Thermally Stratified Compression Ignition: A new advanced low temperature combustion mode with load flexibility☆
Introduction
Low Temperature Combustion (LTC) is a group of advanced combustion concepts that promises simultaneous reductions in fuel consumption and pollutant emissions. Homogeneous Charge Compression Ignition (HCCI) is one of the earliest forms of LTC and possibly the most widely researched [1], [2], [3], [4], [5], [6]. In HCCI, a homogeneous mixture of air and fuel is compressed until it autoignites. HCCI therefore pairs the homogenous, ultra-low soot characteristics of conventional well-mixed Spark Ignition (SI) combustion with the high efficiencies (achieved through lean, unthrottled operation) usually typical of diesel Compression Ignition (CI) combustion. Engine-out NOx emissions are kept low through high levels of dilution with air and/or residuals. Due to these factors, HCCI has demonstrated near-zero NOx and soot emissions at efficiencies similar to, or greater than, conventional diesel combustion [5]. However, HCCI is only achievable over a narrow, part-load operating range due to the lack of direct control over the start and rate of heat release. In order to provide control over heat release in HCCI, a better understanding of the fundamental combustion processes is required.
Recently, the community gained a better understanding of LTC heat release through optical chemiluminescence and planar laser-induced fluorescence (PLIF) images, which showed that while HCCI can be compositionally homogeneous, there is a significant amount of thermal stratification (i.e. a distribution of in-cylinder gas temperatures prior to ignition) that stagger the ignition timing of various regions based on their local temperature [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17]. Depending on the valve strategy and the fuel injection strategy, there can also be compositional gradients of either residual gases or fuel fraction, respectively, that can also stagger the autoignition timing of various regions [18], [19], [20], [21], [22], [23]. With this new information, an improved understanding of LTC was achieved: (1) regions autoignite sequentially based on their local temperature and reactivity, and (2) the early-igniting regions release their energy first, compressing the remaining unburned regions causing their subsequent autoignition. Due to this second underlying phenomenon, the energy release process in LTC is a positive feedback loop, which explains its relatively low high-load limit.
In addition to the insight about thermal stratification gained through the PLIF optical diagnostics by Dec and other researchers around the world, a post-processing technique was previously developed and validated by Lawler et al. [24], [25] that can estimate an unburned temperature distribution prior to ignition in a fired, metal engine. The technique, called the Thermal Stratification Analysis (TSA), uses isentropic ideal gas relations to model the possible gas temperatures in the cylinder and their change over the compression and combustion processes. The autoignition integral [26] and an ignition delay correlation [27] are then used to predict the ignition timing of each temperature trajectory, which are finally coupled to the calculated heat release characteristics to estimate a temperature distribution. The TSA approach has been previously validated against the optical distributions measured by Dec et al. as well as against CFD simulations [25], and has subsequently been applied to study the effects of wall conditions, engine geometry [28], and operating conditions [29] on thermal stratification. The results showed that wall conditions had a surprisingly small influence on the thermal stratification [28]. Instead, the top dead center (TDC) temperature, combustion phasing, and swirl all had a noticeable influence on the level of thermal stratification prior to ignition [29].
In traditional HCCI, neither the compositional gradients, nor the temperature distribution prior to ignition, nor the positive feedback loop are controlled. This lack of control is responsible for the narrow, part-load operating range. To address the control challenges associated with HCCI, a wealth of alternative LTC modes have been proposed. Reactivity Controlled Compression Ignition (RCCI) uses two distinct fuels and a specific range of direct injection (DI) timings to introduce a gradient of fuel properties in the cylinder prior to ignition [30], [31], [32], [33]. By controlling the fraction of each fuel and the DI timing, RCCI has demonstrated the ability to achieve higher loads than pure HCCI with better controllability. However, the requirement of two distinct fuels and fuel systems detracts from RCCI’s commercial viability, especially for the light-duty market. Spark Assisted Compression Ignition (SACI) is a variant of HCCI where a spark discharge is used to help control the start and rate of heat release [17], [34], [35]. In SACI, a flame front burns between 5% and 50% of the mixture, and the remaining 50–95% of the mixture autoignites similarly to pure HCCI. SACI has shown the ability to provide some level of control and help extend the load range. However, the spark discharge introduces a significant amount of variability which can be a challenge for LTC. Gasoline Compression Ignition (GCI) is an advanced combustion concept that uses gasoline in a diesel engine [36], [37]. GCI has its advantages, but its NOx emissions and pressure rise rates pose a challenge. Dec et al. introduced Partial Fuel Stratification (PFS) which uses a second injection of fuel during the compression stroke to provide a gradient of equivalence ratios to stagger the ignition timing of various regions when using a φ-sensitive fuel (i.e. a fuel whose autoignition timing is sensitive to variations in equivalence ratio) [21], [38].
Most of the alternative LTC modes that attempt to provide control over the start and rate of heat release rely on a direct fuel injection event to introduce a stratification of equivalence ratio and reactivity. This approach can be effective; however, the intentional fuel-air mixture inhomogeneities present a risk of higher particulate matter (PM) and NOx emissions due to the locally rich regions which are closer to the soot production island and have higher burned gas temperatures. In fact, recent detailed particulate emissions measurements on these equivalence-ratio-stratified advanced combustion concepts has shown that although a smoke meter measurement may be zero, the particulate emissions are simply smaller in diameter and different in composition (e.g. increased concentrations of polycyclic aromatic hydrocarbons and aldehydes) [39], [40], [41], [42], [43], [44]. This result, in combination with biological research which has shown that smaller particles are more detrimental to human health [45], [46], might suggest that the equivalence-ratio-stratified concepts could shift the distribution of particulate matter to a more harmful region compared to conventional diesel combustion.
Instead of attempting to use a forced fuel-air mixture stratification to control the heat release rates in LTC, we propose a new combustion mode that controls the amount of thermal stratification in LTC. This approach, termed Thermally Stratified Compression Ignition (TSCI), employs direct injection of water to control both the mean temperature and the temperature distribution in the cylinder, thereby offering control over the start and rate of heat release in LTC. This paper first presents experimental data to fundamentally understand the effects of the direct water injection event on thermal stratification in the cylinder, and second, presents a load sweep to demonstrate the load limits that are achievable with and without water injection.
Water injection in internal combustion engines has a long history [47], [48], [49], [50], [51], [52], [53], [54]. Water injection has been used in SI combustion to mitigate knock [47]. Water injection has also been used to lower temperatures and reduce NOx emissions in any combustion mode [48], [49]. Water injection has even been previously investigated in HCCI and Premixed Charge Compression Ignition (PCCI) [50], [51], [52], [53], [54]. Specifically, Lund Institute of Technology port injected water and found that the evaporative cooling in the intake manifold allowed the control of start of combustion by controlling the intake valve closing (IVC) temperature [50]. New A.C.E. Institute [52] and Hokkaido University [53], [54] direct injected water through a diesel fuel injector in HCCI and PCCI, respectively, and found that water injection offers the ability to extend the load range of HCCI and PCCI.
This work expands upon the prior studies utilizing water injection in HCCI combustion by applying the TSA to fundamentally understand the impact of the direct water injection event on the temperature distribution prior to ignition. The improved understanding of the effects of direct water injection on thermal stratification allows the development of the new TSCI combustion strategy. The TSA will be used throughout this paper to gain a better understanding of the effects of direct water injection on thermal stratification. Additionally, this study was conducted on a gasoline engine platform with GDI technology, typical of current light-duty engine hardware, as opposed to the studies in the literature that were conducted on diesel engine platforms.
Section snippets
Experimental Configuration
Experiments were conducted at Oak Ridge National Laboratory at the Fuels, Engines, and Emissions Research Center (FEERC). The engine was a production, gasoline, SI 2.0L General Motors (type LNF) engine. A production, gasoline engine was used to demonstrate the applicability of the proposed concept to the light-duty, passenger car market and because the added hardware costs associated with water injection are more tolerable on a gasoline engine platform compared to a diesel engine platform. Of
Effect of water injection on the start of heat release in LTC
Data was collected with and without water injection to demonstrate the effects of water injection on the start of heat release in LTC. Three cases are considered and the relevant control parameters are shown in Table 1. Case 1 and 2 demonstrate that combustion phasing can be advanced by lengthening the NVO duration. This result agrees well with the literature which has shown that longer NVO durations elevate the IVC temperature by increasing the internal RGF [34], [55], [56]. In Case 3, the NVO
Conclusions
In this paper, we propose a new advanced combustion mode, Thermally Stratified Compression Ignition (TSCI), which uses direct water injection to control the start and rate of heat release in Low Temperature Combustion. Experiments were conducted to better understand the effects of water injection on LTC, with particular focus on comparisons between operation with and without water injection, as well as the effects of water injection amount and timing. Finally, the load limits with and without
Acknowledgements
This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under contract number DE-AC05-00OR22725.
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This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-planhttp://energy.gov/downloads/doe-public-access-plan).