Internal combustion engines are a key aspect of society, and their continued use poses challenges from an environmental standpoint since they emit pollutant and greenhouse gas emissions. This dissertation focuses on experimental analysis of dual-fuel low temperature combustion (LTC), which can be used as a strategy to reduce engine-out emissions and increase engine efficiencies. Dual fuel LTC uses two different fuels, a high reactivity fuel (HRF) and a low reactivity fuel (LRF). The HRF has a higher cetane number than the LRF, which allows for easier auto-ignition in compression ignition engines. Dual fuel engines also utilize high air to fuel ratios to achieve LTC. This, combined with early injection timings of the HRF, helps to reduce oxides of nitrogen (NOx) emissions. At low load conditions, this is a problem since higher cycle-to-cycle variations can increase pollutants such as unburned hydrocarbons (UHC) and carbon monoxide (CO). To combat this, a firm understanding of dual fuel LTC is required, as well as a strategy for reducing the cycle-to-cycle variations. The first part of this work further identifies a combustion heat release 'transformation region' across different HRF injection timings wherein in-cylinder conditions arise that are conducive for ultra-low NOx emissions. This phenomenon occurs for different IC engine platforms and different fueling combinations. An experimental analysis, 0D chemical kinetic analysis, and 3D computation fluid dynamic (CFD) analysis were combined to elucidate the underlying causes for this phenomenon. The local stratification level of the fuel/air mixture was identified as the likely cause of combustion heat release transformation with changing HRF injection timing. The second part of the present work builds upon the findings of the first part by utilizing local stratification to mitigate cycle-to-cycle variations that are present at low loads. A framework of experiments was formulated for both a low engine load of 5 bar gross indicated mean effective pressure (IMEPg) and a high load of 15 bar IMEPg, wherein an injection strategy concept termed Spray TArgeted Reactivity Stratification (STARS) was utilized using both diesel and Polyoxymethelene-dimethyl-ether (POMDME) as HRFs. A steep decrease in UHC and CO emissions (> 80% reductions) as well as improved engine operation stability were demonstrated using both HRFs with dual fuel LTC at 5 bar IMEPg. Further, potential for emissions mitigation and efficiency improvement are discussed, as well as differences in the experimental results shown between the differing HRFs.

Low temperature combustion strategies have demonstrated high thermal efficiency with low emissions of pollutants, including oxides of nitrogen and particulate matter. One such combustion strategy, called Reactivity Controlled Compression Ignition (RCCI), which involves the port injection of a low reactivity fuel such as gasoline, ethanol, or natural gas, and a direct injection of a high reactivity fuel, such as diesel, has demonstrated excellent control over the heat release event due to the introduction of in-cylinder stratification of equivalence ratio and reactivity. The RCCI strategy is inherently fuel flexible, however the direct injection strategy needs to be tailored to the combination of premixed and direct injected fuels. Experimental results demonstrate that, when comparing different premixed fuels, matching combustion phasing with premixed mass percentage or SOI timing is not sufficient to retain baseline efficiency and emissions results. If the bulk characteristics of the heat release event can be matched, however, then the efficiency and emissions can be maintained. A 0-D methodology for predicting the required fuel stratification for a desired heat release for kinetically-controlled stratified-charge combustion strategies is proposed and validated with 3-D reacting and non-reacting CFD simulations performed with KIVA3Vr2 in this work. Various heat release rate shapes, phasing, duration, and premixed and DI fuel chemistries are explored using this analysis. This methodology provides a means by which the combustion process of a stratified-charge, kinetically-controlled combustion strategy could be optimized for any fuel combination, assuming that the fuel chemistry is well characterized.

Low temperature combustion (LTC) strategies can achieve high thermal efficiency with low engine-out NOx and soot emissions. However, controlling the timing and rate of heat release under LTC conditions in a consistent manner has proven to be difficult, and emissions of CO and unburned hydrocarbons (HC) are high. Several fueling strategies have been developed to create stratification of equivalence ratio and ignition delay such that heat release is retarded and lengthened, but these strategies typically result in a trade-off between efficiency and noise, a narrow operating range, or impractical boundary conditions, and the issue of high CO and HC emissions remains. In this research, a new strategy is proposed in which two fuels with different autoignition characteristics are introduced to the cylinder via multiple direct injections, allowing for greater control and range of reactivity and equivalence ratio distribution, as well as the possibility of mixed combustion modes. A series of experiments detail the development of the fueling strategy, an exploration of the parameters contributing to noise and emissions, a pathway to practical high-load operation, and a comparison to existing LTC strategies. By direct-injecting two fuels, it is possible to gain a new level of control over the shape and stability of the heat release event, along with the ability to combine and exploit the practical benefits of existing combustion strategies in a manner that was previously impossible.

This research will demonstrate that Reactivity Controlled Compression Ignition (RCCI) has several advantages over other LTC concepts in regards to fuel flexibility and combustion controllability. RCCI is a dual-fuel partially premixed combustion concept. In this strategy, a low reactivity fuel, such as gasoline or an alcohol fuel, is premixed via port fuel injection (PFI) and a high reactivity fuel, such as diesel fuel, is direct injected (DI) during the compression stroke. Once it was clear that dual-fuel RCCI operation had great potential in terms of combustion controllability, which is a great challenge for LTC strategies, the study focused on alternative fuels with RCCI combustion. The light-duty engine was used to study two different fuel combinations: gasoline/diesel and methanol/diesel. In addition to the fuels comparison, a modified piston bowl geometry was studied and compared to the stock re-entrant bowl. The modified piston featured a wide/shallow bowl with a matched geometric compression ratio to the stock piston of ~17.3. Using the modified piston, the gross indicated efficiency of RCCI combustion was significantly improved at light loads due to increases in combustion efficiency and decreases in heat transfer losses. At higher loads the modified piston also performed better than the stock piston, but the improvements were not as significant. The final portion of this research looks at the effects of cetane improvers on gasoline, ethanol, and methanol's fuel reactivity and the implications for RCCI combustion. In all three base fuels it was found that 2-ethylhexyl nitrate is more effective at increasing fuel reactivity (i.e., suppressing the octane number) compared to di-tert-butyl peroxide. However, 2-ethylhexyl nitrate has a potential disadvantage due its nitrate group, which can manifest itself as NOx emissions in the exhaust. The relationship between the fuel-bound nitrate group and the engine-out NOx emissions was extensively characterized. It was also observed that methanol's response to cetane improvers was better than that of ethanol, in spite of the fact that they have similar reactivities in their neat form.

Abstract : Gasoline compression ignition (GCI) technology has demonstrated great potentials in improving fuel economy and reducing engine-out NOx and particulate matter emissions. Development and application of the GCI technology on multi-cylinder engines require both fundamental understandings of the gasoline spray combustion characteristics and accurate numerical tools. Due to the large differences in the thermo-physical and the chemical properties between gasoline and diesel range fuels, differences in the spray combustion characteristics between gasoline and diesel is expected. Reports on the gasoline spray combustion characteristics under conditions relevant to medium to heavy-duty engines are scarce and this dissertation aims to fill in this knowledge gap. Experimental work were performed in a constant volume combustion vessel. Non-reacting sprays under low and high ambient charge gas temperatures and reacting sprays were performed using a high reactivity gasoline (research octane number 60) and ultra-low sulfur diesel. The experimental work were designed to isolate the effect of several important fuel properties on spray and combustion. The experimentally investigated spray combustion characteristics include spray dispersion, vapor penetration, liquid penetration, ignition, flame lift-off, and natural luminosity. These experiments provided evidence behind the lower particulate matter emissions benefit of gasoline. A transient spray cone angle correlation was developed based on the experimental measurements. The correlation was developed to improve the description of fuel-air mixing in computational fluid dynamic (CFD) simulations. The correlation was integrated with CFD simulations and the benefits of using a transient spray cone angle profile were demonstrated. Reacting spray CFD simulations were performed and validated extensively against the experimental spray characteristics on ignition, flame lift-off, soot natural luminosity, and external published local soot concentration measurements. The CFD simulations provided additional understanding of the soot emission processes to complement experimental measurements.

The goal of this research is to identify how nitrogen oxide (NOx) emissions and flame stability (blowout) are impacted by the use of fuels that are alternatives to typical pipeline natural gas. The research focuses on lean, premixed combustors that are typically used in state-of-the-art natural gas fueled systems. An idealized laboratory lean premixed combustor, specifically the jet-stirred reactor, is used for experimental data. A series of models, including those featuring detailed fluid dynamics and those focusing on detailed chemistry, are used to interpret the data and understand the underlying chemical kinetic reasons for differences in emissions between the various fuel blends. An ultimate goal is to use these data and interpretive tools to develop a way to predict the emission and stability impacts of changing fuels within practical combustors. All experimental results are obtained from a high intensity, single-jet stirred reactor (JSR). Five fuel categories are studied: (1) pure H2, (2) process and refinery gas, including combinations of H2, CH4, C2H6, and C3H8, (3) oxygen blown gasified coal/petcoke composed of H2, CO, and CO2, (4) landfill and digester gas composed of CH4, CO2, and N2, and (5) liquified natural gas (LNG)/shale/associated gases composed of CH4, C2H6, and C3H8. NOx measurements are taken at a nominal combustion temperature of 1800 K, atmospheric pressure, and a reactor residence time of 3 ms. This is done to focus the results on differences caused by fuel chemistry by comparing all fuels at a common temperature, pressure, and residence time. This is one of the few studies in the literature that attempts to remove these effects when studying fuels varying in composition. Additionally, the effects of changing temperature and residence time are investigated for selected fuels. At the nominal temperature and residence time, the experimental and modeling results show the following trends for NOx emissions as a function of fuel type: 1.) NOx emissions decrease with increasing H2 fuel fraction for combustion of CH4/H2 blends. This appears to be caused by a reduction in the amount of NO made by the prompt pathway involving the reaction of N2 with hydrocarbon radicals as the CH4 is replaced by H2. 2.) For category 2 (the process and refinery blend) and category 5 (the LNG, shale, and associated gases), NOx emissions increase with the addition of C2 and C3 hydrocarbons. This could be due to an increased production of free radicals resulting from increasing CO production when higher molecular weight hydrocarbons are broken down. 3.) For category 3 (the O2 blown gasified coal/petcoke), NOx emissions increase with increasing CO fuel fraction. The reason for this is attributed to CO producing more radicals per unit heat release than H2. When CO replaces H2, an increase in NOx emissions is seen due to an increase in the productivity of the N2O, NNH, and Zeldovich pathways. 4.) For category 4 (the landfill gas) the addition of diluents such as CO2 and N2 at constant air flow produces more NOx per kg of CH4 consumed, and N2 is more effective than CO2 in increasing the NOx emission index. The increase in emission index appears to be due to an enhancement of the prompt NOx pathway as the diluents are added and the mixture moves towards stoichiometric. In addition, the presence of CO2 as a diluent catalyzes the loss of flame radicals, leading to less NOx formation than when an equivalent amount of N2 is used as a diluent. For a selected set of fuels, detailed spacial reactor probing is carried out. At the nominal temperature and residence time, the experimental results show the following trends for flame structure as a function of fuel type: 1.) Pure H2 is far more reactive in comparison to CH4 and all other pure alkane fuels. This results in relatively flat NOx and temperature profiles; whereas, the alkane fuels drop in both temperature and NOx production in the jet, where more fresh reactor feed gases are present. 2.) For category 2 (the Process and Refinery blends), H2 addition increases reactivity in the jet while decreasing overall NOx emissions. The increased reactivity is especially evident in the CO profiles where the fuels blended with C2H6 and H2 have CO peaks on jet centerline and CO emissions for pure CH4 peaks slightly off centerline. 3.) For category 3 (the O2 blown gasified coal/petcoke), the temperature profiles for the gasification blend and pure H2 are nearly identical, which is likely due to the high reactivity of H2 dominating the relatively low reactivity of CO. Despite a small temperature difference, the addition of CO causes an increase in NOx production. 4.) For category 4 (the landfill gas), the temperature profiles are virtually indistinguishable. However, the addition of diluent decreases reactivity and spreads out the reaction zone with the CO concentration peaking at 2 mm off of centerline instead of 1 mm. Diluent addition increases NOx production in comparison to pure CH4 for reasons explained above. 5.) For category 5 (the LNG, shale, and associated gases), the temperature profiles are all very similar. The increased reactivity of C2H6 is evident from looking at the CO profiles. Increased C2H6 promotes CO production on jet centerline which is indicative of the hydrocarbon material breaking down earlier in the jet. At temperatures and residence times other than the nominal conditions, the experimental results show the following trends: 1,) The NOx emissions from LPM combustion of pure CH4, H2, C2H6, and C3H8 are shown to vary linearly with residence time and in an Arrhenius fashion with temperature. This occurs because (1) more reaction time leads to more NOx formation, and (2) NOx formation is a strong, non-linear function of temperature. 2.) The addition of both H2 and C2H6 to a LPM CH4 flame is effective at extending its lean blowout limit. The results of both two and three dimensional CFD simulations are presented to illustrate the general flow, temperature, and species structure within the reactor. Since the two dimensional model is far more computationally efficient, it is employed to study various fuel mixtures with more sophisticated chemical mechanisms. The CFD results from the LPM combustion of H2, H2/CO, and CH4 with NOx formation are presented. A three dimensional CFD simulation is run for LPM CH4 combustion that uses a global CH4 oxidation mechanism. While this model does not predict intermediate radicals and NOx, the CO contours and flow field can be used as guidelines to develop a chemical reactor network (CRN), which can incorporate detailed chemistry. In addition, this model runs quickly enough that it is a good way to initialize the temperature and flow field for simulations that do incorporate more complex chemistry. The two dimensional model is used to illustrate the difference in combustion behavior between the various fuels tested. In particular, it illustrates the geometric locations of the super-equilibrium radical fields and shows where and through which pathways NOx is formed. The pathway breakdowns show good agreement with the CRN modeling results. The main goal of the CFD modeling is to use the results of each model to develop Chemical Reactor Networks, CRNs, that are customized for a particular burner. The CRN can then be used to estimate the impacts due to fuel variation.

A compression ignition engine uses two or more fuel charges having two or more reactivities to control the timing and duration of combustion. In a preferred implementation, a lower-reactivity fuel charge is injected or otherwise introduced into the combustion chamber, preferably sufficiently early that it becomes at least substantially homogeneously dispersed within the chamber before a subsequent injection is made. One or more subsequent injections of higher-reactivity fuel charges are then made, and these preferably distribute the higher-reactivity matter within the lower-reactivity chamber space such that combustion begins in the higher-reactivity regions, and with the lower-reactivity regions following thereafter. By appropriately choosing the reactivities of the charges, their relative amounts, and their timing, combustion can be tailored to achieve optimal power output (and thus fuel efficiency), at controlled temperatures (and thus controlled NOx), and with controlled equivalence ratios (and thus controlled soot).