Single-cylinder Optical Diesel Engine Research Articles

Simultaneous high-speed spectroscopy and 2-color pyrometry analysis in an optical compression ignition engine fueled with OMEX-diesel blends

E-fuels are a very attractive way for improving the well-to-wheel emissions of CO2 in internal combustion engines. In the particular case of compression ignition engines, the Oxymethylene dimethyl ether (OMEX), an e-fuel with nearly soot-free combustion under mixing-controlled conditions, is a good candidate for the replacement of fossil fuels. However, the Lower Heating Value of OMEX is nearly half of the diesel fuel, which means that much longer injection durations are required in the real engine. In addition, the very low viscosity and lubricity of OMEX can damage the injection system if used pure, but it can be an interesting fuel when blended with conventional diesel. Thus, the main objective of this paper is to evaluate the potential of OMEX-diesel blends to bypass these OMEX limitations whilst keeping low soot formation trends. For this purpose, a single cylinder optical diesel engine at part load was employed. The soot production for the different fuel blends was analyzed by applying three different high-speed imaging techniques: natural luminosity, flame spectroscopy and 2-color pyrometry. Natural luminosity analysis showed that the flame light intensity scales with diesel fraction up to 30% of diesel in the blend. The spectroscopy analysis has revealed that soot formation of OMEX fuel is almost null. When blended with diesel at 50%, although soot formation is still lower than for pure diesel, higher soot levels are obtained in the last stages of the cycle as a consequence of the longer injections required.

Understanding in-cylinder soot reduction in the use of high pressure fuel injection in a small-bore diesel engine

This study shows how soot particles inside the cylinder of the engine are reduced due to high pressure fuel injection used in a light-duty single-cylinder optical diesel engine fuelled with methyl decanoate, a selected surrogate fuel for the diagnostics. For various injection pressures, planar laser induced incandescence (PLII) imaging and planar laser-induced fluorescence of hydroxyl (OH-PLIF) imaging were performed to understand the temporal and spatial development of soot and high-temperature flames. In addition, a thermophoresis-based particle sampling technique was used to obtain transmission electron microscope (TEM) images of soot aggregates and primary particles for detailed morphology analysis. The OH-PLIF images suggest that an increase in the injection pressure leads to wider distribution of high-temperature flames likely due to better mixing. The enhanced high-temperature reaction can promote soot formation evidenced by both a faster increase of LII signals and larger soot aggregates on the TEM images. However, the increased OH radicals at higher injection pressure accelerates the soot oxidation as shown in a higher decreasing rate of LII signals as well as dramatic reduction of the sampled soot aggregates at later crank angles. The analysis of nanoscale carbon layer fringe structures also shows a consistent trend that, at higher injection pressure, the soot particles are more oxidized to form more graphitic carbon layer structures. Therefore, it is concluded that the in-cylinder soot reduction at higher injection pressure conditions is due to enhanced soot oxidation despite increased soot formation.

Optical Diagnostics Investigation into the Effect of Pilot Injection Dwell Time and Injection Pressure on Combustion Characteristics and Soot Emissions in a Single-Cylinder Optical Diesel Engine

The present work investigates the effect of the injection dwell time and injection pressure on soot reduction potential in an optical single-cylinder light-duty diesel engine. The engine operated under a double-injection scheme under low load and low engine speed conditions. The conducted experiments considered two different dwell times for three different injection pressures. The fuel quantity of the main injection was adjusted to maintain the same indicated mean effective pressure (IMEP) value among all cases considered. Findings were analyzed via means of pressure trace and apparent heat release rate (AHRR) analyses, as well as a series of optical diagnostics techniques, namely high-speed imaging and planar laser-induced incandescence (pLII). The combination of dwell time and injection pressure substantially affects charge reactivity and soot oxidation potential. The analysis suggests that a shorter dwell time combined with a higher injection pressure can lead to an enhanced potential for engine-out particulate reduction by creating an in-cylinder environment that promotes soot oxidation. Overall, results indicate that a close-coupled pilot and main injection scheme can reduce soot levels, albeit while increasing specific fuel consumption by up to 12% to maintain the same engine power output levels.

An investigation of the effect of post-injection schemes on soot reduction potential using optical diagnostics in a single-cylinder optical diesel engine

This work employs a combination of pressure trace analysis, high-speed optical measurements and laser-based techniques for the assessment of the effects of various post-injection schemes on the soo...

Towards Identifying Flame Patterns in Multiple, Late Injection Schemes on a Single-Cylinder Optical Diesel Engine

ABSTRACTThe work investigates the effect of various post-injection strategies on the flame patterns in a Ricardo Hydra optical single-cylinder, light-duty diesel engine, operated in a partially premixed combustion mode, under low load (IMEP: ca. 2.3 bar), low speed (1200 rpm) conditions. The effect of post-injection fuel amount (12% and 24% of the total fuel quantity per cycle) and post-injection timing (0, 5, 10 deg aTDC) are investigated via pressure trace analysis and optical measurements. Flame propagation is captured by means of high-speed flame natural luminosity imaging and of CH*, C2*, and OH* line-of-sight chemiluminescence measurements. Results suggest that post-injections suppress mixture reactivity but enhance oxidation, and that a larger amount of fuel and/or later post-injection, leads to higher levels of natural luminosity, indicating possible higher soot-out emissions, while post-injection close to the main combustion event appears to have a beneficial effect on the soot oxidation processes.

Investigations of closely coupled pilot and main injections as a means to reduce combustion noise in a small-bore direct injection Diesel engine

Various pilot–main injection strategies are investigated in a single-cylinder optical Diesel engine. It is observed that as the dwell between a single pilot and the main injection is decreased toward zero, combustion noise passes through a minimum, and a reduction of 3 dB is possible. The injection schedules employed in the engine are analyzed with a hydraulic injection analyzer to provide rate shapes for each of the dwells tested. Two distinct injection events are observed even at the shortest dwell tested, and various rate shaping effects are observed with the main injection event as the dwell is adjusted. High-speed elastic scattering imaging of liquid fuel is performed in the engine to examine initial spray penetration rates. These compare favorably to the measured rates of injection, thus providing evidence that rate shaping of the initial phase of the main injection occurs in the engine and that this rate shaping is consistent with the injection rate data. However, experimental evidence suggests that these changes are not responsible for the observed trend in combustion noise as dwell changes. The combination of thermodynamic data and rate of injection data supports the theory that the main injection interacts with the pilot mixture field and influences its heat release process, thus playing a role in decreasing combustion noise. The relative phasing of the pilot and main heat release may play a significant role in reducing combustion noise; further studies will focus on combustion phasing effects.

Optical Investigation on Diesel Engine Fuelled by Vegetable Oils

In the present paper the combustion behaviour and soot generation of different neat vegetable oils on a direct injection optical diesel engine have been investigated using in-cylinder pressure based combustion analysis and high-speed two colour method. A single-cylinder optical diesel engine with an electronically controlled common-rail fuel injection system has been used for this study. Four different vegetable oils, Jatropha, Croton, Rapeseed and Sunflower oil, have been tested at 120MPa injection pressure and fuel temperatures of 60 and 90°C. All tests were performed at fixed injection timings and fixed engine speed at 1200 rev/min with pilot and main injection.The experimental results show that although all four bio-oils have very similar physiochemical properties, some differences in their combustion and soot generation behaviour have been observed. It was concluded that sunflower oil (SFO) had very good combustion properties at lower temperatures compared to other fuels. While croton oil (CO) did not have outstanding combustion behaviour at any conditions, the combustion of Jatropha oil (JO) was improved at higher fuel injection temperatures.

Experimental investigation into the effects of two-stage injection on fuel injection quantity, combustion and emissions in a high-speed optical common rail diesel engine

Diesel combustion and the formation of pollutants are directly influenced by the spatial and temporal distributions of the fuel within the combustion chamber of an internal combustion engine. The requirements for more efficient and responsive diesel engines have led to the introduction and implementation of multiple injection strategies. However, the effects of such injection modes on the hydraulic systems, such as the high pressure pipes and fuel injectors, must be thoroughly examined and compensated for. The objective of this study was to investigate the effects of fuel injection equipment characterisation and optimisation on diesel combustion and emissions with two-stage fuel injection. The fuel injection system was characterised and optimised through the measurement of the fuel injection rate and quantity, in particular, the interaction between the two injection events was quantified and compensated for. The effects of twin and variable split two-stage injection and dwell angle on diesel combustion and emissions were investigated in a high-speed direct injection single-cylinder optical diesel engine using heat release analysis and high-speed fuel spray and combustion visualisation technique. The results indicated that two-stage injection has the potential for simultaneous reduction of NOx and soot emissions. Nevertheless, the studied two-stage strategies resulted in higher soot emission, mainly due to the interaction between two consecutive fuel injection events, whereby the fuel sprays during the second injection were injected into burning regions, generating fuel-rich combustion. In addition, the variable two-stage strategies produced high levels of uHC emission in comparison to single and twin split injection cases. This was mainly attributed to firstly greater fuel quantity injected during the second injection and secondly poor mixing and air utilisation during the second fuel injection event.

Experimental investigation of hydraulic effects of two-stage fuel injection on fuel-injection systems and diesel combustion in a high-speed optical common-rail diesel engine

In order to meet the ever more stringent emission standards, significant efforts have been devoted to the research and development of internal combustion engines. The requirements for more efficient and responsive diesel engines have led to the introduction and implementation of multiple injection strategies. However, the effects of such injection modes on the hydraulic systems, such as the high-pressure pipes and fuel injectors, must be thoroughly examined and compensated for since the combustion and the formation of pollutants in direct-injection engines are directly influenced by the spatial and temporal distribution of the injected fuel within the combustion chamber. This study investigated the hydraulic effects of two-stage fuel injection on diesel combustion and emissions. The fuel-injection system was characterised for all the tested strategies through the measurement of the fuel-injection rate and quantity. In particular, the interaction between the two injection events was identified. The effects of two-stage injection, dwell angle and the interactions between two consecutive injection events on the combustion process and the emissions were investigated in a high-speed direct-injection single-cylinder optical diesel engine using heat-release analysis and high-speed fuel spray and combustion visualisation techniques. The results indicated that the two-stage injection strategy has the potential for simultaneous reduction of nitrogen oxide, soot and unburned hydrocarbon emissions. The results suggested that an optimum fuel quantity in the first injection exists, 0–30%, with which simultaneous reduction of nitrogen oxide, soot and unburned hydrocarbon emissions can be achieved with the added benefits of improved engine performance, fuel economy and combustion noise. However, higher soot emissions were produced, mainly due to the interaction between the two consecutive fuel-injection events whereby the fuel sprays during the second injection were injected into burning regions, as well as reduced soot oxidation due to the continuation of the combustion into the expansion stroke.

Measurement of in-cylinder soot particles and their distribution in an optical HSDI diesel engine using time resolved laser induced incandescence (TR-LII)

Soot particles generated during combustion process in a single cylinder optical diesel engine were measured using time-resolved laser induced incandescence technique (TR-LII). Experimentally measured LII data were fitted with theoretically simulated LII data to determine the count median particle diameter and the geometric width of the particle size distribution. TR-LII signals were acquired simultaneously at two different wavelengths for crank angles in the range from 48.4° to 111.4° after top dead centre (aTDC) and for various engine operating conditions. Particle size was found to be dependent on the engine load but no significant change in particle sizes were observed for different fuel injection timings. The average size of the primary particles measured over different crank angles were larger at higher engine loads compared to lower engine loads. A general trend of the soot particle diameter decreasing with crank angle was observed due to oxidation, but towards the end of the expansion stroke a marginal increase in size was noted. For all operating conditions the width of the particle size distribution was found to decrease with crank angle until 61.4°aTDC, and thereafter it increases to reach a well defined size distribution for a given fuel injection quantity despite the differences in the injection timing. The soot particles are randomly transported by the complex fluid motion present within the combustion chamber leading to strong cycle to cycle fluctuations of the measured time resolved LII data. The in-cylinder soot volume fraction derived from TR-LII data revealed that for different fuel injection timings, a relatively higher amount of soot was produced for shorter ignition delay compared to larger ignition delays.

Effect of the dwell angle of split injection in a single-cylinder optical diesel engine

This study investigates the effects of split-injection strategies with high levels of exhaust gas recirculation on the combustion performance and emissions in a single-cylinder direct-injection optical diesel engine. The engine is equipped with high-pressure common-rail fuel injection and designed to provide optical access through the piston window and side windows. The experiments were carried out at a relatively high engine speed of 2000r/min for an optical engine and in-cylinder conditions matched to a multiple-cylinder engine through boost and intake charge heating. High-speed combustion imaging, two-colour temperature, and soot concentration measurements were carried out in conjunction with in-cylinder pressure and heat release analysis and exhaust emissions. The results at two dwell angles and different injection timings are presented and analysed in this paper. The effects of the dwell angle and fuel injection timings on fuel air mixing, combustion, soot formation, and emission are identified and discussed.

Multi-lognormal soot particle size distribution for time-resolved laser induced incandescence in diesel engines

In-cylinder and exhaust soot particle size measurements were carried out using time-resolved laser induced incandescence and electrical mobility spectrometer techniques in a single cylinder optical diesel engine and multi-cylinder high-speed diesel engine. The temporal decay of the laser induced incandescence signal from a polydisperse nanoparticle ensemble of soot during transient diesel combustion is shown to be described by both a single-lognormal distribution as well as multi-lognormal size distribution. However, a multi-lognormal particle size distribution is introduced in the existing model for a comprehensive characterisation and realistic reconstruction of the size distribution. Detailed theoretical analysis of multi-lognormal size distribution along with its application to the experimentally measured soot particle size is validated in this work. These results were also qualitatively compared and independently verified by the experimental results obtained by the electrical mobility spectrometer and published transmission electron microscopy data. These findings reveal that the in-cylinder and the exhaust soot particle size distributions in engines are better represented by a multi-lognormal size distribution.