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.
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.
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.