DENSO explains spark plug technology

Despite the growth in electric drivetrains, the internal combustion engine (ICE) is still the most common propulsion technology visiting workshops across Europe. In addition, with the pivotal role they continue to play in the powertrain of hybrid vehicles, they will remain at the centre for the independent sector for the foreseeable future.

Although many ICEs are powered by diesel, petrol is increasingly common as it is the favoured fuel for most modern hybrids, so understanding spark plug technology and selecting the most appropriate reference for the application is as important as ever.

Construction

To be able to work effectively and efficiently under difficult operating conditions and to achieve the correct performance requirements, spark plugs are constructed in three main sections: the housing, insulator and electrodes, and these sections then contain individual components that are made from carefully selected materials.

The example below shows the construction of a DENSO Iridium Power Spark Plug.

The housing (item 1) forms an outer shell that surrounds and supports the insulator and secures the spark plug assembly to the engine. The ring washer or gasket (item 2) provides an insulating seal between the spark plug housing and engine to prevent any gases escaping during compression and combustion. The ground electrode (item 10) is mounted to the threaded lower section of the housing, which allows electric current to flow through the engine back to the battery.

The ceramic-based insulator (item 3) provides electrical insulation between the terminal, centre shaft, housing and centre electrode. With spark voltages on some modern ignition systems exceeding 40kV, the insulator, with a thickness of only millimetres, must provide the required insulating properties. As a result, DENSO spark plugs incorporate a ceramic material with high purity aluminium oxide to provide superior heat resistant characteristics, mechanical strength and excellent electrical insulation. The two ring gaskets (item 4) provide a secure fit and airtight seal between the housing and the insulator.

The high voltage is supplied through the terminal (item 5), either from a direct fit to the base of the ignition coil or by using a spark plug wire/cord that connects the plug terminal to the coil. Different terminal types are produced, which allows almost any high tension spark plug wire/cord or ignition coil to be connected to the spark plug terminal.

DENSO offers four different terminal types:

• Thread (with no terminal nut, used for motorcycles and older types of cars)
• Terminal nut (thread with nut that is easy to unscrew)
• Crimped (thread with crimped nut for better connection between the nut and thread, but although the nut can be removed, it is more difficult)
• Solid (solid terminal for car applications, which cannot be removed)

The steel centre shaft (item 6) connects the terminal and the centre electrode and allows high voltage current to flow from the terminal to the centre. The resistor (item 7), which is also referred to as a suppressor, reduces the peak current of the spark. Without a resistor, the peak current will create bursts of electromagnetic fields or radio noise, which can interfere with electrical equipment in the car. DENSO Spark Plug resistors are made of a special glass and copper powder mix.

The centre electrode (item 9) is made from materials, such as nickel alloys, that can resist high temperatures. These materials must also be extremely hard and durable to minimise the wear caused by spark erosion. The centre section of the electrode (item 8) often contains a copper core to improve thermal conductivity.

For improved performance and durability, centre electrodes can also be manufactured with an electrode tip made from precious metals that are even tougher than the traditional electrode materials. These harder wearing materials can operate at higher temperatures with reduced wear, but another major benefit is that they allow for the use of finer electrodes, resulting in better ignition performance.

DENSO Spark Plugs feature several unique patented materials:

• Standard 2.5 mm diameter nickel alloy
• New and unique 1.5 mm diameter nickel alloy (used in nickel TT spark plugs) that reduces spark wear by up to 40% compared to standard nickel
• Platinum, a precious metal that can withstand very high temperatures, with 1.1 mm diameter electrodes
• High purity iridium alloy that is resistant to the highest temperatures and is the hardest material ever used in a spark plug. These iridium tips with diameters of 0.4 mm, 0.55 mm or 0.7 mm are laser welded to the centre electrode

These smaller electrodes lower the required voltage, ensure a reliable spark and reduce the quenching effect, so improving ignition performance.

The ground electrode (item 10) is subject to the extreme temperature changes inside the combustion chamber and although a nickel chrome alloy is used for most of these electrodes, platinum can be added to extend their life. Some spark plugs however, are equipped with a copper core ground electrode for improved thermal conductivity.

DENSO uses some special features for the ground electrode to improve ignition performance:

• Patented U-groove, which increases the edge section, allowing sparks to occur easily and the flame to grow faster
• Taper cut ground electrode, shaped to reduce the quenching effect and improve flame growth
• Needle type with either 1.5 mm diameter protruded nickel electrode (nickel TT) or a 0.7 mm diameter welded platinum electrode (SIP and Iridium TT)

As with the centre electrode, the smaller needle type ground electrodes lower the required voltage, ensure a reliable spark, which reduces the quenching effect and improves ignition performance.

The DENSO Spark Plug Range

Frequently Asked Questions

How to select the correct spark plug?

Although there are many factors to consider such as their dimensions, projection and heat range, the easiest way to identify the right spark plug is via the DENSO e-catalogue denso-am.eu/e-catalogue, as this allows technicians to find the correct DENSO option based on make and model information or cross references.

What about spark plugs for LPG and CNG applications?

LPG (liquified petroleum gas) and CNG (compressed natural gas) engines remain popular options throughout Europe, but require a higher performance spark plug than petrol engines. The DENSO TT range provides a suitable upgrade to meet these requirements and details can again be found in the DENSO catalogue.

Should the electrode gap be adjusted?

Although DENSO Spark Plugs are manufactured with a pre-set gap, with nickel plugs, especially for motorcycle applications, the gap may need adjustment. However, any alteration must be made using a specific spark plug gapping tool, but the gap must never be changed on platinum, iridium or Twin Tip spark plugs, because their fine electrodes may be damaged.

What is pre-ignition?

Pre-ignition, or auto-ignition, occurs when the air/fuel mixture combusts before the engine’s electrically timed spark occurs. This can happen because of a hot surface in the combustion chamber and may be caused by:

(1) An overheated tip of the spark plug (the wrong heat range is selected)

(2) The exhaust valve is too hot

(3) Carbon deposits within the cylinder are still glowing from the previous combustion cycle

What is engine knock (detonation)?

Engine knock is often a result of pre-ignition. Instead of progressively burning, the air/fuel mixture explodes in an uncontrolled manner, which can be terminal for the spark plug and could also cause serious engine damage.

Further details of the DENSO Aftermarket programme are available online at: www.denso-am.eu