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Additive Manufacturing (AM) using stereolithography (SLA) was applied to produce engine O-rings using two different flexible polymer printing materials, Flex 80A and Elastic 50A. Print orientation of the O-ring in the SLA 3D printer is important, with the horizontal configuration most commonly providing for the smoothest final O-ring printed surface due to the lack of printing support tabs required. AM printing tabs lead to O-ring ‘marks’ (non-smooth surfaces) that were evaluated using the Society of Automotive Engineers SAE AS871B standard. It was seen that numerous printing approaches produced ‘marks’ that were larger than acceptable, which shows that these studied AM processes can not replace traditional methods of O-ring manufacture. However, further evaluation was pursued to explore possible remote emergency usage of these O-rings. Printed O-rings were next tested-soaked in engine related fluids in order to characterize O-ring swelling behavior.

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.

Partially Premixed Combustion (PPC) approaches were applied in a single cylinder diesel research engine in order to characterize engine power improvements. PPC (dual fuel) and PPC (single fuel) are alternative advanced combustion approaches that generally result in lower engine-out soot and NOx emissions, with a moderate penalty in engine-out unburned hydrocarbon (UHC) and carbon monoxide (CO) emissions. In this study, PPC was accomplished with a minority fraction of fuel (isobutanol, iso-octane and jet JP-5) injected into the intake manifold, while the majority fraction of jet JP-5 fuel was delivered directly to the combustion chamber near the start of combustion (SOC). Four compression ratios (CR) were studied. Exhaust emissions plus exhaust opacity and particulate measurements were performed during the experiments in addition to fast in-cylinder combustion metrics.

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.

The US Navy is in the process of evaluating Catalytic Hydrothermal Conversion Jet fuel (CHCJ-5) for inclusion in the JP-5 specification, MIL-DTL-5624, and evaluating Catalytic Hydrothermal Conversion Diesel fuel (CHCD-76) for inclusion in the F-76 specification, MILDTL-16884. CHC fuels are produced from renewable feedstocks such as triglycerides, plant oils, and fatty acids. A Catalytic Hydrothermolysis process chemically converts these feedstocks into a mixture of paraffins, cycloparaffins, aromatics, olefins, and organic acids. The resulting mixture is then hydroprocessed and fractionated to produce a kerosene (or diesel) product having a distillation profile comparable to traditional petroleum derived fuels. The end product is a fuel that is able to meet the jet (or diesel) chemical and physical MIL-SPEC requirements without blending with conventional petroleum fuels.

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.

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.

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.

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.