The natural gas industry has seen a considerable increase in production recently as the world seeks out new sources of economical, reliable, and more environmentally friendly energy. Moving this natural gas requires a complex network of pipelines and compressors, including reciprocating engines, to keep the gas moving. Many of these engines were designed more than 40 years ago and must be retrofit with modern technologies to improve their performance while simultaneously reducing the harmful emissions that they produce. In this study a directed energy ignition system is tested on a two-stroke, single cylinder, natural gas-fired engine. Stability and emissions will be observed throughout a range of spark waveforms for a single speed and load that enables the most fuel-lean operation of the engine. Improving the combustion process of the legacy pipeline engines is a substantial area of opportunity for reducing emissions output. One means of doing so is by improving an engines ability to operate at leaner conditions. To accomplish this, an ignition system needs to be able to send more energy to the spark plug in a controlled manner than a tradition capacitive-discharge ignition system. Controlling the energy is accomplished by optimizing the structure of the waveform or “profile” for each engine design. With this particular directed energy ignition system, spark profiles are able to be configured by changing the duration and amount of current sent to the spark plug. This study investigates a single operating speed and load for 9 different spark energy configurations. Engine operation at these test conditions will allow for emissions and engine performance data, using directed energy, to be analyzed in contrast to capacitive-discharge ignition.

Lean combustion has proven to be an effective way to improve the efficiency and emissions of the direct injection spark ignition (DISI) engine. However, one of the main problems at the lean stability limit is the major decrease in flame temperature due to dilution, resulting in a low laminar flame speed, especially under low-speed engine operating conditions. The split injection is a potential technology to realize proper air-fuel mixing and achieve different spray distribution that can help in solving such problems. In this study, split injections with different secondary injection timings were tested to achieve homogeneous and homogeneous-stratified modes in a DISI optical engine under lean-burn mode. The split ratio of each strategy was 1:1. The engine was operated at 800 rpm, and a high-energy ignition system was utilized to realize lean combustion at a lambda of 1.55. Engine combustion performance and emissions were tested while performing high-speed color recording to study the characteristics of flame chemiluminescence through a quartz piston combined with a 45-degree mirror installed below. Flame structure during various combustion phases was compared under different selected conditions based on a digital image processing technique. The results show that the pressure and emissions vary with the second injection timing. Proper control of the split injection timing can improve lean combustion performance, including faster flame speed, increased indicated mean effective pressure (IMEP), and lower harmful emissions. Poor fuel evaporation and soot generation from spatial hot spots in the combustion process of split injection are the major challenges for further improvement.

In the present study, the performance and emissions characteristics of three low-temperature plasma (LTP) ignition systems were compared to a more conventional strategy that utilized a high-energy coil (93 mJ) inductive spark igniter. All experiments were performed in a single-cylinder, optically accessible, research engine. In total, three different ignition systems were evaluated: (1) an Advanced Corona Ignition System (ACIS) that used radiofrequency (RF) discharges (0.5–2.0 ms) to create corona streamer emission into the bulk gas via four-prong electrodes, (2) a Barrier Discharge Igniter (BDI) that used the same RF discharge waveform to produce surface LTP along an electrode encapsulated completely by the insulator, and (3) a Nanosecond Repetitive Pulse Discharge (NRPD) ignition system that used a non-resistor spark plug and positive DC pulses (∼10 nanoseconds width) for a fixed frequency of 100 kHz, with the operating voltage-controlled to avoid LTP transition to breakdown. For the LTP ignition systems, pulse energy and duration (or number) were varied to optimize efficiency. A single 1300 revolutions per minute (rpm), 3.5 bar indicated mean effective pressure (IMEP) homogeneous operating point was evaluated. Equivalence ratio (ϕ) sweeps were performed that started at stoichiometric conditions and progressed toward the lean limit. Both the ACIS and NRPD ignition systems extended the lean limit (where the variation of IMEP < 3%) limit (ϕ = 0.65) compared to the inductive spark (ϕ = 0.73). The improvement was attributed to two related factors. For the ACIS, less spark retard was required as compared to spark ignition due to larger initial kernel volumes produced by four distinct plasma streamers that emanate into the bulk gas. For the NRPD ignition system, additional pulses were thought to add expansion energy to the initial kernel. As a result, initial flame propagation was accelerated, which accordingly shortens early burn rates.

Max Fiedler developed a lightweight automotive diesel engine with a novel combustion system, presenting initial results in 1939. A unique injection concept incorporated in the engine created a pre-mixed, semi-homogeneous charge using uniformly low injection pressures and advanced injection timing. His evidence showed: extremely smooth combustion and controlled energy release in the early stages of burning; complete elimination of diesel knock; appropriate timing of the onset of combustion; and a combustion event that remained robust through to its conclusion, likely forming a minimum of particulate. Fiedler’s work was largely ignored by other engineers of the day, but there is significant evidence in later work that both explains and confirms his basic claims. The available knowledge relating to Fiedler’s combustion system provides important information about the conditions required to produce smooth heat release in well mixed compression ignition systems.

This study demonstrates the effects of technologies applied for the development of a gasoline direct injection (GDI) engine for improving the brake thermal efficiency (BTE) over 44%. The GDI engine for the current study is an in-line four-cylinder engine with a displacement of 2156cm 3 , which has relatively high stroke to bore ratio of 1.4 (110mm stroke and 79mm bore). All experiments have been conducted using a gasoline having RON 92 for stoichiometric operation at 2000RPM. First, since compression ratio is directly related to the thermal efficiency, four compression ratios (14.3, 15.2, 15.8 and 17.2) were explored for operation without exhaust gas recirculation (EGR). Then, for the same four compression ratios, EGR was used to suppress the knock occurrence at high loads with high compression ratio (CR), and its effect on initial and main combustion duration was compared. Second, the shape of intake port was revised to increase tumble flow of in-cylinder charge for reducing combustion duration at low and high load, and extending EGR-stability limit further eventually. Then, as an effective method to ensure stable, complete and fast combustion for EGR-diluted stoichiometric operation, the use of twin spark ignition system is examined by modifying both valve diameter of intake and exhaust, and its effect is compared against that of single spark ignition. In addition, the layout of twin spark ignition was also examined for the location of Front-Rear and Intake-Exhaust. To get the maximum BTE at high load, 12V electronic super charger (eSC) was applied. Under the condition of using 12V eSC, the effect of intake cam duration was identified by increasing from 260deg to 280deg. Finally, 48V eSC was applied with the longer intake camshaft duration of 280deg. As a result, the maximum BTE of 44% can be achieved for stoichiometric operation with EGR.

The ignition mechanism of a lean premixed CHVair mixture by a hot turbulent jet issued from the pre-chamber combustion is investigated using 3D combustion CFD. The turbulent jet ignition experiments conducted in the rapid compression machine (RCM) at Michigan State University (MSU) were simulated. A full simulation was carried out first using RANS model for validation, the results of which were then taken as the boundary condition for the detailed simulations using both RANS and LES. To isolate the thermal and chemical kinetic effects from the hot jet, two different inlet conditions of the chamber were considered: inert case (including thermal effects only) and reactive case (accounting for both thermal and chemical kinetic effects). It is found that the chemical kinetic effects are important for the ignition in the main chamber. Comparison of OH and HRR (heat release rate) computed by RANS and LES shows that RANS predicts slightly faster combustion, which implies higher predicted turbulent flame speed. Correlations between vorticity, mixing field, and temperature field are observed, which indicate that the flow dynamics strongly influence the mixing process near the flame front, and consequently affect flame propagation.

Ultra-lean burn with high turbulence has high potential for improving thermal efficiency and reducing NO x emissions in spark-ignition engines. Formation of initial flame kernel in high-turbulence flow by advanced ignition technologies is crucial for successful implementation of the ultra-lean burn concept. In this study, a four-coil ignition system is designed to enable temporally flexible discharge, including the single strike, multi-strike and continuous discharge with the discharge energy range from 100 to 300 mJ. The performance of the different discharge strategies on igniting the lean methane-air mixture is evaluated in an optically accessible constant volume vessel. The initial mixture pressure of 3.0 MPa and temperature of 388 K are set to simulate typical conditions near TDC (top dead center) of turbocharged large-bore natural gas engines. Both the flow and quiescent conditions around the spark plug are taken into account with and without gas flows in the vessel. The flame kernel formation and developing processes are captured by using the Schlieren imaging technique with a high-speed CMOS video camera, while evolution of both the voltage and current in the circuit are well monitored by the high-voltage probe and current clamp. With the continuous discharge ignition, the lean limit is remarkably extended in the case of the flow condition, while it is changed only slightly under the quiescent condition, compared with the other strategies. Analysis of the current and voltage waveforms shows that the continuous discharge strategy can enable a steadier and longer discharging period than the other strategies, regardless of conditions with and without gas flow. Besides, the continuous discharge strategy can accelerate the initial flame propagation compared with the other strategies. Once the flame kernel is successfully established, an increase in the discharge energy of single strike has no obvious effects on the flame development, but it is necessary for maintaining the lean limit. Although, in principle, the multi-strike discharge strategy can increase the ignition energy released to the mixture, the current waveform is prone to be interrupted with the discharge channel strongly distorted by the gas flow under the high-pressure condition. The flame propagation speed of the ultra-lean mixture is rather slow under the high ambient pressure quiescent condition compared with the high ambient pressure flow condition. Enhancement of turbulent flow in the mixture is very crucial for realizing the highly efficient and stable combustion of the lean mixture.

Lean-burn combustion dominates the current reciprocating engine R&D efforts due to its inherent benefits of high BTE and low emissions. The ever-increasing push for high power densities necessitates high boost pressures. Therefore, the reliability and durability of ignition systems face greater challenges. In this study, four ignition systems, namely, stock Capacitive discharge ignition (CDI), Laser ignition, Flame jet ignition (FJI), and Nano-pulse delivery (NPD) ignition were tested using a single cylinder natural gas engine. Engine performance and emissions characteristics are presented highlighting the benefits and limitations of respective ignition systems. Optical tools enabled delving into the ignition delay period and assisted with some characterization of the spark and its impact on subsequent processes. It is evident that advanced ignition systems such as Lasers, Flame-jets and Nano-pulse delivery enable extension of the lean ignition limits of fuel/air mixtures compared to base CDI system.

Non-premixed combustion of directly-injected natural gas offers diesel-like performance and efficiency with lower fuel costs and reduced greenhouse gas emissions. To ignite the fuel, a separate ignition source is needed. This work reports on the initial development of a new hot-surface based ignitor, where a small quantity of natural gas is injected and ignited by a hot element. This generates a robust pilot flame to ignite the main gas injection. A series of experimental tests were conducted to evaluate the sensitivity of the pilot flame formation process to hot surface temperature and geometry and to gas pilot injection geometry. Tests were conducted in a constant-volume combustion chamber at up to 6 bar with hot surface temperatures up to 1750 K. Reacting-flow computational fluid dynamics (CFD) evaluation is used to help interpret the results and to extrapolate to engine-relevant pressures. The results show that hot surface temperatures around 1500 K can minimize the pilot ignition time. An injector geometry where the pilot gas jets are angled such that they impinge on the hot surface but retain sufficient momentum to convect mass into the main chamber helps to ensure rapid and stable ignition. The CFD results indicate that, at engine pressures, a stable gas pilot flame could be established within 1–2 ms using the proposed injector geometry. These results will be used to underpin further development activities on this concept.

Lean or diluted combustion has been considered as an effective strategy to improve the thermal efficiency of spark ignition engines. Under lean or diluted conditions, the combustion speed is reduced by the diluting gas. In order to speed up the combustion, in-cylinder flow is intentionally enhanced to promote the flame propagation. However, it is observed that the flow may make the spark ignition process more challenging due to the shortened discharge duration, the frequent re-strikes of spark plasma and the more complicated interactions between the flow and the flame. In this research, the effects of spark discharge current level and discharge duration on flame kernel development and flame propagation of lean methane air mixture are investigated under flow velocity of about 25 m/s and background pressure of 4 bar abs in an optical combustion chamber. A dual coil ignition system and an in-house developed current management module are used to create different discharge current levels. The average discharge current levels range from 55 mA, 190 mA, up to 250 mA. Detached flame kernel is observed under some test conditions. The flame propagation speed with the detached flame is generally slower than the flame developed from a flame kernel attached to the spark plug. The flame detachment is related to both the discharge current level and the discharge duration. When the discharge current level is high at 250 mA, the detached flame is observed at shorter discharge duration of 0.8 ms, while when the discharge current is low at 190 mA, detached flame can happen at longer discharge duration of 1.3 ms. Various discharge current and discharge durations are adopted to initiate the combustion in a single-cylinder engine operating with lean gasoline air mixture. It is shown from the results that a higher discharge current level and longer discharge duration are beneficial for controlling the combustion phasing and improving the operation stability of the engine.

In the present paper, a comprehensive ignition system model (VTF ignition model) accounting for the practical module and working mechanism of a spark plug was developed, aiming to provide enhanced capability for the 3D combustion simulation of spark ignition engines. In this model, an electrical circuitry model is used to represent the ignition coil, spark plug, and air column. The air column is represented by a set of Lagrangian particles that move with the local flow field. Flame propagation is directly calculated using SAGE model with a reduced isooctane reaction mechanism. The new ignition system model is further implemented into CONVERGE through user defined functions and is verified by comparing with the conventional DPIK model. It is found that the VTF ignition model predicts slower combustion than the DPIK model, mainly due to more realistic energy deposit method and energy discharging rate. Furthermore, the VTF model also has the capability of predicting the arc motion and restrike phenomena associated with spark ignition processes. It is expected that with more validation with experiments, the new VTF model has the great potential to better serve the needs of engine combustion simulation.

With the advancement of spark ignition engines, lean or diluted in-cylinder charge is often used to improve the engine performance. Enhanced in-cylinder charge motion is widely applied under such conditions to promote the flame propagation, which raise challenges for the spark ignition system. In this work, the spark discharging process is investigated under different flow conditions via both optical diagnosis and electrical measurement. Results show that the spark plasma channel is stretched under flow conditions. A higher discharge current can maintain the stretched spark plasma for a longer duration. Re-strikes are observed when the spark plasma is stretched to a certain extent. The frequency of re-strikes increases with increased flow velocity and decreased discharge current level. The discharge duration reduces with the increased flow velocity. The effects of gas flow on the ignition and flame kernel development are studied in a constant volume optical combustion chamber with premixed lean and stoichiometric methane air mixture. Two spark strategies with low and high discharge current are used for the ignition. The flame propagation speed of both lean and stoichiometric mixtures increases with the increased gas flow velocity. A higher discharge current level retains a more stable spark channel and improves the flame kernel development for both lean and stoichiometric conditions, especially under the higher gas flow velocity of 20 m/s.

Recent gas engines developments tend to use more excess air to reduce NO x emissions. Under these circumstances the ignition in a single cylinder research gas engine with micro pilot injection of highly ignitable fuels has been investigated. Three igniting fuels, Hydrogenated Vegetable Oil (HVO), 2-ethoxyethyl ether (2-EEE) and a Diesel/2-ethylhexyl nitrate blend have been selected by a systematical assessment and their properties have been analyzed. These fuels have been evaluated concerning their aptitude as igniting fuels and compared with diesel as reference fuel. A higher ignitability of igniting fuel reduced the ignition delay of the injected fuel and enabled the diminution of the igniting fuel fraction. A significant share of NO x emissions have been attributed to the ignition injection, therefore micro pilot injection is necessary to reach emission targets. The micro pilot injection of 2-EEE as a highly ignitable fuel with the highest Cetane Number showed favorably low ignition delay. Depending on the selected fuel and the igniting fuel fraction, the combustion phasing can be controlled directly by the injection timing. In the last section, the results for pilot injection with 2-EEE as an igniting fuel have been compared with the results using a conventional spark plug. Advantages and disadvantages for both ignition systems have been identified at constant Air Fuel Ratio (AFR). A thermodynamical comparison with each ignition system has been performed to explain the different effects on combustion.

This work presents an experimental investigation of advanced combustion of extremely lean natural gas / air mixture in a gas fueled automotive engine with a scavenged pre-chamber. The pre-chamber, which was designed and manufactured in-house, is scavenged with natural gas and is installed into a modified cylinder head of a gas fueled engine for a light duty truck. For initial pre-chamber ignition tests and optimizations, the engine is modified into a single cylinder one. The pre-chamber is equipped with a spark plug, fuel supply and a miniature pressure transducer. This arrangement allows a simultaneous crank angle resolved pressure measurement in the pre-chamber and in the main combustion chamber and provides important validation data for computational fluid dynamics (CFD) simulations. The results of the tests and initial optimizations show that the pre-chamber engine is able to operate within a significantly wider range of mixture composition than the conventional spark ignition engine. Full load operation of the pre-chamber engine is feasible with stoichiometric mixture (compatible with a three-way catalyst), without excessive thermal loading of components. At low load operation, the results show low NO x emissions with a high potential to fulfil current and future NO x limits without lean NO x exhaust gas after-treatment. The scavenged pre-chamber helps to increase the combustion rate mainly in the initial phase of combustion. However, significant unburned hydrocarbons emissions due to incomplete combustion need further optimizations. Thermal efficiency of lean operation of the engine with the pre-chamber compared to the conventional spark ignition system operated in stoichiometric conditions shows approximately 13% improvement.

Dilute combustion is an effective approach to increase the thermal efficiency of spark-ignition (SI) internal combustion engines (ICEs). However, high dilution levels typically result in large cycle-to-cycle variations (CCV) and poor combustion stability, therefore limiting the efficiency improvement. In order to extend the dilution tolerance of SI engines, advanced ignition systems are the subject of extensive research. When simulating the effect of the ignition characteristics on CCV, providing a numerical result matching the measured average in-cylinder pressure trace does not deliver useful information regarding combustion stability. Typically Large Eddy Simulations (LES) are performed to simulate cyclic engine variations, since Reynold-Averaged Navier-Stokes (RANS) modeling is expected to deliver an ensemble-averaged result. In this paper it is shown that, when using RANS, the cyclic perturbations coming from different initial conditions at each cycle are not damped out even after many simulated cycles. As a result, multi-cycle RANS results feature cyclic variability. This allows evaluating the effect of advanced ignition sources on combustion stability but requires validation against the entire cycle-resolved experimental dataset. A single-cylinder GDI research engine is simulated using RANS and the numerical results for 20 consecutive engine cycles are evaluated for several operating conditions, including stoichiometric as well as EGR dilute operation. The effect of the ignition characteristics on CCV is also evaluated. Results show not only that multi-cycle RANS simulations can capture cyclic variability and deliver similar trends as the experimental data, but more importantly that RANS might be an effective, lower-cost alternative to LES for the evaluation of ignition strategies for combustion systems that operate close to the stability limit.

The efficiency improvement and emissions reduction potential of lean and EGR dilute operation of spark-ignition gasoline engines is well understood and documented. However, dilute operation is generally limited by deteriorating combustion stability with increasing inert gas levels. The combustion stability decreases due to reduced mixture flame speeds resulting in significantly increased combustion initiation periods and burn durations. A study was designed and executed to evaluate the potential to extend lean and EGR-dilute limits using a low-energy transient plasma ignition system. The low-energy transient plasma was generated by nano-second pulses and its performance compared to a conventional transistorized coil ignition system operated on an automotive, gasoline direct injection (GDI) single-cylinder research engine. The experimental assessment was focused on steady-state experiments at the part load condition of 1500 rpm 5.6 bar IMEP, where dilution tolerance is particularly critical to improving efficiency and emissions performance. Experimental results suggest that the energy delivery process of the low-energy transient plasma ignition system significantly improves part load dilution tolerance by reducing the early flame development period. Statistical analysis of relevant combustion metrics was performed in order to further investigate the effects of the advanced ignition system on combustion stability. Results confirm that at select operating conditions EGR tolerance and lean limit could be improved by as much as 20% (from 22.7 to 27.1% EGR) and nearly 10% (from λ=1.55 to 1.7) with the low-energy transient plasma ignition system.

This paper discusses the characteristics of EGR dilute GDI engines in terms of combustion stability. A combined approach consisting of RANS numerical simulations integrated with experimental engine testing is used to analyze the effect of the ignition source on flame propagation under dilute operating conditions. A programmable spark-based ignition system is compared to a production spark system in terms of cyclic variability and ultimately indicated efficiency. 3D-CFD simulations are carried out for multiple cycles with the goal of establishing correlations between the characteristics of the ignition system and flame propagation as well as cycle-to-cycle variations. Numerical results are compared to engine data in terms of in-cylinder pressure traces. The results show that an improved control over the energy released to the fluid surrounding the spark domain during the ignition process has beneficial effects on combustion stability. This allows extending the dilution tolerance for fuel/air mixtures. Although affected by cyclic variability, numerical results show good qualitative agreement with experimental data. The result is a simple but promising approach for relatively quick assessment of stability improvements from advanced and alternative ignition strategies.

Direct injection natural gas (DING) engines offer the advantages of high thermal efficiency and high power output compared to spark ignition natural gas engines. Injected natural gas requires some form of ignition assist in order to ignite in the time available in a diesel engine combustion chamber. A glow plug — a heated surface — is one form of ignition assist. Simple experiments show that the thickness of the heat penetration layer of a glow plug is very small (≈10 −5 m) within the time scale of the ignition preparation period (1–2 ms). Meanwhile, the theoretical analyses reveal that only a very thin layer of the surrounding gases (in micrometer scale) can be heated to high temperature to achieve spontaneous ignition. A discretized glow plug model and virtual gas sub-layer model have been developed for CFD modeling of glow plug ignition and combustion for DING diesel engines. In this paper, CFD modeling results are presented. The results were obtained using a KIVA3 code modified to include the above mentioned new developed models. Natural gas ignition over a bare glow plug was simulated. The results were validated against experiments. Simulation of natural gas ignition over a shielded glow plug was also carried out and the results illustrate the necessity of using a shield. This paper shows the success of the discretized glow plug model working together with the virtual gas sub-layer model for modeling glow plug assisted natural gas direct injection engines. The modeling can aid in the design of injection and ignition systems for glow plug assisted DING engines.

This article describes a study involving new spark plug technology, referred to as pulsed energy spark plug, for use in igniting fuel-air mixtures in a spark ignition internal combustion engine. The study involves precisely controlled constant volume combustion bomb tests. The major defining difference between the pulsed energy spark plug and a conventional spark plug is a peaking capacitor that improves the electrical-to-plasma energy transfer efficiency from a conventional plug’s 1% to the pulsed energy plug’s 50%. Such an increase in transfer efficiency is believed to improve spark energy and subsequently the ignition time and burn rate of a homogeneous, or potentially stratified, fuel-air mixture. The study observes the pulsed energy plug to shorten the ignition delay of both stoichiometric and lean mixtures (with equivalence ratio of 0.8), relative to a conventional spark plug, without increasing the burn rate. Additionally, the pulsed energy plug demonstrates a decreased lean flammability limit that is about 14% lower (0.76 for conventional plug and 0.65 for pulsed energy plug) than that of the conventional spark plug. These features — advanced ignition of stoichiometric and lean mixtures and decreased lean flammability limits — might qualify the pulsed energy plugs as an enabling technology to effect the mainstream deployment of advanced, ultra-clean and ultra-efficient, spark ignition internal combustion engines. For example, the pulsed energy plug may improve ignition of stratified-GDI engines. Further, the pulsed energy plug technology may improve the attainability of lean-burn homogeneous charge compression ignition combustion by improving the capabilities of spark-assist. Finally, the pulsed energy plug could improve natural gas spark ignition engine development by improving the ignition system. Future work could center efforts on evaluating this spark plug technology in the context of advanced internal combustion engines, to transition the state of the art to the next level.

Large bore natural gas engines have the perennial challenge to achieve ever higher efficiency with ever lower NOx emissions, while maintaining stable combustion, avoiding misfire and engine knock. A primary strategτy to achieve these goals is to run leaner and leaner. However, leaner mixtures lead to reduced combustion stability and the operating space between misfire and engine knock shrinks. Leaner operation requires a high performance ignition system. This report will highlight the fundamental challenges related to lean operation and the progress Woodward has made to create a novel high performance prechamber spark plug to achieve good combustion stability in a passive prechamber spark plug under lean conditions. The spark plug in combination with the appropriate ignition system enables faster and more stable combustion under increasingly lean conditions, improving fuel efficiency and emissions. Engine simulation modeling is used to demonstrate the benefits of lean gas mixtures and reduced combustion duration to enhance the NOx versus fuel consumption trade-off for a range of air fuel ratios. With this database available, a design requirements flow-down is performed such that combustion performance requirements can be specified a priori, which if met would ensure the high level engine emissions and performance targets would be met. With combustion requirements in hand, CFD simulations are used to identify the mechanisms by which flame propagation is improved with prechamber spark plugs in general, and by the Lean Quality Plug (WW-LQP) prechamber spark plug under development at Woodward. Experimental validation was carried out to confirm the benefits of lean operation and improvement of combustion stability (COV) on the NOx-efficiency trade-off. Operation with Woodward’s WW-LQP spark plug and IC1100 AC ignition system showed improved fuel efficiency at constant NOx on a high BMEP engine. Additionally, the enhanced stability and low COV of the WW-LQP enables extension of the natural gas lean limit closer to λ = 2.00 for an open chamber engine.