Primary & Secondary Reference Fuel Effects as a Function of Compression Ratio in a CFR Diesel Engine

Primary Reference Fuels (PRFs) and Secondary Reference Fuels (SRFs) in the range of cetane from 30 to 60 were operated in a Waukesha Diesel Cooperative Fuels Research (CFR) engine under operating conditions that emulate the cetane rating test. Due to the large number of test points in this study, the exact ASTM cetane rating protocol was not followed precisely, however these results are representative of cetane characterization testing with very similar equivalence ratio and combustion phasing across a broad range of Ignition Delays (IGDs) that varied as a result of Compression Ratio (CR) changes in the eleven to twenty-two range. Intake air temperature was operated both heated, as in the cetane rating test, as well as at ambient laboratory conditions. Additional research instrumentation was added beyond the standard CFR equipment for advanced combustion analysis. Combustion analysis shows that engine torque and efficiency increase significantly with increases in CR. At longer IGDs representative of the cetane rating test (13 deg IGD), the increase in IGD with reduced cetane number is relatively linear. For all of the fuels tested, IGD steadily monotonically decreases with increased CR significantly by more than a factor of two. Shorter IGDs lead to longer burn durations; fuel effect differences become less important at very high CRs. Associated companion analysis shows that at the time of fuel injection (Start Of Injection – SOI), cylinder pressure roughly doubles over the CRs studied, however, cylinder charge temperature only moderately increases. This effect leads to a doubling in cylinder air charge concentration at the highest CRs showing an important effect on the fast kinetics at high CRs. A common IGD correlation was evaluated showing good agreement except for the high CN fuel. New IGD correlations are also presented.

Coordination of fuel reactivity and injection timing to achieve highly efficient and stable gasoline compression ignition combustion in a wide load range

Gasoline compression ignition combustion is a new combustion mode with great development potential and is highly influenced by fuel reactivity and injection strategy. This paper coordinates the fuel octane number and single-injection timing to operate gasoline compression ignition combustion with high efficiency in a wide load range, with the speed fixed at 1500 r/min. The primary reference fuels with octane numbers 60, 70, 80, and 90 were used, labeled as PRF60, PRF70, PRF80, and PRF90, respectively. The results proved that under steady-state conditions where the speed and load changed slightly, taking the fuel economy and combustion and emission performance into account, PRF60 and PRF70 should be applied at a load lower than 2 bar and 2–8 bar, respectively, and the start of injection timing should be set at 13 °CA before top dead center. When the load is higher than 8 bar, PRF90 should be applied at the start of injection timing of 11 °CA before top dead center. It is noteworthy that PRF70 under medium-load conditions could achieve the indicated thermal efficiency of up to 47%. The injection timing of PRF90 was limited to 9–1711 °CA before top dead center due to the limit of the peak value of pressure rise rate, whereas PRF60 had a wider injection timing boundary than PRF90.

Influences of isomeric butanol addition on anti-knock tendency of primary reference fuel and toluene primary reference fuel gasoline surrogates

The anti-knock tendency of blends of butanol isomers and two gasoline surrogates (primary reference fuels and toluene primary reference fuels) was studied on a single-cylinder cooperative fuel research engine. The effects of butanol molecular structure (n-butanol, i-butanol, s-butanol and t-butanol) and butanol addition percentage on fuel research octane numbers were investigated. The experimental results revealed that butanol addition to either PRF80 or TPRF80 increased research octane numbers, and the research octane numbers of fuel blends showed higher linearity with the molar percentage than with the volumetric percentage of butanol addition. Furthermore, the research octane number boosting effects of butanol isomers were observed to change with the fuel compositions, that is, i-butanol >s-butanol >n-butanol >t-butanol for primary reference fuels and i-butanol >s-butanol >t-butanol >n-butanol for toluene primary reference fuels. In addition, butanol/primary reference fuel blends exhibited higher research octane numbers than butanol/toluene primary reference fuel blends. We thereafter tried to elucidate the underlying fuel combustion kinetics for the observed anti-knock quality of different butanol/gasoline surrogate blends. It was found that the measured research octane numbers of fuel blends showed the best correlation with the calculated ignition delay times at the thermodynamic condition of 770 K and 2 MPa, and the reaction sensitivity analysis in auto-ignition at this condition revealed that the H-abstraction reactions of butanol isomers by OH radical suppressed fuel reactivity, thus elevating the fuel research octane numbers when butanol was added to the gasoline surrogates. Compared with the butanol/primary reference fuel blends, the positive sensitive reactions related to n-heptane were of higher importance, while the inhibitive effects of sensitive reactions related to butanol and iso-octane decreased for the toluene primary reference fuel/butanol blends, thus leading to lower research octane numbers of the toluene primary reference fuel/butanol blends than those of the butanol/primary reference fuel blends.