More on running on

Following the ' sparking plug thread fix' brieffeatured in YWA/TCMMay 2020, page 80, two readers have asked about the ' nuts and bolts' of what is happening when an engine runs as implied, with fuel vapour flaming caused by red hot protruding sparking plug thread repair wire insert. Another correspondent claimed this was pre-ignition rather than fuel mix detonation, as I'd written.

Correctly, petrol used in internal combustion engines is ignited by ignition sparks, not detonated to flame as I recorded in the feature. The word ' detonate' was chosen in this case to differentiate what was happening from conventional ignition.

The type of internal combustion engine running achieved when a glowing spark plug thread insert protrudes into the combustion chamber is akin to that of hot tube ignition employed by some makers during the early pioneering days of motor vehicles, and also some industrial engines. Before Georges Bouton of De Dion Bouton introduced his mechanical ' make and break' (first ignition contact breaker) in 1895, makers of internal combustion engines had three main choices (plus a few less successful systems) of igniting fuel vapour: flame, wipe type electric ignition and hot tube.

Of the three, hot tube was arguably the favourite among pioneer vehicle makers and was originally fitted to a number of vehicles, including some Leon Bollees and early Beeston tricycles.

Precise detail of hot tube ignition varied, but, in essence, comprises an external to the engine burner heating a heat conductive wire or thin rod, usually platinum, to red hot. The wire passes through a protective shield or tube into the engine, with the wire glowing red internally within the combustion chamber, to ignite the compressed fuel mix. Engines using this ignition are slow revving, with some running as slowly as 240-280rpm, others up to about lO00rpm.

Sometimes, these engines are known as fixed speed engines. This isn't strictly true in many cases, but certainly such engines have a limited rev range - more limited than if the engine was fitted with a make and break controlled electric system. Makers attempted to refine the system, including Georges Bouton before he concentrated on his make and break.

Whether makers of hot tube ignition realised the bonus of using platinum wire/rod is unclear to me, but, much later, the makers of glow plug model aero engines did. I know precious little about model aircraft engines, but surmise the glow plug model aero engines which run on methanol (or methanol with a small percentage ofnitro-methane added) rely on three factors for ignition: heat from compression, the glow (heat) of the glow plug and the catalytic effect of platinum. And one assume the catalytic effect of platinum assisted hot tube ignition, too.

History informs the German engineer Ray Arden stumbled on the phenomenon of glow plug ignition after the First World War. Friends running model aero engines with electric ignition found one engine continued to run after the ignition had been cut. Investigation revealed a faulty sparking plug, which, after running, for some time glowed red at the tip. And as compression remained constant, the combination of glowing red plug tip within the combustion chamber and compression enabled the engine to continue running.

In effect, the same is happening in motorcycle engines when a wire insert protrudes into the combustion chamber and after running glows red.

Experience shows the wire insert doesn't straighten to protrude at depth into the combustion chamber, which in extreme could result in pre- ignition ( also caused by glowing carbon build up which forms on piston crowns and around the top perimeter of the cylinder liner) but positions itself as a pig's tail around the plug's tip. And, on glowing, it ignites petrol vapour under compression, enabling the engine to run after the ignition has been cut.

Pre-ignition results in an engine running on in a jerky fashion (my father likened this to death throes ... ) developing no power, while engines running from a glowing wire insert or similar run evenly at low revs, develop limited power but won't rev to more than 1000-lS00rpm. I had never seen this phenomenon, but it is well known to some in the motor/ motorcycle trade. Then, about 10 years ago, a friend appeared with his motorcycle doing exactly this. Me being me, I had to try riding it. It pulled away in bottom gear, managed a change up to second and third gear but stalled when fourth gear was engaged.

Beeston initially favoured hot tube ignition over the electrical make and break system most were adoptingfrom 1896 onwards, then offered it as an option with an electrical system, stating in publicity material it was favourable and simpler, and then dropped it circa late 1898 or 1899.

The thought of bouncing along bumpy roads with an open burner heating a length of platinum wire and the required engine and burner slopping about is an interesting one. Makers must have been in league with local hospital burns units and undertakers ...