Effects of Amorphous Ti–Al–B Nanopowder Additives on Combustion in a Single-Cylinder Diesel Engine

As global energy demand continues to rise, concern increases regarding the supply of petroleum. Although bioderived alternative fuels have become the subject of much research and development, these fuels typically have lower volumetric energy density, due to the oxygenation of the feedstock. For example, lower heating values (LHVs) are approximately 15% lower for typical biodiesel fuels than for standard #2 diesel fuel (see Table 10 in Ref. [1]). One method to counteract this is to use additives, and metals/metalloids are particularly attractive due to their high volumetric energy densities [2,3]. In fact, research on slurry fuels, consisting of conventional liquid fuels containing large percentages of various metals to significantly increase the energy density, dates back several decades [4–6]. There are, however, several issues that have so far prevented widespread use of metal particle additives in mobility fuels. For example, aluminum particles combust and release energy much slower than typical liquid hydrocarbon fuels that readily vaporize and mix with oxidizer gas. Boron, which has very high theoretical energy content of approximately 138 kJ/cm³, is more refractory than aluminum, which makes ignition even more difficult and prevents gasification during combustion.

Recent research has been devoted to study of nanoscale particles, not only to alleviate the ignition and combustion issues discussed above but also to make it easier to suspend the particles in liquid fuel. Studies of nanoboron have shown evidence of decreased ignition times, but have not shown convincing evidence that the particles are releasing their theoretical energy content [7–11]. There also has been considerable research on nano-aluminum (nano-Al), including in suspensions with liquid fuels [12–15]. One of the main drawbacks of nano-Al is the passivation layer (oxide shell) that reduces the active metal content, which is a significant issue for all nanoparticles, since the oxide shell becomes an increasing portion of the overall fuel mass as particle size is reduced.

We have taken a novel approach to creating nanoparticles with high energy content and favorable combustion characteristics. Using a sonochemical synthesis process, we have developed mixed-metal materials that incorporate Ti, Al, and B into amorphous nanoparticles, and after low-temperature heat treatment at 150 °C under vacuum, these RMNPs are highly air-stable and can have energy content as high as 34 kJ/g or 85 kJ/cm³ [16]. Furthermore, we have shown these RMNPs to combust well when entrained in a methane–air flow in an aerosolized-powder burner [17] and when suspended in decane in a liquid-fuel spray burner [18,19]. In the present work, we examined the combustion behavior of liquid/powder composite fuels consisting of RMNPs suspended in JP-5 in a single-cylinder diesel engine. Suspensions of nano-Al in JP-5 also were tested for comparison. Analysis of in-cylinder pressure data showed that both additives led to shorter ignition delays, retarded peak pressure locations, lower maximum heat-release rates, and longer burn durations, although the effects were modest. In subsequent engine testing, RMNPs led to increased CO₂, CO, and NO_x emissions and a slightly higher fuel consumption rate, relative to baseline JP-5; these effects were attributed to fouling of the injector tip and orifices by unburned RMNP deposits. For nano-Al, in contrast, emissions levels were similar to those for JP-5 and fuel consumption rate was approximately 17% lower. Unfortunately, within less than 5 min nano-Al particles agglomerated in the base of the fuel reservoir, thereby preventing flow to the fuel pump and stalling the engine. The results suggest that metal additives can, in fact, provide additional useful energy in an engine, but the liquid–solid powder fuel mixtures would require formulation improvements for long-term suspension stability and the engine fuel system would need to be adapted to these new fuels.

RMNPs were synthesized using sonochemical reaction methods previously described in detail [16,20,21], thus only a brief description is provided here. Both LiAlH₄ (LAH) and LiBH₄ (LBH) are dissolved in diethyl ether (Et₂O) and slowly added to a reaction flask containing TiCl₄ also dissolved in Et₂O. With the flask held in an ultrasonic bath, the reaction is allowed to proceed for 18 h under sonication while byproduct H₂ gas is continuously vented. The entire process is carried out under inert N₂ atmosphere. The RMNPs, black in appearance, are removed from the reaction flask, dried, washed with tetrahydrofuran (THF) to remove the LiCl byproduct, and heat treated at 150 °C under vacuum (10–20 mTorr) for 16–18 h. Low-temperature heat treatment makes the RMNPs more air-stable while retaining as much energy content as possible. Reaction stoichiometry can be controlled to produce RMNPs with various Al:B ratios; for this work, the product RMNPs were tuned to contain approximately 20 at % each Ti and B, and approximately 10 at % Al, as well as 20 at % H, 10 at % C, with the balance being Li and Cl. The elemental composition was determined using combustion gas analysis and inductively coupled plasma-optical emission spectroscopy [16]. RMNPs treated at 150 °C typically have energy content, as determined by bomb calorimetry, of approximately 30 kJ/g or 75 kJ/cm³, although higher energy content has been retained in some cases [16]. Finally, a cryogenic ball mill is used to grind the larger flakes into nanoscale particles. Measurements in a dynamic light scattering (DLS) instrument show that the milled powder has a relatively narrow size distribution with average particle diameter of approximately 380 nm.

Suspensions were made by mixing RMNPs with jet fuel JP-5, and using a nonionic surfactant (sorbitane monooleate, sold commercially as Span^® 80) to disperse the powders. JP-5 was chosen as the base fuel primarily because it was readily available in large quantity, and it performs well as a compression-ignition fuel. The mass ratio of surfactant to powder was 0.5, as higher surfactant levels did not substantially improve the suspension stability. A high-shear homogenizing mixer was used for 15 min to improve dispersion and break up any powder agglomerates. Light-extinction experiments, although only qualitative, showed that larger particles settled to the bottom within minutes while smaller particles appeared to remain suspended indefinitely. When ready for testing, each suspension was hand-swirled and poured into the glass burette that serves as the engine fuel reservoir. It was assumed that vibration of the engine and fuel reservoir would aid in maintaining stable suspensions.

The same procedure was used to create suspensions of commercial nano-Al powder (purchased from SkySpring) in JP-5, with the same surfactant fraction. The nano-Al powder used contained 80.7% active metal content, as determined by bomb calorimetry, with the remainder already oxidized [16]. Similar to the RMNPs, DLS measurements showed that the nano-Al powder has a narrow size distribution with average particle diameter of approximately 370 nm. For both RMNPs and nano-Al, engine tests were conducted for suspensions containing additive weight fractions of 1%, 2%, 4%, and 8%.

Two engines were used in this study, one to examine the effect of the additives on basic combustion behavior and the other to measure fuel consumption rates and emissions. Small single-cylinder engines were chosen primarily to minimize fuel consumption, since RMNPs are highly specialized materials that are synthesized in very small quantities. The engine used for the basic combustion study was a Yanmar L48V four-stroke single-cylinder diesel engine rated at 3.5 kW at 3600 RPM. The engine used for the consumption and emissions measurements was a Yanmar L100V four-stroke single-cylinder diesel engine rated at 6.2 kW at 3600 RPM. Specifications for both engines are listed in Table 1. Both engines utilize Bosch Unit Pump Systems with injector opening pressure of nominally 200 bar, and nominally fixed start-of-injection (SOI) timing. Each engine was equipped with a fast in-cylinder pressure sensor (Kistler 6052C in the L48V; AVL GH14D in the L100V) and a high-pressure injection line sensor (Kistler 4067C2000) located at the fuel injector inlet. Further equipment details can be found in previous papers [22,23].

The L48V is coupled to a Dyne Systems Midwest MW66 dry gap eddy-current dynamometer controlled by an Inter-Lok control unit using constant speed control while varying the fuel mass injected to vary load for this study. Fuel mass injected varies with duration of injection (DOI), which is controlled by changing end-of-injection (EOI) timing while maintaining constant SOI. The L100V is in a genset configuration and is mechanically governed to a constant speed of 3600 RPM. A portion of the exhaust from the L100V is pumped through a portable emissions analyzer (Infrared Industries FGA4000XDS) to measure total (unburned) hydrocarbons (THC), CO₂, CO, O₂, and NO_x. According to the manufacturer, exhaust gas emissions measurements have accuracy of ±1% of full scale, where full-scale is 10,000 ppm for THC, 20% for CO₂, 10% for CO, 25% for O₂, and 5000 ppm for NO_x. Another portion of the exhaust is pumped through a long stainless steel line (to cool the sample) and a fiberglass filter to collect particulates for analysis.

The first set of experiments was the basic combustion study in the Yanmar L48V engine. Suspensions of RMNPs and nano-Al were tested for additive mass fractions of 1–4%. When interpreting results, however, the order in which the tests were done is important. Tests with nano-Al were conducted first, followed by tests with RMNPs. Table 2 shows the order in which tests were done for each additive. Testing was also attempted with a suspension containing 8% nano-Al, but the crankshaft broke during these tests. Also, although the surfactant fraction varied with the fraction of nano-Al or RMNPs (to maintain the constant 0.5 ratio of surfactant to metal additive), we tested only the highest surfactant mass fraction of 4% (corresponding to 8% additive) to assess the effect of the surfactant alone. For both additives, the engine was run at a constant speed of 3000 RPM and load was varied from low to high. At each of five load levels, after conditions equilibrated data were recorded for 30 s, or approximately 750 cycles for the engine speed of 3000 RPM.

The second set of experiments involved measurement of fuel consumption rate and emissions for the Yanmar L100V engine. In this portion of the study, the engine was operated at constant speed of 3600 RPM and at a nominally constant midload condition with gross indicated mean effective pressure (gIMEP) of approximately 6.6 bar on average. Furthermore, only the single weight fraction of 8% was tested for both RMNPs and nano-Al. Table 3 shows the order in which these tests were performed. For each fuel (or fuel suspension), the amount of time required for the fuel volume in the reservoir, which is actually a graduated burette, to decrease from 225 mL to 50 mL was determined using a stopwatch in order to calculate average fuel consumption rate. During that time, pressure data were recorded for 30 s (approximately 900 cycles for the engine speed of 3600 RPM) at 1 min, 3 min, and 5 min after start of the consumption measurement. Also during that time, emissions levels were recorded every 1 min, with a single representative value of each species read from the Infrared Industries portable analyzer and written down at each interval. Finally, particulates were collected on a fiberglass filter for approximately 1 min in the middle of each fuel test, with a new filter used for each case.

Both engines are outfitted with shaft encoders that provide resolution of 1 crank angle degree. Fast in-cylinder pressure, fuel pressure, and crank angle data were collected at 50 kHz during all tests in this study using National Instruments hardware and matlab-based data acquisition. The 50-kHz acquisition rate is equivalent to a resolution of 0.36 crank angle degrees at an engine speed of 3000 RPM and 0.43 crank angle degrees at 3600 RPM. Following testing, a single zone heat release analysis [24,25] was applied to the data to characterize start of combustion (SOC) and burn duration (10–90%). In this first-law analysis, the ratio of specific heats (γ) is treated as a linear function of the bulk cylinder temperature. Bulk temperature is determined from the ideal gas law, with volume calculated using the standard slider–crank equation. SOI was determined from the fuel injector pressure sensor. SOC was determined analytically as the crank angle at which 10% of the cumulative heat release has occurred. Ignition delay was the difference between SOI and SOC.

In-cylinder pressure measurements are shown in Fig. 1 for midload conditions, where each trace is from a single representative cycle at that condition. Although modest, an increasing delay in combustion was seen with increasing nano-Al fraction. As testing with 8% nano-Al (not shown) was nearing completion, the crankshaft broke at the coupler to the dynamometer. The bottom portion of the engine was rebuilt with a new crankshaft, and testing resumed with the RMNP suspensions. Figure 1(b) shows that, after the rebuild, peak pressures were slightly lower due to a slightly lower compression ratio. Performing a baseline JP-5 test both before and after the rebuild, however, made it possible to examine the relative effects of each additive with respect to its own baseline. Peak pressure locations (PPLOCs), however, remained nearly the same, which is expected since overall combustion phasing is typically fairly stable for mechanically injected direct-injection diesel engines. Also, although Fig. 1 only shows one load condition, we found that PPLOC was relatively insensitive to load. Similar to nano-Al, Fig. 1(b) shows a modest delay in combustion for RMNPs relative to the base fuel. The lack of clear trend with respect to RMNP content is likely due to buildup of injector deposits, as discussed below.

Cycle-averaged combustion metrics derived from in-cylinder pressure data are shown in Fig. 2 as functions of engine load. Ignition delay (IGD) results are shown in the top row. In general, it can be seen that IGD lengthened with increasing load, likely due to the increasing importance of charge cooling with increasing injected fuel mass. IGD modestly shortened at very high loads, likely due to the increased thermal state of the engine at the highest load raising the overall engine and charge temperatures. RMNP results showed a moderate overall lengthening of IGD relative to the nano-Al results, noting that the bottom portion of the engine was rebuilt between nano-Al and RMNP testing. A more detailed analysis shows that, after the rebuild, in-cylinder pressure at top dead center (TDC) was on average 1.5 bar lower than before. The lower pressure at TDC resulted in slightly cooler temperatures at end of compression, and thus moderately longer IGDs.

Figure 2 shows that the base fuel generally resulted in the longest IGD over the entire load range. IGD decreased slightly due to the surfactant, and continued to decrease with increasing nano-Al content. Thus, the surfactant and nano-Al both appear to have a modest cetane increase effect. IGD also modestly shortened with RMNPs in the fuel. At the highest RMNP level of 4%, IGD increased toward the baseline level. This was due to lower compression and in-cylinder pressure at TDC, which was approximately 1 bar lower than for the baseline and 1% and 2% RMNP cases. As the 4% case was run in-between the other two RMNP cases, the cause of this behavior is unclear.

Peak pressure location (PPLOC) is shown in the second row of Fig. 2. In general, it can be seen that the peak pressure location was relatively insensitive to load, particularly for the nano-Al measurements. Thus, despite the fixed SOI timing, the bulk combustion phasing of the engine was generally stable. The PPLOC trends were the opposite of the IGD trends with respect to increasing nano-Al content. IGD shortened with increasing nano-Al fraction, leading to less fuel–air premixing before SOC. The result was a longer overall burn duration (classic diffusion phase), as will be seen below. This extension of burn duration then moves the bulk combustion phasing later by a few degrees for the nano-Al suspensions. Although the behavior is more scattered for the RMNPs, similar overall trends were observed, namely, later PPLOC (i.e., bulk combustion phasing) resulting from shorter IGDs.

The third row of Fig. 2 shows the cycle-averaged maximum rate of heat release (maxROHR). Instantaneous heat release rate, also known as apparent heat release rate because it does not distinguish heat release from other heat transfer effects, was evaluated over the entire burn duration to find the maximum for each operating condition. In general, it can be seen that maxROHR increased with load, which is expected since more fuel was injected with increasing load. MaxROHR decreased with increasing nano-Al content, likely due to shorter IGDs with the additives. Shorter IGD leads to less initial fuel–air premixing, and thus a longer, less intense combustion event. MaxROHR was lower for RMNPs than for baseline JP-5 at all loads. The trends with respect to RMNP content, however, are scattered and not monotonic, most likely due to buildup of injector deposits as discussed below.

Cycle-averaged burn duration results are shown in the bottom row of Fig. 2. Burn duration was defined as the crank angle duration from the point of 10% to the point of 90% cumulative fuel burned. Burn duration initially shortened with increasing load due to lengthening IGDs and thus more fuel–air premixing. With increasing load, burn duration lengthened due to the increasing importance of more fuel and energy release with higher loads. With increasing nano-Al content, burn duration lengthened across the load range due to shorter IGDs that led to longer combustion events, since there was less fuel–air premixing before SOC. Similar trends were observed for the RMNP suspensions, although changes in burn duration were larger than for nano Al. The 1% RMNP case (tested last), however, had the longest burn durations. This is believed due to the buildup of injector tip deposits that led to poor spray characteristics in the combustion chamber.

In general, the trends in all combustion metrics were monotonic with respect to nano-Al content and all differences were small, especially when taking into account the variance in the data (i.e., the size of the error bars). For RMNPs, however, differences were larger and behavior was more erratic. At the conclusion of RMNP testing, which was also the end of all testing for this portion of the overall study, the injector was removed from the engine. Inspection of the injector revealed the presence of black deposits on the injector tip, specifically in the area of the orifices, which suggests agglomeration and clogging of powder material. Since the injector was only inspected at the conclusion of all testing, it is not clear whether the source of the deposits was nano-Al, RMNPs, or both. The erratic nature of the combustion behavior for the RMNP tests, however, coupled with the fact that the deposits were the same color as unburned RMNPs (black), rather than unburned nano-Al (gray) or combusted metal oxides (white), suggests that agglomerated and unburned RMNPs were the source of the issue. Therefore, it is not clear whether the observed differences in combustion behavior in the RMNP tests were due to real effects of RMNP combustion or simply due to deterioration of fuel-spray characteristics and fuel–air mixing due to agglomeration and deposit formation.

In-cylinder pressure measurements are shown in Fig. 3, where each trace is from a single representative cycle during a 30 s acquisition initiated at the indicated time after start of the relevant consumption test. For example, the trace labeled “3 min” in Fig. 3(a) was recorded during a 30 s acquisition period that began at 3 min after the start of the RMNP consumption test. It is immediately clear from these data that RMNPs caused issues with operation of the engine. Whereas the pressure traces for JP-5 and 8% nano-Al were highly consistent over time, there were significant changes for the 8% RMNP case, particularly between 3 min and 5 min.

Corresponding emissions measurements are shown in Fig. 4. The trends that are most obvious and noteworthy are those for O₂, CO₂, and CO. During the time when the RMNP suspension was fueling the engine, O₂ dipped from approximately 11% to 9%, suggesting a transient increase in fuel that consumed a portion of the excess O₂. At the same time, CO₂ increased from approximately 710 g/kW hr to 860 g/kW hr due to the increased delivery of hydrocarbon fuel (JP-5) and thus increased CO₂ production. The large increase in CO is attributed to greater incomplete combustion due to enrichment from the apparent surge in fuel delivery. During the subsequent re-establishment of the baseline with JP-5, followed by nano-Al testing, O₂, CO₂, and CO emissions returned to their baseline levels. Similar to the pressure data (Fig. 3), this suggests that the nano-Al additives did not cause the same problems as the RMNPs. Trends in NO_x and total (unburned) hydrocarbons (THCs) were less obvious and perhaps not entirely related to the fuels or fuel additives. These trends are at least partially attributed to thermal effects, considering that the engine was shut down and allowed to cool for a significant period of time to examine the injector after the RMNP tests. Upon subsequent startup, lower temperatures in the cylinder wall likely led to lower thermal NO_x production, while also causing an increase in THC emissions due to increased quenching of the flame at the wall.

Because of the abnormalities in the RMNP tests, the engine was stopped and the injector inspected before moving on to nano-Al tests. As discussed in detail below, black deposits were once again found on the injector tip. The injector was cleaned and reinstalled and engine testing continued with JP-5 to re-establish normal behavior in the engine and obtain new baseline data. The emission results in Fig. 4 show that the engine was still experiencing issues at the end of this second JP-5 test, but testing with nano-Al was conducted nevertheless. Whether because of the nano-Al or simply by coincidence, emission measurements quickly returned to the levels observed for the first JP-5 test. Considering the consistency of the pressure traces throughout the second JP-5 test and the nano-Al test (see Fig. 3), it is possible that the abnormal emissions during the second JP-5 test were a residual effect of the RMNP testing combined with a long residence time in the exhaust sampling system.

At the conclusion of nano-Al testing, the engine was stopped and the injector once again removed and inspected. Photographs of the injector tip are shown in Fig. 5, both after RMNP testing and nano-Al testing. A second injector, which was used in tests with standard diesel fuel (F-76), is shown in both photographs for comparison. The RMNPs clearly led to significant buildup of black deposits on the injector tip, including around the fuel-spray orifices. In contrast, the nano-Al did not cause any significant deposits that would cause problems for the fuel sprays, although there does appear to be a thin layer of grayish-white material on the injector tip. This material is either Al₂O₃, which would indicate nano-Al combustion, or unburned nano-Al powder.

Combustion metrics, shown in Fig. 6, were examined to determine what the effects of the additives were on combustion behavior in this portion of the study. Similar to the results of the basic combustion study (see Fig. 2), only small changes in most of the combustion metrics were observed in these tests. There was a slightly increasing trend in gIMEP (i.e., load), but the overall standard deviation was approximately 0.10 bar, which represents only 1.6% of the average gIMEP value of 6.62 bar. RMNPs led to later PPLOCs and longer burn durations, but these changes certainly could have been caused by poor spray characteristics. Other than for the RMNPs, PPLOC was nominally constant as expected for this engine. For the 8% nano-Al suspension, however, IGD was shorter, burn duration was longer, and maxROHR was lower. Although these changes were small, the trends were consistent with observations in the basic combustion study (see Fig. 2).

The primary objective of the L100V study was to determine if the additives offer any advantage in terms of fuel consumption rate. Thus, the amount of time required for the engine to consume 175 mL of fuel was measured and an average rate of consumption was calculated for each fuel. Each consumption measurement took nominally 5 min, with the exact time depending on the fuel. Results are shown in Fig. 7, where each value represents a single measurement taken simultaneously with the emissions data shown in Fig. 4 and the combustion data shown in Fig. 6. As expected, 4% Span^® 80 in JP-5 was consumed at nominally the same rate as JP-5 itself. For the 8% RMNP suspension, however, consumption rate was ∼5% higher, despite the higher volumetric energy density. In concert, the emissions (Fig. 4), combustion (Fig. 6), and consumption (Fig. 7) measurements suggest that the primary effect of the RMNPs was buildup of agglomerated powder deposits on the injector that degraded the spray characteristics and combustion behavior. Unfortunately, it is not clear if any RMNPs actually made it into the combustion chamber and burned. Analysis of the exhaust collection filter using X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), Raman spectroscopy, and Fourier transform infrared (FTIR) spectroscopy did not show any evidence to suggest the presence of oxidized products of RMNP combustion, such as TiO₂ or Al₂O₃. It is noted, however, that the collection system was not tested or optimized prior to this study.

As shown in Fig. 7, consumption rate further increased for the second JP-5 test. It is not clear why this occurred, since CO₂, CO, and O₂ emissions improved (see Fig. 4), combustion metrics returned almost to the same values as for the first JP-5 test (see Fig. 6), and in-cylinder pressure traces were normal (see Fig. 3). On the other hand, THC emissions increased significantly when this second JP-5 test was started and remained high throughout the remainder of the measurements, indicating that something was abnormal in the engine operation. Regardless, the most compelling result was observed with the subsequent nano-Al test, which showed a fuel consumption rate that was ∼17% lower than the first JP-5 measurement. Interestingly, the expected volumetric energy density of the nano-Al suspension is only 7% higher than that of JP-5. This suggests that the particles might have rapidly settled at the bottom of the fuel burette, resulting in an actual nano-Al concentration that far exceeded the nominal value of the suspension. Such behavior is not surprising, since we have shown in other work, in which similar suspensions were studied in a liquid-fuel spray burner, that the concentration of metal delivered to the burner varied considerably from the nominal value over the course of an experiment [19]. The likelihood of rapid particle settling is further supported by the fact that an agglomerated plug of nano-Al powder apparently became lodged in the bottom of the fuel burette and obstructed flow to the fuel pump. Approximately 4.5 min into the measurement of fuel consumption rate, the engine stalled due to lack of fuel flow. Thus, the consumption experiment was unexpectedly shortened and the fuel volume consumed was limited to 125 mL for the nano-Al test.

Regardless of the issue with suspension stability and the shortened test, experimental evidence seems to show that nano-Al actually combusted and provided a significant energy benefit to the engine. Analysis of the exhaust collection filter showed evidence of Al₂O₃, which supports the conclusion that nano-Al burned in these experiments. Given that RMNPs performed at least as well as nano-Al in both an aerosolized-powder burner [17] and a liquid-fuel spray burner [18,19], it is likely that RMNPs would burn and provide an energy benefit in the diesel engine that is similar to or greater than that of nano-Al if the RMNPs were adequately delivered through the fuel system and fuel injector without agglomerating and building up deposits.

Sonochemically generated, amorphous, RMNPs that combine Ti, Al, and B were incorporated into suspensions with JP-5, with surfactant (Span^® 80) to aid in dispersion. These RMNP suspensions, as well as nano-Al suspensions for comparison, were tested in a Yanmar L48V single-cylinder diesel engine to examine how the additives affect combustion behavior, for additive concentrations up to 4% by weight. In addition, RMNP and nano-Al suspensions with 8% additive by weight were tested in a Yanmar L100V single-cylinder diesel engine to examine how the additives affect emissions and fuel consumption rate. Below are the important results from this work:

• (i)

  Though differences were small, both RMNP and nano-Al suspensions resulted in shorter ignition delays, retarded peak pressure locations, decreased maximum rates of heat release, and longer burn durations compared with baseline JP-5.

• (ii)

  For nano-Al, differences in combustion characteristics increased monotonically with increasing additive concentration. For RMNPs, differences were more scattered and trends were not monotonic with additive concentration.

• (iii)

  RMNPs caused an increase in NO_x, CO₂, and CO and a decrease in O₂ in the exhaust compared to results with baseline JP-5. In contrast, nano-Al caused no significant changes in the emissions levels compared to those with JP-5, with the exception of total (unburned) hydrocarbons (THCs). The increase in THC emissions, which began with a second JP-5 baseline test immediately before nano-Al testing, was likely caused by increased flame quenching at the cylinder wall, since the engine was stopped and allowed to cool after the RMNP test.

• (iv)

  Compared to baseline JP-5, an 8% RMNP suspension resulted in a 5% increase in fuel consumption rate, while an 8% nano-Al suspension resulted in a 17% decrease in the fuel consumption rate. The 17% decrease for nano-Al was nearly 2.5× the expected benefit, since the volumetric energy density was only 7% higher than for JP-5. There was nearly zero change for a mixture of 4% Span^® 80 in JP-5.

• (v)

  After RMNP testing in both engines, inspection of the injector revealed significant presence of black deposits. Inspection of the injector after nano-Al emissions/consumption testing in the L100V revealed no significant deposit buildup.

The most important conclusion from this work is that high-energy metal-based additives can burn within the short time scales required to provide an energy benefit in a diesel engine. Although there were problems with suspension stability, nano-Al resulted in a significantly decreased fuel consumption rate with minimal impact on combustion characteristics or emissions. These results, combined with comparison studies of nano-Al and RMNPs in laboratory burners, suggest that RMNPs have the potential to provide a greater energy benefit in an engine than nano-Al, if the issues of RMNP agglomeration and clogging can be overcome.

We thank the Office of Naval Research for financial support under the Naval Research Laboratory (NRL) Base Program, as well as the National Research Council for postdoctoral support. Dr. Weismiller and Dr. Huba performed the research as National Research Council postdoctoral associates under Contract No. N00173-13-2-C002 with the Naval Research Laboratory. Contributions of Dr. Fisher, Dr. Epshteyn, and Dr. Cowart were performed as part of their duties as employees of the U.S. Government.

degATC =

degrees after top (dead) center

DLS =

dynamic light scattering

gIMEP =

gross indicated mean effective pressure

IGD =

ignition delay

maxROHR =

maximum rate of heat release

PPLOC =

peak pressure location

RMNP =

reactive mixed-metal nanopowder

RPM =

revolutions per minute

SOC =

start of combustion

SOI =

start of injection

TDC =

top dead center

THC =

total (unburned) hydrocarbons