The residual gas fraction is an important parameter to get good performance with high efficiency and low emissions in the HCCI-engine. The goal in this thesis is to formulate an algorithm for estimation of the residual gas fraction based on the cylinder pressure. The estimation is improved if also the exhaust gas temperature is used together with the cylinder pressure. The formulated algorithm has then been tested on data from a single cylinder engine running in HCCI-mode during steady state conditions. An error of 4% was noted compared with the residual gas fraction obtained from simulations. The thesis also investigates the effects of some possible error sources.

With growing concerns over emissions, homogeneous charge compression ignition (HCCI) engines offer a promising solution through reducing NOx and particulate emissions and increasing efficiency. However, this technology is not without its challenges and numerical modeling of these engines can offer some insight into addressing these challenges. This study uses domain decomposition with FORTRAN MPI to subdivide computationally intensive sections of a 10 zone simulation model. Using an Intel i7 quadcore workstation the parallelized model reduced runtimes by half compared to serial computations. From here, two sets of biofuel experimental data were used to improve the validation base of the model. The fuels used were a simulated biomass derived gas (consisting of H2, CH4, CO, CO2, and N2) and a butanol/n-heptane blend. Once calibrated, the model showed good pressure, heat release, and products of incomplete combustion prediction for biogas. NOx emissions were high, however the overall trend was captured. Similarly, once calibrated to the butanol/n-heptane data to account for some of the effects of negative valve overlap (NVO), excellent pressure and heat release predictions were obtained. However, products of incomplete combustion and NOx were low and this was attributed to the inability of the model to properly account for inhomogeneity and all the effects of NVO. Once again though, the overall trend in NOx levels was captured by the model. It was also found that the model does not operate very well near the misfire limit of the engine as it cannot capture the cyclic variability that can occur here. Based on the two new validation cases, it is concluded that once calibrated, the model can be used as a predictive tool for pressure, heat release, and combustion phasing of biofuelled HCCI engines. Furthermore, to improve its predictive capabilities, it is recommended that the model be restructured to incorporate mass transfer between zones, a fixed crevice volume and variable thermal boundary layer, and a CFD solver to improve emissions predictions and reduce reliance on calibration. Finally, changing the zone distribution from ring like zones to lumped stirred reactors is recommended to allow for more realistic modeling of actual experimental HCCI conditions.

Low temperature combustion (LTC) is an appealing new method of combustion that promises low nitric oxides and soot emissions while maintaining or improving on engine performance. The three main points of this study were to develop and validate an engine model in GT-Power capable of implementing LTC, to study parametrically exhaust gas recirculation (EGR) and injection timing effects on performance and emissions, and to investigate methods to decrease pressure rise rates during LTC operation. The model was validated at nine different operating points, 3 speeds and 3 loads, while the parametric studies were conducted on 6 of the 9 operating points, 3 speeds and 2 loads. The model consists of sections that include: cylinders, ports, intake and exhaust manifolds, EGR system, and turbocharger. For this model, GT-Power calculates the combustion using a multi-zone, quasi-dimensional model and a knock-induced combustion model. The main difference between them is that the multi-zone model is directly injected while the knock model is port injected. A variety of sub models calculate the fluid flow and heat transfer. A parametric study varying the EGR and the injection timing to determine the optimal combination was conducted using the multi-zone model while a parametric study that just varies EGR is carried out using the knock model. The first parametric study showed that the optimal EGR and injection timing combination for the low loads occurred at high levels of EGR (60 percent) and advanced injection timings (30 to 40 crank angle degrees before top dead center). The optimal EGR and injection timing combination for the high loads occurred at low levels of EGR (30 percent to 40 percent) and retarded injection timings (7.5 to 5 crank angle degrees before top dead center). The knock model determined that the ideal EGR ratio for homogeneous charge compression ignition (HCCI) operation varied from 30 percent to 45 percent, depending on the operating condition. Three methods were investigated as possible ways to reduce pressure rise rates during LTC operation. The only feasible method was the multiple injection strategy which provided dramatically reduced pressure rise rates across all EGR levels and injection timings.

No description available.

With growing concerns over emissions, homogeneous charge compression ignition (HCCI) engines offer a promising solution through reducing NOx and particulate emissions and increasing efficiency. However, this technology is not without its challenges and numerical modeling of these engines can offer some insight into addressing these challenges. This study uses domain decomposition with FORTRAN MPI to subdivide computationally intensive sections of a 10 zone simulation model. Using an Intel i7 quadcore workstation the parallelized model reduced runtimes by half compared to serial computations. From here, two sets of biofuel experimental data were used to improve the validation base of the model. The fuels used were a simulated biomass derived gas (consisting of H2, CH4, CO, CO2, and N2) and a butanol/n-heptane blend. Once calibrated, the model showed good pressure, heat release, and products of incomplete combustion prediction for biogas. NOx emissions were high, however the overall trend was captured. Similarly, once calibrated to the butanol/n-heptane data to account for some of the effects of negative valve overlap (NVO), excellent pressure and heat release predictions were obtained. However, products of incomplete combustion and NOx were low and this was attributed to the inability of the model to properly account for inhomogeneity and all the effects of NVO. Once again though, the overall trend in NOx levels was captured by the model. It was also found that the model does not operate very well near the misfire limit of the engine as it cannot capture the cyclic variability that can occur here. Based on the two new validation cases, it is concluded that once calibrated, the model can be used as a predictive tool for pressure, heat release, and combustion phasing of biofuelled HCCI engines. Furthermore, to improve its predictive capabilities, it is recommended that the model be restructured to incorporate mass transfer between zones, a fixed crevice volume and variable thermal boundary layer, and a CFD solver to improve emissions predictions and reduce reliance on calibration. Finally, changing the zone distribution from ring like zones to lumped stirred reactors is recommended to allow for more realistic modeling of actual experimental HCCI conditions.

Zugl.: Karlsruhe, Univ., Diss., 2009

No description available.

Future internal combustion engines demand higher efficiency but progression towards this is limited by the phenomenon called knock. A possible solution for reaching high efficiency will be to improve the anti-knock quality of the fuels by blending high-octane fuel with a low-octane fuel. In this study, the non-linear blending effect by blending oxygenated/non-oxygenated fuels of high octane number with low octane fuels were studied in three different combustion modes: Spark ignition (SI), Homogeneous Charge Compression ignition (HCCI) and Compression Ignition (CI). For SI combustion, RON and MON was used for the fuel rating, for HCCI combustion, Lund Chevron HCCI fuel number and for rich combustion conditions, Derived Cetane Number (DCN) was used to understand the fuel auto-ignition behavior. A Cooperative Fuel Research (CFR) engine was used for SI and HCCI mode whereas Ignition Quality Tester (IQT) was used for CI mode. The non-linear blending behavior was described using the concept of blending octane number. Five octane additives including ethanol, methanol, 1-butanol, toluene and iso-octane were used in this study, of which ethanol and methanol gave the strongest octane enhancement effect whereas iso-octane resulted in the weakest octane enhancement. The base fuel composition and octane number also had an important role in the blending behavior of the fuels. The non-linear blending of fuels highlighted that some of the blended fuels behaved similarly in both SI and HCCI combustion mode, therefore the study was further extended to understand the pre-spark heat release or Low temperature heat release (LTHR) in both the combustion modes. Knock occurs in SI due to end-gas auto-ignition and for HCCI, the combustion is controlled by auto-igniting of the complete charge inside the cylinder. Therefore fundamentally the combustion process in the end gas region of SI and HCCI combustion modes is controlled by auto-ignition. In this respect, HCCI combustion was used as an alternative path to understand the end-gas auto-ignition in SI engine using the standard CFR engine. Pre-spark heat release or low temperature reactions were detected in both the combustion modes.

Ignition and combustion characteristics of n-butanol/air, 2-butanol.air and isobutanol/air mixtures at stoichiometric (ϕ = 1) and lean (ϕ = 0.5) conditions were investigated in a micro flow reactor with a controlled temperature profile from 323 K to 1313 K, under atmospheric pressure. Sole distinctive weak flame was observed for each mixture, with inlet fuel/air mixture velocity set low at 2 cm/s. One-dimensional computation with comprehensive chemistry and transport was conducted. At low mixture velocities, one-stage oxidation was confirmed from heat release rate profiles, which was broadly in agreement with the experimental results. The weak flame positions were congruent with literature describing reactivity of the butanol isomers. These weak flame responses were also found to mirror the trend in Anti-Knock Indexes of the butanol isomers. Flux and sensitivity analyses were performed to investigate the fuel oxidation pathways at low and high temperatures. Further computational investigations on oxidation of butanol isomers at higher pressure of 5 atm indicated two-stage oxidation through the heat release rate profiles. Low temperature chemistry is accentuated in the region near the first weak cool flame for oxidation under higher pressure, and its impact on key species – such as hydroxyl radical, hydrogen peroxide and carbon monoxide – were considered. Both experimental and computational findings demonstrate the advantage of employing the micro flow reactor in investigating oxidation processes in the temperature region of interest along the reactor channel. By varying physical conditions such as pressure, the micro flow reactor system is proven to be highly beneficial in elucidating oxidation behavior of butanol isomers in conditions in engines such as those that mirror HCCI operations.

No description available.