Ignition leads – technical information
The ignition system must produce about 20,000 V across the spark plug gap at the right moment in the right cylinder; ignition leads carry this voltage from the coil to the plugs while suppressing radio interference. SENTECH leads use a stainless conductor spirally wound on a ferromagnetic core, combining inductive and resistive interference suppression. Lead classes A to F define temperature ranges from 80 °C to 180 °C.
The ignition system
The ignition system has two tasks:
- to create a spark of sufficient voltage (20,000 V) across the spark plug gap to start combustion of the air-fuel mixture in the combustion chamber,
- to make sure the spark occurs at the right time and in the right cylinder.
The basic operating principle of the electrical ignition system has not changed for 75 years. What has changed is the method of producing the spark. Cars (engines) have evolved from a mechanical system (distributor) to an electronic one. Both systems control the low voltage in the coil's primary circuit, which induces a high voltage in the secondary circuit.
What are ignition leads for?
Ignition leads carry high-voltage current from the coil to the spark plugs, in the order needed to produce the spark that ignites the fuel, while preventing harmful high-frequency electromagnetic fields (radio interference).
SENTECH ignition leads – advanced technology
SENTECH technology is the result of laboratory research, and every element of it is innovative. Every component of the manufacturing process – cable, terminals and boots – is inspected and tested.
Cable
The high demands of vehicle manufacturers and the aim of "zero defects" over a long service life call for the best materials:
- Electrical insulation (insulating layer) – a layer of plastic, thermoplastic elastomer or synthetic elastomer that provides suitable dielectric properties and minimises the voltage drop between the coil and the spark plugs.
- Outer jacket – provides mechanical and thermal protection of the cable against the engine environment and external conditions (temperature, oil, fuel, salt water, humidity, cold).
- Braid – made of polyester (polyaramid) or glass fibre, which withstands mechanical stress. This ensures that the terminal crimped onto the cable holds properly.
Metal terminals cannot be crimped directly onto the outer jacket without a braid, because:
- the metal terminals could tear the insulation when pulled,
- the outer diameter of the insulation or dielectric could shrink (or expand), allowing the terminal to come off.
The braid binds the insulating layers together and stops the insulation from shrinking in diameter. As a result, the cable cannot stretch, yet it stays flexible.
Terminals
- Their shape is designed for the application (spark plug nut, male terminal, female terminal) and the dimensions. The terminals are designed for automatic assembly and automatic crimping.
- Their job is to provide a mechanical and electrical connection between the core of the ignition lead and the relevant ignition system components (coil, distributor, spark plugs).
Boots
- The main purpose of the boots is to protect and dielectrically insulate the high-voltage connections. Their material is carefully selected and their shape is designed specifically for each application to ensure a tight grip on the connecting parts (spark plug, distributor or coil).
See also: construction of a SENTECH ignition lead.
SENTECH cable – original technology
SENTECH high-voltage lead technology is unique. A very thin synthetic core is covered with a ferromagnetic material that absorbs electromagnetic interference and conducts high-voltage current without energy loss. This prevents interference in the low-voltage electrical system.
This technology makes it possible to meet the strictest radio interference standards. Interference is reduced along the entire length of the cable, eliminating interference with the radio, TV, mobile phone and other electronic equipment in the car.
SENTECH – cable suited to different types of engine
During production, SENTECH leads are marked with ink that does not wear off in use. This marking allows full identification and indicates the cable class according to its operating temperature.
| Cable class | A | B | C | D | E | F |
|---|---|---|---|---|---|---|
| Maximum temperature (°C) | 80 | 100 | 120 | 140 | 160 | 180 |
| Minimum temperature (°C) | -20 | -30 | -30 | -30 | -30 | -30 |
| Overload temperature (°C) | 105 | 120 | 155 | 180 | 220 | 250 |
Comparison of technologies available on the market
There are three technologies for high-voltage cables that can suppress the electromagnetic interference generated during combustion in a spark-ignition engine.
Copper core with resistors
The copper core is the oldest ignition lead technology. It is used mainly by German manufacturers and works as follows: a copper core (whose resistance can be taken as zero) is combined with a resistor placed as close to the spark plug as possible (in the boot). The resistor reflects part of the electromagnetic waves back (like an echo), another part is radiated into the environment, and the rest is carried to the vehicle earth. Because the waves are weakened, they cause less interference in the environment.
However, this resistance has a negative effect on combustion: not all the energy passed from the coil through the distributor reaches the spark plug. See also: SENTECH copper core lead.
Carbon core
This technology is used mainly by British and American manufacturers. It uses a carbon core whose resistance is distributed along the length of the cable. Carbon core ignition leads have a short service life and do not guarantee consistent conductivity over time – in practice, resistance can vary by a factor of 1 to 10. Because of their short life, these leads need frequent replacement.
These two solutions are reasonably effective at low frequencies (a few MHz). In reality, however, they are unsuitable for high frequencies, because their resistance is the same regardless of frequency.
Remember: high resistance = energy loss = extra pollution.
Spiral core (SENTECH technology)
A stainless electrical conductor is wound onto a ferromagnetic core. A magnetic field forms inside the lead when high-voltage current flows through it – a unique construction. Excellent interference suppression results from two effects:
- Inductive effect – electromagnetic interference is concentrated and then absorbed by the ferromagnetic material.
- Resistive effect – the spirally wound conductor has a resistance of 5,600 Ω/m. The resistive effect is therefore fully effective at low frequencies and is further supported by the inductive effect. This technology is ideal across the whole frequency range, especially at high frequencies.
Spiral core technology combines the advantages of the other two technologies without repeating their drawbacks.
In this section
- SENTECH lead constructionA SENTECH ignition lead consists of a synthetic elastomer outer jacket, a reinforcing braid or tape, an insulating layer with excellent dielectric properties, a stainless conductor wound at 50 turns/cm on a ferromagnetic core, and a glass-fibre and polyaramid support core. The conductor and ferromagnetic material together suppress interference along the whole cable and give a stronger, longer spark.
- Copper core leadThe SENTECH copper core ignition lead has four layers: a synthetic elastomer outer jacket resistant to heat, cold, water, brine, oils and fuel; a reinforcing braid or tape; an insulating layer with excellent dielectric properties and a high breakdown voltage; and a copper core.
- Reading an oscilloscope patternThe secondary high-voltage waveform at the spark plug shows coil charging (E'–A'), voltage build-up until the spark jumps (B'), the spark line or arc phase (C'–D') and the decaying oscillations. The area under the curve between A' and D' is a measure of the spark discharge energy, and the firing voltage depends on the electrode gap, mixture composition, cylinder temperature and electrode shape.
- Breakdown after replacementInsulation breakdown on new ignition leads is caused by abnormally high voltage in the secondary circuit. The three main causes are too large a spark plug gap, an open circuit due to mismatched terminals (e.g. M4 vs DIN or barrel vs threaded), and a faulty ignition coil or module. Oscilloscope patterns show how mismatched connections reduce spark energy and can stop the coil discharging.
- LaboratoryThe SENTECH laboratory can carry out all tests required by ISO 3808, ISO 6856, SAE J2031 and SAE J2032, as well as other tests agreed with customers. Its test stations measure resistance, capacitance, terminal pull-off force, insulation resistance up to 5 kV and high-voltage strength up to 120 kV. Thermal chambers cover +300 °C to −50 °C for ageing and thermal shock tests.