The way diesel engines operate is being changed from operating in a high temperature to a low temperature regime in order to suppress the formation of both NO_x and soot. … Although there are several acronyms for the new diesel combustion systems, viz., premixed charge compression ignition (PCCI), homogenous charge compression ignition (HCCI), reactivity controlled compression ignition (RCCI), stratified charge compression ignition (SCCI), high efficiency clean combustion (HECC), etc., all of these strategies operate under low temperature combustion (LTC) conditions. It has been demonstrated that both NO_x and soot formation can be avoided even under rich fuel−air conditions by maintaining temperatures lower than 1700 K. However, important challenges in LTC conditions include a detailed understanding of fuel effects for precise combustion control to provide a wider operating load range.

… Since the molecular composition of diesel fuels is much more complex due to the inclusion of thousands of hydrocarbons, modeling diesel composition effects on the combustion process is usually realized using a few representative hydrocarbon species referred to as “surrogates”. A variety of surrogates for diesel fuels have been suggested in the literature depending on the intended application targets, viz., spray, ignition, combustion chemistry, and emissions. … The major challenge in diesel surrogate modeling includes capturing both the spray and combustion characteristics using a single surrogate mixture. Further, modeling diesel chemistry within the realistic computer time limits can only be realized by developing more accurate multicomponent reduced reaction mechanisms.

Although research efforts have been made in the past to understand fuel effects through experiments, corresponding modeling studies are very limited. Also, there is a need to study the fuel effects in modern technology engines, which may need to be operated in advanced combustion modes to meet future emissions standards. A deeper insight into fuel effects is possible through the development and application of more realistic surrogate fuel models.

—Anand et al.

Precise combustion control and a wider operating load range are the two major challenges in the application of LTC, as the combustion process is chemical kinetically driven and thus is sensitive to the fuel composition. The use of simple single- or two-component surrogate models leads to inaccuracies when modeling advanced combustion systems due to differences between the model and real fuel compositions, they explained.

In the study, they considered three diesel fuels with varying saturate and aromatic compositions, representing high-, medium- and low-cetane fuels. The high-cetane fuel composition and properties meet the EC1 Swedish diesel fuel specifications (>51 cetane, < 5% vol. aromatics) and the mid- and low-cetane fuels have properties typical for North American diesel fuel (>40 cetane, <35% vol. aromatics).

The researchers took a hybrid surrogate model approach, developing two groups of surrogates for the fuels; one group for representing the physical/spray properties of a fuel, the other for the chemistry calculations.

They arrived at their surrogate compositions by first modeling the distillation profiles of the three fuels using the KIVA-ERC code incorporated with a Discrete Multicomponent (DMC) fuel model. They judged the fidelity of the surrogate compositions based on validation of important properties, such as density; H/C ratio; heating value; and cetane number. These developed surrogate compositions were used only for representing the physical/spray properties of diesel fuels.

For the chemistry calculations, they arrived at a separate group of surrogates based on a Group Chemistry Representation (GCR) method. They then applied the two groups of surrogates to predict the combustion and emission characteristics of a single cylinder diesel engine operated with the three diesel fuels under varying conditions including conventional and LTC modes.

Among the conclusions were:

• Modeling the distillation profiles of real fuels was found to be an effective tool for accurately representing their composition and properties.

• A hybrid surrogate modeling approach with the use of two separate groups of surrogates to model the spray and chemistry of the three diesel fuels provided flexibility in terms of the choice of surrogates with a lower computational load.

• The developed surrogate models captured well the combustion and emission trends of the three fuels under conventional and low-temperature combustion conditions.

• Low-temperature combustion conditions showed much higher sensitivity to fuel changes, while the changes in combustion and emission parameters with fuel type was very marginal in conventional combustion.

• Changes in EGR rate controlled the start and progress of combustion and emission formation under LTC conditions with the low-CN fuel showing much higher sensitivity than the high-CN fuel, while conventional combustion was less sensitive to up to 10% changes in EGR.

• The locations of unburned emissions in the combustion chamber were significantly different in LTC and conventional combustion. They were mostly located along the cylinder wall regions in LTC, while they originated along the cylinder axis under mixing dominated conventional combustion.

Resources

• K. Anand, R. D. Reitz, E. Kurtz, and W. Willems (2013) “Modeling Fuel and EGR Effects under Conventional and Low Temperature Combustion Conditions.” Energy & Fuels doi: 10.1021/ef401989c