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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.

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.

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.