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Recent Advances in Aircraft Ignition Systems

Recent Advances in Aircraft Ignition Systems ELECTRICS Recent Advances in Aircraft Ignition Systems The Possibilities of a Low-Voltage Ultra-High Frequency Installation to Avoid Radio Interference By M. M. Comber ITH the introduction of airborne wire­ 200-300 megacycles per second are outside the less equipment it became apparent, natural frequency ranges used for communica­ even with the low frequency and volt­ tion and allied purposes, but it is only in recent age ignition systems which were then used, that years that such frequencies have been capable the prevention of radio interference caused by of generation on a commercial scale and by the ignition impulses presented a problem of con­ equipment of a reasonable size and weight. De­ siderable proportions. As engine development finite indications exist that low voltage ultra­ was pursued it was observed that the use of high frequency ignition systems for aircraft are higher ignition voltages and frequency resulted an accomplished fact, but it is not widely known in more efficient combustion; today high voltage in what form such progress in electronics can be and frequency ignition systems involving the use harnessed to the requirements obtaining in the of oscillatory circuits are a common feature in aeronautical field. piston-engined aircraft. Such an ignition system may be built up by To combat the difficulties that have been, and using an ultra-high frequency generator such as still are, associated with radio interference, re­ a magnetron that is possessed of a low but con­ course has been had to one or more of a wide tinuous power rating and which is connected to range of radio shielding devices. While much of special sparking plugs either by means of low this preventative apparatus has been quite effec­ impedance connexions, such as co-axial cables, tive, nearly all the particular embodiments seem or by wave guides. The power supply may be to have exhibited similar inherent defects which provided.by an engine-driven alternator which were attributed to the wide variations in humid­ is coupled, through a transformer with a tapped manner, but differ considerably from the stan­ ity, pressure and temperature normally encoun­ and earthed secondary, to the electrodes of the dard kind in which the central electrode is in­ tered under flight conditions. Various means U.H.F. generator, the latter incorporating some sulated from the earthed body by a ceramic such as air pumps and/or dehydrators have been form of resonant device. In order to supply the mass. Each ultra-high frequency sparking plug adapted for increasing the air pressure and/or sparking plugs, at the appropriate times, with in­ actually comprises a resonant line and consists extracting the moisture within such shielded stantaneous power of a high value, the U.H.F. of a central electrode of a length which is either parts of the ignition system as the sparking generator output is subjected to a pulsing or electrically representative of, or physically equal plugs, distributor, magneto and leads. Although surging operation which is conveniently carried to, one quarter of the wave length of the ultra­ these devices have reached a high degree of effi­ out by the use of a commutator driven from the high frequency impulses and an earthed outer ciency they add unwanted weight in addition to engine and timed in relation to the cylinder fir­ electrode which at one end is joined to the cen­ materially increasing the difficulties of planned ing order in the same way as an ordinary distri­ tral electrode, whilst the other extremity is maintenance. butor contact breaker. From this point the spaced from it to form the sparking gap. It is, power surges are led to a distributor switch therefore, seen to be unnecessary to provide an Until quite recently it appeared as if there which is synchronized with the commutator; the insulator between the central and earthed elec­ existed little hope of immediate mitigation of ultra-high frequency pulses are then transmitted trodes. the above problems, but it now seems as if the through the co-axial cables to the sparking application of recent advances in the field of A low voltage ultra-high frequency ignition plugs. electronics may necessitate an entirely new ap­ system of this general form will, to a very large proach to the problems connected with the de­ With such an ignition system it is necessary extent, reduce interference with the many pieces of now standard equipment which can be classi­ sign of aircraft engine ignition systems. It is well to employ sparking plugs of a type that are cap­ fied under the general heading of radio. known that ultra-high frequencies of the order of able of being secured to the engine in the usual 282 Aircraft Engineering http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Aircraft Engineering and Aerospace Technology Emerald Publishing

Recent Advances in Aircraft Ignition Systems

Aircraft Engineering and Aerospace Technology , Volume 20 (9): 1 – Sep 1, 1948

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Publisher
Emerald Publishing
Copyright
Copyright © Emerald Group Publishing Limited
ISSN
0002-2667
DOI
10.1108/eb031669
Publisher site
See Article on Publisher Site

Abstract

ELECTRICS Recent Advances in Aircraft Ignition Systems The Possibilities of a Low-Voltage Ultra-High Frequency Installation to Avoid Radio Interference By M. M. Comber ITH the introduction of airborne wire­ 200-300 megacycles per second are outside the less equipment it became apparent, natural frequency ranges used for communica­ even with the low frequency and volt­ tion and allied purposes, but it is only in recent age ignition systems which were then used, that years that such frequencies have been capable the prevention of radio interference caused by of generation on a commercial scale and by the ignition impulses presented a problem of con­ equipment of a reasonable size and weight. De­ siderable proportions. As engine development finite indications exist that low voltage ultra­ was pursued it was observed that the use of high frequency ignition systems for aircraft are higher ignition voltages and frequency resulted an accomplished fact, but it is not widely known in more efficient combustion; today high voltage in what form such progress in electronics can be and frequency ignition systems involving the use harnessed to the requirements obtaining in the of oscillatory circuits are a common feature in aeronautical field. piston-engined aircraft. Such an ignition system may be built up by To combat the difficulties that have been, and using an ultra-high frequency generator such as still are, associated with radio interference, re­ a magnetron that is possessed of a low but con­ course has been had to one or more of a wide tinuous power rating and which is connected to range of radio shielding devices. While much of special sparking plugs either by means of low this preventative apparatus has been quite effec­ impedance connexions, such as co-axial cables, tive, nearly all the particular embodiments seem or by wave guides. The power supply may be to have exhibited similar inherent defects which provided.by an engine-driven alternator which were attributed to the wide variations in humid­ is coupled, through a transformer with a tapped manner, but differ considerably from the stan­ ity, pressure and temperature normally encoun­ and earthed secondary, to the electrodes of the dard kind in which the central electrode is in­ tered under flight conditions. Various means U.H.F. generator, the latter incorporating some sulated from the earthed body by a ceramic such as air pumps and/or dehydrators have been form of resonant device. In order to supply the mass. Each ultra-high frequency sparking plug adapted for increasing the air pressure and/or sparking plugs, at the appropriate times, with in­ actually comprises a resonant line and consists extracting the moisture within such shielded stantaneous power of a high value, the U.H.F. of a central electrode of a length which is either parts of the ignition system as the sparking generator output is subjected to a pulsing or electrically representative of, or physically equal plugs, distributor, magneto and leads. Although surging operation which is conveniently carried to, one quarter of the wave length of the ultra­ these devices have reached a high degree of effi­ out by the use of a commutator driven from the high frequency impulses and an earthed outer ciency they add unwanted weight in addition to engine and timed in relation to the cylinder fir­ electrode which at one end is joined to the cen­ materially increasing the difficulties of planned ing order in the same way as an ordinary distri­ tral electrode, whilst the other extremity is maintenance. butor contact breaker. From this point the spaced from it to form the sparking gap. It is, power surges are led to a distributor switch therefore, seen to be unnecessary to provide an Until quite recently it appeared as if there which is synchronized with the commutator; the insulator between the central and earthed elec­ existed little hope of immediate mitigation of ultra-high frequency pulses are then transmitted trodes. the above problems, but it now seems as if the through the co-axial cables to the sparking application of recent advances in the field of A low voltage ultra-high frequency ignition plugs. electronics may necessitate an entirely new ap­ system of this general form will, to a very large proach to the problems connected with the de­ With such an ignition system it is necessary extent, reduce interference with the many pieces of now standard equipment which can be classi­ sign of aircraft engine ignition systems. It is well to employ sparking plugs of a type that are cap­ fied under the general heading of radio. known that ultra-high frequencies of the order of able of being secured to the engine in the usual 282 Aircraft Engineering

Journal

Aircraft Engineering and Aerospace TechnologyEmerald Publishing

Published: Sep 1, 1948

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