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Future synthetic diesel fuels will likely involve mixtures of straight and branched alkanes, possibly with aromatic additives to improve lubricity and durability. To simulate these future fuels, this study examined the combustion characteristics of binary mixtures of 50%, 70%, and 90% isododecane in hexadecane, and of 50%, 70%, and 80% toluene in hexadecane using a single-cylinder research diesel engine with variable injection timing. These binary blends were also compared to operation with commercial petroleum diesel fuel, military petroleum jet fuel, and five current synthetic Fischer-Tropsch diesel and jet fuels. The synthetic diesel and jet fuels showed reasonable similarity with many of the combustion metrics to mid-range blends of isododecane in hexadecane. Stable diesel combustion was possible even with the 80% toluene and 90% isododecane blends; in fact, operation with 100% isododecane was achieved, although with significantly advanced injection timing.

Primary diesel and gasoline reference fuels, along with secondary reference diesel fuels across a very broad cetane range were tested in an Ignition Quality Tester (IQT) unit using the ASTM D6890 protocol. Additionally, numerous pure component fuels across a range of hydrocarbon size and structure were evaluated. The reference fuels’ ignition delay (IGD) followed expected trends, however, the diesel PRF fuels in the low cetane range produced DCNs (derived cetane numbers) that were moderately higher (shorter IGDs) than their cetane reference values. From the perspective of fuel size, IGD shows a significant ‘shortening’ - faster nature with increased fuel carbon number. For a given carbon number fuel molecule, normal alkanes showed the ‘fastest’ IGD, with alkenes and branched alkyl aromatics leading to moderately longer IGDs. Cyclo-paraffins had the ‘slowest’ - longest IGDs. Various methods were used to determine the IGD of the various fuels.

Synthetic diesel fuels from Fischer-Tropsch or hydrotreating processes have high cetane numbers with respect to conventional diesel fuel. This study investigates diesel combustion characteristics with these high cetane fuels. A military jet fuel (JP-5 specification), a Fischer-Tropsch (FT) synthetic diesel, and normal hexadecane (C16), a pure component fuel with defined cetane number of 100, are compared with operation of conventional military diesel fuel (F-76 specification). The fuels are tested in a AM General GEP HMMWV engine, an indirect-injection, largely mechanically-controlled diesel engine. Hundreds of thousands of these are in current use and are projected to be in service for many years to come. Experimental testing showed that satisfactory operation could be achieved across the speed-load operating map even for the highest cetane fuel (normal hexadecane). The JP-5, FT, and C16 fuels all showed later injection timing.

A new alternative fuel has been tested in a number of engines and compared to conventional Navy diesel fuel performance using in-cylinder based diagnostics and brake performance comparisons. This new fuel is derived from a Direct Sugar to Hydrocarbon (DSH) process in which sugar and yeast produce a farnesene type hydrocarbon molecule (branched hydrocarbon with multiple double bonds) which is then processed into a moderately branched single alkane molecule (> 98% purity) with a moderately higher cetane number than conventional diesel fuels. This new fuel was extensively characterized and has a lower density, viscosity and bulk modulus as compared to conventional diesel fuel. These physical property differences lead to later Start of Injection times in three diesel engines (AM General GEP, Waukesha CFR and Yanmar). However, due to the increased reactivity of DSH, ignition delay is reduced - faster across most of the speeds and loads tested.

Fast emissions analysis, soot analysis, and pressure sensing is utilized to examine the first few seconds before, and after startup in a single-cylinder CFR diesel engine. The equivalence ratio, compression ratio, and injection timing are varied. The data show that UHC and CO emissions are highest at the highest and lowest fueling conditions, while NOx emissions peaked at intermediate fueling conditions. Leaner operating conditions show delayed starting but reduced ignition delay. Oil vapor causes soot emissions prior to first combustion, and soot particle size shifts higher during the first few seconds after combustion begins. Injection timing has little effect except at the leanest equivalence ratios, where a retarded injection timing increases the delay until a successful combustion event. At lower compression ratios, large IMEP oscillations occurred during startup. The data suggest possible strategies to optimize diesel startup.

This study considers the relatively high fuel consumption of small-displacement Diesel engines and seeks to improve it through thin ceramic thermal barrier coatings. A small displacement (219 cc) single-cylinder direct-injection production Diesel engine is utilized. A Ricardo WAVE simulation is developed and suggests that through simultaneous application of the coatings and reduction of compression ratio, the fuel consumption can be improved through a reduction in thermal losses. At the stock compression ratio, the application of thermal barrier coatings does not improve fuel consumption unless injection timing is carefully controlled. When injection timing is also adjusted, fuel consumption can be improved by up to 10%, particularly at low loads, with application of the thermal barrier coatings. The data show higher rates of energy release, higher peak pressures, leading to the lower fuel consumption.

Alternative diesel fuels from various renewable sources have recently been achieving high volume production status. These fuels are generally paraffinic in nature, and are notably absent of aromatic and cyclo-paraffinic hydrocarbon compounds. Combustion differences exist with these new fuels. Ignition delay and combustion duration are often different than conventional fuels leading to changes in combustion phasing and thus differences in engine brake metrics. How much of an indicated combustion change is acceptable? Currently no alternative fuel combustion acceptance criteria or metrics exist for new alternative fuels in diesel engines. In this paper a proposed set of indicated combustion acceptance criteria is presented with companion data from two new hydro-treated renewable fuels in a legacy military diesel engine. The three combustion criteria are: 1. relative change in ignition delay, 2. Angle of Peak pressure (AOP location) and 3. relative maximum rate of heat release.

A diesel engine electrical generator set (‘gen-set’) was instrumented with in-cylinder pressure indicating sensors as well as a nearby microphone. Conventional jet fuel plus high (Cetane Number CN55) and low (CN35) secondary reference fuels were operated during which comprehensive engine and acoustic data were collected. Fast Fourier Transforms (FFTs) were analyzed on the acoustic data. FFT peaks were then applied to machine learning neural network analysis with MATLAB based tools. Detection of the low and high cetane fuel operation was audibly determined with correlation coefficients greater than 98% on test data sets. Further, unsupervised machine learning Self Organizing Maps (SOMs) were produced during normal-baseline operation of the engine with jet fuel. Application of the high and low cetane fuel operational acoustic data was then applied to the normal SOM.

A new Alcohol To Jet (ATJ) fuel has been developed using a process which takes biomass feedstock to produce a branched butanol molecule. Further dehydration, reforming and hydro-treating produced principally a highly branched C12 iso-paraffin molecule. This ATJ fuel with a low cetane value (DCN = 18) was blended with Navy jet fuel (JP5) in various quantities and tested in order to determine how much ATJ could be blended before diesel engine operation became problematic (the US Navy and Marine Corps may use jet fuel in their diesel engines). Blends of 20%, 30% and 40% ATJ (by volume) were tested with jet fuel. The Derived Cetane Number (DCN) falls from 45 for the base JP5 to 38 with the 40% ATJ component blended in. Engine start performance was evaluated on two Yanmar engines and a Waukesha CFR diesel engine and showed that engine start times increased steadily with increasing ATJ content.