Joshua Lacey

Effects of refinery stream gasoline property variation on the auto-ignition quality of a fuel and homogeneous charge compression ignition combustion

The combination of in-cylinder thermal environment and fuel ignition properties plays a critical role in the homogeneous charge compression ignition engine combustion process. The properties of fuels available in the automotive market vary considerably and display different auto-ignition behaviors for the same intake charge conditions. Thus, in order for homogeneous charge compression ignition (HCCI) technology to become practically viable, it is necessary to characterize the impact of differences in fuel properties as a source of ignition/combustion variability. To quantify the differences, 15 gasolines composed of blends made from refinery streams were investigated in a single-cylinder homogeneous charge compression ignition engine. The properties of the refinery stream blends were varied according to research octane number, sensitivity (S = research octane number − motor octane number) and volumetric contents of aromatics and olefins. Nine fuels contained 10% ethanol by volume, and six more were blended with 20% ethanol. Pure ethanol (E100) and an un-oxygenated baseline fuel (RD3-87) were included too. For each fuel, a sweep of intake temperature at a consistent load and engine speed was conducted, and the combustion phasing given by the crank angle of 50% mass fraction burned was tracked to assess the sensitivity of auto-ignition to fuel chemical kinetics. The experimental results provided a wealth of information for predicting the HCCI combustion phasing from the given properties of a fuel. In this study, the original octane index correlation proposed by Kalghatgi based solely on fuel research octane number and motor octane number was found to be insufficient for characterizing homogeneous charge compression ignition combustion of refinery stream fuels. A new correlation was developed for estimation of auto-ignition properties of practical fuels in the typical HCCI engine. Fuel composition, captured by terms indicating the fraction of aromatics, olefins, saturates and ethanol, was added to generate the following formula: O I JKZ = RON − K ′ · S + κ · ( Aromatic s 2 ) ( Olefins + Saturates ) + ε · ( Aromatics · Ethanol ) . The results indicate a significantly improved estimation of combustion phasing for gasoline fuels of varying chemical composition under low-temperature combustion conditions. Quantitative findings of this investigation and the new octane index correlation can be used for designing robust HCCI control strategies, capable of handling the wide spectrum of fuel chemical compositions found in pump gasoline.

Understanding the Effect of Wall Conditions and Engine Geometry on Thermal Stratification and HCCI Combustion

Thermal stratification of the unburned charge in the cylinder has a profound effect on the burn characteristics of a Homogeneous Charge Compression Ignition (HCCI) engine. Experimental data was collected in a single cylinder, gasoline-fueled, HCCI engine in order to determine the effects of combustion chamber geometry and wall conditions on thermal stratification and HCCI combustion. The study includes a wall temperature sweep and variations of piston top surface material, piston top geometry, and compression ratio. The data is processed with a traditional heat release routine, as well as a post-processing tool termed the Thermal Stratification Analysis, which calculates an unburned temperature distribution from heat release. For all of the sweeps, the 50% burned point was kept constant by varying the intake temperature. Keeping the combustion phasing constant ensures the separation of the effects of combustion phasing from the effects of wall conditions alone on HCCI and thermal stratification.The results for the wall temperature sweep show no changes to the burn characteristics once the combustion phasing has been matched with intake temperature. This result suggests that the effects of wall temperature on HCCI are mostly during the gas-exchange portion of the cycle. The ceramic coatings were able to very slightly decrease the thermal width, increase the burn rate, increase the combustion efficiency, and decrease the cumulative heat loss. The combustion efficiency increased with the lower surface area to volume ratio piston and the lower compression ratio. Lastly, the compression ratio comparison showed a noticeable effect on the temperature distribution due to the effect of pressure on ignition delay, and the variation of TDC temperature required to match combustion phasing.Copyright

Impact of Refinery Stream Gasoline Property Variation on Load Sensitivity of the HCCI Combustion

The HCCI combustion process is highly reliant upon a favorable in-cylinder thermal environment in an engine, for a given fuel. Commercial fuels can differ considerably in composition and auto-ignition chemistry, hence strategies intended to bring HCCI to market must account for this fuel variability.To this end, a test matrix consisting of eight gasoline fuels comprised of blends made solely from refinery streams were run in an experimental, single cylinder HCCI engine. All fuels contained 10% ethanol by volume and were representative of a cross-section of fuels one would expect to find at gasoline pumps across the United States. The properties of the fuels were varied according to research octane number (RON), sensitivity (S=RON-MON) and volumetric content of aromatics and olefins.For each fuel, a sweep of load (mass of fuel injected per cycle) was conducted and the intake air temperature was adjusted in order to keep the crank angle of the 50% mass fraction burned point (CA50) constant. By analyzing the amount of temperature compensation required to maintain constant combustion phasing, it was possible to determine the sensitivity of HCCI to changes in load for various fuels.In addition, the deviation of fuel properties brought about variations in important engine performance metrics like specific fuel consumption. Though the injected energy content per cycle was matched at the baseline point across the test fuel matrix, thermodynamic differences resulted in a spread of specific fuel consumption for the fuels tested.Copyright

HCCI Operability Limits: The Impact of Refinery Stream Gasoline Property Variation

HCCI combustion is highly dependent on in-cylinder thermal conditions favorable to auto-ignition, for a given fuel. Fuels available at the pump can differ considerably in composition and auto-ignition chemistry, hence strategies intended to bring HCCI to market must account for the fuel variability.To this end, a test matrix consisting of eight gasoline fuels composed of blends made solely from refinery streams was investigated in an experimental, single cylinder HCCI engine. The base compositions were largely representative of gasoline one would expect to find across the United States, although some of the fuels had slightly lower average octane values than the ASTM minimum specification of 87. All fuels had 10% ethanol by volume included in the blend. The properties of the fuels were varied according to research octane number (RON), sensitivity (S=RON−MON) and the volumetric fractions of aromatics and olefins.For each fuel, a sweep of the fuelling was carried out at each speed from the level of instability to excessive ringing to determine the limits of HCCI operation. This was repeated for a range of speeds to determine the overall operability zone. The fuels were kept at a constant intake air temperature during these tests.The variation of fuel properties brought about changes in the overall operating range of each fuel, as some fuels had more favorable low load limits, whereas others enabled more benefit at the high load limit. The extent to which the combustion event changed from the low load limit to the high load limit was examined as well, to provide a relative criterion indicating the sensitivity of HCCI range to particular fuel properties.Copyright