Naphtha. When crude oil comes into a refinery, it is first passed into a distillation process, which separates the crude oil into its different fractions by boiling range. Naphtha refers to the light fraction produced by distillation that is roughly in the gasoline boiling range of ~30 °C to ~200 °C.

Naphta is characterized as light or heavy depending upon its distillation cut, and is used as a feedstock of high-octane gasoline. Light naphtha has a boiling range between 30 °C and 90 °C and 5-6 carbon atoms. Heavy naphtha has a boiling range between 90 °C and 200 °C and 7-11 carbon atoms. While light naphtha has mostly paraffins, heavy naphtha has around 12% of aromatics and olefins.

The Research Octane Number (RON) and Motor Octane Number (MON) are 66 and 62 for light naphtha and 62 and 58 for heavy naphtha, respectively, note Chang et al. in their paper published in the Saudi Aramco Journal of Technology. The derived cetane numbers (DCN) for light and heavy naphtha are about 34 and 41, respectively. (A range of Cetane numbers can be obtained by taking different cuts in different boiling ranges.) This is lower than typical diesel fuel (CN = 51 for EN590 European diesel, CN = 45 for US ultra-low sulfur diesel No. 2) and will provide longer ignition delay than market diesel fuels.

Heavy naphta is often upgraded further. Light naphta is thus the least processed product of a refinery; as a result, less energy is required to produce it, and it accordingly carries a smaller carbon footprint than gasoline or diesel.

With its high volatility, light naphtha has a different characteristic on fuel spray distribution compared to gasoline fuel; spray tends to evaporate faster than gasoline so that it has less chance of wall fuel impingement. This leads to lower particulate matters (PM) emission in conventional diesel combustion, as shown by a number of other studies.

Why naphtha? Naphtha, suggest the researchers who are investigating its use, could prove a useful transportation fuel for several reasons: one, cost and availability; and two, as a useful enabling fuel for advanced combustion regimes.

In a paper presented at the SAE 2013 World Congress, Kalghatgi and his colleagues argued that:

Demand for transport energy is growing, but this growth is skewed heavily toward commercial transport, such as heavy road, aviation, marine and rail which uses heavier fuels like diesel and kerosene. This is likely to lead to an abundance and easy availability of lighter fractions like naphtha, which is the product of the initial distillation of crude oil. Naphtha will also require lower energy to produce and hence will have a lower CO₂ impact compared to diesel or gasoline. It would be desirable to develop engine combustion systems that could run on naphtha.

—Chang et al. (2013a)

Leermakers et al. from Eindhoven University of Technology note that Partially Premixed Combustion (PPC) has shown the potential to decrease emissions as well as fuel consumption. While the use of conventional diesel and gasoline pose several challenges for this approach, the Eindhoven researchers observed, other fuels in the gasoline boiling range have been shown to very suitable for the concept. The load range over which the concept can be applied depends on the reactivity of the fuel.

However, such refinery streams are not expected to be commercially available on the short term. For certain applications, the use of currently commercially available naphtha blend of relatively high volatility could provide [a] practical solution. That is, if such a blend can give similar emission advantages as the earlier mentioned refinery streams and has a suitable load range.

—Leermakers et al.

The Eindhoven team went on to test three naphtha blends; they found that the low reactivity blend achieved Euro VI emissions levels with a peak gross indicated efficiency of 50% when run in a one-cylinder test engine based on a 6-cylinder heavy-duty DAF Diesel engine. The results, they suggested, “clearly indicate the potential of this concept.”

Emissions. In the paper presented at the 2013 World Congress, Kalghatgi and his team investigated if and how an existing modern diesel engine could be run on low Cetane fuels in general and naphtha in particular.

They showed that a single-cylinder research engine could be run on narrow-cut naphtha with a derived Cetane (DCN) number of 38, at all relevant speed and load conditions, while meeting or exceeding the efficiency and emissions requirements.

Based on these engine studies, they installed a downsized four-cylinder 1.6-liter diesel engine adapted with piezoelectric fuel injectors, EGR cooling and two-stage air-boosting system in a demonstration vehicle equipped with an on-board combustion control system. The vehicle was successfully run under cold NEDC (New European Driving Cycle) operation at high efficiency with good transient operation and acceptable noise levels while achieving engine-out NO_x emission below EURO6 levels using naphtha. Engine-out particulate matter emissions were also lower than those generated with conventional diesel fuel on the base engine.

More specifically, they found that:

• Naphtha achieved the same level of full power as diesel fuel, but with a significant reduction in CO₂: 122 g/km during the cold NEDC cycle.

• There is potential for reducing after-treatment cost (specifically the de-NO_x catalyst) since the Euro6 NO_x level can be met at the engine exhaust. Tailpipe PM emission of naphtha is at the same level as for diesel after the diesel particulate filter and is also within Euro 6 levels.

  Because the engine-out PM level of naphtha is much lower than that of diesel, however, the filter will become less loaded. This could lead to further benefits in fuel economy by reducing the frequency of regen.

• Tailpipe HC and CO emissions are slightly higher than Euro6 levels. This indicates that a different selection of diesel oxidation catalyst is required.

• Naphtha provides a well-to-tank reduction in CO₂, since it requires less refinery processing.

If both the fuel and the powertrain are regarded as one system and synergistically optimized there is a significant potential to reduce the cost of future powertrains while achieving high efficiency and low emissions. The main objective of the paper was to demonstrate that a naphtha cut can be run in a vehicle (not just in a single-cylinder research engine) while meeting all the requirements for emissions, transient operation and noise while maintaining efficiency.

This is important since it opens a way for bridging the gap between future global demand and supply for different transport fuels. On the current trajectory, huge investments have to be made in refineries to meet the expected increase in global diesel demand (in preference to gasoline). Meanwhile compression ignition engines will become more expensive and complicated to meet stringent requirements on soot and NO_x as a direct consequence of using high-cetane diesel.

A futre solution might be to run CI engines on low-cetane fuels (even lower cetane than that of naphtha used in the current paper) like full boiling-range naphtha to enable low soot/low NO_x operation. The current work represents one milestone along this path.

Reducing the cetane further most likely would have yielded more benefits in some areas but would have required much more development work to enable a practical vehicle to be run as we have been able to do. Our future work will aim to demonstrate that the cost of the diesel engine could be reduced, e.g., by further reducing the Cetane number of the fuel, e.g., b using the light fractions of Naphtha or full boiling range naphtha, the injection and emission control systems could be simplified while meeting the requirements for efficiency, emissions, transient operations and noise.

—Chang et al. (2013a)

Fuel economy. In the paper to be presented in Korea at the end of this month, Kalghatgi and team developed a more optimized combustion chamber design to improve idle and light load combustion stability. They will report that with a newly designed 14:1 CR piston and light naphtha, they achieved 26% average fuel consumption reduction over a range of part load operating points which represents federal test procedure (FTP) city cycle, compared to running the base SI engine on gasoline while engine out NO_x & PM emissions were within Tier II Bin 5 levels.

They identified load range windows for partially premixed combustion operation without compromising emission and pressure rise rate thresholds. They also demonstrated stable naphtha combustion under cold conditions.

Therefore, this work shows another way of developing a highly efficient fuel and engine system starting from a SI engine platform and optimized synergistically using naphtha fuel.

—Chang et al. (2013b)

Intricacies of managing the combustion. Advancing such a concept toward commercial viability across a full load range would require “significant development work” (Chang 2013a). One key measure to improve low-load operation is an increase in compression ratio (CR). Such an increase, however, needs to be done in such as way so as not to increase soot emissions.

To alleviate the higher CO and HC at low loads and higher heat release rates at high loads that PPCI (Partially Premixed Compression Ignition) operation can deliver, the Kalghatgi team investigated managing mixing through injector design (e.g. nozzle size and centerline spray angle) and also changing the CR.

In the second paper to be presented at the Korea meeting, the Kalghatgi team ran a single-cylinder diesel engine on fuel blends by using three different nozzle designs (nozzle size: 0.13 mm and 0.17 mm, centerline spray angle: 153° and 120°) and two different CRs (15.9:1 and 18:1). The engine could be run on such blends with extremely low smoke and low NO_x at speeds and loads of up to 4000 rpm and 10 bar IMEP.

The smoke at comparable NO_x levels was extremely high with diesel fuel at these conditions. The engine could also be run at near-idle conditions on these blends with levels of HC and CO emissions comparable with the diesel fuel.

The wider volatility range with high compression ratio shows a benefit in avoiding over-mixing and over-leaning which could lead to poor combustion stability, they concluded.

The efficiency of PCCI combustion vehicle is expected to be about the same as for a diesel engine. PPCI combustion can be achieved more easily by using fuels with higher resistance to auto-ignition than conventional diesel fuel. Additionally, a successful commercial development of PCCI combustion vehicles could open the way for more efficient vehicles in markets where diesel fuel is not widely available for passenger car applications.

—Won et al.

Similarly, a team from Tsinghua University (Shuai et al.) report on their investigation of double injection strategies with straight-run naphtha in a single-cylinder diesel engine from low to high loads in a paper in the International Journal of Engine Research.

The Tsinghua team realized a two-stage combustion strategy by split spray and combustion events around the compression top dead center with a dominant feature of “Combust After Injection End, Inject After Combustion End” to ensure the premixed compression ignition. (Single-stage combustion was realized by a “spray–spray–combustion” process with the start of combustion separated from the end of injection.)

The straight-run naphtha had a RON of 58.8, and the compression ratio and displacement of the test engine were 16.7 and 0.5 L.

They found that NO_x and total hydrocarbon emissions of the two-stage combustion mode were lower than that of single heat release mode—and that it was much easier to produce two-stage combustion mode at the higher engine load.

Resources

• Chang, J., Kalghatgi, G., Amer, A., Adomeit, P. et al. (2013a) “Vehicle Demonstration of Naphtha Fuel Achieving Both High Efficiency and Drivability with EURO6 Engine-Out NO_x Emission”, SAE Int. J. Engines 6(1):101-119, doi: 10.4271/2013-01-0267

• Junseok Chang, Yoann Viollet, Amer Amer, Gautam Kalghatgi (2013b) “Fuel Economy Potential of Partially Premixed Compression Ignition (PPCI) Combustion with Naphtha Fuel” (SAE 2013-01-2701)

• Hyun Woo Won, Norbert Peters, Heinz Pitsch, Nigel Tait, Gautam Kalghatgi (2013) “Partially Premixed Combustion of Gasoline Type Fuels Using Larger Size Nozzle and Higher Compression Ratio in a Diesel Engine” (SAE 2013-01-2539)

• Shijin Shuai, Zhi Wang, Jianxin Wang, Hongming Xu (2013) “Performance of straight-run naphtha single and two-stage combustion modes from low to high load”, International Journal of Engine Research doi: 10.1177/1468087413488113

• Leermakers, C., Bakker, P., Somers, L., de Goey, L. et al. (2013) “Commercial Naphtha Blends for Partially Premixed Combustion,” SAE Int. J. Fuels Lubr. 6(1) doi: 10.4271/2013-01-1681

• Junseok Chang, Yoann Violet, Amer A. Amer and Gautam T. Kalghatgi (2012) “Enabling High Efficiency Direct Injection Engine with Naphtha Fuel through Partially Premixed Charge Compression Ignition Combustion” Saudi Aramco Journal of Technology (Summer 2012)