Spark plugs, like most automotive components these days, are experiencing a revolution in technology, with advanced new materials and designs that promise to increase engine performance while lengthening service life. However, as with any other aspect of automotive service, the problem is to get car owners to pay attention to recommended service and replacement intervals, and to ensure that replacement guidelines are followed.
“We find that people do not typically change their spark plugs at these intervals as preventative maintenance,” says Tim Stumpff, product manager, spark plugs with the automotive aftermarket division of Robert Bosch. “Additionally, people sometimes put a lower quality spark plug in their vehicle than what they should – copper when the original equipment is iridium, for example. This leads to a decrease in engine performance, as well as affecting the life of the spark plug.” The key is to use OE-quality or better metallurgy spark plugs, and to replace them at the recommended service interval for proper engine performance, he adds.
“With the advent of vehicle technologies since the early 2000s, it has now become critical to use OEM type, or at least an OEM equivalent spark plug, as a replacement,” says Sukhneet Mavi, product manager with NGK Spark Plugs Canada Limited.
Mavi continues that the more advanced materials and designs of today’s spark plugs do extend the useful service life, but at the same time increasingly harsh engine operating conditions also have an effect.
For the most part, the replacement interval recommended by the OEM depends on the type of metal used in the product, says Jeffrey Boehler, chief engineer at Autolite. “For aftermarket products, the spark plug makers warranty/recommend the change intervals based on their testing,” Boehler says. He notes that the OEMs typically have two maintenance schedules – one for standard driving and one for severe duty.
While there have been major advances in spark plug materials and design, basic sound practice is still critical. The best way to tell if spark plug electrodes are worn is still by measuring the gap and visually inspecting the condition of the electrodes, says Boehler. “The wear on a spark plug is on the electrodes. Over the life of the plug, the gap will grow as the electrodes erode. A gap measurement is the best way to tell if the electrodes are worn. If the engine is burning oil, or is tuned extremely rich, there will be deposits formed on the firing end of the spark plug that can lead to fouling and engine misfire [which leads to] reduced performance, reduced fuel efficiency, and increased emissions.”
The advent of direct injection and smaller displacement engines with turbochargers calls for a slightly different approach, and these trends have spawned the development of engine-specific spark plug designs, says NGK’s Mavi. “These spark plugs have some unique features, like longer reach – some are up to 26.5mm – narrow thread diameter (12mm and 10mm) and other features such as 12-point bi-hex, hybrid multi-ground, dual fine electrode firing end, etc.” These changes represent a challenge for the aftermarket, as special tools are increasingly required to service plugs, while some engine layouts require removal of multiple components in order to access the spark plugs, leading to increased labour time.
Get it right the first time
With direct injection engines, spark plugs have to be installed perfectly to ensure proper engine performance, says Stumpff. “If the spark plug is installed so that the ground electrode blocks the injection, the fuel won’t be ignited by the spark and will cause a misfire. To ensure this doesn’t happen, it is critical that the spark plug is tightened to the correct torque specifications.” This ensures that the plug stays tight in the cylinder, but also that that the ground electrode is in the correct position to allow ignition to occur.
“Direct injection engines typically have slightly higher compression,” says Autolite’s Boehler. “This, as well as turbochargers, increases the cylinder pressure. With increased pressure, the firing voltage of the spark plug increases. This translates into high dielectric strength designs or materials being necessary. It’s very hard to see a punctured ceramic with the naked eye; specialized test equipment is required.”
NGK’s Mavi cites the specific example of the new 1.4-litre turbocharged engine in the Dodge Dart, where even a precious-metal spark plug has a relatively short replacement window of 48,000 km, simply because of the harsh engine operating environment. “Vehicle owners have to follow a very strict maintenance schedule for their vehicle – it’s important for technicians to learn and understand these changing operating conditions of the newer engine designs.”
Due to increasingly stringent government standards for emissions, as well as consumer demand for increased performance without sacrificing fuel economy, direct injection and forced induction systems are becoming very popular, says James Miyamoto, senior product specialist, powertrain product management with Denso Corporation. “To maintain the optimal engine efficiency, it is even more important to install a high quality OE specification spark plug or better.”
Deviating from the OE specs brings a range of dangers, Miyamoto adds. Downgrading spark plugs from OE specifications can significantly alter the vehicle’s performance, fuel economy, and replacement interval. “For example,” he says, “a vehicle that comes equipped with a long-life Iridium spark plug should not be replaced with a lower specification metal technology spark plug.”
For its part, Autolite has a technology backstory that includes unique small diameter designs, employment of precious metal enhancements to optimize durability, and a high-durability resistor formulation and processes that suppress noise and improve the efficiency of sealing. “We are continually evolving the materials, design, and manufacturing processes in order to provide advanced solutions for the most challenging engine applications,” Boehler says.
Mavi says that over the last four model years (2010-2014) NGK has been introducing 70 new OEM type spark plug part numbers to its program. “Most of these designs are so unique that we don’t offer any aftermarket substitutes – only the OEM type plug is available and recommended for replacement,” Mavi says. Most of these designs have newer firing end designs which could be broadly classified as DFE (Dual Fine Electrodes), SPE (Square Projected Electrodes) and PSPE (Projected Square Platinum Electrode). Each design differs in how efficiently it combusts the fuel mixture. “Most of these plugs types are very specific to engine configurations and cannot be replaced by standard design spark plugs.”
Miyamoto highlights Denso’s iridium-rhodium alloy products, which he says boast the longest durability on the market in addition to high resistance to wear and corrosion, while enabling smaller centre electrode design. The company is also working with “needle-to-needle” technology, where a second platinum tip is employed on the ground electrode to optimize spark propagation and reduce heat quenching. “The combination of this design technology and Denso’s iridium-rhodium alloy creates a longer lasting spark plug with more power, more torque and cleaner emissions,” Miyamoto says.
Stumpff at Bosch elaborates on the needle-to-needle design concept, which Bosch designates as “pin-to-pin” and which the company is also working with. This design, which is catching on in Asian vehicles in particular, enables the spark plug to use less voltage to fire, which decreases spark plug wear and enhances the service life. “At the same time, this allows the flame kernel – the initial ignition of the fuel – to grow more easily, and provides better engine performance,” Stumpff says. But he also warns that if a pin-to-pin unit is replaced with a lower-grade standard plug, the vehicle will experience a decrease in engine performance.