Reading the Spark plug

Keith Pickersgill

Paramotors are somewhat different to bikes. We often spend extended periods at full power, and cruise for sometimes hours on end at a constant RPM.

This means we have different problems to bikes, and need to change how we tune our carbs, and monitor whether they are running correctly. 

Firstly, you need to be aware of the following: 

Too lean or not? Are you referring to:
a) Fuel/Air ratio, or
b) Oil/petrol ratio

Both can and will cause problems if too lean, but also if too rich as well ! If you are concerned with the fuel/air ratio, then you must ask yourself, "at what throttle setting?" as it will change across the range of the carb's throttle, engine RPM and engine load. 

Obviously your motor develops the most heat at wide-open-throttle (WOT) and therefore this is of most concern, as your motor will seize quickly if running too lean at full power. 

However, it could have the perfect fuel/air ratio at full power, and yet run too lean at cruise power setting, which will also damage the motor, and this is usually the case in most engine seizures that we see. There are a few ways of determining how your carb is tuned. The most accurate, which responds instantly to throttle and load changes, is an 

EGO meter (Exhaust Gas Oxygen). This is a Lambda sensor in your exhaust, which measures the Residual Oxygen (unburnt portion) in the exhaust gases, and feeds this data to some sort of instrument. 

The correct ratio is 14.7 parts air to one part fuel, measured by weight. This is very easy to determine and accurately measure with an EGO. The simplest instruments have an LED bar-graph, with a few Red LED's to show the lean side, a few Green for perfect ratio, and a few Orange for the rich side. These LED's change instantly in direct response
to changes in throttle position, RPM and load on the engine (airspeed in our case, but also different propellers will change the load).  Unfortunately they are not cheap, and unfortunately the Lambda Sensors do not last very long on a
two-stroke engine, due to contamination from the oil. A wide-band sensor can set you back as much as R1200 every 20 to 60 hours of flight! Narrow-band sensors are cheaper, last longer, but less useful, and they require quite
sophisticated programming to interpret the data to deliver a meaningful display to the pilot. The better sensors have a heating element to get them up to operating temperature very quickly (minimising damage from contaminants too),
but require on-board 12V power supply (or a very large battery!) 

In the absence of an EGO, the next best is to do a plug-chop, one at full power, and another at cruise power. 

To do a plug-chop.... Start the motor up, get it thoroughly warm, then quickly fit a brand new plug, re-start as soon as possible before the engine cools down, and launch as soon as you possibly can (i.e. launch within 60 seconds of
killing the engine). Then immediately go to FULL POWER, and stay there at full power for about 3 minutes or so....then while holding the throttle wide-open, kill the engine. It is absolutely important that you do not back off the
throttle at all until the engine is dead. Now land, pull the plug and inspect the coating on the white ceramic part of the plug, and look for traces of aluminium deposits anywhere else on the exposed parts of the plug. 

Then take another brand new plug, and repeat the above process, but this time, as soon as you launch, go to your normal cruise power level (level flight at your normal trim setting), and hold it there for about 5 to 10 minutes, without
any changes in throttle setting, and chop the plug (kill the engine while at that throttle setting). This longer time is required due to the lower running temperature at lower power level. Land, pull the plug and inspect. 

A "normal" paramotor should be slightly leaner in the mid-range at cruise power than at full power, as it is more likely to overheat at full-power, hence a bit more fuel to quench the engine, with a bit more oil being brought in too, to
help lubricate the stressed components (rings, bearings, etc) at full power. 

Any chemical fuel, whether it is petrol, diesel, paraffin, LPG, wood, coal, or whatever for mechanical systems, also food-fuel for biological systems (sugar, starch, fat, etc), should contain Carbon and Hydrogen. Those are the only two useful elements in any fuel, hence the terms "Hydro-carbons" for engines, or "Carbo-Hydrates" for people and other living organisms. Carbon oxidises to form Carbon Di-oxide (H2O) and Hydrogen oxidises to form water (H2O) in the form of steam, which in the ideal situation, should be the only two gases exiting the exhaust. 

Unfortunately most fuels contain complex compounds containing other elements which are not conducive to generating the heat required to fuel the system. These are the "contaminants" which build up on your spark plug in the form of
partially-burnt products of combustion, usually a mixture of soot, caramel and varnish with many other elements mixed in. With a relatively clean fuel such as petrol, these should leave a thin layer of coffee colour or caramel colour on
your plug, most easily witnessed on the white ceramic surface of the plug.

Now you go and blend in 2.5% oil into your petrol as a sacrificial lubricant. This oil is going to burn black, no matter what, and confuses the whole issue of reading a spark-plug, which is why you need to do a proper plug-chop with a
brand new plug, and read the very thin layer built-up on the ceramic during a constant RPM run. 

That plug picture you posted has many hours on it, across a wide range of throttle and RPM, including most likely a lot of engine idling while you prepare to launch and on your glide down to land. It cannot tell you much at all, except if your engine was running VERY, VERY lean, then you would see small specs of aluminium pieces from your piston speckled on the plug, but then you would probably have burnt a hole through your piston crown anyway,
resulting in an engine-out. 

Better to remove the cylinder-head and inspect the crown of the piston to see if it still has the perfectly smooth convex surface without a flat-spot or concave section near the center. 

The heat range of the spark plug can not affect the running temperature of your engine. A hotter plug does not make your engine run hotter. You cannot save your engine from over heating or seizing by fitting a cooler plug.

About spark plug heat range (July 2013)

The heat range of a plug is designed to keep the central electrode at operating temperature.

The outer electrode, the curved tip, is an extension of the metal thread, which is in direct contact with the cylinder head which carries its heat away.

However the central electrode needs to be electrically insulated from the outer electrode, cylinder-head and all other metal, as the entire engine is electrically negative (connected to the negative of the ignition coil). If there were any metal connection to the central electrode to help carry its heat away, then the ignition will not work.

As the heat builds up, so the temperature of the central electrode and anything touching it, risers.

Read that previous paragraph again. Temperature is the CONCENTRATION of heat. Just like pressure is the concentration of fluid in a container. Add more fluid and the pressure goes up. Make the container smaller and the pressure goes up. Same with heat flowing into something, raising its temperature.

Now... How do you electrically insulate the central electrode from the engine, yet conduct away most of the heat? Its a challenging problem. Metal is a great heat conductor (as well as a great electrical conductor). So we need a material that is a good heat conductor, but does not conduct electricity at all. Can you think of one? Ever noticed how a sheet of glass always feels cool to the touch no matter how warm the weather is? That is because the glass conducts the heat away from your fingertips very quickly and efficiently. Glass is a fantastic conductor of heat, but does not conduct electricity, i.e. it is an electrical insulator.

So they use glass to hold the central electrode of a spark plug in place, to electrically insulate it from the negatively charged metal, yet to thermally connect it to the metal to conduct the heat from the central electrode, to the metal jacket of the plug, then on to the cylinder-head and its radiating cooling-fins.

The white ceramic insulator is just a stronger form of glass, otherwise they would be too fragile.

Now... That central electrode must not be kept too cool, as it will foul up with the residue of products of combustion, that black gunk, which will cause the plug to malfunction.

At the same time, it must not get too hot, as steel burns readily if you get it hot enough, which will burn away the central electrode.

You don't thing that steel can burn? Google "Thermic Lance" which is where steel rods are used as fuel to burn through and cut up ship hills underwater. Place some Thermite on any chunk of steel, which quickly brings steel to its own ignition temperature and the steel burns faster than firewood. But that is dangerous, so here is an experiment to show you can burn steel with a humble Lion Match or cigarette lighter. Take very fine steelwool and tease it out, Making the ball larger and less dense. Put a flame to it and stand back. Amazingly, steel burns! The fine steelwool cannot conduct the heat away from the flame fast enough, so its temperature quickly reaches the ignition point of steel. Mild Steel ignites at 1300C. The only reason a match cannot get a chunk of solid steel to ignite, is that steel is such a good heat conductor that the chunk soaks away the heat so fast, that the point where your flame touches, does not reach 1300C despite a butane lighter producing a 3000C flame, more than twice the ignition temperature of the steel.

OK, so the spark plug's central electrode can and will burn away if it permitted to get too not.

But too much cooling and it fouls up.

So now what?

The clever engineers who design spark plugs, design the "heat path" from the central electrode, to the metal jacket, by changing the shape of the ceramic insulator.

On a high performance engine that produces a lot of heat, they create a short heat path, with the central electrode just poking out of the ceramic and the ceramic entirely filling the gap to the metal jacket.

On a low performance engine producing much less heat (think of a 3hp four-stroke lawnmower engine), they cut a very deep groove in the ceramic jacket, all the way around the central electrode. The heat must flow a long way to reach the cooler metal jacket. This helps to retain more heat, and tries to keep the central electrode at operating temperature.

Now you have the tools to decide for yourself if your plug is running perhaps too hot (gap widens over time) or running too cool (plug keeps fouling up even if your oil/petrol mixture or fuel/air mixture is not too rich), or just right like Goldilocks' third bowl of porridge which was neither too hot nor tool cool.

If you do a lot of thermalling with your engine idling, the plug will foul up as an idling engine cannot keep the plug hot enough to burn away the gunk. You could fit a hotter plug, but then you may have to close the plug gap again occasionally, until the central electrode becomes too short for practical use. Also you need to monitor whether the hotter plug perhaps gets so hot that it ignites the charge before the actual spark fires. This can be seen where the kill switch does not kill a hot engine. This problem can and will damage the engine as the pre-ignition is too early, trying to drive down a piston that is still rising, placing enormous load on the crank, conrod, piston-bearing and piston crown.

So unless you know what you ate doing, stick to the engine manufacturers specified plug and get expert advice before changing.

One change we all do in SA though, is replace the original number 10 plugs with number 9 when the original becomes due for replacement, as the 10 plugs are simply not available in SA for various reasons. This is why most of us prefer to use a 9 plug that does not have a mild-steel central electrode. The S for steel is replaced with a G for Platinum, or a I for iridium. So a B10ES is replaced with a BR9EG or BR9EIX. The R is to add a RF (Radio Frequency) supressor for better radio communications. The X is for a booster-gap on the iridium plug

I hope this clears up the matter. 

All the best, 

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Keith Pickersgill.  Cape Town, South Africa