Can soeone explain hotness and coldness of spark plugs.

sizemoremk

Registered User
I tried to search, but couldn't find an explanation...

Also what is the significance of the length of the plugs?

Thanks!
 
Spark plugs are one of the most misunderstood components of an engine. Numerous questions have surfaced over the years, leaving many people confused.

This guide was designed to assist the technician, hobbyist, or race mechanic in understanding, using, and troubleshooting spark plugs. The information contained in this guide applies to all types of internal combustion engines: two stroke engines, rotary engines, high performance/racing engines and street vehicles.


Spark plugs are the "window" into your engine (your only eyewitness to the combustion chamber), and can be used as a valuable diagnostic tool. Like a patient's thermometer, the spark plug displays symptoms and conditions of the engine's performance. The experienced tuner can analyze these symptoms to track down the root cause of many problems, or to determine air/fuel ratios.

SPARK PLUG BASICS:
The spark plug has two primary functions:

To ignite the air/fuel mixture
To remove heat from the combustion chamber
Spark plugs transmit electrical energy that turns fuel into working energy. A sufficient amount of voltage must be supplied by the ignition system to cause it to spark across the spark plug's gap. This is called "Electrical Performance."

The temperature of the spark plug's firing end must be kept low enough to prevent pre-ignition, but high enough to prevent fouling. This is called "Thermal Performance", and is determined by the heat range selected.

It is important to remember that spark plugs do not create heat, they can only remove heat. The spark plug works as a heat exchanger by pulling unwanted thermal energy away from the combustion chamber, and transferring the heat to the engine's cooling system. The heat range is defined as a plug's ability to dissipate heat.

The rate of heat transfer is determined by:

The insulator nose length
Gas volume around the insulator nose
The materials/construction of the center electrode and porcelain insulator
 
A spark plug's heat range has no relationship to the actual voltage transferred though the spark plug. Rather, the heat range is a measure of the spark plug's ability to remove heat from the combustion chamber. The heat range measurement is determined by several factors; the length of the ceramic center insulator nose and its' ability to absorb and transfer combustion heat, the material composition of the insulator and center electrode material.

Heat rating and heat flow path of NGK Spark Plugs
chartheatratingflowpath.gif


The insulator nose length is the distance from the firing tip of the insulator to the point where insulator meets the metal shell. Since the insulator tip is the hottest part of the spark plug, the tip temperature is a primary factor in pre-ignition and fouling. Whether the spark plugs are fitted in a lawnmower, boat, or a race car, the spark plug tip temperature must remain between 500C-850°C. If the tip temperature is lower than 500°C, the insulator area surrounding the center electrode will not be hot enough to burn off carbon and combustion chamber deposits. These accumulated deposits can result in spark plug fouling leading to misfire. If the tip temperature is higher than 850°C the spark plug will overheat which may cause the ceramic around the center electrode to blister and the electrodes to melt. This may lead to pre-ignition/detonation and expensive engine damage. In identical spark plug types, the difference from one heat range to the next is the ability to remove approximately 70°C to 100°C from the combustion chamber. A projected style spark plug firing tip temperature is increased by 10°C to 20°C.
 
Tip Temperature and Firing End Appearance

charttempfiringend.gif



The firing end appearance also depends on the spark plug tip temperature. There are three basic diagnostic criteria for spark plugs: good, fouled and overheated. The borderline between the fouling and optimum operating regions (500&def;C) is called the spark plug self-cleaning temperature. The temperature at this point is where the accumulated carbon and combustion deposits are burned off.

Bearing in mind that the insulator nose length is a determining factor in the heat range of a spark plug, the longer the insulator nose, the less heat is absorbed, and the further the heat must travel into the cylinder head water journals. This means the plug has a higher internal temperature, and is said to be a hot plug. A hot spark plug maintains a higher internal operating temperature to burn off oil and carbon deposits, and has no relationship to spark quality or intensity.

Conversely, a cold spark plug has a shorter insulator nose and absorbs more combustion chamber heat. This heat travels a shorter distance, and allows the plug to operate at a lower internal temperature. A colder heat range is necessary when the engine is modified for performance, subjected to heavy loads, or is run at high rpms for a significant period of time. The colder type removes heat more quickly, and will reduce the chance of pre-ignition/detonation and melting or damage to the firing end. (Engine temperature can affect the spark plug's operating temperature, but not the spark plugs heat range).
 
Below is a list of some of the possible external influences on a spark plug's operating temperatures. The following symptoms or conditions may have an effect on the actual temperature of the spark plug. The spark plug cannot create these conditions, but it must be able to cope with the levels of heat...if not, the performance will suffer and engine damage can occur.

Air/Fuel Mixtures seriously affect engine performance and spark plug operating temperatures.

Rich air/fuel mixtures cause tip temperature to drop, causing fouling and poor driveability
Lean air/fuel mixtures cause plug tip and cylinder temperature to increase, resulting in pre-ignition, detonation, and possibly serious spark plug and engine damage
It is important to read spark plugs many times during the tuning process to achieve the optimum air/ fuel mixture
Higher Compression Ratios/Forced Induction will elevate spark plug tip and in-cylinder temperatures

Compression can be increased by performing any one of the following modifications:

a) reducing combustion chamber volume (i.e.: domed pistons, smaller chamber heads, mill ing heads, etc.)

b) adding forced induction (Nitrous, Turbocharging or Supercharging)

c) camshaft change
As compression increases, a colder heat range plug, higher fuel octane, and careful attention to igni-tion timing and air/fuel ratios are necessary. Failure to select a colder spark plug can lead to spark plug/engine damage
Advancing Ignition Timing

Advancing ignition timing by 10° causes tip temperature to increase by approx. 70°-100° C
Engine Speed and Load

Increases in firing-end temperature are proportional to engine speed and load. When traveling at a consistent high rate of speed, or carrying/pushing very heavy loads, a colder heat range spark plug should be installed
Ambient Air Temperature

As air temperature falls, air density/air volume becomes greater, resulting in leaner air/fuel mixtures.
This creates higher cylinder pressures/temperatures and causes an increase in the spark plug's tip temperature. So, fuel delivery should be increased.
As temperature increases, air density decreases, as does intake volume, and fuel delivery should be decreased
Humidity

As humidity increases, air intake volume decreases
Result is lower combustion pressures and temperatures, causing a decrease in the spark plug's tem-perature and a reduction in available power.
Air/fuel mixture should be leaner, depending upon ambient temperature.
Barometric Pressure/Altitude

Also affects the spark plug's tip temperature
The higher the altitude, the lower cylinder pressure becomes. As the cylinder temperature de-creases, so does the plug tip temperature
Many mechanics attempt to "chase" tuning by changing spark plug heat ranges
The real answer is to adjust jetting or air/fuel mixtures in an effort to put more air back into the en-gine
 
WOW! thatnks for the great question and the awesome answer!

this should be submitted to the FAQ. Someone email george or the other guy (dave,don)? i can never remember :eek: :eek: please dont hate me :p
 
Types of Abnormal Combustion

Pre-ignition

Defined as: ignition of the air/fuel mixture before the pre-set ignition timing mark
Caused by hot spots in the combustion chamber...can be caused
(or amplified) by over advanced timing, too hot a spark plug, low octane fuel, lean air/fuel mixture, too high compression, or insufficient engine cooling
A change to a higher octane fuel, a colder plug, richer fuel mixture,
or lower compression may be in order
You may also need to retard ignition timing, and check vehicle's cooling system
Pre-ignition usually leads to detonation; pre-ignition an detonation are two separate events
Detonation

The spark plug's worst enemy! (Besides fouling)
Can break insulators or break off ground electrodes
Pre-ignition most often leads to detonation
Plug tip temperatures can spike to over 3000°F during the combustion process (in a racing engine)
Most frequently caused by hot spots in the combustion chamber.
Hot spots will allow the air/fuel mixture to pre-ignite. As the piston is being forced upward by mechanical action of the connecting rod, the pre-ignited explosion will try to force the piston downward. If the piston can't go up (because of the force of the premature explosion) and it can't go down (because of the upward mo-tion of the connecting rod), the piston will rattle from side to side. The resulting shock wave causes an audible pinging sound. This is detonation.
Most of the damage than an engine sustains when "detonating" is from excessive heat
The spark plug is damaged by both the elevated temperatures and the accompanying shock wave, or concussion
Misfires

A spark plug is said to have misfired when enough voltage has not been delivered to light off all fuel present in the combustion chamber at the proper moment of the power stroke (a few degrees before top dead center)
A spark plug can deliver a weak spark (or no spark at all) for a variety of reasons...defective coil, too much compression with incorrect
plug gap, dry fouled or wet fouled spark plugs, insufficient ignition timing, etc.
Slight misfires can cause a loss of performance for obvious reasons (if fuel is not lit, no energy is be-ing created)
Severe misfires will cause poor fuel economy, poor driveability, and can lead to engine damage
Fouling

Will occur when spark plug tip temperature is insufficient to burn off carbon, fuel, oil or other deposits
Will cause spark to leach to metal shell...no spark across plug gap will cause a misfire
Wet-fouled spark plugs must be changed...spark plugs will not fire
Dry-fouled spark plugs can sometimes be cleaned by bringing engine up to operating temperature
Before changing fouled spark plugs, be sure to eliminate root
cause of fouling
 
Well I'm running the autlite 103s, probabbly becaue seomone here told me they were stock equivalent. I thoguht they were about stock.

That may explain why the engine doesn't start as quickly when it has been shutoff for a short period of time? Perhaps they are just a tad bit fouled. I did see where some folks run this plug to help against pinging.

I also read that if you run N2O, that you may want to use a colder plug.

So this may work out great, as I want to hit the bottle in the near future!
 
My only heartburn with all that is the implication that the spark plug is what is cooling the engine :eek: :eek: That's just plain wrong! The ceramic is not a good conductor of heat. Its benefit is its hard and stands up combustion pressures and doesn't conduct electricity.. again, the primary purpose of a spark plugs ceramic is as a high voltage electrical insulator, not to pull heat out of the combustion chamber. In fact, ceramic is a mediocre to poor conductor of heat, and the more ceramic, the slower the heat travels through it. Aluminum and copper, and metals are materials which transfer heat energy more effectively. e.g.: http://230nsc1.phy-astr.gsu.edu/hbase/tables/thrcn.html#c1

The heat primarily travels not through the spark plug, but through the walls of the cylinder (that's why coolant flows around them) and through the heads into the coolant.. the amount of heat which is conducted out that teensy little spark plug is miniscule.

OK, just wanted to clarify that just in case anyone was misled.
 
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The NKG article perhaps isn't clear, but the cooling they discuss is related to the temperature at the tip of the spark plug. This temperature is controled by how much of the insulating material is in contact with the steel body of the spark plug which is in contact with the head.

It is the head that removes the heat from the spark plug. The length of the insulating conductor not in contact with the steel plug body determines the speed at which the tip of the plug can have it's heat removed. Thus a plug with a longer ceramic center section is considered to be "hotter" than a plug with a shorter ceramic center section.

The actual length of the ceramic center section not in contact with the plug body may not always be visible due to the plug construction.
 
The article clearly states that the spark plug helps to cool the combustion chamber, not the engine. You have the metal of the spark plug inside the combustion chamber-the metal transfers the heat of the combustion chamber up through the head and into the cooling jackets.

If the cooling was taking place through the cylinder walls and the head as TbirdSCfan suggests, then engines would be designed with water jackets right over the combustion chamber-and that would mean that you'd have to restrict the airflow into the cylinder to provide adequate cooling.

I sure as heck didn't see anyone else jump up and take a shot at finding the answer! :rolleyes:
 
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Mike8675309 said:
The NKG article perhaps isn't clear, but the cooling they discuss is related to the temperature at the tip of the spark plug. This temperature is controled by how much of the insulating material is in contact with the steel body of the spark plug which is in contact with the head.

It is the head that removes the heat from the spark plug. The length of the insulating conductor not in contact with the steel plug body determines the speed at which the tip of the plug can have it's heat removed. Thus a plug with a longer ceramic center section is considered to be "hotter" than a plug with a shorter ceramic center section.

The actual length of the ceramic center section not in contact with the plug body may not always be visible due to the plug construction.
That is likewise my understanding. The beginning of the article seemed to start down another path indicating that the plug is what was cooling the combustion chamber which makes 0 sense. The title of the thread was to eliminate confusion, but the manufacturer themselves only created more by wording such as:
SPARK PLUG BASICS:
The spark plug has two primary functions:

To ignite the air/fuel mixture
To remove heat from the combustion chamber
.. OK.. looks to me like the implication is that the plug is what is cooling the combustion chamber which is a bogus statement... the heat from combustion travels primarily out of the chamber in the form of exhaust gas.. thats the bulk of the heat. The remaining built up heat is transferred to the piston, the clinder walls, the valves, the heads, and our teensy little highly insulated (read low heat conductivity) device called the spark plug.

Alas, the article continues to foster confusion. :rolleyes: :rolleyes:
 
Perhaps the confusion comes from a lack of clarity in the statement:
It is important to remember that spark plugs do not create heat, they can only remove heat. The spark plug works as a heat exchanger by pulling unwanted thermal energy away from the combustion chamber, and transferring the heat to the engine's cooling system.

Yes, a spark plug does act as a heat exchanger pulling unwanted thermal energy out of the combustion chamber. But it does so only in a very specific region. The tip of the spark plug. This is significant as the spark plug tip is the hottest region within the combustion chamber.

In the NKG piece above they reference this only vaugly and imply that it's role is greater than what it truly is.
 
****-from now on I don't try to help these guys with their questions-you wanna-be engineers can spread your lack of knowledge around.

Bloody amatuers.
 
Well, I for one got some good knowledge out of it, and certainly appreciate it.

I just didn't think about searching spark plug manufacterers website...

The questions were answered pretty well I thought, and then some.

There was some complaints about the wording, but I think the main points were explained well...

Thanks!
 
Birdman93 said:
****-from now on I don't try to help these guys with their questions-you wanna-be engineers can spread your lack of knowledge around.

Bloody amatuers.
In my case, I'm not a wanna-be, I am. ;) ;)I have taken courses in materials, fluid dynamics, heat transfer, etc. but you don't need very much of that to understand basic heat transfer concepts.

And I'm not knocking you for posting what you found, but what you found has problems; that's all.. and assuming you weren't the author, I don't see why you would be offended when the article is criticised.

The only reason I brought it up was because someone reading could wrongly assume that a spark plug could make their engine run hotter or colder. It is your Air/Fuel/Foul-gas (EGR, PCV gas) ratios, spark timing and duration, and compression ratio which determine how hot the engine gets. :D
 
I got a lot of good info out of that article as well, and would like to thank Birdman93 for going through the trouble of finding and posting it. Also, thanks to Mike and TbirdSCFan for clarifying some of the points. When I read the article, I didn't really get the impression that the author was stating that the plugs help cool the engine, but understood that the design of the plug helps cool the heat generated at the tip of the plug.

So now I can say I learned something new today! :)
 
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