3. FACTORS AFFECTING PLUG PERFORMANCE

1. FACTORS AFFECTING THE TEMP. OF THE PLUG

Engine speed and load - as you increase RPM and load on an engine the temp. of the plug increases

Ignition Timing - great effect on plug temp., even after power curve has dropped off, increasing the timing with the engine under a load will result in a hotter plug temp.

Horsepower/Torque Increase - most changes that increases the power will result in an increase in plug temp. (ex. increasing comp.)

Cylinder Head Temp. - insulator tip temps. vary almost directly with cylinder head temp.  When changing cylinder heads the cooling capability around the spark plug should be considered.  Some later design heads have no water jacket around the plug hole.

Denotation - causes extreme and rapid rise in plug temp. which can then lead to pre-ignition and plug damage.

2. FACTORS AFFECTING THE VOLTAGE REQUIREMENT OF THE PLUG

Gap Spacing - With increases in gap comes an increase in "start up" voltage, but the "lock in" voltage remains relatively the same regardless of gap.  Ex. fluorescent light - a 1' long bulb or a 6' long bulb use a very similar amount of voltage to keep the gases ionized but the larger bulb requires a substantially larger voltage (ballast/transfmr) to initiate the ionization.

Electrode Temp. - as the insulator temp. and electrode decreases the voltage required to create a spark increases. This is not so critical at higher RPM where the plug temp. is relatively high regardless of the plugs heat range, but it could be critical on a cold plug/low RPM situation where a weak spark could promote fouling.  Note * lower power race engines require as much ignition as their higher power cousins because though igniting the mixture may be easier due to lower compression it's made more difficult because of the lower cylinder temp. and too good a fuel quality.

Fuels - fuel conductivity will affect the voltage requirement in that a very conductive fuel can, when it wets the spark plug, bleed off a portion of the electrical charge. A good example is methanol which is a very conductive and requires and very strong ignition system.

3. FACTORS AFFECTING THE EFFICIENCY OF THE PLUG

Plug Location - most important factor concerning the plug next to the heat range. The first 10% of flame burn greatly affects the remaining 90%, i.e. if the flame burn begins very slowly the remaining comb. process will take place at a slow rate, therefore the manner in which the flame front is initiated is very critical to the engine's performance. With that said it is obvious that the plug should be unshrouded to promote a larger initial flame kernel.

Some theories on plug location:

1. Plug should be centrally located in the combustion chamber so the flame can spread omni-directionally resulting in a faster burn rate. Minimal length of flame travel = faster burn rate = quicker pres. rise = less denotation tendencies

2. Plug should be located near the exhaust valve - having a hot exhaust valve located near the end gases promotes denotation, therefore locating the plug near the exhaust valve places the end gases in a cooler environment resulting in a decrease in denotation, this plug location also promotes rapid combustion because the heat from the exhaust valve promotes the 10% burn rate resulting in less ignition timing needed with a more rapid pressure rise.

3. Plug location relative to the swirl within the combustion chamber - there are two schools of thoughts on this:

A) Consider a turbulent mixture of vortices, if the spark occurs at the center of a vortex (a), the flame must spread without the aid of turbulence until it reaches the vortex boundary, on the other hand, ignition at the vortex boundary (b), will immediately aid the spread of flame because of the shearing action encountered.

B) A plug located on the vortex boundary can become so beaded with fuel wet fouling can occur (most common at lower RPM) ex. restart hesitations experienced by methanol fueled sprint cars

Multiple spark plugs - can reduce cyclic variation in the 10% fuel burn at lower RPM because there is a better opportunity for the ignition point to be in a proper air/fuel mixture and be near a vortex edge to propagate the flame. Also used in applications where large amounts of fuel are making it difficult to ignite the mixture, in this case 2 plugs are used with 2 separate ignition systems to create enough heat (current density) to reduce misfires. Can also reduce denotation because of reduced combustion time, provided the spark is retarded enough to hold peak pressure at optimum value. The negative side to this arrangement is that the multiple flame fronts create a lean spot where they meet thus creating less power and potentially harming the engine. In an ideal engine a multiple plug arrangement would show no benefit over a centrally located single plug but there is no ideal engine.