Spark plugs use an electric-to-plasma energy transfer to ignite the air-fuel mixture in the cylinder; the effectiveness of a spark plug can be described by an electric-to-plasma energy efficiency. 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 up to 50% for the pulsed energy plug.

The increase in transfer can theoretically improve spark energy and subsequently the ignition time and burn rate of a homogeneous—or potentially a stratified—mixture.

...future engine technology is likely to rely on exploiting the thermodynamic advantages of lean mixtures. Historically, lean mixtures were not attainable in spark ignition engines for two reasons: 1. The lean mixture is typically more difficult to ignite than the stoichiometric mixture and 2. even if ignition occurs, the flame speed and turbulent entrainment of a lean mixture are much slower than that of stoichiometric, resulting in overall slower burn. This latter complication (slower burn rate of a lean mixture) degrades engine efficiency (poorly phased combustion) and can cause exhaust valve burnout.

...One engine technology that may correspondingly resolve the old problem of poor lean-mixture ignition and burn profiles is stratified gas direction injection (stratified-GDI). With such technology, a stratified charge with ideally near-stoichiometric ratio within the stratification (but an overall lean mixture in the cylinder) is centered on the spark plug enabling the spark to properly ignite it. The near-stoichiometric mixture burns with an acceptable profile to release the chemical energy; the overall lean mixture, however, provides the thermodynamic benefit of possessing high ratio of specific heats which results in high efficiency. Further, the stratified charge enables higher compression ratios—which also lead to high engine efficiency—as no ignitable mixture exists near the cylinder walls where fuel knock typically commences and correspondingly causes engine damage. Finally, the stratified charge may decrease heat transfer, further improving engine efficiency.

The challenge of stratified GDI is ensuring the stoichiometric mixture near spark plug; variable load and environmental conditions can make the ordinary mechanical methods of doing this unreliable. Another approach is to develop a plug that can easily ignite a mixture of highly variable equivalence ratio.

Given the potential promise pulsed energy plugs may offer in improving flame kernel development and subsequent burning rate, the objective of this study is to isolate and explain the differences between conventional spark plug and pulsed energy spark plug in a controlled combustion bomb apparatus with varying equivalence ratio and mixture turbulence.

—Jacobs et al.

In the work reported at the ASME conference, the team used combustion bomb testing to isolate the effects of plugs on start and progression of combustion, using a constant volume combustion reactor at a nationally recognized engineering research laboratory. In addition to a conventional fine-wire iridium spark plug, they tested six different designs of the pulse energy plug, with variations in the capacitor and electrode angle.

The combustion bomb apparatus simulated engine in-cylinder conditions similar to those at the time of spark—i.e., generally initial pressure of around 6.9 bar and an initial temperature of around 400 K. Fuel-air equivalence ratio was held to stoichiometric, except for conditions to test the lean flammability limit of the plugs.

They measured in-cylinder pressure during the reaction to calculate rate of heat release. High-speed video Schlieren technique was applied to analyze the initial flame development for each plug. Each plug was tested at four fan speeds (0, 5, 10, and 15 Hz), intended to simulate different levels of mixture turbulence, and φ of 1.0. Further, each plug was tested with 0 Hz fan speed at φ of 0.8. Finally, each plug was evaluated at 0 Hz for its lean flammability limit.

Among their findings were:

• Generally, the pressure rise as a function of time after ignition of the pulsed energy plug typically occurs sooner (if not the same) as the conventional plug.

• In addition to the reduced flame initiation period, burn durations are basically the same between the spark plug and those pulsed energy plugs showing sooner pressure rise (i.e., earlier ignition).

• Particular improvements in the timing of pressure rise as a function of time occur under relatively lean conditions (φ equal to 0.8).

• The lean flammability limit of the pulsed energy plug is about 14% lower (0.76 for conventional plug and 0.65 for pulsed energy plug) using surface gas geometry with the pulsed energy plug.

Such improvements in igniting relatively leaner mixtures might create opportunities to improve lean-burn gas direct injection and homogeneous charge compression ignition technology, as well as improve natural gas combustion with an improved ignition system.

—Jacobs et al.

Preliminary data also suggests an improvement in BSFC (brake specific fuel consumption. The team suggested that 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.

Resources

• Timothy Jacobs, Louis Camilli and Joseph Gonnella (2012) Improvement In Lean Homogenous Spark-Ignition Combustion With Pulsed Energy Spark Plug (ICEF2012-92165)

• Louis Camilli, Joseph Gonnella and Timothy Jacobs (2012) Improvement in Spark-Ignition Engine Fuel Consumption and Cyclic Variability with Pulsed Energy Spark Plug (SAE 2012-01-1151)