Spark Plugs (III): Types of Abnormal Combustion

Note: This is the last posting for Spark Plugs, see my previous posting for spark plugs.

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 cause (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.

Source: NGK Tech Info

Spark Plugs (II): External Influences

Below is a list of external influences on a spark plug's operating temperature. 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:
• reducing combustion chamber volume (i.e.: domed pistons, smaller chamber heads, mill ing heads, etc.)
• adding forced induction (Nitrous, Turbocharging or Supercharging)
• camshaft change
• As compression increases, a colder heat range plug, higher fuel octane, and careful attention to ignition 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, 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 temperature 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 decreases, so does the plugs tip temperature
• Many mechanics attempt to "chase" tuning by changing spark plug heat ranges
• The real answer is to adjust air/fuel mixtures by rejetting in an effort to put more air back into the engine

Source: NGK Tech Info

Spark Plugs (1): The Basic

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

I quote the above sentence from the famous spark plugs manufacture NGK site.

The spark plug has three primary functions:

• Ignite air/fuel mixture
• Transfer heat from the combustion chamber
• And transmit the electrical energy that converts fuel into working energy.

It is important to note that spark plugs never create heat, only remove it. The temperature at the end of the armature has to be kept low enough to prevent pre-ignition, but at the same time high enough to keep from fouling. The plug also works as a heat exchanger by transporting thermal energy from the combustion chamber to the cooling system. The term heat range is the plug’s ability to dissipate heat from the tip, but has nothing to do with the actual voltage transfer.

The plug’s efficiency is also determined by variety of factors such as insulator tip length, the absorption and transfer of combustion heat, the gas volume surrounding the insulator tip, and the construction/materials used in the center electrode and insulator. Regardless of spark plug preference, the difference in the heat range is the measured ability to remove 70 to 100 degrees from the combustion chamber. The longer the spark plug tip, the longer the travel of heat before it is absorbed by the cylinder head, retaining more heat equaling a hotter plug. Conversely, a shorter tip equals a colder one.

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. Conversely, if the tip temperature exceeds 850 degrees, the ceramic material around its tip will blister, causing the electrode to melt. The temperature range between 450 and 850 degrees is loosely defined as the spark plug cleaning region. At these temperatures, the accumulated deposits burn away from the center isolator. In addition, this region has no bearing on spark quality or its intensity.

Spark Plugs Selection and Installation

• Select a plug heat range that operates near the center of the normal operation range.
• Check with the owner’s manual for recommended gap settings.
• Always use a wire-type feeler gauge to check the gap
• If adjustment is necessary, use a gapping tool to carefully bend the armature. Never hit, pry or use pliers against the center insulator or armature to widen the gap.
• When installing spark plugs, clean both sets of threads, and always use torque wrench at the correct setting.