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Ignition Coil Developments

Lifting the bonnet of today’s cars presents a totally different view from that of less than two decades ago. The first view is of a much tidier looking engine, usually shrouded in plastic and surrounded by many unfamiliar looking components.


Gone is the carburettor, replaced by electronic fuel injection. Gone too is the distributor and ignition amplifier to be replaced by the ECU.

The ignition coil is still there but not in the familiar design and location – and what about the plug leads? Distributor-less ignition systems have been around for many years, with the spark being supplied to the cylinders by a single coil, but in recent years the trend has seen a move towards multi-coil systems.

Multi-coil or Coil on Plug systems have become the way forward for a number of reasons, in particular reliability, performance and cost reduction. Removing the plug leads not only reduces cost and assembly line time but also eliminates a potential source of trouble and possible radio interference.


Benefits of distributor-less ignition systems include:

  • Improved fuel economy over conventional ignition systems
  • Improved Exhaust Gas Recirculation tolerance
  • Lower emissions over conventional ignition systems
  • Reduced electrical system load
  • Increased resistance to spark plug fouling
  • Improved combustion stability at idle speed
  • Improved idle stability


Modern Ignition Systems

There are several types of direct ignition coils currently used on vehicles. The first is the “cassette” or “rail” type of coil, which runs along the top of the engine block and has an outlet for each cylinder that goes directly on to the spark plug.

The second type is the pencil or mushroom coil which is an individual coil for each cylinder. Each coil is individually connected to the ECU, allowing the spark to each cylinder to be controlled independently.

The third and less common type is the “wasted spark” or “semi-direct” coil, where a coil on plug with two outlets is used, one of which feeds a HT lead connection to another cylinder. Effectively, one coil is shared between two cylinders. This coil fires a spark to both cylinders simultaneously. One cylinder will be on the combustion stroke of its cycle and ignition of the fuel/air mixture will take place, whilst the other cylinder will be on its exhaust stroke resulting in the spark being “wasted” into the exhaust gases.

Using a separate coil for each cylinder gives each coil more time between firing, increasing the “coil saturation” time. This increases the coil’s output voltage leading to greater spark energy at the plug. Coil outputs in excess of 20,000 Volts are sometimes specified by engine designers to help improve combustion and reduce the risk of misfire at high engine speeds with lean fuel mixtures.

The coil switching on multi-coil ignition systems is handled by the engine management control unit (ECU). The ECU receives a timing signal from the crankshaft or camshaft position sensor to determine engine speed, firing order and ignition timing. Inputs are also monitored by the ECU from the throttle position sensor, airflow or MAP sensor and coolant temperature sensor among others. The ECU uses this information to control the ignition timing to each individual plug and is able to make timing adjustments between cylinder firings, adapting the ignition strategy almost instantly to changing engine loads and driving conditions.

Unlike engines fitted with a single ignition coil, where coil failure affects all cylinders, failure of direct mounted coils only affects the individual cylinder and any misfire may be less pronounced than earlier systems. Be aware that prolonged running with a faulty coil may cause expensive damage to the ECU.


Quality Replacement Parts

When fitting a replacement coil it is essential that a high quality unit is used to avoid further premature failure. It is also good practice to replace all of the coils when one has failed as experience has shown that further failures of similarly aged units may not be far away.

Intermotor ignition coils are rigorously tested to ensure that they meet the OE specification and give installers confidence that they are fitting a quality product that will offer high levels of performance and an optimum lifetime.


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