Publications

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%.

Homogeneous charge compression ignition (HCCI) combustion with fully premixed charge is severely limited at high-load operation due to the rapid pressure-rise rates (PRR) which can lead to engine knock and potential engine damage. Recent studies have shown that two-stage ignition fuels possess a significant potential to reduce the combustion heat release rate, thus enabling higher load without knock. This study focuses on three factors, engine speed, intake temperature, and fuel composition, that can affect the pre-ignition processes of two-stage fuels and consequently affect their performance with partial fuel stratification. A model fuel consisting of 73 vol.% isooctane and 27 vol.% of n-heptane (PRF73), which was previously compared against neat isooctane to demonstrate the superior performance of two-stage fuels over single-stage fuels with partial fuel stratification, was first used to study the effects of engine speed and intake temperature. The results for PRF73 show that increasing engine speed from 1200 to 1600 rpm causes almost no change in φ-sensitivity, which is defined by the advancement of combustion phasing for an increase in equivalence ratio. Consequently, the maximum combustion pressure rise rate (PRRmax) can be reduced substantially with partial fuel stratification at this higher speed as it was at 1200 rpm. In contrast, increasing intake temperature from 60°C to 174°C eliminates the low temperature heat release of PRF73. Despite the single-stage ignition at this temperature, PRF73 still shows a weak but definitive φ-sensitivity, likely due to the relatively strong intermediate temperature heat release before hot ignition. As a result, PRR max was reduced modestly with partial fuel stratification. This PRF73 result is distinctively different from that of isooctane at the same intake temperature. To study the importance of fuel composition, PRF73 is compared with a low-octane, gasoline-like distillate fuel, termed Hydrobate, which could be readily produced from petroleum feedstocks. With the similar HCCI reactivity to PRF73, Hydrobate shows little difference in φ-sensitivity and performs similarly with partial fuel stratification compared to PRF73. This result indicates that it is the overall fuel HCCI reactivity, rather than the exact fuel composition, that determines the φ-sensitivity and the consequent performance with partial fuel stratification. © 2011 SAE International.

Abstract not provided.

Long chain alcohols possess major advantages over ethanol as bio-components for gasoline, including higher energy content, better engine compatibility, and less water solubility. Rapid developments in biofuel technology have made it possible to produce C{sub 4}-C{sub 5} alcohols efficiently. These higher alcohols could significantly expand the biofuel content and potentially replace ethanol in future gasoline mixtures. This study characterizes some fundamental properties of a C{sub 5} alcohol, isopentanol, as a fuel for homogeneous-charge compression-ignition (HCCI) engines. Wide ranges of engine speed, intake temperature, intake pressure, and equivalence ratio are investigated. The elementary autoignition reactions of isopentanol is investigated by analyzing product formation from laser-photolytic Cl-initiated isopentanol oxidation. Carbon-carbon bond-scission reactions in the low-temperature oxidation chemistry may provide an explanation for the intermediate-temperature heat release observed in the engine experiments. Overall, the results indicate that isopentanol has a good potential as a HCCI fuel, either in neat form or in blend with gasoline.

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.