Cheng et al. explain that this early direct-injection strategy achieves LTC by employing more premixed, fuel-lean (global equivalence ratio of approximately 0.4) conditions as well as high EGR levels targeted at eliminating the high-temperature NO_x-formation regions associated with conventional direct-injected diesel combustion. However, other work (Martin et al.) has shown a strong connection between liquid-fuel impingement on in-cylinder surfaces and increased fuel consumption and emissions during early direct-injection, LTC operation.

In cases where liquid fuel impingement produced surface fuel films that persisted beyond the onset of combustion, “pool fires” could occur and generate high levels of soot. Increased NO_x emissions were also associated with vigorous pool fires and attributed to the near-stoichiometric regions around the fuel-rich pool-fire region. In addition, the level of pool-fire activity was linked to hydrocarbon (HC) and carbon monoxide (CO) emissions. The results emphasize the significant negative consequences of liquid-phase fuel impingement on in-cylinder surfaces during LTC operation.

—Cheng et al.

Cheng et al. investigated the addition of high-volatility fuel components as a way to reduce or eliminate in-cylinder liquid-fuel impingement and fuel-film accumulation, using a single-cylinder version of a Caterpillar heavy-duty engine modified to provide extensive optical access to the combustion chamber. They tested five blends of conventional no. 2 diesel fuel (the baseline) and a high-volatility (HV) fuel mixture of 80 vol % n-heptane with balance toluene with approximately the same ignition quality as the baseline fuel over a range of injection timings at steady-state speed-load operating conditions.

The high-volatility fuel was denoted HV100, while the baseline diesel fuel was denoted HV0. The blends of intermediate fuels with varying levels of high-volatility content (HV content) were also prepared by mixing 28, 56, and 78 vol % HV100 in HV0—denoted as HV28, HV56, and HV78, respectively.

Diagnostics included conventional heat-release analysis; the measurement of spatially integrated broadband light emitted during the combustion process (natural luminosity); and high-speed, in-cylinder imaging of both natural luminosity and laser light elastically scattered from liquid-phase fuel in the charge gas. Engine-out emissions of nitrogen oxides (NO_x), smoke, unburned hydrocarbons (HC), and carbon monoxide (CO) also were monitored.

The results of the study affirmed the earlier work by Martin et al. in showing that as the injection timing is advanced during LTC operation, liquid-fuel films on in-cylinder surfaces are likely to form because of low in-cylinder gas and surface temperatures, particularly for the lower-volatility fuels. Such liquid-fuel films can lead to pool fires and higher smoke, HC, and CO emissions, as well as lower fuel-conversion efficiencies.

Increasing HV fuel content was found to be an effective means of reducing or eliminating liquid-fuel films and pool fires, as well as their undesirable effects on efficiency and emissions. Small increases in the HV content produced large changes under conditions where pool-fire activity was significant. For the LTC conditions studied, an HV content of 78% eliminated pool fires and reduced smoke emissions to near-zero levels.

On the basis of this work, it appears that the detrimental effects of liquid-fuel films will make it extremely challenging to achieve desired efficiency and emissions targets with early direct-injection, LTC strategies using conventional diesel fuel. The use of higher-volatility blendstocks or fuels may be an important means of limiting the problems resulting from fuel film formation.

—Cheng et al.

Resources

• A. S. (Ed) Cheng, Brian T. Fisher, Glen C. Martin and Charles J. Mueller (2010) Effects of Fuel Volatility on Early Direct-Injection, Low-Temperature Combustion in an Optical Diesel Engine. Energy Fuels, Article ASAP doi: 10.1021/ef9011142

• Martin, G. C.; Mueller, C. J.; Milam, D. M.; Radovanovic, M. S.; Gehrke., C. R. (2008) Early Direct-Injection, Low-Temperature Combustion of Diesel Fuel in an Optical Engine Utilizing a 15-hole, Dual-Row, Narrow-Included-Angle Nozzle. SAE Int. J. Engines 1 (1), 1057– 1082 (SAE Technical Paper 2008-01-2400)