Environmental pressures and total cost of ownership are beginning to change the types of engines used for off-highway applications, and this trend is likely to accelerate. The off-highway sector includes many niche markets, such as stationary engines for power generation including combined heat-power applications, natural gas pumping, and transit engines for marine, rail, construction and mining applications. Historically, the preferred choice for most of these has been diesel power, but a combination of factors is leading some to switch to natural gas-fueled engines, requiring spark ignition. Federal-Mogul Powertrain has developed a number of spark plug technologies that meet the specific needs of this new generation of engines.
Pressures on diesel engines
Diesel engines face growing pressures on several fronts. Emissions reduction is making diesel engines more expensive in many of the off-highway sectors because increasingly sophisticated combustion control, allied to more complex aftertreatment, is becoming necessary to meet the mandatory limits for NOx and particulate emissions. At the same time, cost pressures on operators and fleet owners mean demand continues for further reductions in fuel consumption and extended intervals between servicing.
Natural gas has been well-known as an environmentally friendly fuel for many years, but gaseous-fueled engines have traditionally been unable to match the efficiency of diesels. Now that advances in system and component technologies enable advanced combustion strategies like Miller or Atkinson cycles and higher cylinder pressures, they can reach a thermal efficiency closer to a diesel.
While diesel engines still remain dominant, in many sectors a migration to natural gas and other alternative gaseous fuels began with the use of dual-fuel engines able to run on diesel and gas. Some operators have already completed the transition to full spark ignition operation where the business economics support the change, such as marine systems operating at a site with a plentiful gas supply. We anticipate rapid growth in marine in the next 10 years in the coastal regions of Scandinavia, Australia and North America, but the challenge of mid-ocean refueling is likely to limit the potential for inter-continental applications.
Beside economic considerations, infrastructure requirements also influence the choice of fuel. For power generation applications, there is often an unlimited natural gas supply on-site; for rail transport and mining applications, the infrastructure is less mature in its development. With mining, there is no fuel available onsite, so it must be transported and refilled constantly, while for rail applications, only one or two railroad cars can carry the gas needed to operate a large train.
The challenges of designing for natural gas
Natural gas or alternative gaseous fuels include a wide range of variations from different geological regions and derived from alternative sources, leading to considerable differences in composition that can have a major influence on combustion and consequently on spark plug design. For example, while the main constituent, methane, is very clean, some of the compositions of digester gas or biogas contain significant hydrogen sulphide levels even after sulphur scrubbing, from which highly corrosive compounds are formed during the combustion process. This necessitates design changes on spark plugs, such as switching from Platinum to Iridium electrode materials for greater corrosion resistance.
The higher efficiency, lower emissions and reduced fuel consumption of the latest engine designs have been achieved through various measures that, in themselves, create challenges for the spark plug. To reduce NOx and fuel consumption, air:fuel ratios as high as Lambda 2.0 are being introduced; satisfactory combustion of these ultra-lean mixtures requires turbulent pre-combustion conditions combined with higher in-cylinder pressures. Older plug technology such as J-plug designs do not work effectively with such high levels of turbulence—akin to blowing out a candle. As cylinder pressures and turbulent kinetic energy rise to new levels at the time of ignition, we also need higher ignition energy to ensure consistent combustion initiation.
New, more aggressive combustion strategies also drive the need for advanced ignition systems featuring multi-strike modes and high energy output. A two- or three-strike ignition mode produces faster combustion and better efficiency, but increases erosion of the electrodes. Whereas a single strike creates a single erosion event, multiple strikes combined with longer spark duration cause several events with higher erosion rates per event. We simulate in the laboratory the different ignition modes and their characteristics using rig tests, in order to investigate their effect on electrode materials and to develop suitable solutions.
Differences in the duty cycles for alternative applications have a major impact on spark plug design as well. An engine used for power generation will operate for prolonged periods at a steady load; a transit application, such as a ferry or tugboat, will alternate between full-load and no-load running much more often. This introduces another potential failure mode through cyclic fatigue.
Unique solutions for natural gas operation
The combined effects of higher voltage demands, increased in-cylinder pressures/temperatures and turbulent charge flow conditions, coupled with longer service life requirements, presents challenges to spark plug design and stimulates the evolution of new technologies. This has enabled developments in both product design and manufacturing processes, including the choice of physical geometry, base metallurgy and precious metal content, with optimum solutions tailored to the size of the plug and to the application.
The engines for transit applications typically use smaller plugs (M14 thread) than the power generation sector (M18 thread), which subjects the plug to greater challenges of strength, dielectric and durability because of the inherently smaller cross sections. This drives the development of enhanced materials and unique electrode designs.
To increase ignitability, Federal-Mogul Powertrain has added inherent shielding around the electrodes, such as an annular gap with a ring electrode to reduce the risk of spark blow-out during combustion initiation. Another means of igniting ultra-lean mixtures is the use of a pre-chamber type plug with an enclosed cap to optimize mixture and flow condition around the spark gap at time of ignition. Successful initiation of ignition in the pre-chamber results in flame torches blowing over via jets into the main combustion chamber. These torches carry an ultra-high energy source, allowing for reliable ignition, which results in highest efficiency and lowest NOx.
Another good example of a specific technology that addresses natural gas-related challenges is Federal-Mogul Powertrain’s Pokal plug, specifically designed for 14-mm applications. As engine manufacturers are pushing combustion closer and more frequently towards the knock threshold, we developed a unique ceramic insulator geometry to resist the loads associated with operating under these conditions. Conventional insulators have a conical nose, the shape of which defines the heat range of the plug and ultimately limits the thermal and electrical properties. The nose of the Pokal spark plug incorporates a cup-shaped cavity around the center electrode, improving both the electrical and especially mechanical strength. This shape allows the plug to better withstand increased voltage demands and allows for safe operation under high peak cylinder pressure or abnormal combustion conditions.
To reduce electrode temperatures and resist erosion in the 18-mm size for fuel-fed pre-chamber engines, our “cold” plug technology is a solution which reduces ground electrode temperatures by more than 200°C (360°F). It produces an exceptionally large spark surface area and provides up to four times the service life of a standard iridium J-gap spark plug in active pre-chamber gaseous-fueled engines operating at over 23 bar (334 psi) BMEP. To ensure the required electrode durability, we applied our specialized laser welding technology, which enables the combination of nickel alloys with iridium-rhodium alloys that would normally present significant manufacturing challenges.
These developments illustrate Federal-Mogul Powertrain’s approach to the industry’s challenges. As a leading supplier of base engine component technology to the world’s OEMs, we understand the commercial and regulatory pressures facing our customers and continue to focus on innovative technologies that provide cost-effective solutions to their challenges.
Gian Maria Olivetti, Chief Technology Officer, Federal-Mogul Powertrain, wrote this article for Truck & Off-Highway Engineering as part of our annual Executive Viewpoints series.