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Study suggests pulsed energy spark plug may support mainstream deployment of advanced, ultra-efficient spark-ignition GDI engines

1 October 2012

Plug-comparison
The Enerpulse pulseplug (“Pulstar”) features a peaking capacitor to improve energy transfer efficiency. The external form factors are the same. Click to enlarge.

Findings in a study by a team from Texas A&M and Enerpulse (earlier post) suggest that pulsed energy spark plugs may be able to serve as enabling technology to support the mainstream deployment of advanced, ultra-clean and ultra-efficient spark ignition internal combustion engines. The pulsed energy plug may improve ignition of stratified-GDI (gasoline direct injection) engines; and further 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.

The paper—and newer data not included in the paper—were presented by Dr. Timothy Jacobs, Texas A&M, at the ASME 2012 Internal Combustion Engine Division Fall Technical Conference last week in Vancouver, BC. (Jacobs and his colleagues from Enerpulse, the developer of the pulse energy plug, had presented an earlier paper on improvements in fuel consumption in conventional combustion with the plug at SAE World Congress in April.)

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)

October 1, 2012 in Engines, Fuel Efficiency, Vehicle Systems | Permalink | Comments (7) | TrackBack (0)

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Comments

Sounds interesting, if a little technical for me.
Better gasoline engines are generally a good thing, particularly if hybridized.

What we want is a paper from GM or Ford, using these and reporting mpg (or emissions) improvement (or not [!] ).

Igniting the charge is not the only problem. I would say that the main problem with the lean-burn technology is NOx aftertreatment. In contrast to the three-way catalyst (TWC), a NOx storage catalyst has to be used. Some European (and a few Japanese) manufacturers have used this technology with various success but most of them have now reverted to homogenous GDI. Probably the most advanced were the BMW 4 and 6-cylinder engines. However, BMW and most others have abandoned this technology and are now employing homogenous GDI and turbocharging (downsizing) instead. In the BMW case, also fully variable valve timing and lift (Valvetronic) is used and EGR (optionally cooled) is another additional option to consider. Ultimately, lean-burn GDI could provide the greatest reduction in pumping losses but the measures mentioned for homogenous combustion can reduce this potential to a mere few per cent (I have seen one study indicating <2%). The diminishing return in efficiency has to be weighed against a more complex, and intermittently operating, NOx aftertreatment. I would not say that lean-burn with gasoline is impossible but this does not appear to be the main trend in the automotive business for the moment. Large gaseous-fuelled engines might be a better application.

The technology route for some selected engines from BMW over a few years can be summarized in 3 steps:

1. Stoichiometric (lambda=1) port injection with Valvetronic

2. Lean-burn GDI with conventional valve lift (cam shift only) and increased engine speed compared to option 1

3. Turbocharging, downsizing (6→4 cyl), GDI and Valvetronic and decreased engine speed compared to option 2 (downspeeding)

Fuel consumption went down and further down for options 2 and 3 compared to the previous options (and so did power density). Note that there is a limitation in engine speed for Valvetronic compared to conventional valve mechanism so partly, the desired rated engine speed also determines if this feature can be used or not.

150 year old technology. Very glad they have finally gotten around to making improvements. I guess the battery industry and the CAFE standards are having some effect. Someone will likely figure out that instead of fuel economy they can make it the same fuel usage with just more horse power. Because, by golly, we just can't drive fast enough in these vehicle nowadays.

"the old problem of poor lean-mixture ignition"
To me it appears they are trying to create a spark with more energy to ignite the mixture. The added capacitive element probably makes the spark last a bit longer - more igniting power. I saw some attempts to make stronger spark - Spark plugs with V-groove in the center electrode, Twin-Spark ie 2 spark plugs in Alfa Romeo, Mercedes also used 2 in some engines earlier. I don't know about what racing engines use.

I wonder if instead of single tip spark plug, a better option would be using a two concentric rings, one having at least 4 small bulges that would act as mini spark plugs to create "ring of fire". It all would require a larger/wider spark plug, and space in cylinder head is already crowded with several valves.
Could the valves be moved to the edge of cylinder head, could the cylinder be widened a little bit near top to accommodate wider spark plug and (still round) valves?
Say if cylinder bore is 75 mm, spark plug's new "ring of fire" to have a diameter 18-25 mm. Much larger surface would get initially ignited, but if this is what's needed, I don't know, or how it would reflect on NOx emissions. For sure it would ensure mixture is better ignited, I guess HC emissions would be lower.
Another possible benefit: Currently, with single point (just center) ignition, if mixture happen to be too lean in centre (as a result of all the turbulence - uneven mixture), it may fail to ignite properly. But if sparks are fired around the ring, somewhere there will be mixture rich enough to ignite.
I wonder if "ring of fire" would help to direct shock wave mostly downwards. With centre-pin spark, I guess the shock wave also travels towards cylinder walls (ie goes in all directions), then bounces.

Very Interesting Article! I'm planning on re-reading this a few more times to digest it further.

One thing that stood out was the two pointed case of the problem this product addresses.. "complication degrades engine efficiency (poorly phased combustion) and can cause exhaust valve burnout." The poorly phased combustion could largely being addressed in modern engines with GDI which are currently programmed to lambda ~1 mixtures.

This plug would allow for ignition of stratified mixtures down to lambda=.65! This technology could enable the combination of lean stratified combustion in a GDI in a High-compression mild-turbocharged and intercooled, down-sized and down-sped engine.

In such an engine the intercooling could be replaced or augmented by using direct water injection at two points; ~30 deg ABDC on the compression stroke (or just after intake valves close in an engine with a high bypass ratio) and ~45 deg ATDC on power stroke. This would allow higher boost pressures but with lower NOx while keeping the engine cleaner and exhaust valves cooler. The late injection water would still be expanding through the exhaust stroke and the turbine increasing efficiency while reducing exhaust temperatures.

Such a win-win sounds like an obvious path forward. The biggest problem with water injection is you'd need to engineer the engine to withstand the worst case (when the water runs out and low octane fuel is used in stop and go summer traffic). The compromises to accommodate such a scenario may mitigate the dramatic power, efficiency and emissions improvements otherwise possible.

I do not know how steam impacts catalytic converter performance.

"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."

If it sounds too good to be true, it probably is.

It has been known for 20+ years that improved controlled plugs and injectors can significantly improve ICEs efficiency and reduce fuel consumption. The technologies required to to it have also been around for 20+ years. The same applies to CVTs, on-board high efficiency heat pumps, LED lights, low resistance tires, improved aerodynamics, lower weight bodies, ICE, etc, waste heat recovery systems, braking energy recovery system and many other low cost possibilities.

Why wasn't it done before?

Who is not interested to have more efficient ICEVs consuming less fuel?

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