This study systematically investigates the effects of variousengine operating parameters on the thermal efficiency of a boostedHCCI engine, and the potential of E10 to extend the high-load limitbeyond that obtained with conventional gasoline. Understanding howthese parameters can be adjusted and the trade-offs involved iscritical for optimizing engine operation and for determining thehighest efficiencies for a given engine geometry. Data wereacquired in a 0.98 liter, single-cylinder HCCI research engine witha compression-ratio of 14:1, and the engine facility was configuredto allow precise control over the relevant operating parameters.The study focuses on boosted operation with intake pressures(Pin) ≥ 2 bar, but some data for Pin< 2bar are also presented. Two fuels are considered: 1) an 87-octanegasoline, and 2) E10 (10% ethanol in this same gasoline) which hasa lower autoignition reactivity for boosted operation. This study considers several engine operating parameters,including: intake temperature, fueling rate, engine speed, fueltype, and the effect of various amounts of mixture stratificationusing three fueling methods: fully premixed, early-DI, and premixed+ late-DI (termed partial fuel stratification, PFS). The effects ofthese operating parameters on the factors affecting thermalefficiency, such as combustion phasing (CA50), amount of EGRrequired, ringing intensity, combustion efficiency, γ =cp/cv, and heat transfer are also exploredand discussed. The study showed that in general, thermal efficiencyimproves when parameters are adjusted for lower intaketemperatures, less CA50 retard, and less EGR, as long as theringing intensity is ≤ 5 MW/m2to prevent knock, andcombustion efficiency is good (i.e., ≥ about 96%). Additionally,applying a small amount of mixture stratification (using PFS orearly-DI fueling) improves efficiency by allowing more CA50 advancewhen boost levels are sufficient for these fuels to be ϕ-sensitive.E10 gives a small increase in thermal efficiency because EGRrequirements are reduced. E10 is also effective for increasing themaximum load for Pin≥ 2.4 bar, and increasing thehigh-load limit to IMEPg = 18.1 bar, with no engine knock andultra-low NOx and soot emissions, compared to IMEPg = 16.3 bar forgasoline. Overall, this study showed that the efficiencies forboosted HCCI can be increased above their already good baselinevalues. For our engine configuration, improvements of 3 - 5thermal-efficiency percentage units were achieved corresponding toa reduction in fuel consumption of 7 - 11%.
Long chain alcohols possess major advantages over the currently used ethanol as bio-components for gasoline, including higher energy content, better engine compatibility, and less water solubility. The rapid developments in biofuel technology have made it possible to produce C{sub 4}-C{sub 5} alcohols cost effectively. These higher alcohols could significantly expand the biofuel content and potentially substitute ethanol in future gasoline mixtures. This study characterizes some fundamental properties of a C{sub 5} alcohol, isopentanol, as a fuel for HCCI engines. Wide ranges of engine speed, intake temperature, intake pressure, and equivalence ratio are investigated. Results are presented in comparison with gasoline or ethanol data previously reported. For a given combustion phasing, isopentanol requires lower intake temperatures than gasoline or ethanol at all tested speeds, indicating a higher HCCI reactivity. Similar to ethanol but unlike gasoline, isopentanol does not show two-stage ignition even at very low engine speed (350 rpm) or with considerable intake pressure boost (200 kPa abs.). However, isopentanol does show considerable intermediate temperature heat release (ITHR) that is comparable to gasoline. Our previous work has found that ITHR is critical for maintaining combustion stability at the retarded combustion phasings required to achieve high loads without knock. The stronger ITHR causes the combustion phasing of isopentanol to be less sensitive to intake temperature variations than ethanol. With the capability to retard combustion phasing, a maximum IMEP{sub g} of 5.4 and 11.6 bar was achieved with isopentanol at 100 and 200 kPa intake pressure, respectively. These loads are even slightly higher than those achieved with gasoline. The ITHR of isopentanol depends on operating conditions and is enhanced by simultaneously increasing pressures and reducing temperatures. However, increasing the temperature seems to have little effect on ITHR at atmospheric pressure, but it does promote hot ignition. Finally, the dependence of ignition timing on equivalence ratio, here called {phi}-sensitivity, is measured at atmospheric intake pressure, showing that the ignition of isopentanol is nearly insensitive to equivalence ratio when thermal effects are removed. This suggests that partial fuel stratification, which has been found effective to control the HRR with two-stage ignition fuels, may not work well with isopentanol at these conditions. Overall, these results indicate that isopentanol has a good potential as a HCCI fuel, either in neat form or in blend with gasoline.