A pathway to gasoline compressed ignition using naphtha fuels; higher efficiency, lower cost
02 May 2014
A new study by Dr. Gautam Kalghatgi and his colleagues at Saudi Aramco provides further support a pathway for significant improvements in the efficiency of a gasoline engine (i.e., spark ignited, SI) by running it in compression ignition mode with naphtha fuels. (Earlier post.) This latest work, presented at SAE 2014 World Congress, shows that moving to higher compression ratios (CR) and lower fuel cetane numbers (DCN) from an SI base engine offers a better trade-off than increasing DCN with a lower CR. In other words, using only cetane to improve CI fuel consumption is less beneficial than relying on a low cetane fuel and higher compression ratio.
Past work done by Kalghatgi and his team, as well as by Prof. Bengt Johansson at Lund University and others, has demonstrated that using low-octane gasoline in diesel engines has the potential to achieve very high efficiency while reducing the cost of diesel engines by lowering injection pressures and requiring less expensive exhaust aftertreatment. Broadly, this approach is termed Gasoline Compression Ignition (GCI).
Initially, GCI technology has to be developed using existing market fuels. In general, practical gasoline fuels have RON > 90 (cetane number, CN < ~19) while diesel fuels have CN >40 (RON < ~40) in most markets. Sellnau and co-workers [earlier post] have been developing a GCI system to run on US regular gasoline—(RON+MON)/2 of 87. They have changed the base SI engine by increasing the compression ratio to 15, designing a new injection system and injectors and redesigning the combustion chamber. As a result, they have demonstrated significant improvement in fuel efficiency compared to the base SI engine.
Reitz and co-workers have been using a dual-fuel approach, which they term Reactivity Controlled CI or RCCI [e.g., earlier post]. In this concept, the injection strategy is changed by using port-fuel injection of a market gasoline, and direct injection of a market diesel and indicated efficiencies of greater than 50% are achieved, while around 90% of the fuel used in gasoline rather than diesel.
… All these concepts will allow “diesel” engines to mostly use gasoline rather than diesel fuel and thus open a path to mitigate the expected demand imbalance between gasoline and diesel. Other research suggests that for the GCI concept, the optimum fuel is a much less processed fuel—e.g., a “low quality” gasoline, with RON between 70 and 85, which does no need to have a final boiling point that is as low as today’s gasolines.
—Viollet et al.
Naphtha refers to the light fraction produced by distillation in a refinery that is roughly in the gasoline boiling range of ~30 °C to ~200 °C. Conventionally, naphtha is processed further—requiring more energy and emitting more CO2—to increase its octane number for use as a gasoline component. Thus, in addition to delivering higher engine efficiencies, the use of naphtha as a fuel would reduce the energy used in fuels manufacture, Kalghatgi notes. Well-to-tank CO2 emissions from the production of naphtha are lower than any other fuel produced in the refinery due to its lower processing requisites.
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| Representative process chart for petroleum refinery (OSHA). 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—after further processing. Forecasts estimate a 40% increase in global transport energy consumption by 2040, primarily in non-OECD countries. About 90% of that transport energy is still expected to come from petroleum by then. Demand growth is expected to skew toward commercial transport, which uses heavier fuels such as diesel and jet. This could lead to a possible surplus of light fuels in the future as well as further investment in fuel manufacture. In addition to enabling higher fuel efficiency at lower costs, Kalghatgi’s naphtha scheme could help address the potential imbalance between light and heavy fuels using a fuel with a lower well-to-tank carbon footprint. |
Most prominent GCI schemes—with the notable exception of Mark Sellnau at Delphi with partners University of Wisconsin-Madison and Hyundai—start with a diesel (CI) engine. Kalghatgi and his team earlier started with a single-cylinder direct-injection spark-ignited (DISI) research engine with a maximum injection pressure about 10x lower than that of a typical diesel, and showed a 19% fuel efficiency benefit over an FTP cycle by running the engine in CI mode on heavy naphtha. They then boosted the efficiency benefit to 26% by upping the CR to 14 with a redesigned piston and using light naphtha.
In the study presented this year at World Congress, they developed three new pistons with a diesel bowl-type feature for naphtha compression ignition combustion; the piston each enabled a different compression ratio (12, 14 and 14). They ran the study in a single-cylinder 4-valve SI engine with a 12:1 geometric compression ratio. The combustion chamber originally was designed for stratified charge spark ignition combustion. For the GCI tests, the team disabled the spark plug.
They defined a GCI load and speed map (from 1000 to 3000 rpm) for six different engine/fuel combinations. Light and heavy naphtha cuts with a DCN of 31 and 41 were run over the same operating points covering the FTP cycle with low soot and NOx with each piston.
They used two distinct control strategies: multiple injection for light and heavy load conditions, and variable intake valve closing timing to change the effective compression ratio.
Among their findings were:
Fuel economy improvements over SI operation vary from 15% with CR12 to 26% with CR14 with light naphtha. Heavy Naphtha did not benefit as much from the increased compression ratio and the greatest fuel economy improvement in this case was 22% at CR13.
The maximum load achieved was 10bar NMEP with light naphtha at CR12, however it did not provide good combustion stability at loads below 6bar NMEP. At all three CRs, heavy naphtha allows operating with good combustion stability on the load range below 2bar NMEP but fails to reach loads above 7bar NMEP on the high end due to high pressure rise rates.
Light naphtha and CR14 provided the best fuel and engine matching.
Double injections were required to improve stability at light load and control smoke emissions at medium and high load.
They found a clear trend with respect to CR and cetane number versus the load range that can be covered. Higher compression ratio is better at light load but cannot go to higher load. This holds for both light and heavy naphtha, although light naphtha provides wider load coverage.
The fuel’s autoignition property could be directly compensated by compression ratio.
Resources
Viollet, Y., Chang, J., and Kalghatgi, G. (2014) “Compression Ratio and Derived Cetane Number Effects on Gasoline Compression Ignition Engine Running with Naphtha Fuels,” SAE Int. J. Fuels Lubr. 7(2) doi: 10.4271/2014-01-130110.4271/2014-01-1301.
Note: naptha competes w. diesel, as the two are largely from different distillation fractions. The relative costs of producing the two on a per gallon basis would converge and cause one product to compete with the other, tumbling all automotive fuel costs. But not quite as fast as the costs of petroleum and other feedstock, as the new automotive efficiency will create an oil glut.
Would oil companies allow this? Will refineries fight back?
Posted by: kalendjay | 16 May 2014 at 03:52 PM