A team at Oak Ridge National Laboratory (ORNL) has achieved diesel-like efficiency in a single-cylinder spark-ignition (SI) research engine while maintaining compatibility with stoichiometric 3-way catalyst (TWC) systems. A paper on their work is published in the International Journal of Engine Research.

The ORNL team conducted engine experiments with both 91 RON E10 gasoline and liquified petroleum gas (LPG). The unique high stroke-to-bore engine design (1.5:1) with cooled exhaust gas recirculation (EGR) and high compression ratio (13.3:1 for both fuels, with additional runs at 16.8:1 for LPG only) improved engine efficiency by up to 30% compared with a production turbocharged gasoline direct injection spark ignition engine.

Comparison of the derived brake thermal efficiency of the ORNL research engine (LSE) running LPG to 2019 production Cummins ISB diesel. Splitter et al.

A major benefit of SI engines is that they allow the use of low carbon content fuels like methane and propane. Additionally, criteria pollutant control, like unburned hydrocarbons, carbon monoxide, and nitrogen oxides (UHC, CO, and NO_x) is extremely efficient and does not require an additional reducer, as in selective catalytic reduction (SCR) systems, characteristic of diesel engines. Consequently, it is of interest to develop ways of closing the efficiency gap between diesel and stoichiometric SI engines combining the best characteristics of each system.

… diesel engines are expected to meet significant technical barriers in being able to meet future NO_x regulations. Recently the California Air Resources Board (CARB) has announced its intention to reduce U.S. domestic NO_x emission regulations for medium- and heavy-duty vehicles by an order of magnitude by 2027. While SCR systems are very effective at reducing NO_x at steady state conditions, they require the use of urea as a reducer, which decreases the overall system fluid efficiency. Additionally, transient control requires complex control strategies to optimize urea consumption and formation of urea crystal deposits. The major challenge with upcoming MD/HD regulations is the requirement to maintain NO_x control under extremely low engine loads.

… Currently, SI engines can achieve the criteria emissions regulation targets proposed for diesel engines by California Air Resource Board (CARB). However, due to their relatively low fuel efficiency, achieving phase 1 and phase 2 CO₂ emission regulations is a challenge that is typically overcome by using fuels with higher hydrogen content (i.e. higher H/C ratio) than gasoline. While gasoline shows a marginal CO₂ improvement over diesel fuel for a given engine efficiency, liquefied petroleum gas (LPG) and compressed natural gas (CNG) can achieve significant reductions simply due to their lower carbon content. For engines with brake thermal efficiency (BTE) parity, LPG and CNG can reduce CO₂ emissions when compared to a diesel fuelled engine by 11.7% and 32.6%, respectively. Consequently, if an engine can achieve diesel-like efficiencies with LPG or CNG, there can be a simultaneous reduction in CO₂ emissions as well as a simplification, and cost reduction of the emissions control system.

… To achieve diesel-like efficiencies, loss mechanisms of stoichiometric SI engines must be identified and addressed. Additionally, technologies need to take advantage of the unique fuel properties to maximize engine performance. To that end, ORNL has recently commissioned a custom-built long stroke single-cylinder research engine (LSE) targeting high efficiency SI combustion.

—Splitter et al.

The LSE engine—based on the GM Ecotec LNF family of engines—is fitted with EGR and a variable valve actuation system, and uniquely features a significantly larger stroke-to-bore ratio of 1.5:1, compared to the stock 1:1. The increase in stroke-to-bore ratio results in increased in cylinder turbulent kinetic energy (TKE), increasing turbulent flame speeds and reducing combustion durations, the ORNL team explained.

LSE billet block, crankshaft and connecting rod. Cylinder head is equipped with a fully flexible valve actuation system by Sturman Industries. Engine was built for single cylinder operation. Custom crankshaft was built with two counterweights for overall engine balance. Splitter et al.

Major findings from the study include:

• Regardless of liquid petroleum gas (LPG) or gasoline fueling the LSE engine exhibited significantly improved combustion durations, which were ~20% faster than the production GM LNF engine, when using the same piston and TDC geometry. When using the similar compression ratios (i.e. a “dome” piston in the LNF was required), the LSE exhibited even further combustion duration benefits. The team attributed these differences to significantly increased turbulence from the LSE’s increased stroke.

• With gasoline the LSE was found to offer significantly improved HC and combustion efficiency improvements when high compression ratio was used. However, with DI LPG the advantages were reduced as HC emissions were extremely low with LPG overall.

• LPG was found to enable increased EGR rate compared to gasoline (20%–50% relative increase depending on load). The results were attributed to fundamental flame dynamics and combustion properties such as flame stretch differences of LPG versus gasoline when EGR is used.

• A combination of DI LPG with LIVC (110°CA bTDC_f), EGR (~30%) and high r_c (16.8:1) was found to yield more than 45% net thermal efficiency (NTE) with diesel-like power density. This was an improvement of ~30% relative to the production engine with DI LPG.

• DI LPG was found to have effectively zero measurable FSN and soot, but similar PN to gasoline. Particle size distribution (PSD) as a function of load showed that as load increased regardless of 0% or 15% EGR, DI LPG exhibited smaller diameter and lower number counts of particles. This translated to reductions in total PN overall. These combined results were postulated to be an effect of significantly reduced wall wetting with DI LPG as compared to gasoline.

Resources

• Splitter D, Boronat V, Chuahy F, Storey J. (2021) “Performance of direct injected propane and gasoline in a high stroke-to-bore ratio SI engine: Pathways to diesel efficiency parity with ultra low soot.” International Journal of Engine Research doi: 10.1177/14680874211006981