Ignition
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Graphic: Top view of ROTAX 912 engine, propeller hub left, cylinder numbering. |
The ROTAX 912 has an integrated generator flange-mounted to the rear of the engine. The voltage generated by the charging coils is used to charge the on-board battery (instruments, starter, starter motor). Separate ignition coils are provided to generate the ignition voltage. The entire units have a dual design, i.e. it is redundant. One system can fail without the engine stopping. If an ignition unit fails, there is a small drop in speed, which can be checked during run-up by switching off one unit.
If the complete charging coils fail, the battery is no longer charged and is drained by connected consumers such as instruments. If the battery is empty, it is not possible to start the engine, all consumers no longer in function, but the engine would continue to run.
A 4-stroke engine must be ignited once every two crankshaft revolutions per cylinder. As the ROTAX 912 has 4 cylinders on one crankshaft, 2 ignition sparks are required per crankshaft revolution so that one spark plug per cylinder can be fired.
Double ignition is achieved by duplicating the trigger coils (A1/2 and A3/4) and trigger cam (1) in the ignition unit (B1/2 and B3/4) and (2). Coils (A/B) and cams (1/2) are staggered in depth so that pulses are only triggered if the levels of coil and cam are identical. This means that, as mentioned above, the ignition system is duplicated.
The staggered coils and cams are not visible in the top view. A 5th Trigger coil is in charge for the rev. counter. The firing order is 1 4 2 3 - see picture above. Although there are standards for the numbering of the cylinders, there are many different numberings. Whether old or new standard BRD DDR US or engine manufacturer, seen from the power take-off side or vice versa - the only decisive factor is what the engine manufacturer specifies.
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Graphic: ROTAX 912 at MMH Sep 2022 74-00-00 IGNITION UNIT - the offset of the ignition pick-ups (Trigger coil) and sensors (Trigger cam) are not visible. |
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Picture: with kind permission David R. Walker |
The ignition times for start and continuous operation are determined by the cams. Once when the cam hits a coil in its level (operating) and once when the cam leaves this coil (start). The control unit (ignition box) determines how long the trailing edge of the cam is relevant at the start and then switches to the leading edge (signal on and off are processed).
Due to engine design, ignition occurs also at the TDC of the exhaust stroke, but this has no effect on engine operation. To understand how the two ignition modules "interlock" the cylinders and the upper and lower plugs, it is essential to study the documentation.See e.g. MMH 74-00-00 and 74-20-00.
The ignition timing and any soft-start device and its function can be found in the engine's technical manuals. This can be, for example, the current MMH (Maintenance Manual Heavy) for the relevant engine, which can be found at
https://www.flyrotax.com/p/service/technical-documentationSpark Plug Comparison
As the name suggests, spark plugs generate the spark that ignites the air/gas mixture. They protrude into the combustion chamber and are therefore exposed to high thermal and pressure loads.They are also affected by the burnt residues. It is therefore not surprising that these components are also optimized according to certain properties. Replacing worn, sooted spark plugs can achieve a noticeable improvement in running behavior.
Spark plugs optimized for the engine withstand continuous load and burn free during operation so that no oil carbon can build up. oil carbon can build up. The burning of the electrodes (which causes an increase in the gap ) should be minimal to ensure a long service life and short maintenance intervals.
In addition to the design / thread / length / material, the thermal value, i.e. the ability to dissipate heat is relevant. On the one hand, the spark plug itself should not overheat and thus be damaged and, on the other hand, it should "free-burn" during operation, i.e. no build up oil carbon. It is therefore not surprising that there is a vast number of different spark plugs of different spark plugs, optimized for the engine and the combustion process.
There are, or rather "were", recommendations for the ROTAX engine family as well for the optimized spark plug. How relevant is the right choice of spark plug? Assuming The spark plug thread and length match the cylinder head and the previous plugs were intact and not sooted up - this is a basic requirement. What do you notice after changing to a new set of spark plugs? Not much, possibly no difference to before. Does it then matter whether conventional electrode material, Iridium, air sparks or sliding sparks, heat value and what else? Examinations of the spark pattern show that there are clear differences in the spark pattern of the various types of spark plug, but they don't seem to make any difference. This is a phenomenon that you should never fall for after changing a spark plug. Only at the beginning of the new plugs it doesn't seem to matter what the spark looks like and what the actual technical parameters of the spark plug are - e.g. the heat value. But when the engine comes under load, it becomes clear whether the heat value is correct. If the spark plug soots up prematurely, if the spark plug overheats, etc., the "right" spark plug is still important for a long engine life.
So why the extensive discussion about the "right" spark plug? While explicit instructions about the right spark plug for a ROTAX have been given for almost decades were made - and I don't want to know how many complaints were rejected because supposedly the wrong spark plug was in the engine - now it doesn't seem to matter whether 80 hp or 100 hp or more. Even with the 2-stroke engine, the cylinder head was modified to fit the universal ROTAX spark plug in all engines of the ROTAX-Line. One spark plug for everything, heat value no idea, doesn't matter. Can that be? Of course not. Engines with less power develop less heat heat, so they need a "hotter" plug, i.e. the plug should not dissipate the heat so quickly so that it can still free-burn, while engines with more power need the opposite. Spark plugs that can do everything and adapt independently to the engine, combustion and heat development have not yet been invented.
To download the subjective comparison of spark plugs click here:
Spark Plug ComparisonEngine ⇐ | ⇒ TRT800 ADS-B out