Lighting the fire

Auto racing legend Smokey Yunick once performed a simple experiment. After filling a 55 gallon steel drum with gasoline vapours, he set a spark plug over the bung hole and connected it to a coil with a live wire. Sparks flew — but nothing happened. He repeated the experiment with a propane torch on a long pole. The drum, violently, turned itself into a large piece of sheet steel. The conclusion Yunick drew boiled down to a simple fact about combustion of air/fuel mixtures: an electric spark isn’t the optimum way to light a fire. It is, however, the only technology available. Making cheap, reliable and adaptable spark plugs to various driving conditions is everyone’s goal. As a result, spark plugs get lots of attention from owner and mechanics alike. Proper plug performance requires enough energy from the coil (and ignition wires) making that component at least as important for good system performance.

Giving the plug the juice

Giving the spark plugs what they want is the coil’s only job. But like everything in life, it takes time. The coil has internal resistance, as does the circuit that feeds it. This resistance affects the amount of time it takes to fully charge, or “saturate,” the coil’s primary windings. A typical coil might charge in one-to-five milliseconds (thousandths of a second). That’s fast. Only, how fast is it relative to piston position in the cylinder? The math is simple:

rpm x 360 degrees/rpm? /60 s/min = degrees crankshaft rotation/second

So at a typical idle speed of 800 rpm, the crankshaft rotates at:

800 x 360/60 = 4,800 degrees/second

Expressed in milliseconds, that’s 4.8 degrees per millisecond, so every thousandth of a second that the coil spends charging, the crank is rotating almost five degrees towards top dead centre. At 3,500 rpm, that’s 21 degrees of crank rotation per millisecond! With the pistons moving that fast, the ECU can’t afford to wait until optimal coil saturation has been reached, so it fires the plugs with whatever electrical/magnetic energy is at hand. At high rpms, it’s even worse, as the need for significant timing advance requires an even earlier spark during acceleration. And this model assumes one coil per cylinder. Share a coil between two or more cylinders and the challenge increases proportionately.

The tuner community often adds “high energy” coils and coil packs that claim astronomical peak voltages. But it’s the amount of current delivered to the spark plug that counts, not the theoretical peak voltage of a bench test. That’s why it’s important to work backwards, from the plugs back through the ignition wires, before installing “high-performance” parts into an ignition system. For high rpm work (which is becoming more common as manufacturers continue to raise redlines on small engines to generate higher horsepower figures), it is a reliable triggering of the coil’s field collapse that matters and is a tough-to-troubleshoot contributor to high-rpm miss. High energy coils attempt to fight back by driving more current into the windings in the hope that the lower potential available by the time the coil is commanded to fire, is enough to drive the plugs. Sixty-thousand volt coils are out there, but if it takes eight-15,000 volts to initiate the spark event, the “reserve” will only be truly useful during wide open throttle, high rpm conditions.

How long does it take to discharge the coil? On a modern system it can be as short as a tenth of a millisecond, perhaps three degrees of crank rotation at highway speeds. That’s fine to initiate the spark, but there’s more to the spark event than just jumping the gap. After the potential (voltage) difference across the plug gap reaches a critical “breakdown” value, the plug fires and draws a slowly (relatively-we’re talking thousandths of a second here) decreasing amount of current until the arcing stops. It’s this duration of the spark event and the current draw at the gap that’s the most overlooked, important factor and it’s a major reason why the “hold the screwdriver near the block” spark test method isn’t good enough for modern troubleshooting.

What can you test?

With distributorless coil-on-plug engines now reaching their post-warranty service years in rapidly increasing numbers, the ability to tap into ignition secondary systems is fast disappearing. Without a simple method to “scope” spark plug firing conditions on the secondary side during basic troubleshooting, emphasis is shifting to the primary side. Modern systems are capable of drawing currents into the double-digit range, and that current can be used to infer what’s going on in the coil. Non-intrusive clamp-type or remote testing (at the fuse block or connectors) is best, and common waveform libraries are available for comparison. For coil primaries, voltage should increase in the familiar ramp fashion, plateau as the coil magnetically “saturates” then drop off sharply when the plug fires. If it’s suspect, resist the temptation to go diving into the coil pack immediately. Not only does the ECU determine when to fire the plugs in modern engines, it may also have a current limiting function, so codes are still a first step check, and even then, it’s hard to rule out the computer. What about the plugs? Obviously, damaged, contaminated or highly worn plugs are drivability killers, but modern engines often perform well up to the time of the first scheduled plug inspection, which can take many drivers years to reach if measured by kilometers driven.

Is plug reading still useful?

Reading spark plugs is a useful way to determine everything from worn oil rings to poor mixture control. But it’s no longer the best way to diagnose an ignition-related drivability issue on modern engines. Why not? U.S. EPA emissions durability requirements have driven OEM’s into platinum or iridium-tipped long-life spark plugs, often encased by coil-on-plug modules and sealed from outside elements. With sufficient energy to fire wet or lightly fouled plugs, modern ignition systems can cover up mild mixture or oil fouling issues, but if the plug isn’t firing, it’s likely most efficient to determine that fact before pulling the coil and plug. Read the plugs, sure, but only to confirm what you already know from meter, scope and scan tool work. And when should they be changed? Regardless of the manufacturer’s inspect-check-replace schedule, plugs should be replaced with new units every time they’re removed regardless of mileage, for two reasons. The first is to create a new baseline condition in the combustion chamber allowing useful diagnosis at higher mileages. If you’ve performed a fuel system cleaning, for example, fresh plugs let you assess injector performance at the next service interval. The second, and perhaps more important, reason is that the old plugs are useful tools for communicating information about an engine to the car owner. A set of removed plugs, racked neatly in the order they came out of the engine, can be shown to the customer to illustrate the engine’s internal condition. Add a poster showing normal and abnormal plug tip illustrations and the potential for upsell service increases dramatically. This process also reduces skepticism when you break bad news. Valve seals gone? Fouled plugs can show where the oil goes and why the problem needs to be corrected. Is this unnecessary service? When the cost of new plugs is compared to the door rate used to establish the cost to “re and re,” there’s no more economic justification for reinstalling old plugs than there is for doctors to re-use wooden tongue depressors. Look at them, and then throw them away. It’s cheap insurance.

Will Star Wars replace the spark plug?

Spark plug-based electrical ignition of gasoline engines has been around for a century and has been developed to the point where many engines will see only one or two plug changes in their expected lifetime. With mixtures getting leaner and redlines moving upward, however, research into alternatives is ongoing. The U.S. Department of Energy has proposed laser-based ignition for internal combustion engines and tested prototypes on LPG engines in 2002. Results suggest that laser-based ignitions could fire leaner mixtures, propagate combustion faster and allow less advance in ignition timing for better efficiency. Drawbacks are mainly centred on mass production of lasers that are cheap, powerful and compact enough for the application, although with fibre optics piping the energy to the individual cylinders, remote mounting of a relatively large unit might be possible. Will lasers and optical fibres replace coils, wires and plugs? Not in the immediate future, but for spark ignition designs to fight back against the efficiency of modern direct injection diesels, a better way to ignite super-lean mixtures is needed. More on the U.S. DOE research can be found on the Internet at: www.netl.doe.gov/publications/proceedings/02/naturalgas/3-1.pdf

Do outside factors affect plug performance?

Several factors outside of the control of the ignition system affect plug performance. Some of these as:

* High compression, nitrous oxide or turbo/supercharging elevates plug operating temperatures.

* Advanced ignition timing. A 10 C increase in lead causes tip temperature to increase by approx. 70-100 C.

* Engine speed and load increases firing-end temperatures proportionately. When traveling at a consistent high rate of speed, or towing/hauling very heavy loads, a colder heat range spark plug should be installed.

* High humidity. As humidity increases, air intake volume decreases, lowering combustion pressures and temperatures, causing a decrease in the spark plug tip temperature.

* Pre-ignition/detonation. In a stock engine, poor cooling system maintenance, improper timing control or use of low-octane fuels can overwhelm the PCM’s ability to control pre-ignition or the more damaging detonation. Plugs can be physically destroyed by detonation.

Information courtesy NGK