Always consider your SAFETY FIRST!

The Hit and Miss engine - the basics of how it ( should ) work

In the early 1900's the "Hit and Miss engine" was used to power many pieces of equipment. With a big fly wheel(s) the engine only fired when it needed to, that is when the fly wheel slowed down and thus the engine ran at an almost constant speed.

They had an intake valve operated by suction and an exhaust valve that could be held open.

With the exhaust valve open there would be no suction and the engine would free wheel.

Then as the engine slowed down a governor allowed the exhaust valve to close.

With the exhaust valve closed it allowed fuel to be draw into the cylinder, the spark plug to fire and the engine would speed up. A magneto was used to create the spark and a container of water on top to keep the cylinder cool.

So by comparison to the standard 4 stroke action of SUCK SQUEEZE BURN BLOW which occurs in just two revolutions of the crank shaft with the inlet and exhaust valves both operated by cams and push rods the hit and miss operate like this:-

On the HIT cycles

SUCK, the piston going down the cylinder, the intake valve would be free to open and the exhaust valve held closed

SQUEEZE, the piston rising up the cylinder, the intake held closed by the squeeze pressure and exhaust valve kept closed by the spring

BURN, the piston going down the cylinder, the spark plug fires with inlet valve held closed by a light spring and by the burn pressure and exhaust valve kept closed by its valve spring.

BLOW, the piston going up the cylinder, the intake held closed by the BLOW pressure ,the exhaust valve is opened by the push rod.

On the miss cycles

SUCK, the piston going down the cylinder, the intake valve would be free to open BUT the exhaust valve held open no suction thus no intake of fuel

SQUEEZE, the piston rising up the cylinder, the intake held closed by the squeeze pressure and exhaust valve kept open to allow free wheeling

BURN, the piston going down the cylinder, the spark plug does not fire so no the burn pressure, exhaust valve kept open to allow free wheeling

BLOW, the piston going up the cylinder, just the same as in a "hit" stroke but no gases to exhaust.

The fuel line

The fuel system of a hit-and-miss engine consisted of a fuel tank, fuel line, check valve and fuel mixer. The fuel tank most typically held the petrol. A fuel line connected the fuel tank to the mixer, a rudimentary carburetor to create the correct fuel/air mixture. A check valve was inserted into the fuel line to prevent the petrol from running back to the tank between combustion strokes.

The ignition

Early Economy Engines ( 1909 - 1913) were equipped with LT battery and induction coil ignition but 1913 to 1924 the Webster Tri-Polar Oscillator magneto LT ignition was adopted. The Webster Tri-Polar Oscillator magneto did not have continuous rotation but instead an armature of this magneto is "twanged" by a "trip lever" and was returned to its neutral position by the action of the two tension springs. To make a spark, the magneto required only one twelfth of a revolution, the remainder of the revolution the magneto is at rest the result is minimum wear on all parts. The Webster's windings are stationary which are fitted over the ends of the magnet it is an armature which moves. Further research has revealed that the "Webster Tri-Polar Oscillator" used a type "M" unit and is a right handed engine as the magneto is on the right hand side as you look at the cylinder head towards the flywheel ( f the magneto had been on the other side it would have been a left handed engine). Sadly the size of the unit if made 1/2 scale is unlikely to produce the necessary spark so an alternative method to produce the spark is being considered.

HEATH WARNING -- DO NOT PLAY with HIGH VOLTS

NOW BE CAREFUL --- you experiment at your own risk as you will be creating many many 1000's of volts to cause a spark and a DANGER OF DEATH

It is thought that this is what happens with an HT coil... or magneto come to that we plan to prove this imperially in due course.

The ignition points are closed for a much longer time than they are open per revolution of the crank shaft. This closed circuit allows nearly a full revolution of TIME for the current to build up in the primary circuit. The inductance of the primary stopping any INSTANTANEOUS build up to maximum current allowed by Ohms Law (R=V/I).

So, assuming the current reaches maximum in this TIME period, the points then open. The capacitor across the points stops the back emf in the primary from causing a spark big enough to jump the points gap (keeping the current going) so effectively creating an instantaneous break in the current.

Because the current is broken as good as instantly, Lenzes Law dictates (in a perfect world) that the back emf across the primary must rise to an infinite value to keep the current flowing. This colossal back emf across the primary (say 300v) is enough if you were to touch the points gap without a capacitor in place, to RISK DEATH, is multiplied by the turns ratio of the coil and so produces the biggest arc going across the spark plug...even at the raised pressure inside the cylinder.

What does this mean if you want to try and build a coil? Well it is likely that one does not need a giant turns ratio step up HT coil as might have been thought making construction of an ignition coil easier. A turns ratio of 60 to 1 may well provide 15kV plus output to make a spark! Thus 75 turns primary 4 or 5000 turns on the secondary may well do it.

the crank shaft carries the following pieces up to the first web:-

• a Flywheel
• a crank spacer
• a main bearing

then from the second web :-

• a main bearing
• a timing gear spacer
• crank gear (a 21T gear of 18DP which meshes with a 42T 18DP Timing gear which gives a 2:1 reduction ratio. The Timing Gear also meshes with a 17T 18DP Governor Gear which give a ratio of 1 to 2.47)
• a flywheel spacer

More to come in due course !!!!