Ruthenium spark plugs: should you use them on your EcoBoost?

Boost makes the mixture in the cylinder denser, and a denser mixture takes more voltage to jump the spark plug gap. That is why EcoBoost engines, particularly under high boost, place high demands on the ignition system, and why the NGK Ruthenium HX is a plug worth evaluating for them.

The electrodes are the metal tips the spark jumps between, and NGK makes the Ruthenium HX in two electrode designs. PSPE, a projected square platinum electrode, is the one NGK recommends for turbocharged and supercharged engines. DFE, a double fine electrode, is recommended for engines without a turbo.

NGK’s own charts show two tests of the Ruthenium HX, one of combustion stability and one of flame kernel growth. Each shows something useful, and each leaves something out.

The combustion pressure test charts the pressure in the cylinder as a band, for the Ruthenium HX and for a plug NGK labels conventional. NGK marks the Ruthenium HX band as little dispersion and the conventional plug’s as more, and the conventional band is wider at the peak.

A narrower band means less variation from one cycle to the next. Engineers measure it as the coefficient of variation of IMEP, where IMEP, the indicated mean effective pressure, is the work each cycle does on the piston expressed as an average pressure. It matters because ignition timing has to be safe for the fastest-burning cycles, which are the ones most likely to knock. The less the cycles vary, the closer timing can sit to its best on average, and the more even the torque.

NGK gives no engine, speed or load for this test. Both plugs peak near 4,000 kPa, about 580 psi, far below the peak pressure a boosted direct injection engine reaches at full load. The chart shows how consistent each plug was at that test point, not how either behaves at wide open throttle on boost.

The spark starts a small ball of flame, the flame kernel, which grows into the flame that burns the rest of the mixture. NGK’s photos show the kernel at ignition and at 1, 2 and 3 milliseconds after it, for the Ruthenium HX, an iridium plug and a nickel plug.

At 2 milliseconds the Ruthenium HX kernel is about the size the iridium and nickel kernels reach at 3 milliseconds. A millisecond matters here: at 3,000 rpm the crankshaft turns 18 degrees in one. A flame that grows faster in its first milliseconds brings peak cylinder pressure sooner after the spark.

The comparison changes the electrode design as well as the tip material, so the photos show the plug as a whole. They do not separate how much of the difference comes from ruthenium.

Spark timing is how far before top dead center the plug fires, measured in degrees of crankshaft rotation. Best-torque timing, MBT, is the timing that puts peak pressure where the piston turns it into the most torque. When the burn starts faster, peak pressure arrives sooner, so MBT moves later and less spark advance is needed to reach it.

This matches what I have seen in logs. Final timing often reads slightly later with the Ruthenium HX, while torque output and peak achievable air load, the air each cylinder can take in, hold steady or improve. The later timing is not a sign of knock or lost efficiency. It reflects the faster burn and the pressure curve moving with it.

Which effect matters more depends on the fuel. On gasoline at high boost, timing is usually limited by knock rather than set at MBT, so the steadier combustion matters most. Ethanol blends resist knock well enough that timing can often reach MBT, so there the later MBT matters most. Either way, the differences are small.

1. Ruthenium HX spark plugs, NGK Spark Plugs. The electrode designs and what each is recommended for. The pressure chart and flame kernel photos are NGK’s.
2. John B. Heywood, Internal Combustion Engine Fundamentals, 2nd edition, McGraw-Hill Education, 2018. Spark timing and MBT, flame development, cycle-to-cycle variation and knock.

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