There are two types of ignition management system, those triggered by a distributor and those triggered from a crank position sensor, often called distributorless. The adoption of the term distributorless can be misleading since many crank-triggered systems still use a distributor cap and rotor arm to dispatch the spark to the appropriate cylinder. With these systems a crank sensor and not the distributor does the triggering to the EMS.
Distributor based systems
Distributor-based systems use a conventional distributor to trigger the EMS but the distributor will have no in-built advance mechanism. Typically the trigger will come well before the ignition point and the EMS will work out when to fire the ignition coil. The spark is then carried to the appropriate cylinder in the conventional way via the rotor arm and HT leads.
Crank trigger based
Since crank-triggered systems only know the engine position and not the cycle position they need a way of ensuring that the correct cylinder receives the spark. There are three common ways of achieving this.
The first is to use a conventional distributor cap and rotor arm that is normally attached to the end of one of the camshafts and routes the spark to the appropriate cylinder.
The second method is to use two coils that are paired to fire cylinders 1 & 4 and 2 & 3 respectively. When one of the coils fires it sends the spark to both of its cylinders. One of these will be on the firing stroke and will fire normally, the other will be on the scavenge part of the cycle (exhaust stroke) where the spark will be wasted, for this reason these systems are known generically as ‘wasted spark’.
The third method is to use an additional sensor on one of the camshafts so that the EMS is aware of the engine's cycle position and can fire the appropriate cylinder at the correct time using individual coils for each cylinder.
This type of arrangement is used with early EMS such as the Ford ESC system. It is also popular for after-market applications since it enables the installer maximum re-use of existing components. Any inaccuracies in the distributor manufacture are reflected in the distribution of timing between the cylinders, since the spark is always relative to the trigger points given by the distributor. Typically the distributor will trigger four times per engine cycle i.e. twice per engine revolution.
The distributor will have no advance mechanism installed or will have the advance mechanism rendered inoperative since the EMS provides for the engine's needs.
How it works
Normally the distributor will ‘trigger’ at around 65-70 degrees before TDC since this is greater than the expected maximum advance. The EMS will then look up the ignition map to calculate the appropriate timing figure for the engine's speed and load, then using the engine's speed as a factor will calculate how long to wait before firing the spark. The initial trigger point must be at least the maximum advance figure plus a few degrees latency to allow the microprocessor to do its work.
The conventional distributor cap and rotor arm ensure that the spark goes to the correct cylinder since the EMS will produce a spark every time the distributor pulses. Given that the system is given a pulse from the distributor for each of the appropriate cylinders, it would not be difficult to use the triggering information for the injection system to provide sequential injection. However it would be necessary to have additional feedback to determine which of the pulses belonged to cylinder number '1'. I have seen this done by attaching an inductive pickup onto number '1' spark plug lead.
This type of system is a halfway house toward a totally distributorless system. Only the distributor cap and rotor arm are retained; the rest of the distributor is not present. Typically the rotor arm is installed on the end of the camshaft and the distributor cap is bolted over. It has most of the advantages of a totally distributorless system in that it uses a crank sensor. The EMS however is unaware of the engine's cycle position so it can only provide batched or grouped injection.
The Rover ‘K’ series MEMS uses this system for its basic implementation and therefore can only provide grouped injection. Some of the Vauxhall engines use this system also. It is a very popular and low-cost way of implementing managed ignition. It allows the manufacturer to re-use many of the constituent parts of earlier distributor-based systems.
The EMS is aware of the TDC position from the crank sensor and by counting teeth can tell exactly what the engine position is at any time. It uses this information together with the information from the throttle position sensor/MAP sensor to look up the appropriate ignition timing settings from the ignition map. It is then able to determine exactly when to fire the coil. The coil is fired twice per engine revolution at exactly opposite positions in the engine's rotation because when cylinder 1 & 4 are at TDC, cylinders 2 & 3 are at BDC and vice versa. The spark is then routed to the appropriate cylinder by the rotor arm and cap.
This type of system does away with the distributor altogether. It uses the crank sensor to indicate where TDC is and then uses the signals from the sensor and the map information to determine when to fire the spark (twice per revolution). It groups the signals to two separate coils which provide the spark to pairs of cylinders that are at the same relative crank position. One of these cylinders will be on the firing stroke and will ignite; the other will be in the scavenge stroke and therefore the spark will be ‘wasted’. This is why these systems are known generically as ‘wasted spark’ systems. In practice the coils are usually double-ended with a high-tension lead running from either end to each of the cylinders in the pairing.
The Ford Zetec and Vauxhall XE 16V engines use this type of system.
There are variations to the wasted spark system which uses individual coils for each cylinder that are paired together in parallel rather than using a pair of coils, each serving two cylinders.
Variations on a theme - Distributorless & Sequential
The EMS is aware of the TDC position from the crank sensor and by counting teeth can tell exactly where the engine position is at any time. It uses this information together with the information from the throttle position sensor/MAP sensor to look up the appropriate ignition timing settings from the ignition map. It is then able to determine exactly when to fire the coils. Each coil is fired once per engine revolution at exactly opposite positions in the engine's rotation, because when cylinder 1 & 4 are at TDC, cylinders 2 & 3 are at BDC and vice versa. The spark travels to both of the paired cylinders.
This type of system is similar to the ‘wasted spark’ system in that it is distributorless and multiple coil. It has a cam phase sensor in addition to the crank sensor which allows the EMS to determine where in the engine's cycle each individual cylinder is. There is a discrete coil per cylinder and the EMS is then able to fire the appropriate coil.
The cam phase sensor can also be used by the injection system to provide proper sequential injection. The Rover MEMS as fitted to the VVC engine uses this kind of system, but just uses two coils as per the wasted spark set-up. The cam phase sensor is also used by the EMS to help drive the VVC mechanism. The EMS on the Subaru Imprezza uses this type of system.