Sunday, November 26, 2006
AV - Distributorless Ignition System
"DIS" stands for 'Distributor-less Ignition System,' of which the Compu-fire DIS-IX is a good example. Which causes a lot of non-mechanics to start jumping up & down, pointing at the body of the Bosch -009 distributor and insisting Compu-fire should be sued for false advertising.
The problem here has to do more with language than automotive engineering.
On the Volkswagen engine (and all other cars of its era) the device we commonly refer to as a 'distributor' fulfills three roles, only one of which has anything to do with distributing the spark from the coil to the proper plug. That chore is accomplished using a high-voltage rotary switch, a special cap having terminals for the spark plug wires and a graphite button against which the rotary switch presses, and the high voltage leads connected to the spark plugs, as well as the lead between the coil and the distributor cap.
Due to erosion of the high-voltage rotor, wear of the center contact and break-down of the cap's dielectric over time, the distribution function requires frequent periodic maintenance. When we do away with the distribution function we automatically eliminate the need for a significant amount of maintenance which increases the inherent reliability of the ignition system. That is why all modern engines use some form of distributorless ignition system. A good example of this is modern-day automobile engines that can go 100,000 miles without a tune-up.
The two other functions performed by the thing we call a distributor are ignition switching and ignition timing. Ignition-switching is just that: a switch that turns the system on and off. Historically, this chore was handled by a mechanical switch having tungsten-alloy contacts commonly called points having a service life between 100 and 500 hours. In modern-day systems the points have been replaced with a transistorized switch, typically having a service-life in excess of 5,000 hours.
The final task of the thing we call a distributor was timing: telling the points or transistor when to turn itself on and off. The nature of the Otto-cycle engine is quite critical to ignition timing since the precise moment of ignition varies according to the speed of the engine. Why this is so has to do with the fact that the process of combustion takes approximately the same amount of time regardless of the speed of the engine, forcing us to light the fire a bit earlier as the speed of the engine increases.
With the Compu-Fire DIS-IX ignition system the distribution function has been eliminated, replaced by a pair of two-cylinder waste-spark ignition systems. Ditto for the switching function, which is now accomplished by a pair of high-powered transistorized switches. But we still have need for the timing function and the body of a Bosch -009 'distributor' contains a robust centrifugal-advance mechanism that fulfills that role in a practical, inexpensive and reliable manner.
It's worth noting that every magneto contains a 'distributor.' Magnetos continue to use the same arrangement of a mechanical rotary switch to distribute the spark voltage to the spark plugs as found in automobile distributors. In doing so, the magneto is subject to exactly the same ills which plague automobile distributors and require the same type of periodic maintenance. Inside the magneto you will also find a set of ignition points and a 'condenser,' functionally identical to similar components found in the ignition systems of older automobiles, susceptable to the same wear and requiring the same maintenance.
The typical magneto used on Volkswagen conversions presently costs about a thousand dollars, has a service life of about 500 hours, and requires a special mount. The DIS-IX system costs less than three hundred dollars, plugs right in to the VW engine and has a useful life conservatively rated at fifteen hundred hours. But even more important is the fact it is easier to start and provides a better spark than the typical magneto.
To install the DIS-IX system, remove the distributor cap, rotor and points from your -009 distributor and toss them. Replace the rotor with the DIS-IX magnetic disk then attach the DIS-IX cap. The 'brick' of two coils can be mounted on the firewall. When properly mounted and wired, the DIS-IX allows even the largest VW conversion to be started with a flip of the prop.
SOP when using this unit in an airplane is to install the grub-screw mod, locking the distributor body in place, and to safety-wire the clips securing the cap to the distributor body. These items have been discussed in most of the Groups devoted to VW-powered aircraft and the mods have even been illustrated on some of them.