Alfa Romeo Twin Spark engine

The Alfa Romeo Twin Spark (TS) technology was developed by Alfa in the early to mid 1980s to enable it to achieve a higher power output from its engines, incorporate newly developed technology and most importantly to enable their cars to comply with stricter emission controls.

The 'Twin Spark'

The TS engines are all '4 cylinder inline' with twin cam (DOHC) shafts, and are developed with either 8 or 16 valves. All cars featuring the Twin Spark engines since the Alfa Romeo 155 use the 16 valve models over the 8v ones. The Twin Spark name comes from the fact that there are two spark plugs per cylinder, the second small spark helps create a larger flamefront ensuring less unburnt fuel in the exhaust as well as a higher thermodynamic efficiency. The common TS engines, 1.6, 1.8 and 2.0, all use a 10mm diameter and a 14mm diameter plug per cylinder.

The engines also incorporate two devices to improve the performance under normal operation, the Camshaft Phase Variator and the air intake Variable Inlet Control. On the two litre engine there is also a variable outlet.

Phase Variator

The Phase Variator, also simply known as the 'Variator', adjusts the angle (or 'phase') of the air intake camshaft to allow the valve open and close time to advance or retard as the engine RPM changes. The angle of adjustment is retarded 25 degrees when energised. The cam runs in an advanced configuration when idling or when the engine is in its high-RPM, full-power state.

The variator's action is controlled by the flow of engine oil into the barrel of the variator device. An electro-mechanical solenoid fitted above the air intake plenum to the rear of the engine controls this flow. The solenoid remains de-energised, or in its 'closed phase', at idle, blocking the flow of oil into the variator. Once the engine RPM rises above idle, the solenoid is energised and oil flows through oil-ducts in the engine head into the variator. This causes the variator to shift its phase through the movement of an internal piston and associated helical teeth.

The variator is de-energised once the engine enters its high-RPM, full power area; that is when its RPM rises above 3500 to 4000 RPM. The camshaft's timing is advanced as a result, and this provides a surge in power. This a similar process to that which is used in Honda's VTec and Toyota's VVT-i technology.

Variable Inlet Control

The Variable Inlet Control causes the air intake (plenum) to shorten its path from the intake trumpet to the valve bores, again when the engine reaches a pre-defined RPM. This works on the principal that the air intake length is a tuned pipe that when operating at its ideal frequency-in tune with the valves, will allow a smooth and even air flow, and will assist with filling the cylinders efficiently. This works in a similar method to the tuned exhaust system on all modern cars, where the exhaust system creates back pressure to pull spent gasses from the cylinder bores. Modifying this tuned system in any way and failing to re-tune it will create performance drops.

The notable effect that these two devices have is that the engine will surge powerfully forward on hitting the ~4300 RPM mark, right up to the RPM limiter.