Scooter CDI
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Scooter CDI

CDI (Capacitor Discharger Ignition)

Roller CDI
The Capacitor Discharger Ignition or short CDI, is a contactless, microelectronic control unit. The signal from the CDI unit is used to determine the ignition point of the Spark plug electronically controlled, thus initiating fuel combustion in the cylinder. The principle of electronic ignition by CDIs has been in use since the 1960s, but it was not until the early 1990s that it became established, especially for scooters. The capacitor of the CDI is continuously charged and supplied with voltage via a battery or coil.

Static vs. dynamic CDI

In the engine, the piston compresses the fuel-air mixture in the cylinder to a maximum. Shortly before the piston has reached the top position (top dead centre OT / piston reversal point) the mixture is ignited and the piston is pushed down again. A static CDI has a fixed ignition point, which is always close to TDC. (A Look into the cylinder interior shows the processes described above). A dynamic CDI unit, on the other hand, not only gives the spark plug the electrical charge it needs for ignition, but also regulates the time of ignition. By adapting the ignition timing to the speed of the scooter engine, the CDI ensures that the mixture is always ignited at the "right" moment. This point is not always shortly before TDC, but shifts with 2-stroke engines at higher speed in the direction of a later ignition (late ignition). The dynamic CDI makes a corresponding adjustment in the various engine speed ranges in order to optimize combustion. Product descriptions of dynamic CDIs often contain phrases such as "modified" or "performance-optimized ignition curve".

Especially for more powerful scooters and mopeds the ignition timing adjustment is important. The reason for this is simple: at higher engine speeds, the 2-stroke engine produces regular turbulence, the effect of which is stronger the more power the engine has. So that the ignition timing is somewhat outside this wild explosion turbulence, it is moved to the rear and the fuel can burn better at high engine speeds. The situation is different for scooters with 4-stroke engines. Here, with increasing engine speeds, it should be ignited continuously earlier. Accordingly, dynamic CDIs for 2-stroke and 4-stroke scooters are not interchangeable because they work in the opposite direction. Static CDIs, on the other hand, can often be used on both types if the surrounding data is correct.

Advantages of a CDI ignition system compared to other ignition systems

The main advantage of CDIs - as found in scooters or maxi scooters - is that they are absolutely maintenance free and low-wear. In addition, they produce a stronger ignition spark than Coil ignitions, but have a higher switching effort.

The adjustment of the ignition curve for dynamic CDIs results in a considerably better torque, especially in the lower and medium speed range. The separation of the energetic charge and the voltage into different circuits also proves to be a particular advantage, as these can be individually optimized. Unfortunately, the disadvantage of the compact design without various individual parts is that a defective CDI unit on the scooter is usually no longer possible. If the CDI has been identified as a fault-causing part with no ignition spark, the only solution is to replace it with a new CDI.

Ignition timing and engine run

The effects of the ignition timing on engine running are often underestimated. A too late point in time provides for a pleasantly calm idle throttle and good cold start behaviour, however, the engine does not tour up well, comes only with difficulty into the higher speeds and has noticeably little achievement. This is due to the fact that the piston has already run too far down again and the compression of the fuel mixture has decreased too much at the ignition point. Subsequently, the engine becomes dirty due to partly unburned fuel and later ignitions manifest themselves as misfires. Early ignition, on the other hand, is indicated by the fact that the engine is powerless at low engine speeds, but turns up with a single blow at a certain engine speed. Ignition should still take place in the upward movement of the piston (before TDC), but the explosion wave should hit the piston only in its downward movement. If the ignition is too early, however, the explosion still hits the piston in its upward movement. Engine parts, in particular the Krankshaft, as well as drive parts can be permanently damaged as a result because they are exposed to very high loads and the combustion chamber becomes too hot.

The CDI as scooter throttle (45km/h or 25 km/h throttle)

A CDI can also be used as a scooter throttle. It is possible to adjust the crossover frequency of the CDI so that it does not transmit an electrical impulse at a certain speed. The CDI thus functions as a speed limiter. However, such a manipulation also requires a high degree of experience and technical know-how. With some scooter models the 45km/h throttling is realized exclusively by the CDI, but mostly the speed limitation is realized by a combination with mechanical throttling (e.g. stops in the Variomatik, Variomatik sleeves with collar).

Open, Sport or Tuning CDIs

If the 45km/h throttling on the scooter is to be lifted (e.g. for racing or test drives on private terrain), an "open" ignition unit without speed limitation is required in addition to the removal of other mechanical or electronic throttles. With many original ignition units the limitation is approx. 8,000 - 9,000 rpm. With many scooters, this speed limit is no longer necessary after simply plugging in an unthrottled CDI - the engine can now work freely up to its power limit and the top speed usually rises significantly above 45 km/h. The speed limit is also set by the engine. By adjusting the variomatic (e.g. the variomatic weights) it is now possible to further optimise the engine. When using Tuning Exhaust Systems or Tuning Zylinderkits an open CDI is necessary, because the tuning components develop their power only in high speed ranges. If the engine speed remains limited, no good result is to be expected.

Unthrottled CDIs are also available for some vehicles with programmable / adjustable ignition behaviour. These ignition units provide an additional performance plus through an optimal design of the ignition curve. The CDIs with freely definable ignition curve can also be used for untuned engines to de-throttle the vehicle - but this only makes sense if the de-throttled engine manages to turn into the speed limit of the original CDI. The manufacturer "Naraku", for example, offers a large selection for almost every scooter model, especially for the scooters made in China and throttled by the CDI. Also other brand manufacturers like e.g. Malossi, offer with their CDIs the possibility for dethrottling and performance improvement. The ignition box Force Master 2 offers a real luxury version. This highend CDI injection module is available for different rollers and can even be set without software or computer.

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