Question about fine-wire spark plugs...

Why don't they use tungsten instead of platinum/gold/palladium for fine-wire electrodes? It melts at a higher temperature, plus it has better thermal conductivity than platinum and palladium.

I don't know if I just answered my own question, but would the tungsten absorb the electrical current and convert it into heat? That's not good...

tungsten is extremly resistive iirc.

From what I could find, gold, platinum and palladium are less reactive than other metals so they won't erode easily. This would be important when using such thin wires. You're on the right track though. One of the advantages of the new iridium wires is that it has a higher melting point. Here's an excerpt I found:

"A fine wire center electrode. This makes the best use of the expensive platinum metal. The fine wire requires less voltage to fire, at any engine speed, optimizing cold starts, hill climbing and passing lane power."

So, it's easier for the spark to jump from a sharp point rather than a big round wire, BUT, this means that the spark can jump sooner, before a larger current can be built up. Therefore, the spark is thinner and weaker than one carrying a lot of current.

So are you saying that the spark won't ignite the mixture?

Well, even a weakish spark must work most of the time or they couldn't sell the plugs but I'm guessing there must be cases where igniting the fuel must be more difficult. I don't know what those conditions would be though. Actually, I haven't heard of this particular problem, but I have heard that these kinds of plugs have more of a tendency to get fouled by carbon forming a conductive path on the ceramic insulator and by deposits clogging the tiny hole the spark has to emerge from.

I thought that spark plugs were supposed burn off deposits at the correct operating temperature? Excuse my ignorance - I'm kind of new to cars.

Also, what determines the temperature of the spark? Energy? Current? Voltage?

You're right, plugs are supposed to burn off deposits at operating temp, but if the deposits form faster than they can be burned off, such as during slow speed driving and short trips where the engine doesn't have time to warm up completely, then the deposits will build up. I haven't heard of any plug that was immune to deposit formation or any of the other problems that can develop, it's just that some are more prone to develop certain problems than others. As with many things, you have to compromise and try to pick a plug that'll work best for your type of driving.

Just in case you're wondering, plugs don't add heat to the cylinder, they only take it away. A hotter plug does not produce a hotter combustion. That only means that the tip of the plug is a few degrees hotter than the next cooler plug. If you are getting deposits on your plugs, a hotter plug may help burn them off but it could also cause preignition of the fuel/air mixture which hurts the engine and robs you of power. I do a lot of slow driving around town and everyone I've ever asked about going to a hotter plug to make sure I didn't get deposits forming has recommended that I stick with the factory suggested plug heat range. This is also what I've seen in my own research online. Plugs are designed for a variety of driving conditions so while deposits may form when driving slowly, they'll also burn off when you get on the expressway. The tip of the plug should be able to heat up fast to burn off deposits as soon as possible, but it shouldn't get too hot or preignition will occur. Copper core plugs are especially good at staying at a good operating temp regardless of driving speed.

As for the temp of the spark, I think voltage, current and time are important. Voltage times current = watts which is an overall measure of energy. A spark that lasts a relatively long time also helps. A high voltage spark with low current is a very skinny spark. This is like a static electricity spark - many thousands of volts but only microamps of current and safe to touch. A spark with high current is very fat and can burn you easily if touched. But if the voltage is too low it will have trouble jumping the plug gap. The voltage produced for plugs is usually around 10,000 to 30,000 volts (there's a lot of variation). While this may seem to be more than enough to jump the tiny plug gap, it is needed inside the cylinder. The high pressures in there make it harder for the spark to form. When it does, a fatter spark, which lasts long enough, and which is well exposed to the fuel/air mixture and not shielded by ground electrodes or laying up against an insulator will have an easier time igniting the fuel.

If you have any more questions, just keep asking. Jim.

Thanks for the info. :)