Chapter 7
SUMMARY AND CONCLUSIONS
7.1. INTRODUCTION
Due to the stringent emission regulations the interest in always more efficient combustion processes and after-treatment systems is getting stronger. In recent years, low temperature combustion strategies, like HCCI combustion, have been studied for their low level of pollutant emission.
HCCI combustion, using gasoline or diesel fuels, is being widely studied as an attractive alternative to conventional spark-ignition or diesel compression ignition engine combustion for its reduced emission production and low fuel consumption. In this combustion strategy, fuel is injected and well premixed with air before ignition, thus avoiding fuel-rich regions leading to reduction of particulate matter emissions. Combustion gradually takes place at the same time in the whole combustion chamber, since lean mixture is used and charge is diluted with exhaust products. NOx emissions, strongly dependent on the gas temperature, are greatly reduced because the burned gas temperatures are relatively low due to the adopted mixture composition.
However, HCCI combustion has several difficulties that must be overcome for practical use, such as ignition timing controllability and the preparation of homogeneous mixtures. The combustion phasing has to be accurately controlled to avoid thermal NOx emission formation or incomplete combustion. Due to the previous aspects, currently HCCI combustion is not able to cover the whole engine operating field but is feasible only in low/medium load condition.
Multidimensional CFD modeling represents a fundamental resource for engine research and development. This tool provides detailed information of the in-cylinder physics helping visualization and analysis processes. In the present work some focus was turned on implementation and development of advanced ignition and combustion models.
In recent years, with considerable achievements in the fundamental combustion chemical kinetics and rapid advances in computer technology, elementary chemical kinetic mechanisms have been coupled with multidimensional CFD codes for IC engine simulations. These advanced models can help to obtain a much clearer picture of the in-cylinder combustion chemistry. Therefore, the coupling between fluid dynamics and chemical kinetics has become an important trend in engine research.
7.2. SUMMARY AND CONCLUSIONS
The research activity can be divided in three sections, in which possible solutions to HCCI combustion limitations, as mixture formation, ignition phasing control and operating range extension, have been analyzed:
• numerical and experimental analysis of gasoline HCCI and PCCI combustion, to understand the influence of mixture in-homogeneity on combustion behavior and emission production;
• numerical analysis of dual fuel combustion through advanced combustion models, to avoid ignition phasing control uncertainties;
• numerical analysis of an innovative diesel HCCI combustion solution, to control and extend HCCI combustion to the whole engine operating range.
7.2.1. HCCI/PCCI COMBUSTION ANALISYS
In the first part of the research gasoline HCCI and PCCI combustion have been analyzed to observe the effect of mixture in-homogeneity on ignition phasing controllability.
An accurate modeling of spray characteristics is essential for a correct prediction of the in-cylinder mixture distribution, especially in direct injection PCCI combustion. Thus, the spray model was validated with well-designed experimental measurements before proceeding with engine simulations. Results in terms of spray tip penetration, overall and local SMD distribution have been obtained and compared with experimental data. The predictions capture the transient spray shapes and behaviors excellently, giving confidence to the spray model used in the study.
A numerical study of gasoline HCCI/PCCI combustion, with comparisons with experimental data was performed. The reference engine is a Caterpillar 3401 single cylinder engine usually used for conventional diesel combustion applications. The KIVA3V code implemented with two combustion models (Shell-CTC and CHEMKIN) with different accuracy and required computational time, has been used to evaluate the importance of detailed chemistry prediction for low temperature combustion. Three different set of experiments related to HCCI and PCCI combustion, with different engine speed, injection timing and overall mixture equivalence ratio have been taken as a reference. A heater was used to create ignitable initial conditions at IVC for the lean mixtures. Auto-ignition of lean gasoline/air mixtures is hard to obtain due to its long ignition delay with naturally aspirated air conditions at room temperature. Therefore, the initial temperature has to be increased to ensure ignition with the compression ratio and speed of the engine considered in this study. The range of SOI timings used in these experiments is very wide and covers the range needed for analysis of HCCI and PCCI cases. The equivalence ratios also cover the usual conditions for these combustion regimes. Results in terms of pressure, heat release rate and emission production have been obtained from numerical and experimental point of view.
From the results analysis emerges that both the Shell-CTC (turbulence-controlled) combustion model and CHEMKIN (chemistry-(turbulence-controlled) can adequately predict pressure and apparent heat release profiles for HCCI and PCCI combustion at various mixture conditions, engine speeds and injection timings, without case-by-case model constant tuning. The detailed chemistry combustion model performs better than the Shell-CTC model in the prediction of emissions for gasoline fueled HCCI/PCCI operation. This is due to the equilibrium assumption on which the Shell-CTC model is based. On the other hand the detailed chemistry model needs consistently larger computational time than the Shell-CTC. Comparisons with a set of experiments with very early injection timings during the intake process show that NOx emissions are under-predicted as the mixture equivalence ratio is increased. This is attributed to 3-D flow effects in the cylinder which are not considered in the present study. The other emission outcomes suggest that the assumption of initial in-cylinder homogeneous charge is incorrect, since it precludes the capturing of initial charge in-homogeneity effects that can play an important role in emission production for low temperature combustion regimes.
7.2.2. DIESEL/GASOLINE DUAL FUEL COMBUSTION
In the second part of the research activity diesel/gasoline dual-fuel combustion has been proposed as a way to solve the combustion phasing control problem. In this operation, a small amount of diesel fuel is injected as a pilot injection to ignite a pre-mixture of gasoline and air.
Although dual-fuel combustion is an attractive way to achieve controllable HCCI operation, few studies are available to help the understanding of its in-cylinder combustion behavior. This study describes a numerical study of diesel-gasoline dual-fuel combustion, to understand the influence of fuel mixture composition and injection timing on combustion behavior.
A primary analysis was performed to investigate the feasibility of the concept. Diesel/gasoline dual-fuel combustion was simulated using a modified version of the Shell-CTC model in order to test the capability of this new approach of controlling combustion phasing while maintaining low levels of pollutant emissions. Several relative in-cylinder mixture compositions were tested in order to observe the influence of several diesel injection amount. For a fixed injection timing of diesel fuel, the variation of the diesel/gasoline proportion is seen to change the ignition timing of the mixture, which indicates ignition timing controllability of the dual-fuel operation. As expected, increasing the percentage of premixed gasoline caused both soot and NOx emissions to decrease. The decreased amount of injected diesel allows the mixture to become closer to a homogeneous mixture, which lowers the local burned gas temperatures and, consequently, reduces the soot and NOx emissions.
Before proceeding with the dual-fuel analysis, some model validation for a newly proposed combustion strategy was necessary. This strategy, implemented in the KIVA3V code introduces a new Damkohler number, ratio between a laminar flame propagation timescale and a chemical timescale, to determine whether in combustion could be locally controlled by flame propagation or by volumetric heat release. This is then considered in combustion calculation. KIVA3V code using Damkohler approach was first tested with available diesel flame liftoff length experiments showing good accuracy on predicting liftoff length transient and typical
diesel triple flame structure. Subsequently available diesel/natural gas combustion experiments were used to test the code on dual fuel combustion. Good accuracy in pressure, HRR and emission calculations were observed.
Then, as a main object of this part, an accurate numerical analysis was performed using the KIVA 3V code, integrated with CHEMKIN and G-equation combustion models on diesel/gasoline dual-fuel combustion in several load and engine speed conditions. Several injection amounts of diesel fuel were tested in order to study their influence on combustion phasing and controllability. The percentages of diesel and gasoline introduced were changed while keeping the total amount of released energy constant. Several diesel injection timings were tested with two different injectors (multihole injectors with six and eight holes equally spaced) in order to consider the influence of the mixture formation on the combustion process. Four different combustion strategies, with different calculation approaches in the burning zone, have been used for this study to test their predictability. All these operations were performed for two engine operative points with low and medium engine speed and load. The reference engine is the Caterpillar 3401,as in the previous section.
Pressure, HRR, mean and peak temperature profiles were analyzed for the whole set of considered cases as well as emission results. From the results analysis it emerges that the details of the mixture composition totally influence the dual-fuel combustion and emission production. Decreasing the amount of injected diesel, dual-fuel combustion becomes closer to HCCI combustion. It is interesting that, with less than 25% diesel, the diesel injection plays a controlling role as the ignition source and combustion initially takes place by a flame propagation mechanism. Then, due to the high in-cylinder pressure and temperature conditions, auto-ignition spreads throughout the entire combustion chamber. The mixture preparation details are important for determining the ignition timing and start of combustion. Exhaust emissions are affected as well. The adoption of an eight-hole injector leads to more efficient combustion and reduced emissions for large amounts of injected diesel. Decreasing the amount of injected fuel, the effects of the mixture formation details become smaller. Injection timing variation strongly influences the ignition and combustion processes when more than 10% diesel fuel is injected. With smaller percentages, combustion is controlled by gasoline auto-ignition and the injection timing effect is reduced. All of the examined combustion models give similar predictions for the very early combustion stage. However, the flame propagation G-Equation model predicts a lower heat release rate since the first part of combustion is guided by flame front propagation. Subsequently chemistry effects become dominant. The other three combustion models give similar results for the whole combustion event.
The present computational tools successfully demonstrated the capability of describing dual-fuel combustion operation. The results are useful to better understand the physics and to guide experimental test selection options.
7.2.3. NEW CONCEPT DIESEL HCCI COMBUSTION – PRELIMINARY STUDY
The third and last part of the research concerns an innovative concept to control HCCI combustion in diesel-fuelled engines. The main purpose of this strategy is the obtaining of diesel HCCI combustion also with high mean effective
pressures rendering the combustion behavior more controllable as well. The concept consists in forming a pre-compressed homogenous charge outside the cylinder and in gradually admitting it into the cylinder during the combustion process. In this way, combustion can be controlled by the flow rate transfer. High pressure gradients, typical of common HCCI combustion, can be limited as well.
A first numerical analysis has been done, using the AVL FIRE 8.5 code, to test the validity of the concept, regardless of which effective solution will be adopted. A single cylinder filled with a perfectly stirred mixture of air and diesel fuel through a transfer duct has been considered in the simulation. The results show that homogeneous combustion occurs with almost constant pressure (oscillations are in the range of a couple of bar). The heat is released in a gradual way, as it was expected. The obtaining of almost constant pressure combustion allows using high engine compression ratio and consequent high initial pressure and temperature conditions.
Subsequently, two applicative solutions, for two and four stroke cycle operations, have been proposed and analyzed. The two stroke solution is based on the split cycle concept, with intake and compression phases performed outside the cylinder. Compression is realized by an external volumetric compressor and the high-temperature compressed air is transferred to the cylinder through an inlet valve during the engine combustion phase. During the air transfer, fuel is injected into the transfer duct, evaporates and mixes with air, bringing about the conditions for homogeneous combustion. In the four stroke solution, the air is directly compressed by the piston during the compression stroke. The maximum pressure is reached before the piston arrives to TDC then compressed air begins entering a secondary chamber which is in constant communication with the cylinder. The volume of this secondary chamber, theoretically starting from zero, varies keeping pressure constant during the last part of the compression stroke and during combustion, thanks to a moving wall over which a constant pressure acts. When the piston begins its down-stroke, the air is transferred back into the cylinder while fuel is injected, evaporates and mixes with air, bringing about the conditions for homogeneous combustion.
Results in terms of pressure, HRR, temperature and emissions production have been obtained. Likewise in the concept validation results, pressure remains almost constant during the combustion phase and the heat is released in a gradual way as well even though there is a longer combustion for the two stroke solution. However bigger oscillations are present at the beginning of the transfer due to pressure losses in the transfer duct. Emission results have then been compared with conventional diesel emissions. From the results analysis it emerges that the two solutions produce more soot than the concept validation case (and than the diesel case as well), pointing out that the air-fuel mixing is probably not optimized, as it appears also from equivalence ratio maps. NOx emissions are instead very low due to delayed combustion of part of the fuel and consequent low combustion efficiency. A successive injection process optimization has been performed on both the proposed solutions. Different injectors have been used in several positions of the admission duct. Results shown that the mixture formation represents a fundamental process for the obtaining of a gradual combustion encouraging further research, including experimental activity.
