Publications

Improving Efficiency and Using E10 for Higher Loads in Boosted HCCI Engines

Dec, John E.; Yang, Yi Y.; Dronniou, Nicolas D.

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%.