Effect of Gasoline Surrogate Composition and Properties on Engine Knock and Soot Emissions

The growing concern about the impact of emissions on environment and human health has become a prime driver for combustion research in recent years. Development of advanced combustion engines (ACEs) and identification of potential alternative/renewable fuels are two strategies being widely examined within the combustion community. With the advent of several ACEs and unconventional biofuels, identifying optimum fuel-engine conditions is highly challenging. Gasoline is one of the most commonly used fuels in the transportation sector, whose combustion and emission characteristics to a large extent are governed by its physical and chemical properties. Further, blending gasoline with biofuels adds complexity in characterizing its performance, due to any possible changes to physical and chemical properties. Hence, an in-depth understanding of these properties and their effects on combustion performance and emissions is essential for the development of next-generation fuels which can maximize engine performance and minimize emissions. With this motivation, the present work aims to investigate the central fuel property hypothesis (CFPH) which states that “certain fuel properties are sufficient to characterize the combustion performance irrespective of fuel chemical composition.” This hypothesis is investigated with respect to two performance parameters – knock limits under boosted spark-ignition conditions and soot emissions under advanced compression ignition conditions – by considering three different combustion systems. These include a spark-ignition engine (SI) for investigating the knock limits, laminar counterflow flames for examining polycyclic aromatic hydrocarbon (PAH) and soot emissions for gasoline-alcohol surrogates, and a gasoline compression ignition engine (GCI) for investigating efficiency, PAHs, and soot emissions. The fuel properties of interest are Research Octane Number (RON) and Octane Sensitivity (S). The first part of the thesis examines the validity of CFPH in terms of S and RON, for knock limited performance. In other words, the objective is to examine if these two fuel properties are sufficient to describe the fuel's knock-limited performance under boosted spark ignition (SI) conditions. Four TPRF-biofuel blends having different compositions, but the same RON (= 98) and S (= 8) were examined through numerical simulations. Three unconventional biofuels included in the analysis were: di-isobutylene (DIB), iso-butanol, and anisole. Numerical simulations for these fuel surrogates were performed using respective skeletal mechanisms and in a virtual cooperative fuel research (CFR) engine model under a representative boosted operating condition. In the computational fluid dynamics (CFD) model, the G-equation approach was employed to track the turbulent flame front, and the well-stirred reactor (WSR) model combined with multi-zone binning strategy was used to capture auto-ignition in the end-gas. In addition, laminar flame speed was tabulated for each blend as a function of pressure, temperature, and equivalence ratio a priori, and the lookup tables were used to prescribe laminar flame speed as an input to the G-equation model. Parametric spark timing sweeps were conducted for each fuel blend to determine the corresponding knock-limited spark advance (KLSA) and 50% burn points (CA50) at the respective KLSA timing, which is further used to evaluate knocking performance. It was observed that despite the same RON and S, and engine operating conditions, the TPRF-Anisole blend exhibited markedly different knock-limited performance from the other three blends. This deviation from the octane index (OI) expectation was shown to be caused by differences in laminar flame speed (LFS). However, it was found that relatively large fuel-specific differences in LFS (> 20%) would have to be present to cause any appreciable deviation from the OI framework. Otherwise, RON and MON would still be robust enough to predict a fuel’s knock-limited performance. The second half of the thesis examines the validity of CFPH in terms of RON and S, for soot emissions under two combustion configurations. The first combustion system is a counterflow configuration where laminar diffusion and partially premixed flames are simulated to examine the effects of fuel composition, RON, and S on PAHs and soot emissions, using four-component gasoline-alcohol blend surrogates comprised of iso-octane, n-heptane, toluene, and three different alcohols – methanol, ethanol and n-butanol. A total of 320 TPRF-alcohol mixtures, with S in the range of 1-10, are considered. A detailed chemical mechanism coupled with a comprehensive soot model, that includes processes – soot inception, surface growth, PAH condensation, and oxidation, is adopted for this investigation. The analysis indicates that toluene content in the fuel mixture has a prominent effect while alcohol content and S of the fuel have a weak correlation with the PAHs and soot. However, it is not clear if any of these three variables – toluene and alcohol content in fuel, and S - are individually sufficient to characterize the PAHs and soot across various blends. For this reason, a new variable (XCHO) based on the elemental composition of the fuel mixture has been identified. XCHO along with S of the fuel are identified as key parameters in characterizing soot emissions satisfactorily. Further, a comparison of three blends containing same amounts of alcohol (45%) and toluene (16%), indicates that the efficacy of alcohols in reducing soot emissions follows the order: methanol>ethanol>n-butanol. The third configuration considered is a heavy-duty (HD) GCI engine operating under low load conditions, where the investigation is extended beyond laminar flame conditions to characterize the effects of fuel composition and S on the trade-off between engine efficiency and soot emissions. The objective is to capture the fuel composition and property effects under real engine conditions, characterized by turbulent reacting two-phase flows at high pressures and with thermal stratification. The 3D engine CFD model employs adaptive mesh refinement and finite-rate chemistry approach to capture auto-ignition. A reduced mechanism with PAH chemistry coupled with a detailed soot modeling strategy – the method of moments (MOM), is adopted. First, the CFD model is validated against experimental data from a heavy-duty GCI engine for 87AKI E10 gasoline (RD5-87), with respect to in-cylinder pressure, heat release rate, combustion phasing, and soot emissions. The CFD model using TPRF-ethanol (E20) blend with 20% ethanol (by mole) as a surrogate for RD5-87 predicts the experimental data satisfactorily for a broad range of start-of-injection (SOI) timing (-21/-27/-36/-45oaTDC). Thereafter, consistent with the laminar flame study, the model is exercised to investigate two other gasoline surrogates (E45 and B45) with ~45% ethanol/ n-butanol by mole. This captures the impacts of both chemical and physical properties of the fuels on autoignition and soot emissions. While the ignition delay time monotonically increases with advancing SOI, the -27oaTDC SOI condition produces the lowest amount of soot emissions, despite having lower mixing times than -36 /-45 oaTDC SOI conditions. Further, the soot emissions for all SOI timings shows the trend: B45>E20>E45. Overall, the results indicate that autoignition and soot emissions are strongly coupled to both chemical and physical properties of the fuel. Moreover, the effect of physical properties on autoignition and soot emissions becomes more pronounced with advancing SOI.