a limited range of injection and ambient conditions. Especially, the prediction of spray geometry in multi-hole injectors remains a great challenge due to the lack of understanding of the complicated flow dynamics. To overcome the challenges, a series of spray experiments were carried out in a constant volume spray chamber (CVSC) coupled with high-speed Mie-scattering imaging to obtain the spray characteristics at various injection and ambient conditions.

With low-temperature combustion engine research reaching an applicable level, physics-based control-oriented models regain attention. For reactivity controlled combustion concepts, chemical kinetics-based multizone models have been proven to reproduce the governing physics for performance-oriented simulations. They offer accuracy levels similar to high-fidelity computational fluid dynamics (CFD) models but with a fraction of their computational effort. Nevertheless, state-of-the-art reactivity controlled compression ignition (RCCI) simulations with multizone model toolchains still face challenges related to predictivity and calculation speed. This study introduces a new multizone modelling framework that addresses these challenges. It includes a C++ code, deeply integrated with open-source, thermo-kinetic libraries, and coupled to an industry standard 1-D modelling framework.

Crank shaft torsional vibration has impact on the crank angle measurement accuracy in large-bore Internal Combustion Engines (ICE). In large bore engine, the torsional vibration angular displacement can be up to 1 degree, which in turn can cause a fault of 2 bar in Indicated Mean Effective Pressure (IMEP) and a fault of 0.6 degrees in the Crank Angle of 50% burned (CA50). IMEP and CA50 are critical feedback parameters for closed-loop combustion control, therefore to compensate torsional vibration effect in real-time engine control system can not only provide higher accuracy crank angle data but especially improve the combustion analysis and closed-loop control accuracy. Thus, in this work, a torsional vibration dynamic model is established to improve the accuracy of the crank angle measurement.

With the tightening requirements on engine emission and performance, pressure based combustion controls are becoming common in medium speed large bore reciprocating internal combustion engines. The accuracy of the cylinder pressure data including the raw pressure value at its corresponding crank angle, has a vital impact on engine controllability. For instance, this work shows that a 1-bar pressure offset leads to a 0.6% variation in the total heat release (THR) while the 50% heat release crank angle (CA50) can be shifted by 1.5 degrees. Similarly, with a single degree error in the crank position, the indicated mean effective pressure (IMEP) gets a 1.8 bar error. Thus, in this work the typical errors for cylinder pressure measurement are reviewed and analyzed for large bore four stroke marine and power plant production engines. The main sources of error for pressure measurement are thermal shock and installation defects.

In this work the pre- to main chamber ignition process is studied in a Wärtsilä 34SG spark-ignited lean burn four-stroke large bore optical engine (bore 340 mm) operating on natural gas. Unburnt and burnt gas regions in planar cross-sections of the combustion chamber are identified by means of planar laser induced fluorescence (PLIF) from acetone seeded to the fuel. The emerging jets from the pre-chamber, the ignition process and early flame propagation are studied. Measurements reveal the presence of a significant temporal delay between the occurrence of a pressure difference across the pre-chamber holes and the appearance of hot burnt/burning gases at the nozzle exit. Variations in the delay affect the combustion timing and duration. The combustion rate in the pre-chamber does not influence the jet propagation speed, although it still has an effect on the overall combustion duration.

In HCCI you do not have the same control of the combustion like in SI and Diesel engines. Controlling the start of a combustion event is a difficult task and requires feedback from previous cycles. This feedback can be retrieved from ion current measurements. By applying a voltage over the spark gap, ions will lead a current and a signal that represents the combustion in the cylinder will be retrieved. Voltages of 450 V were used. The paper describes a new method to enhance the combustion phasing from the Ion current trace in HCCI engines. The method is using the knowledge of how the signal should look. This is known due to the fact that the shape of the ion current signal is similar from cycle to cycle. This new observation is shown in the paper. Also the correlation between the ion current and CA50 was studied. Later the signals have been used for combustion feedback.

The main benefit of HCCI engines compared to SI engines is improved fuel economy. The drawback is the diluted combustion with a substantially smaller operating range if not some kind of supercharging is used. The reasons for the higher brake efficiency in HCCI engines can be summarized in lower pumping losses and higher thermodynamic efficiency, due to higher compression ratio and higher ratio of specific heats if air is used as dilution. In the low load operating range, where HCCI today is mainly used, other parameters as friction losses, and cooling losses have a large impact on the achieved brake efficiency. To initiate the auto ignition of the in-cylinder charge a certain temperature and pressure have to be reached for a specific fuel. In an engine with high in-cylinder cooling losses the initial charge temperature before compression has to be higher than on an engine with less heat transfer.

The Homogenous Charge Compression Ignition (HCCI) operating range in terms of speed and load does not cover contemporary driving cycles, e.g. the European driving cycle EC2000, without increased engine displacement, supercharging, or without excessive noise and high NOx emissions. Hence, the maximum achievable load with HCCI is too low for high load vehicle operation and a combustion mode transfer from HCCI to spark ignited (SI) has to be done. At some operating conditions spark assisted HCCI combustion is possible, which makes a mixed combustion mode and controlled combustion mode transfers possible. The mixed combustion region and the operating conditions are investigated in this paper from lean SI limit to pure HCCI without SI assistance. Parameters as compression ratio, inlet air pressure, inlet air temperature, and lambda are used for controlling the mixed combustion mode. A strategy for closed-loop combustion mode transfer is discussed.

Combustion initiation in an HCCI engine is dependent of several parameters that are not easily controlled like the temperature and pressure history in the cylinder. So achieving the same ignition condition in all the cylinders in a multi-cylinder engine is difficult. Factors as gas exchange, compression ratio, cylinder cooling, fuel supply, and inlet air temperature can differ from cylinder-to-cylinder. These differences cause both combustion phasing and load variations between the cylinders, which in the end affect the engine performance. Operating range in terms of speed and load is also affected by the cylinder imbalance, since misfiring or too fast combustion in the worst cylinders limits the load. The cylinder-to-cylinder variations are investigated in a multi-cylinder Variable Compression Ratio (VCR) engine, and the effect it has on the engine performance.

This study applies Closed-Loop Combustion Control (CLCC) using Fast Thermal Management (FTM) on a multi cylinder Variable Compression Ratio (VCR) engine together with load control, to achieve a favorable combustion phasing and load at all times. Step changes of set points for combustion phasing, Compression Ratio (CR), and load together with ramps of engine speed with either constant load, i.e. load control enabled, or constant fuel amount are investigated. Performances of the controllers are investigated by running the engine and comparing the result with CLCC using VCR, which was used in an earlier test. Commercial RON/MON 92/82 gasoline, which corresponds to US regular, is used in the transient tests. Limitations to the speed ramps are further examined and it is found that choice of fuel and its low temperature reaction properties has large impact on how the CLCC perform.

The operating range in terms of speed and load for a natural aspirated Homogenous Charge Compression Ignition (HCCI) engine is restricted by a lack of dilution at high load and a poor combustion at low load. Today HCCI is seen as a part load concept, but the operating range for vehicle applications should at least cover contemporary driving cycles, without switching to a conventional Spark Ignited (SI) combustion mode. Dilution with air at high load can be increased by supercharging, but with the drawback of parasitic losses or pumping losses decreasing the brake efficiency and by that one of the benefits of HCCI. The effect of advancing the combustion phasing and throttling the inlet air on the combustion efficiency at zero loads is investigated. The combustion efficiency increases drastically in both cases. The increase in the combustion efficiency overcomes the drawbacks of the early combustion phasing in the first case and the pumping losses in the second case.

This study applies Closed-Loop Combustion Control (CLCC) using Variable Compression Ratio (VCR) and cylinder balancing using variable lambda to solve the problem. Step changes of set points for combustion phasing, Compression Ratio (CR), and load together with ramps of engine speed and inlet air temperature are investigated. Performances of the controllers are investigated by running the engine at either a constant amount of injected fuel corresponding to an approximate load of 1.5 or 2.5 bar BMEP and/or constant speed of 2000 rpm. Commercial RON 92 gasoline is used in the test. The CLCC is found to be fast and effective and has a potential of handling step changes in a matter of cycles, while the speed and temperature ramps need some more optimization of the CLCC. The CR controller is very fast and has a time constant corresponding to three engine cycles at 2000 rpm.

Combustion phasing in a Homogeneous Charge Compression Ignition (HCCI) engine can be achieved by affecting the time history of pressure and temperature in the cylinder. The most common way has been to control the inlet air temperature and thereby the temperature in the cylinder at the end of the compression stroke. However this is a slow parameter to control, especially cycle to cycle. A multi cylinder engine using Variable Compression Ratio (VCR) for controlling the compression temperature and consequently the combustion phasing is used in the experiments. Operating range in terms of speed and load is investigated in naturally aspirated mode. Trade-off between inlet air temperature and Compression Ratio (CR) is evaluated. Primary reference fuels, isooctane / n-heptane, are used during the tests. High speed HCCI operation up to 5000 rpm is possible with a fuel corresponding to RON 60. The effect of octane number with and without exhaust cam phasing is also investigated.