Article pubs.acs.org/EF
Modeling Fuel and EGR Effects under Conventional and Low Temperature Combustion Conditions K. Anand,†,* R. D. Reitz,† E. Kurtz,‡ and W. Willems§ †
Engine Research Center, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States of America Ford Motor Company, Dearborn, Michigan 48126, United States of America § Ford Forschungszentrum Aachen GmbH 52072 Aachen, Germany ‡
ABSTRACT: Precise combustion control and a wider operating load range are the two major challenges in the application of advanced combustion modes, as the combustion process is chemical kinetically driven and thus is sensitive to the fuel composition. Although diesel fuels are complex multicomponent mixtures, most previous research has been carried out using simple single- or two-component surrogate models. This assumption leads to inaccuracies when modeling advanced combustion systems due to differences between the model and real fuel compositions. The present study proposes multicomponent surrogate models for three different diesel fuels that mimic the compositions and property variations of European and American diesel fuels. The composition of the surrogate fuels were arrived at by modeling the distillation data of the three fuels to within 1.5% maximum absolute error. The developed surrogate models were then applied to predict the combustion and emission characteristics of the three fuels tested in a single cylinder diesel engine operated under various conditions, including conventional and low temperature combustion (LTC) conditions. The computations were performed using the multidimensional computational fluid dynamics code, KIVA-ERC, incorporated with suitable reduced reaction mechanisms for the surrogate fuels. The predicted combustion and emission characteristics showed good agreement with measured data. The peak pressures are predicted to within 9.8% mean absolute error. Sensitivities to fuel type and EGR concentration were also explored in conventional and LTC modes using the surrogate models. The results showed that the combustion trends in conventional combustion are less affected by fuel or EGR changes, while a much higher sensitivity was observed under LTC conditions, thus demanding more realistic fuel models to precisely describe advanced combustion modes.
1. INTRODUCTION
The study of the effect of diesel fuel composition/properties on diesel engine performance and emissions has been an active research area for more than three decades.9 Karonis et al.10 carried out experimental investigations using a matrix of 68 fuels with different properties and concluded that the most significant fuel parameters were cetane number, density, and distillation temperatures. The fuel aromatic content was found to affect through its correlation with cetane number, density, and 90% distillation temperature (T90). Based on experimental investigations using 20 different fuels in a diesel engine, Li and Gulder11 concluded that the effects of cetane number on NOx was a physical effect through the changes in fuel mass burnt in the premixed phase, while the effects of aromatics on NOx was chemical through a higher adiabatic flame temperature. Fuel property effects on combustion and emissions were investigated by Kidoguchi et al.12 by independently varying the cetane number and aromatic content. Their results showed that with a constant cetane number, increasing the aromatic content had little effect on combustion but increased both NOx and particulate emissions due to locally rich high temperature combustion. Similar studies on fuel property effects under low temperature combustion conditions were conducted by Cho et al.13 using nine different FACE (Fuels for Advanced Combustion Engines) fuels in a light duty diesel engine and
Internal combustion engine research is driven by the need for near zero pollutant emissions and also higher thermal efficiency targets for improved fuel economy and lower greenhouse gas emissions. Over the past two decades, significant research efforts have been made to achieve high efficiency and low emission engines through proper control of fuel−air mixture quality and temperature by means of several advanced combustion strategies. The way diesel engines operate is being changed from operating in a high temperature to a low temperature regime in order to suppress the formation of both NOx and soot. The conceptual model of conventional diesel combustion proposed by Dec1 improved the understanding of nitric oxide and soot formation mechanisms in high temperature diesel combustion. Although there are several acronyms for the new diesel combustion systems, viz., premixed charge compression ignition2 (PCCI), homogenous charge compression ignition3 (HCCI), reactivity controlled compression ignition4 (RCCI), stratified charge compression ignition5 (SCCI), high efficiency clean combustion6 (HECC), etc., all of these strategies operate under low temperature combustion (LTC) conditions. It has been demonstrated that both NOx and soot formation can be avoided even under rich fuel−air conditions by maintaining temperatures lower than 1700 K.7 However, important challenges in LTC conditions include a detailed understanding of fuel effects for precise combustion control to provide a wider operating load range.8 © 2013 American Chemical Society
Received: October 3, 2013 Revised: November 24, 2013 Published: November 25, 2013 7827
dx.doi.org/10.1021/ef401989c | Energy Fuels 2013, 27, 7827−7842
Energy & Fuels
Article
of the present work was to develop suitable surrogate models for typical Swedish and American diesel fuels and to examine their fidelity to capture the characteristics of a single cylinder diesel engine operated under various conditions, including conventional and LTC modes. The low temperature combustion was realized in the present work by maintaining higher levels of EGR. Fuel and EGR effects were explored in the two different combustion modes using the developed surrogate models.
their conclusion was that the cetane number had a more dominant effect on combustion than fuel aromatics. From experimental investigations using 13 different diesel-, kerosene-, and gasoline-like fuels in a Euro 6 single cylinder diesel engine under the advanced combustion operation, Cracknell et al.14 concluded that a fuel with a lower cetane number and higher volatility was beneficial at low part-load operating conditions. They had also observed that by matching CA50 with an adjustment of injection and boost pressures, the indicated efficiency were similar for all the fuels at the same load and speed conditions. Based on experimental investigation using the FACE fuels in a HCCI engine, Bunting et al.15 established that fuels with lower cetane number along with a lower 90% distillation temperature were desirable for better fuel economy and lower emissions. The effects of fuel cetane number were investigated by Chen et al.16 by coupling Ditert-butyl peroxide (DTBP) with a reduced diesel surrogate mechanism. They concluded that an increase in cetane number advanced the ignition of the pilot injection with a negligible effect on the combustion processes of the main and post injections and on the engine-out NOx and soot emissions. The experimental investigations conducted by Szybist et al.9 showed that the high load operating point using pilot and main injections showed reduced sensitivity to fuel ignitability as compared to using a single injection at the same load condition. Studies by Stark et al.17 showed that an improved understanding of fuel property effects was beneficial to increase the operating load range of an HCCI engine by more than 30%. Research by Ryan and Mathews18 showed that there were fundamental limitations on the use of current diesel fuels for HCCI operation. An alternative strategy to use diesel fuels under low temperature combustion conditions was achieved through the use of high levels of exhaust gas recirculation8 (EGR). Unlike in HCCI, the start of combustion was closely coupled with the fuel injection event in this LTC strategy, and the autoignition and the subsequent combustion were controlled by a combination of chemical kinetics and mixing process.19 The use of EGR was found to provide control of ignition timing and combustion rate in addition to reducing the combustion temperatures.20 The LTC strategy using EGR was found to be beneficial to reduce both NOx and soot simultaneously, regardless of the fuel−air equivalence ratio by maintaining temperatures below 1650 K.19 Since the molecular composition of diesel fuels is much more complex due to the inclusion of thousands of hydrocarbons,21−24 modeling diesel composition effects on the combustion process is usually realized using a few representative hydrocarbon species referred to as “surrogates”.25 A variety of surrogates for diesel fuels have been suggested in the literature depending on the intended application targets, viz., spray, ignition, combustion chemistry, and emissions. Existing surrogate models for diesel fuels were reviewed in refs 26 and 27. The major challenge in diesel surrogate modeling includes capturing both the spray and combustion characteristics using a single surrogate mixture. Further, modeling diesel chemistry within the realistic computer time limits can only be realized by developing more accurate multicomponent reduced reaction mechanisms.28 Although research efforts have been made in the past to understand fuel effects through experiments, corresponding modeling studies are very limited. Also, there is a need to study the fuel effects in modern technology engines, which may need to be operated in advanced combustion modes to meet future emissions standards. A deeper insight into fuel effects is possible through the development and application of more realistic surrogate fuel models. Based on the above premises, the objective
2. METHODOLOGY The diesel fuels are complex multicomponent mixtures, which may include thousands of different hydrocarbon species but which can be grouped under four major hydrocarbon classes, viz., n-paraffins, naphthenes, iso-paraffins, and aromatics.21 The composition and properties of diesel fuels are highly variable depending on the crude oil source as observed from the differences between European and American diesel fuels. To study fuel effects, three diesel fuels with varying saturate and aromatic compositions were considered in the present work. The measured hydrocarbon class compositions and the cetane number for the three fuels are provided in Table 1.27 Table 1. Measured Hydrocarbon Classes of the Three Diesel Fuels fuel type/hydrocarbon class
high-CN diesel
mid-CN diesel
low-CN diesel
cetane number saturates total aromatics polycyclic aromatics olefins
56.9 97.7 1.8 0.5 0.5
43.0 70.1 28.6 6.6 1.3
40.9 65.7 32.4 4.8 1.9
The compositions of the three fuels are varied to capture the cetane number variations between Swedish and American diesel fuels. The high cetane fuel composition and properties meet the EC1 Swedish diesel fuel specifications (>51 cetane, < 5% vol. aromatics29) and the mid and low cetane fuels have properties typical for North American diesel fuel (>40 cetane,
