Alfa Romeo Twin Spark
Posted 17 February 2011 - 08:31 PM
The TS series engines are all '4 cylinder inline' with twin cam (DOHC) shafts. The original 8 valve engine featured a light alloy (Si enhanced alu alloy) block + head, wet cooled iron cylinder liners and the camshafts were driven by double row timing chains (long and short). Similar design to the earlier and famous Alfa Romeo DOHC engines, but with narrower valve angle on this model.
The later 16 valve engines had a heavier cast iron block engine, with an alloy head, and the camshafts were belt driven. The Twin Spark name comes from the fact that there are two spark plugs per cylinder. The block was Fiat sourced. It was cast iron for its higher beam strength, less complexity and hence lower production costs. When new, these engines were notable for their high efficiency as demonstrated by the BMEP (brake mean effective pressures) exerted upon the piston crowns.
The two sparks on the 8V Alfa Twin Spark engines do not fire at exactly the same time (true at least of the 75 which uses traditional ignition coils with king leads and distributors). This was deliberate to avoid the most powerful part of the flame fronts meeting at the centre (weakest part) of the piston. As there are 2 symmetrically placed sparks plugs, the flame front must travel less distance which allows less ignition advance to be used and it is accepted ignition timing can be closer to optimal than a single spark plug would allow. Also, leaner mixtures can also be tolerated for better fuel economy. The 8V engine also has 8 identical spark plugs. There is no room for a centrally positioned spark plug due to the 2 valve design which uses a 44mm diameter inlet valve on the 2.0 engine.
On 16V engines there is room for a spark plug in the centre of the cylinder as in all 4-valve configuration engines, but also a second smaller plug (off to the side on the axis) is installed. Both of the plugs fire at exactly the same time on compression and exhaust stroke, due to the way in which the coils are paired (1&4 and 2&3). This production economy allows the use of 4 coils, instead of eight, which would normally be required to fire eight plugs, and is common practice called "wasted spark" system. (used also in Ford EDIS system as well as in some Alfa Romeo V6 engines 3 coils for the 6 cylinders). The main reason for the wasted spark system is cost. As both plugs are connected to the same coil the spark one of them operates with reversed polarity and requires decreased breakdown voltage. On the later CF3 (2001 on Euro 3 emissions standard) 16v TS the four coils fire the same cylinder spark plugs (so not 1 and 4 and 2 and 3 as pairs but coil #1 fires the big and small plug of cylinder 1, and so on). The TS 16V engines, 1.6, 1.8 and 2.0, all use a 10 mm diameter and a 14 mm diameter long life platinum electrode spark plug per cylinder. The spark plugs have a replacement interval of 100,000 kilometres (62,000 mi). The operation of the Twin spark engine has been subject to much debate but this is the general theory of operation, as described by Auto Italia magazine.
16 valve Twin Spark with older cover.
The engines also incorporate two other devices to enhance the performance under operation, the Camshaft Phase Variator and the Variable Intake Length Control on the later (plastic cam cover) 1.8 and 2.0 litre versions. On lower rpms the intake length is increased which increases the intake air speed and promotes better combustion and mixing under these conditions, also changes the intake resonance frequencies.
The advancing of the inlet camshaft by 30 degrees (or 15 camshaft degrees) which makes the intake valves open and close earlier, when the predetermined rpm and load range is reached, makes the compression phase of the combustion process to begin earlier than when the camshaft is not in its advanced state and in this way the dynamic effective compression ratio is increased which produces more torque at the given engine speed. Also as the intake valve is also opened earlier the valve overlap is also increased at that mode. On some similar inlet cam phasing systems like BMW VANOS the phasing is returned to the retarded state at higher rpm.
On 8V engines the valve overlap and intake open duration are quite big. These engines hardly idle with the variator at On position so on these models it had the meaning also to enhance lower speed operation. On the newer 16V engines the camshaft variator is used to enhance the performance/emissions but also might be the source to the common 'diesel noise' problem often seen on high mileage used models which used the earlier internals of the variator. The same variator system is also used in many Fiat/Lancia engines like one used in Lancia Kappa 5-cylinder engine, some Fiat Bravo/Fiat Marea engines, Fiat Barchetta, Fiat Coupe, Fiat Stilo etc. models.
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