Double ignition

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Ignition distributor for double ignition of a six-cylinder engine ( Porsche 993 )

Double ignition in connection with gasoline engines means that two spark plugs are used per cylinder in order to ensure more even, more complete and thus more energy-efficient combustion of the fuel-air mixture. The two spark plugs can also be of different types.

Basics

Double ignition is being used more and more on motorcycles with large individual displacement , as is the case with in-line four-cylinder Alfa Romeo with the designation Twin Spark. In the 1920s and 1930s, Daimler-Benz used double ignition in the W 06 series supercharged cars and the “Big Mercedes” W 07 / W 150 . The two Porsche 911 types 964 and 993 (without turbo ) and the Porsche 550 were also equipped with such a system. Mercedes-Benz used them again from 1997 in V6 ( M 112 ), V8 ( M 113 ) and V12 engines ( M 137 ), but did not use them again in their successors. In contrast to the Alfa Romeo and Porsche, the ignition was carried out with different phases, with the aim of minimizing the noise level and pollutant emissions. The 700 cc petrol engines from Smart ( Fortwo and Roadster ) as well as various V engines in Honda motorcycles also have dual ignition.

aviation

A Pratt & Whitney R-2800 aircraft engine. The two identical distributors of the double ignition system can be seen on the gray-blue painted gearbox, pointing diagonally upwards, with the tandem ignition magnet in the middle. The ignition distributor on the right fires the spark plugs located in the front of the cylinder heads, the left the rear spark plugs.

A double ignition system has been used in aircraft engine construction since the 1910s (e.g. in the Mercedes D III ) and is now standard in aircraft with piston engines. There are two self-sufficient ignitions with two magnetos, distributors and two candles per cylinder, one of which receives the ignition voltage from the "left" and one from the "right" magneto. To increase operational safety and create redundancy , these motors are able to continue working if one of the ignition systems fails.

Magnet check

A characteristic of this is the magnetic check immediately before take-off to detect ignition defects: for this purpose, the pilot stops before rolling onto the runway with the wheel brakes held, both ignition systems are switched on, a specified engine speed is switched on and one after the other switches off one and the other ignition system . This is done in the run-up area away from airfield traffic so as not to endanger other aircraft from the propeller air flow. Alternatively, the variant is also common to take a slanted position for the magnet check in the queue of waiting aircraft on the taxiway ; This also prevents the other backers from being hit by the air stream.

During the magnet check, attention must be paid to an identical, perceptible drop in speed of usually approx. 50 rpm on both ignition circuits when switching off: If there is no difference in speed, this is the sign that the switched off ignition circuit is not working at all by default In this case, the engine will shut down completely when the ignition circuit is still intact, which must always result in the start preparation being aborted. The engine and the ignition must also be checked in the event of greater or different speed differences that indicate a defect in individual plugs. During the "run-up", following the magnet check, the controllable pitch propeller (s) and in particular their sail position are checked, if available . After the magnet check, the ignition selector switch is set back to "Both" (both magnets in operation) for normal operation . The speed loss of approx. 50 / min is explained by the poorer and slower combustion of the gasoline-air mixture in the large individual cubic capacities that are common in aircraft engines, if ignition is only carried out at one point.

The magnet check takes its name from the magneto ignition , an ignition system from the earlier days of engine construction, which is simple, works independently of the on-board network and is therefore still used in aviation.

literature

  • Jürgen Kasedorf: Motor vehicle engine test. Petrol engines. 7th revised edition. Vogel Buchverlag, Würzburg 1997, ISBN 3-8023-0461-6 .

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