Browsing by Author "Larmi, Martti, Prof., Aalto University, Department of Mechanical Engineering, Finland"

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Alternative Fuels and Emission Control Methods in Compression Ignition Engines
(2019) Tilli, Aki
School of Engineering | Doctoral dissertation (article-based)
The focus of this doctoral thesis is on the effect of alternative diesel fuels on engine performance, emissions, and emission reduction technologies in diesel engines. The studies focused on normal engine operation, enhanced engine parameters for biofuel use, and on modern aftertreatment technologies. The research included engine experiments, simulations, and optical studies. Especially hydrotreated vegetable oil (HVO) and its blends were studied. For comparison, standard EN590 fulfilling regular diesel and its blends with traditional biodiesel (FAME) were also investigated. FAME and HVO represent different biofuel generations. FAME is a first generation biofuel with typically small-scale production and varying product quality. It consists of oxygen-containing esters not found in pure fossil diesel. Additionally, FAME has lower energy content. The maximum amount of FAME is 7% in standard EN590. HVO is a second generation biofuel with typically large-scale production and high quality. HVO consists of paraffinic hydrocarbons - compounds found also in regular diesel. High proportions of HVO can typically be used in diesel engines without any modifications. The first part of this thesis involves biofuels and their effects on medium-speed diesel engine performance and emissions. The biofuels were evaluated with simulations, and the effects of 100% HVO on engine performance and emissions were investigated with engine experiments. Internal exhaust gas recirculation (iEGR) and Miller timing were studied with regular diesel and HVO. The latter part of the thesis focuses on late post-injections (LPI) with 30% blends of HVO and FAME, used for diesel particulate filter (DPF) regeneration in an off-road diesel engine. During the LPI mode, exhaust gas temperature rise in the diesel oxidation catalyst (DOC), emissions, and oil dilution were investigated. Additionally, the corresponding fuel sprays were investigated in optical engines. With 100% HVO, the compatibility, performance and emissions were first investigated with no engine parameter changes. With Miller timing and iEGR, the high ignitability lead to significant nitrogen oxide (NOx) decrease, with no fuel consumption or particulate matter (PM) emission increase. In contrast to the typical NOx-PM -trade-off, both emissions were lowered. In the LPI studies, no drawbacks were found with 30% HVO blend. 30% FAME blend resulted in worse oil dilution related to LPI's, and increased emissions during the LPI mode. No significant differences in the LPI spray lengths were measured between the fuels. Thus, the differences in the oil dilution were contributed to the distillation characteristics of the fuels.
Diesel Spray Studies in Modern Diesel Engines
(2020) Hulkkonen, Tuomo
School of Engineering | Doctoral dissertation (article-based)
In this doctoral thesis, diesel injection and spray formation in modern diesel engines were studied. The goal of this thesis is to answer some fundamental questions and hypotheses about injection and spray formation in modern diesel engines. First, the fundamental spray characteristics of renewable diesel were studied under non-evaporative conditions. Second, these spray characteristics were studied under extremely high cylinder pressure. Third, the spray characteristics of conical nozzle orifice geometry were studied. Finally, the spray characteristics of biofuel blends were studied in an optical engine during late-post-injection, which is relevant to exhaust gas after-treatment. Studies showed that the spray tip penetrations with renewable diesel and petroleum diesel were similar under non-evaporative conditions. The spray angle was slightly wider, spray tip velocities were higher, and the inner delay of the injector was shorter with renewable diesel. The conclusion of the study was that there is no need to redesign the combustion chamber or readjust the injection parameters due to wall impact or spray collision. Very high in-cylinder pressure and density have a significant effect on spray penetration. Higher gas phase mixing was observed with higher in-cylinder density. No negative aspects were found for extremely high gas density. When the spray tip penetration was compared between different conical geometries and a cylindrical nozzle orifice geometry, a clear difference was not found under non-evaporative conditions. This result is inconsistent with earlier studies. The main reasons for this inconsistency may be the different approach and high injection pressure. The spray angle was smaller, and the mass flow rate higher, with conical nozzle orifice geometry. Standard hydraulic flow measurement with an injection pressure of 100 bar underestimates the flow rate of conical orifices due to lack of cavitation. Different hypotheses about higher spray tip penetration and cylinder wall-wetting during late post-injection were studied. A clear difference in the spray tip penetration was not observed when three different fuel blends were compared. The conditions that would be needed for droplets to evaporate before reaching the cylinder wall are not attained with very late injection. Hence, hypotheses that the amount of fuel ending up on the cylinder walls is higher with biofuels are unlikely. The main reason for oil dilution rate differences between fuel blends is probably related to the volatility of the fuel fraction, or because the control unit increases the volume of the post-injections due to the lower volumetric heat value of renewable diesel.
Experimental Studies on Fuel Effects in Dual-Fuel Combustion
(2022) Ahmad, Zeeshan
School of Engineering | Doctoral dissertation (article-based)
The present dissertation belongs to the research field of experimental physics and emissions reduction from a heavy-duty engine. Various alternative fuels such as diesel-like liquid fuels, methane, ethane, hydrogen, and methanol are employed in a single-cylinder research engine to experimentally investigate their combustion characteristics at varying engine conditions. Dual-fuel (DF) combustion is the focal point in this dissertation in which port-injected low reactivity fuel is ignited by a diesel-pilot directly injected close to the top-dead center. The research includes optical study and engine experiments, which aim at extending the fundamental understanding of DF ignition and subsequent combustion progression, improving engine efficiency, performance, and reducing engine-out emissions. DF combustion is a promising engine combustion technology for adopting various fuels, however, there are still operational limitations that need further improvements to meet future clean-energy goals in transportation. The present dissertation consists of 5 journal publications. Four of them investigate primarily the use of methane and one is focusing on methanol DF combustion. Publication 1 investigates diesel-methane DF ignition and progression of subsequent combustion in an optical engine using high-speed imaging of natural luminosity. Publication 2 investigates the effects of pilot fuel properties on DF ignition, engine performance, and engine-out emissions. Publications 3 and 4 study ethane/hydrogen enriched methane DF combustion to improve combustion efficiency, stability, and engine performance at lean conditions. Publication 5 investigates methanol DF combustion with negative valve overlap (NVO) to attain high engine efficiency together with ultra-low pollutant emissions at wide range of engine operating loads. The main findings of this dissertation can be summarized as follows: 1) In diesel-methane DF combustion, premixed flames grow and propagate towards the center of the chamber and at high equivalence ratios, flame propagation is more prominent. The ignition delay time is found to be a function of methane equivalence ratio, charge-air temperature, and pilot-diesel amount. In addition, DF combustion progresses as three overlapping stages with premixed combustion as a dominant mechanism. 2) For small pilot quantities, in general, a higher cetane number fuel with higher aromatic content improves DF combustion. Additionally, high viscosity, density, and distillation temperatures avoid possible leaning out of a pilot spray within methane-air mixture. 3) Ethane or hydrogen enrichment improves the reactivity of pure methane and helps to extend the range of lean operability and attain high thermal efficiency (>50%). 4) It is found that methanol DF combustion under NVO mode can produce high efficiency (>50%) with ultra-low emissions at wide range of engine operating loads.
Modeling the effects of fuel properties on end-use performance in light-duty road transport and aviation
(2022) Kroyan, Yuri
School of Engineering | Doctoral dissertation (article-based)
The present Doctoral Dissertation belongs to the field of Energy Technology with a focus on System-Level modeling and end-use analysis of renewable fuels in the transport sector. The scope includes on-road transportation and aviation, where the impact of alternative fuel properties was investigated for the regular fleet of spark-ignition engines, flex-fuel engines, and aircraft jet engines. Based on literature data the matrices containing fuel properties as independent variables and fuel consumption as an output property were constructed for the modeling purpose. The Multiple Linear Regression with incorporated quantitative analysis was employed to develop state-of-the-art mathematical models representing the impact of fuel properties exclusively. The current work consists of 4 journal publications. The first publication is focused on the development of the Fuel Consumption (FC) model for Spark-Ignition Light-Duty Vehicles (SI-LDVs). Tested fuels were blends of ethanol and isomers of butanol with gasoline. The most important parameters for FC in regular SI-LDVs turned out to be Research Octane Number (RON), calorific content, density, and oxygen content. The model achieved high accuracy reflected by an R-Square of 0.989, and an average absolute error of 1.1% in external validation. The second publication extended the scope to Flex-Fuel Vehicles (FFV) engines, which are better optimized for non-drop-in fuels such as E85. In the case of FFVs, besides ethanol and butanol, blends containing methanol and ethyl tertbutyl ether were investigated, including their binary and tertiary combinations. The results show that for FFV engines, octane sensitivity, calorific content, density, and vapor pressure were the most significant fuel properties. The high accuracy of the model expressed by R-Square of 0.994 was confirmed in external validation by an average absolute error of 1.9%. While further findings indicated that FFV engines utilize alternative fuels more efficiently than regular SI engines. The third publication was focused on jet engines and Sustainable Aviation Fuels (SAF), where a model containing the effect of viscosity, density, and calorific content was developed for end-use analysis. The achieved high R-Square of 0.993 translated into 0.68% error in external validation. The fourth publication studied various challenges to the successful market uptake of renewable fuels highlighting that the current tank-to-wheel (TTW) approach in emissions estimation should be extended to more robust well-to-wheel or cradle-to-grave type of assessments. The present work showed that the collective impact of fuel properties could successfully be applied to model and simulate fuel consumption for various alternative fuels in end-use sectors. The results show that although some alternative fuels might increase volumetric fuel consumption, they tend to reduce TTW carbon dioxide emissions and energy consumption such as in the case of alcohols or specific SAF.
Numerical studies for charge formation in combustion engines
(2018) Keskinen, Karri
School of Engineering | Doctoral dissertation (article-based)
Modern lean and low-temperature combustion (LTC) techniques form a pathway towards increased efficiency and reduced emissions in internal combustion engines. However, combustion abnormalities (associated with efficiency detriments and unburned hydrocarbon emissions) constitute a major engineering challenge augmented by the inherent cyclic variation in engines. Fuel-air charge combustibility is highly dependent on local in-cylinder metrics such as flow turbulence, mixture composition and temperature, for which experimental investigations are difficult. Thereby, it would be highly beneficial to develop fast and accurate computational tools capable of reliably describing charge formation phenomena such as turbulent mixing, wall heat transfer and thermal stratification. The primary aim of this dissertation is to enhance understanding and predictive accuracy of scale-resolving wall-bounded simulations pertinent to charge formation, hoping to facilitate improved comprehension and mitigation of combustion abnormalities in future studies. As a secondary objective, mixture formation trends are assessed in the context of gas direct injection, a modern fuel supply technique for lean charges. The present computational fluid dynamics (CFD) studies include statistical (Reynolds-averaged simulation; RANS) and filtered (large eddy simulation; LES) turbulence modelling approaches. Model-centric near-wall approaches are emphasised due to their reduced computational load compared to direct numerical simulation (DNS) and wall-resolved LES. In particular, implementation and validation is carried out for a zonal hybrid LES/RANS method with a specific near-wall treatment (HLR-WT). Simplified engine setups and academic flow configurations are employed as test cases, with reference data including (1) measurements and DNS of academic and engine-like flows, in addition to (2) planar laser induced fluorescence (PLIF) imaging and high-resolution LES of gas jets. Direct gas injection investigations highlight both independent and combined effects of injection pressure, timing and nozzle type. In scale-resolving simulations, hybrid LES/RANS methods provide varying improvements on coarse-grid LES, while methodology-specific characteristics should be acknowledged in practical utilisation. The novel methodological combination HLR-WT displays enhanced grid flexibility in academic configurations and promising results in engine-like flows. In particular, turbulent heat transfer characteristics, near-wall scaling and thermal stratification trends are relatively accurately reproduced in the highly demanding compression stroke with mild grid sensitivity. This is a positive indication of applicability in engineering-relevant, higher Reynolds number configurations.
Renewable fuels for compression ignition engines in various transport modes: effect of fuel properties on end-use performance
(2024) Wojcieszyk, Michał
School of Engineering | Doctoral dissertation (article-based)
This doctoral dissertation belongs to the field of energy technology and focuses on the system level analysis of renewable liquid fuels for transportation purposes. Light-duty, heavy-duty and marine segments were in the scope of the study while compression ignition (CI) engine technology was the common powertrain. Based on statistical methods, the research linked engine performance with fuel properties, which has not been demonstrated yet. In this approach, data-driven black box modeling exploiting the multilinear regression method together with significance analysis and validations was successfully applied. The dissertation consists of four publications in peer-reviewed journals. Publication I concentrates on passenger cars and renewable drop-in fuels like hydrotreated vegetable oil (HVO) and traditional biodiesel (FAME). The experimental results from driving cycle test procedures are utilized in the model development. As an outcome, volumetric fuel consumption and carbon dioxide (CO2) emissions are predicted for light-duty fleet based exclusively on lower heating value, cetane number and density. In Publication II, the performance of heavy-duty vehicles is analyzed and predicted for various neat alternative fuels and their blends with fossil diesel. Simulations for new blending components are executed using heating value, density and cetane number. Publication III focuses on marine transport and suggests a model capable of predicting CO2 emissions from a medium-speed marine CI engine based on heating value, density and viscosity turned out to be significant properties. In Publication IV, the end-use challenges for the deployment of renewable fuels in the market are discussed and recommended interventions suggested. The novel approach presented in this doctoral dissertation enabled to link fuel properties with engine performance indicators like fuel consumption and tailpipe CO2 emissions. Three state-of-the-art models with high accuracy (coefficient of determination over 0.9 and all p-values below 0.05) were demonstrated in Publications I-III. The results from the modeling work represent the collective effect of fuel properties on end-use performance from the fleet perspective. Furthermore, all four publications support the deployment of renewable fuels in transportation by providing relevant modeling tools (Publications I-III) and recommendations (Publication IV) to decision makers. The outcomes of the dissertation serve for an unbiased comparison of the most promising fuel options that could be fitted to the existing infrastructure with minimum effort.
Studies on the reduction of nitrogen oxides emission in a large-bore diesel engine
(2016) Imperato, Matteo
School of Engineering | Doctoral dissertation (article-based)
This experimental research studied different technologies for reducing nitrogen oxides (NOx) in the exhaust gases, running with a large-bore medium-speed diesel research engine. NOx mainly form during combustion in local high temperature zones. This study considered primary methods, avoiding high combustion temperatures. In particular, the Miller cycle, which is a proven concept for NOx reduction, was deeply studied. This technology was applied by closing early the intake valves to create a first gas expansion before the compression stroke, thus reducing the effective compression ratio. At high load, a detailed analysis of the mixing-controlled combustion, which constitutes the most influent part of the combustion process, was carried out, reaching in-cylinder pressure of 300 bar. Same NOx level and no soot were achieved increasing the output power, while keeping the same fuel injection pressure.At partial load, a more extensive study of different Miller rates was performed. An advanced Miller rate resulted in NOx reduction up to 55%. Since the Miller cycle presented some limitations, other techniques were implemented together with the Miller cycle. First, a split injection strategy was tested with low fuel injection pressure. Then, a paraffinic fuel, hydrotreated vegetable oil (HVO), was tested, while keeping lower oxygen content in the combustion chamber, obtained by retaining a part of the exhaust gases. The results of the experiments with split injection showed that it was difficult to decrease further NOx emissions compared to the values obtained with the Miller cycle alone, but specific fuel consumption decreased when a small pilot injection was used. However, a later injection timing could be used to obtain lower NOx values with a small drawback in fuel consumption. HVO's ignitability allowed running with very advanced Miller cycle and low oxygen content. It was possible to obtain low NOx figures, but a certain increase of specific fuel consumption took place.This thesis showed that a unique tool for obtaining a significant NOx reduction without drawbacks in the whole engine load spectrum could not be found. Due to lower combustion temperatures, a higher specific fuel consumption was a common downside. Optimization of injection strategy, use of alternative fuels and dilution with inert mass are valuable tools to implement for achieving low engine-out NOx in large-bore engines. However, the investment costs the possible drawbacks must be accurately evaluated in each case.
Utopia or Opportunity? - Predicted Performance of 21st Century Technology Steam Locomotives
(2021) Hirvensalo, Iiro
School of Engineering | Doctoral dissertation (monograph)
The objective of this study is to predict the performance of liquid fuel burning steam locomotives based on the technology available today. Diesel engine tribology, electronic valve control, and substantially higher steam parameters than those of classic steam locomotives have been applied. Such locomotives have not been built yet but appear to offer promising possibilities. A hypothetic pattern locomotive called Hs1 has been configured, to predict the performance of such locomotives. Several methods have been used for virtual creating and testing of the Hs1. Firstly, the evolution of classic steam locomotives has been discussed in terms of their strengths and weaknesses, with resulting pre-requisites from potential customers for any steam power. Secondly, classic, and recent literature has been studied in search for advances in steam- and other applicable technologies, materials, and practices. Thirdly, innovative experimental locomotive designs have been discussed. Fourthly, properties and adaptability of bio-oil vs. fossil oil as fuels have been discussed. Finally, enginemen's know-how has been exploited, to include practical and operational views in the design of the Hs1. Conclusions of the findings within the discussions constitute the basic specifications of the Hs1. A radial reciprocating steam engine is opted for the prime mover, with a turbine to recover the energy of the exhaust steam of the radial engine, and with an electrical transmission to power the driving wheels. The power chain downstream of the prime mover is thus identical with that of diesel-electric locomotives. Diesel components have been exploited to unify the operational and maintenance characteristics of the Hs1 with those of diesel-electric locomotives. Electric transmission enables recovery of energy generated by the traction motors in dynamic braking by storing it into accumulators or by feeding resistors that heat the feedwater. A simulation program has been created for assessing the performance of the Hs1, and for comparing the figures with those of a reference diesel-electric locomotive as well as of a classic steam locomotive. Road test simulations predict a drawbar efficiency of 21-27 % for the Hs1, depending on whether and in which way the braking energy is recovered, and 33 % for the reference diesel-electric locomotive. The simulated classic steam locomotive attains 6,5-7 % at the maximum in the same assignment, the round-the-year figures being 3-4 %. Yard work involves a lot of braking and thus potential for recovery of braking energy, resulting in predicted drawbar efficiency of 29-36 % for the Hs1 vs. 33% of the diesel electric locomotive. In light passenger trains with frequent stops, the Hs1 is predicted to attain an efficiency of 18-29% vs. 33 % of the diesel-electric locomotive. Bio-oil combustion enhances the sustainability of the Hs1 as motive power when compared with engines dependent on fossil fuel. External combustion enables exploiting lower grade pyrolysis oil that would require further refining to make it fit for internal combustion engines. The CO2 emissions of diesel engines originate from fossil fuel unless 100 % bio-oil is used whereas the fuel of Hs1 is made of forest residue releasing its CO2 content even if left in the woods.