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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.

Optimization of in-cylinder air-fuel mixture preparation in Port Fuel Injected (PFI) engines during all phases of operation is critical for maximizing engine performance while minimizing harmful emissions. In this study, a Cooperative Fuels Research (CFR) gasoline engine is used to evaluate torque and measure in-cylinder and exhaust CO, CO2 and unburned hydrocarbons under various fueling and spark conditions during crank and startup phases. Fast Flame Ionization Detectors (FFID) and Non-Dispersive Infra-Red (NDIR) fast CO and CO2 detectors are used as the principle diagnostics. Additionally, detailed cycle resolved fuel accounting is performed to elucidate the fuel vaporization process from injection to exhaust. The majority of liquid fuel accumulation in the engine puddles occurs within 3 engine cycles after cranking begins. Post combustion UHCs were seen to reach levels of 40-80% of pre-combustion UHC values.

Ethanol based fuels have seen increased use in recent years due to their renewable nature as well as increased governmental regulatory mandates. While offering performance advantages over gasoline, especially at high compression ratios, these fuels are more sensitive to pre-ignition (PI). Pre-ignition experiments using ethanol (E100) and E85 were performed in a CFR spark ignition engine using a diesel glow plug “hot spot” to induce PI. PI is found to occur over a specific air-fuel ratio range based on hot spot temperature. Additionally, increasing ethanol content or compression ratio (CR) decreases glow plug temperature thresholds for PI. A kinetics-based model was used to simulate pre-ignition of E100 and to elucidate sensitivities of pre-ignition to various operating parameters, including initial charge temperature, air dilution, and residual dilution. The model shows that the most violent cases of PI can be mitigated by switching to either lean or rich operation.

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.

Seven biofuel-diesel fuel configurations were tested in a single-cylinder research diesel CFR engine that allowed variable injection timing. These seven configurations included three biodiesel-diesel blends (20% and 100%); two ethanol-diesel blends (15% and 20%), and two cases in which ethanol was injected into the intake air flow (20% and 33%). Combustion characteristics, NOx emissions, and soot emissions were compared with diesel operation across a range of injection timings. The effect of fuel compressibility affected the timing of injection, with biodiesel-diesel blends having advanced injection and ethanol-diesel blends having delayed injection. Biodiesel-diesel blends showed reduced ignition delay with only modest changes in combustion duration, while ethanol-diesel mixtures showed longer ignition delay but much shorter combustion duration and earlier phasing.

In the near future increasing use of ethanol in motor fuels will occur due to legislative mandates. E10 (Gasohol) and E85 will see more widespread use in spark ignition engines. This study looks at the performance and knock characteristics of E10 and E85 in comparison to regular gasoline. Detailed experimental engine data and analysis as a function of compression ratio, ignition timing and fueling are presented with associated physical explanations. Comparative results are presented. Increasing ethanol content provides for greater engine torque, efficiency and knock tolerance, yet fuel consumption worsens. Knock limited trends and sensitivities are presented, for example, 5 degrees of spark retard are required with E10 and gasoline for each compression ratio increase, while the much less sensitive E85 requires only 2 degrees of retard for each compression ratio increase. Trends with efficiency and torque are described amongst the fuels tested.

Intake tuning is a relatively simple alternative to turbochargers and superchargers as a means of augmenting engine performance. Capitalizing on air flow harmonics at specific engine speeds, intake tuning forces more air into the engine cylinders, resulting in greater torque and power. Concepts such as Helmholtz Resonance Theory and Reflective Wave Theory help to describe the physical phenomena that contribute to intake tuning, but previous studies have generally found that computer models utilizing computational fluid dynamics (CFD) are needed to accurately predict performance effects. The current research involves testing various intake runner lengths and cross section geometries on a Honda CBR600 F4i gasoline engine typically used to power a Formula SAE car. Also, the effect of adding 180 degree bends to intake runners is evaluated.

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.

Fischer-Tropsch (FT) synthetic fuels have been shown to produce lower soot and oxides of nitrogen emissions than petroleum-based diesel #2 (D2) in previous studies. This performance is frequently attributed to the very low aromatic content as well as essentially zero sulfur content. The objective of this empirical study was to investigate the high engine load regime using a military FT and D2 fuel in a CFR diesel engine at fueling levels approaching stoichiometric. A testing matrix comprised of various injection advance set points, fueling amounts (e.g. load) above 6 bar gross indicated mean effective pressure (IMEPg), and three different compression ratios (CR) was pursued. The results show that oxides of nitrogen emissions are always equal to or lower running FT compared to diesel. This result is attributed to the higher cetane number of FT leading to lower peak in-cylinder pressures as compared to D2.

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.

Universal Exhaust Gas Oxygen (UEGO) sensors are widely used in engine exhaust streams to measure the recently burned air-fuel (A/F) mixture ratio. UEGO sensors are also frequently used to characterize transient A/F behavior during throttle or fuel variations. This study experimentally compares a UEGO exhaust based sensor response to that obtained from an in-cylinder Fast Flame Ionization Detector (FFID) during engine transients. UEGO sensor transient response time is seen as a limitation to the type of transient measurement that can be accurately characterized. A brief comparison of conventional transient fuel x-τ model parameters based upon the different measurement techniques shows that the transient fuel compensation calibration can be a function of measurement technique and method.

Internal combustion engine knock has limited compression ratios of spark ignition engines for most of the history of gasoline engines. This limitation continues to exist today. While knock is generally a low engine speed, high load phenomenon, this operating condition is infrequently used by many vehicle operators, and if the engine is brought to this operating condition generally little time is spent in this knock prone condition. This study seeks to investigate the transition into knock due to throttle changes from part to full load. The experimental results using a CFR engine operating on iso-octane fuel show that knock is delayed by at least one high load engine cycle after the throttle is opened. Optimization of spark timing to account for this effect provides for the best increase of engine load without audible knock occurring.

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.

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.

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.

Cranking and startup fuel control has become increasingly important due to ever tightening emission requirements. Additionally, engine-off strategies during idle will require substantially more engine startup events with the associated need for very clean starts. Thus, knowledge of an engine's Air-Fuel Ratio (AFR) during its early cycles is necessary in order to optimize cranking and startup fueling. This paper examines and compares two methods of measuring an engine's AFR during engine startup (approximately the first second of operation); an in-cylinder technique using a Fast Flame Ionization Detector (FFID) and the conventional exhaust based Universal Exhaust Gas Oxygen (UEGO) sensor method. Engine starts using a Ford Zetec engine were performed at three different temperatures (0, 20 and 90 C) as well as different initial engine starting positions.

The oil aeration process in the crankcase of a V6 engine was visualized with a high-speed video camera and a borescope at engine speeds up to 4500rpm and oil temperatures to 70°C. The video images showed that a high-speed oil droplet stream was flung from the crankshaft counterweight, passed through the oil drainage gap in the windage tray and struck the free surface of the oil in the sump. There was a speed threshold beyond which foam was observed to form on the oil surface. When the engine speed was reduced to below that threshold, the foam disappeared. When the windage tray was removed, there was no foam formation. The results suggested that foam formation was the net result of the balance between the air bubble formation and destruction processes. These two processes were speed dependent.

Bench fuel injection experiments were performed to investigate the levels of generated fuel vapor immediately after fuel injection into a closed vessel. A synthetic fuel mixture was used consisting of six individual fuel components that are representative of gasoline. Vessel (e.g. port) temperature and pressure were varied, as well as sample location and sample delay time after injection. Vessel vapor space samples were collected and processed in a gas chromatograph in order to quantify the contribution to the fuel vapor by the various fuel components. Companion modeling was performed in order to evaluate two fuel vapor mixture preparation models (Raoult's Law and NIST's SUPERTRAPP). Results indicate that approximately 1/6 to 1/3 of the injected fuel mass is in the vapor form immediately after fuel injection (as a function of temperature). SUPERTRAPP modeling indicates that the injected fuel mass is approximately in equilibrium with 6% of the available air.