Joshua A. Bittle

Cylinder-to-cylinder variation sources in diesel low temperature combustion and the influence they have on emissions

Cylinder-to-cylinder variation in a multi-cylinder diesel engine was found to increase substantially when transitioning to a low-temperature combustion mode. This study was started to investigate the potential influence this effect could have on the emissions levels. Initial testing showed an imbalance in the fuel distribution that prompted this article to focus on data from before and after swapping two injectors under both conventional and low-temperature combustion modes. A significant improvement is observed in cylinder variation based both on visual heat release inspection and on mean effective pressure variation. This is likely a result of a changing combination of exhaust gas recirculation and fuel distribution such that less cylinder-to-cylinder variation is present (e.g. high dilution and low fuel, switched to low dilution and low fuel). Interestingly, despite the reduced cylinder-to-cylinder variation, the results show that the emissions levels are actually not affected. Despite the lack of influence on emissions results, the cylinder-to-cylinder variation in low-temperature combustion modes is still a critical factor that could impact its ability to be implemented in a commercial setting. Further cylinder balancing was attempted and achieved by introducing small (microsecond) adjustments to each cylinder start of injection and injection duration. The balancing is effective, but due to exhaust gas recirculation imbalance, a single adjustment setting does not apply to both conventional and low-temperature combustion modes. Additionally, day-to-day ambient conditions also negate the effectiveness. This supports the idea that some type of consumer-based real-time automatic balancing system may be needed in the future.

The role of system responses on biodiesel nitric oxide emissions in a medium-duty diesel engine

The often-observed higher emission of nitrogen oxides with biodiesel, relative to petroleum diesel, is well-reported in the literature. Upon review of the literature, there seem to be two broad contributors that cause such a trend: those that are manifested by the effects of fuel property differences directly on in-cylinder processes and those that are manifested by the effects of fuel property differences on engine systems, thus rendering an indirect effect on in-cylinder processes. In this article, the former manifestations are called fundamental issues while the latter manifestations are called system response issues. Both can have significant impact on the magnitude and direction of nitrogen oxides emission differences between biodiesel and petroleum diesel fuels. This article has the objective to identify the distinction between fundamental and system response issues on nitric oxide emissions of biodiesel combustion in a diesel engine. It is noted that the article focuses mostly on the system response issues of a production-type engine, and will only briefly summarize some of the fundamental issues believed to most strongly contribute toward differences in nitric oxide emissions between fuels. Consequently, it is important to note (in fact, a major theme of the article) that many of the specific observations of this study will not necessarily transcend to generality across all engine platforms due to differences in engine technology and calibration; instead, the emphasis on the important role of potential system responses manifested by the use of different fuels of any engine system is meant to be the general contribution of this study. The study generally observes that biodiesel, in the absence of system response issues, emits higher NO x than petroleum diesel. System response issues, however, can have a dramatic impact on biodiesel NO x emissions. In some cases, system response issues may cause biodiesel NO x to be lower than petroleum diesel. Such system response issues highlight potential opportunities to mitigate or decrease biodiesel NO x emissions relative to petroleum diesel.

Investigation Into the Use of Ignition Delay as an Indicator of Low-Temperature Diesel Combustion Attainment

The authors evaluated the attainment of low-temperature diesel combustion in a medium-duty diesel engine apparatus. Attainment of low temperature combustion is determined by the simultaneous and substantial decreases in nitric oxide (NO) and smoke concentrations. The extreme low-temperature combustion condition results in greater than 50% reductions in both NO and smoke concentrations relative to a baseline conventional combustion mode. Combustion development occurs at a low speed (1400 rev/min) and nominally light load (nominally 68 N-m torque, or 1.9 bar brake mean effective pressure). The authors conclude that determination of low-temperature combustion attainment is not universally possible using phenomenologically based ignition delay calculations. Evaluations of two methods to determine start of combustion, which render a definition for ignition delay and for engine ignition delay, reveal that neither provides a consistent metric to determine low-temperature combustion attainment.

Biodiesel Effects on Influencing Parameters of Brake Fuel Conversion Efficiency in a Medium Duty Diesel Engine

Biodiesel remains an alternative fuel of interest for use in diesel engines. A common characteristic of biodiesel, relative to petroleum diesel, is a lowered heating value (or per mass energy content of the fuel). For same torque engine comparisons, the lower heating value translates into a higher brake specific fuel consumption (amount of fuel consumed per unit of power produced). The efficiency at which fuel energy converts into work energy, however, may remain unchanged. In this experimental study, evaluating nine unique engine operating conditions, the brake fuel conversion efficiency (an assessor of fuel energy to work energy efficiency) remains unchanged between 100% petroleum diesel fuel and 100% biodiesel fuel (palm olein) at all conditions, except for high load conditions. Several parameters may affect the brake fuel conversion efficiency, including heat loss, mixture properties, pumping work, friction, combustion efficiency, and combustion timing. This article describes a study that evaluates how the aforementioned parameters may change with the use of biodiesel and petroleum diesel, and how these parameters may result in differences in the brake fuel conversion efficiency.

Biodiesel Fuel’s Effects on Influencing Parameters of Brake Fuel Conversion Efficiency in a Medium Duty Diesel Engine

Biodiesel remains an alternative fuel of interest for use in diesel engines. A common characteristic of biodiesel, relative to petroleum diesel, is a lowered heating value (or per mass energy content of the fuel). For same-torque engine comparisons, the lower heating value translates into a higher brake specific fuel consumption (amount of fuel consumed per unit of power produced). The efficiency at which fuel energy converts into work energy, however, may remain unchanged. In this experimental study, evaluating nine unique engine operating conditions, brake fuel conversion efficiency (an assessor of fuel energy to work energy efficiency) remains unchanged between 100% petroleum diesel fuel and 100% biodiesel fuel (palm olein) at all conditions except for high load conditions. Several parameters may affect brake fuel conversion efficiency, including heat loss, mixture properties, pumping work, friction, combustion efficiency, and combustion timing. This article describes a study that evaluates how the aforementioned parameters may change with the use of biodiesel and petroleum diesel, and how these parameters may result in differences in brake fuel conversion efficiency.© 2009 ASME

Efficiency Considerations of Later-Phased Low Temperature Diesel Combustion

Low temperature diesel combustion offers an opportunity to simultaneously and substantially reduce exhaust nitrogen oxides and particulate matter emissions. One issue that remains an area of investigation is the improvement of engine efficiency (i.e., specific fuel consumption) for the novel mode of combustion. The objective of this article is to assess the several parameters (i.e., friction, pumping work, combustion phasing, heat transfer rate, and combustion efficiency) that affect the brake fuel conversion efficiencies of a medium-duty diesel engine as its combustion mode is transitioned from conventional to low temperature. The analysis reveals that, in this study’s development of low temperature combustion, late combustion phasing is the primary factor causing a decrease in brake fuel conversion efficiency. To enable low temperature combustion, combustion is retarded to a point where peak rate of heat release occurs at around 24° after top dead center. Such late combustion misses the opportunity to utilize the full expansion stroke of the piston. Although exhaust hydrocarbon and carbon monoxide concentrations increase as a result of the later-phased low temperature combustion mode, combustion efficiency only drops to around 90%. This decrease in combustion efficiency accounts for only about 18.7% of the corresponding decrease in brake fuel conversion efficiency (the balance decrease being caused by the later-phased combustion). Other factors that typically deteriorate brake fuel conversion efficiency (i.e., pumping work, friction, and rate of heat transfer) are all decreased with this study’s development of low temperature combustion. It is important to note that other implementations of low temperature combustion (e.g., advanced timing low temperature combustion) may not necessarily realize the same reductions in brake fuel conversion efficiency, or reductions may not necessarily be caused by the same dominant factors that are observed in this study’s later-phased low temperature combustion mode.Copyright

Two-Stage Ignition as an Indicator of Low-Temperature Diesel Combustion

Two-stage ignition is characterized by an initial cool-flame reaction followed by typical hot ignition. In traditional combustion conditions the ignition is fast such that the cool flame is not observed. By controlling initial conditions (pressure, temperature, and composition) in a rapid compression machine, for example, the creation and duration of the cool-flame event is predictable. This study focuses on linking the results from the rapid compression machine to those of low-temperature combustion behavior in a medium-duty compression ignition diesel engine. A correlation between cool-flame duration, nitric oxide concentration, and engine control settings allows the postulation that a sufficiently long cool-flame reaction results in a combustion event that can be classified as low-temperature combustion. A potential method for identifying low-temperature combustion events using only the rate of heat release profile is theorized which utilizes the cool-flame reaction duration as a metric.

Validation and Results of a Pseudo-Multi-Zone Combustion Trajectory Prediction Model for Capturing Soot and NOx Formation on a Medium Duty Diesel Engine

A pseudo-multi-zone phenomenological model has been created with the ultimate goal of supporting efforts to enable broader commercialization of low temperature combustion modes in diesel engines. The benefits of low temperature combustion are the simultaneous reduction in soot and nitric oxide emissions and increased engine efficiency if combustion is properly controlled. Determining what qualifies as low temperature combustion for any given engine can be difficult without expensive emissions analysis equipment. This determination can be made off-line using computer models or through factory calibration procedures. This process could potentially be simplified if a real-time prediction model could be implemented to run for any engine platform — this is the motivation for this study.The major benefit of this model is the ability for it to predict the combustion trajectory, i.e. local temperature and equivalence ratio in the burning zones. The model successfully captures all the expected trends based on the experimental data and even highlights an opportunity for simply using the average reaction temperature and equivalence ratio as an indicator of emissions levels alone — without solving formation sub-models.This general type of modeling effort is not new, but a major effort was made to minimize the calculation duration to enable implementation as an input to real-time next-cycle engine controller Instead of simply using the predicted engine out soot and NOx levels, control decisions could be made based on the trajectory. This has the potential to save large amounts of calibration time because with minor tuning (the model has only one automatically determined constant) it is hoped that the control algorithm would be generally applicable.Copyright

Cyclic Variability in Diesel/Gasoline Dual-Fuel Combustion on a Medium-Duty Diesel Engine

Most studies comparing diesel/gasoline dual-fuel operation and single-fuel diesel operation in diesel engines center on time-averaged results. It seems few studies discuss differences in cyclic variability. Motivated by this, the present study evaluates the cyclic variability of combustion in both dual-fuel and single-fuel operations of a diesel engine.Steady-state tests were done on a medium duty diesel engine with conventional direct injection timings of diesel fuel into the cylinder at one speed and three loads. In addition to single-fuel (diesel) operation, dual-fuel (gasoline and diesel) operation was studied at increasing levels of gasoline fraction. Gasoline fuel is introduced via a fuel injector at a single location prior to the intake manifold (and EGR mixing location). Crank-angle resolved data including in-cylinder pressure and heat release rate obtained for around 150 consecutive cycles are used to assess cyclic variability.The sources of cyclic variability, namely the factors causing cyclic variability or influencing its magnitude, especially those related to cylinder charge amount and mixture preparation, are analyzed. Fuel spray penetration and cyclic variability of cylinder charging, overall A/F ratio, and fuel injection timing, tend to increase cyclic variability of combustion in dual-fuel operation. On the other hand, fuel type and fuel spray droplet size tend to increase cyclic variability in single-fuel operation.The cyclic variability in dual-fuel operation in this study is more serious than that in single-fuel operation, in terms of magnitude, indicated by metrics chosen to quantify it. Most measures of cyclic variability increase consistently with increasing gasoline fraction. Variations of gasoline amount and possibly gasoline low temperature heat release cause higher combustion variation in dual-fuel operation primarily by affecting premixed burning.Statistical methods such as probability density function, autocorrelation coefficient, return map, and symbol sequence statistics methods are used to check determinism. In general, the parameters studied do not show strong determinism, which suggests other parameters must be identified to establish determinism or the system is inherently stochastic. Regardless, dominant sequences and optimal sequence lengths can be identified.Copyright