grandpaul
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Internal (mechanical) operation of a Norton twin (Commando) engine
Basic description
Air-cooled, carbureted, pushrod-operated overhead valve, vertical twin, 4-stroke configuration, dry sump oiling system, with chain-driven primary system, multi-plate, diaphragm spring clutch, manual foot-shift transmission, and contact breaker point ignition system.
Power Unit (Bottom end)
Utilizing the basic 4-stroke principal design, the pistons are connected in parallel at the crankshaft via plain-bushed connecting rods; the crankshaft is supported on either end with roller bearings. This sub-assembly rotates clockwise when viewed from the “Timing side” (Right side) of the engine.
On the timing side, the crankshaft pinion gear drives the intermediate camshaft pinion/sprocket, which drives the camshaft via an roller chain, timed such that all 4-stroke combustion cycle events occur at the appropriate time. These gears, sprockets & chain must be synchronized in order for the engine to perform as it should, a simple misalignment of one tooth on one pinion or sprocket is sufficient to keep the engine from starting, and could result in damage to the valves, pistons, or both.
Power Unit (Top end)
As the camshaft rotates, the raised lobes force the cam followers (tappets) to raise up in their bores, lifting the associated pushrods; in turn, the pushrods force the rocker arms to pivot on their shafts, which depresses the associated valve off of it’s seat, allowing either the fuel/air mixture to enter the cylinder from the carburetor, or the expended combustion gasses to escape through the exhaust.
Primary Drive (clutch)
On the Primary side, as the crankshaft turns, the primary sprocket turns the clutch basket (with its inner edge grooved to accept the outer spline teeth of the plain steel plates) via a multi-row chain. Within the clutch basket is a multi-faceted assembly composed of the clutch plate “stack” & pressure plate, the diaphragm spring, and the inner hub (with it’s outer edge grooved to accept the inner spline teeth of the friction plates) which drives the transmission mainshaft.
In operation, the clutch basket, being turned by the primary chain from the crankshaft, turns the plain plates that are meshed to the clutch basket’s inner grooves. Pressure from the diaphragm spring at it’s static state forces the friction plates which have a friction substance affixed to metallic plates with their inner teeth meshed to the hub’s outer grooves, to be virtually bound to the plain plates stacked between one another. As the hub rotates, it drives the splined transmission mainshaft.
When the rider pulls on the clutch lever, a lever mechanism at the opposite end of the clutch cable forces a shaft to release the spring pressure on the clutch pressure plate, freeing the connection between the drive (friction) and driven (plain) clutch plates; this disengages the connection between the engine’s “power unit” and transmission. As the clutch lever is released, the pushrod retracts, allowing the spring pressure to be fed through the pressure plate into the clutch plate stack, resulting in the power unit once again driving the transmission mainshaft.
Note that since the kickstarter engagement is through the transmission mainshaft, the clutch must be ENGAGED (static, lever out) for the kickstarter to rotate the engine. If the clutch is actuated, this disengages the link between the power unit and transmission; thus, the bike cannot be kickstarted with the clutch disengaged (lever pulled in).
Oiling system
Driven by a worm gear from the timing end of the crankshaft, the dual gear oil pump first draws oil from the oil tank through the main circuit, to the crankshaft at the timing (right) side end which is drilled though to both connecting rod journals; it is pumped into the mating surface between the crank’s journals and plain bearings of the connecting rod big ends, where it simply squirts out, splashing all of the rotating and reciprocating parts including the underside of the pistons, to keep the cylinder bores lubricated. Oil control rings in the pistons keep excess oil from entering the combustion chamber; thus, if the rings are excessively worn, it becomes apparent from oil-fouled smoky exhaust and excessively oil-fouled spark plugs. This same circuit feeds a smaller capillary tube that runs up to the head where it feeds the four hollow rocker arm pivot shafts. This oil lubricates the rockers at their pivot points, then drains down through the pushrod tube tunnel, lubricating the cam followers (“lifters” or “tappets”) and the cam.
The scavenge (secondary) circuit draws the oil out of the crankcase bottom through a sump screen, and back to the oil tank; there is also a “telltale” scavenge return orifice which allows inspection and verification that oil is indeed returning to the tank.
The last circuit in the oiling system is the oil pressure relief, which is in common with the crankshaft main end feed passageway, and incorporates a spring-loaded plunger, which (on late models) allows excess oil pressure buildup to be “dumped” into the crankcase sump to avoid over-oiling of the rod bearings; on earlier models, excess pressure is re-routed back to the oil pump inlet.
Crankcase Breather system
The pressure buildup within the crankcase as the pistons travel downwards would force oil to leak from every possible point on the engine’s many mating surfaces and assembly hardware unless some form of relief was present.
In the early model twins, the method employed is a tiny disc keyed to the primary side (left) end of the camshaft. The two tiny openings in the disc align at the proper time with companion openings cut into a chamber in the left side crankcase half which has it’s exit through a pipe that runs to the breather inlet pipe at the top of the oil tank.
Later models incorporated a breather trap behind the crankshaft, with a mesh filter to deter excess oil from escaping into the breather system; the vent hose was routed to the same location as in earlier systems.
The final breathing arrangement was via a common opening between the crankcase and the timing chest; the pressure was vented to the upper rear (inside) corner of the timing chest to the same oil tank breather inlet as previous models.
The outlet breather of the oil tank on early models was to a pipe routed into the airbox; on later models it is routed out a hose to the rear of the bike.
Transmission (4-speed)
Power from the engine is fed through the clutch center hub to the transmission mainshaft which has spline-driven gears that can be shifted to various positions resulting in different ratios between the input (mainshaft) and layshaft, to the sleeve gear (output shaft). Power is transmitted from the mainshaft, through whichever gear pair is selected, to the layshaft, which turns the final drive sprocket, which drives the final drive chain, which drives the rear sprocket, turning the rear wheel. (In top gear, power is fed from the mainshaft directly to the sleeve gear).
The gear selector apparatus is a somewhat intricate assembly of pins, springs, pawls, cam and forks that operate thusly:
When the gearshift lever is depressed, a pin in the mechanism, that slips into a roller, that sits captive in a semi-circular “cup” in a gear-like apparatus that pivots on another pin, rotates slightly. The gear-like apparatus’ opposite end causes a camplate gear to rotate on it’s shaft. The front side of each of the two shifter forks has a pin with a roller slipped over it; these pins with their rollers wend their way through two curvaceous grooves cut in the face of the camplate as it rotates.
The two gear selector forks slide left-to-right on a shaft, with the tines of one fork riding captive in a groove of one gear on the mainshaft, and the tines of the other fork riding captive in a groove of another gear on the layshaft. So, as the gearshift camplate rotates, it moves the position of the appropriate gears one way or another, via the shifting forks, to align each matched pair in succession as the gears are selected. Regardless of the gear selected, power comes in through the mainshaft, feeds the selected drive gear, which feeds it’s paired driven gear, which feeds the layshaft, which feeds the drive sprocket.
The kickstarter’s ratchet gear engages another gear on the mainshaft, which turns the clutch, which turns the crankshaft via the primary chain. As the pedal is released, or as the engine spins, the ratchet's teeth release, allowing the pedal to remain free from counter-rotation except as allowed by the return spring, to the point where an internal stop limits it's travel just beyond the point where the dog tooth in it's ratchet gear disengages the starter gear.
Ignition systems
Driven by the timing end of the camshaft, the ignition system is timed to provide spark to the combustion chamber at the exact point in the 4-stroke sequence for proper combustion and operation of the engine.
In a contact breaker points based system, the camshaft turns an automatic advancer plate upon which pivots a points cam; this points cam is advanced as engine speed increases, by means of centrifugal flyweights which vary the points cam’s relative position to the exhaust cam by a pair of springs. The faster the cam spins, the more centrifugal force is exerted on the flyweights, which rotate the points cam via a tooth-in-groove arrangement, effectively advancing the timing to provide the spark earlier in the compression stroke, allowing the mixture in the combustion chamber to ignite and burn appropriately. 12 volts are fed (through the system voltage sources, more later) to the contact breaker points, which when actuated, send a primary voltage pulse to on of the ignition coils, inducing a high secondary voltage pulse through the ignition lead to the spark plug associated with that coil & points set; the same thing happens again 180 degrees of CAMSHAFT rotation (360 degrees crankshaft rotation) later, and the other set of points fire for the other cylinder.
In a magneto ignition system, the cam drives the magneto’s armature and breaker points cam simultaneously; the magneto’s armature produces the primary voltage for the points, which, when triggered, induce a high voltage in an encapsulated coil which feeds both of the ignition leads to the spark plugs. This results in one cylinder firing, and a “wasted spark” once every cycle of 360 degrees of crankshaft (180 degrees of camshaft) rotation.
In an typical modern electronic ignition, the engine’s cam drives a rotor assembly with two magnetic triggers affixed to it; as these magnets pass in proximity to two pickup coils mounted in the stator plate, an electromagnetic pulse is triggered in the primary circuitry. This primary pulse travels into the ignition’s “black box” which in turn magically (well, through the wonders of modern electronics), sends a single12 volt primary pulse through a pair of 6 volt coils, inducing a secondary pulse in both coils, causing a spark at both spark plugs simultaneously, resulting in one cylinder firing, and a “wasted spark” as with the magneto ignition.
Alternator and charging system (single phase)
On the left end (primary side) of the crankshaft, a magnetically polarized alternator rotor is keyed to the shaft; the rotor spins in close proximity to the fixed alternator stator which has multiple copper-wound coils encapsulated in it. These coils are wired in such a manner that an alternating electromagnetic pulse is triggered as the rotor’s magnets spin past the coils. The stator’s output leads are wired such that two wires carrying alternating current (AC) feed into a diode array called a “rectifier”. The rectifier’s diodes are composed so that the alternating current pulses that are sent through the array, exit in a flow of direct current (DC). This voltage is sent to the storage battery, and all of the electrical systems, as well as a Zener diode which regulates the maximum voltage allowed into the electrical system at just above 12 volts; any additional voltage above that (from the alternator spinning at high speeds), is “bled off” in the form of heat. That is why Zener diodes must always be mounted to some form of heat sink, or they will rapidly fail due to excessive heat buildup.
Carburetion
The various makes and models of carburetors operate in essentially the same manner:
Fuel enters the carb and encounters an orifice that is controlled by a float needle; as the bowl fills with fuel, the float rises to a fixed point at which the paired needle rises in it’s bore until it seats at the orifice, block flow of fuel into that carb so that it doesn’t “flood”.
At idle, a fixed system consisting of a restricted passage or a replaceable “low speed” or “idle” jet allows a tiny amount of fuel to bypass the carb’s main circuits, allowing the engine to idle without intervention; the ratio of air to fuel at idle and low engine speeds is controlled by the low speed mixture screw.
When the throttle twistgrip is actuated, the cables pull on the carb’s slides, lifting them out of the airstream, allowing more air into the carb; as the opening increases, a vacuum is formed by relatively low pressure in the “venturi” of the carb, relative to the higher atmospheric pressure in the float bowl. The greater the air pressure differential, the more fuel flows though the carb’s main jet; this flow is restricted, moderated and adjusted by the slide needle affixed to the carb’s slide. The general running condition fuel-to-air ratio (mixture) is controlled by an intricate combination of different sized and shaped slide, main jet and needle jet, and different shaped and positioned needles.
Exhaust
The spent products of combustion cannot simply be dumped out of the combustion chamber into open air without drastically negative effects on the engine’s performance. The exhaust headers exit the head and minimally channel the exhaust to the rear of the bike; a secondary effect of the bends in the exhaust pipes causes a pressure buildup that interacts with the flow in such a way that a return pulse moderates the flow to a point at which nominal performance is gained. Customizing the diameter of these pipes, and the complexity or simplicity of the circuit, can result in increased effectiveness of the system, resulting in increased performance of the engine. The mufflers may or may not contribute to (or detract from) performance, but are mandatory for road use.
Any questions?
Thanx to L.A.B. for several fine-point corrections.
Internal (mechanical) operation of a Norton twin (Commando) engine
Basic description
Air-cooled, carbureted, pushrod-operated overhead valve, vertical twin, 4-stroke configuration, dry sump oiling system, with chain-driven primary system, multi-plate, diaphragm spring clutch, manual foot-shift transmission, and contact breaker point ignition system.
Power Unit (Bottom end)
Utilizing the basic 4-stroke principal design, the pistons are connected in parallel at the crankshaft via plain-bushed connecting rods; the crankshaft is supported on either end with roller bearings. This sub-assembly rotates clockwise when viewed from the “Timing side” (Right side) of the engine.
On the timing side, the crankshaft pinion gear drives the intermediate camshaft pinion/sprocket, which drives the camshaft via an roller chain, timed such that all 4-stroke combustion cycle events occur at the appropriate time. These gears, sprockets & chain must be synchronized in order for the engine to perform as it should, a simple misalignment of one tooth on one pinion or sprocket is sufficient to keep the engine from starting, and could result in damage to the valves, pistons, or both.
Power Unit (Top end)
As the camshaft rotates, the raised lobes force the cam followers (tappets) to raise up in their bores, lifting the associated pushrods; in turn, the pushrods force the rocker arms to pivot on their shafts, which depresses the associated valve off of it’s seat, allowing either the fuel/air mixture to enter the cylinder from the carburetor, or the expended combustion gasses to escape through the exhaust.
Primary Drive (clutch)
On the Primary side, as the crankshaft turns, the primary sprocket turns the clutch basket (with its inner edge grooved to accept the outer spline teeth of the plain steel plates) via a multi-row chain. Within the clutch basket is a multi-faceted assembly composed of the clutch plate “stack” & pressure plate, the diaphragm spring, and the inner hub (with it’s outer edge grooved to accept the inner spline teeth of the friction plates) which drives the transmission mainshaft.
In operation, the clutch basket, being turned by the primary chain from the crankshaft, turns the plain plates that are meshed to the clutch basket’s inner grooves. Pressure from the diaphragm spring at it’s static state forces the friction plates which have a friction substance affixed to metallic plates with their inner teeth meshed to the hub’s outer grooves, to be virtually bound to the plain plates stacked between one another. As the hub rotates, it drives the splined transmission mainshaft.
When the rider pulls on the clutch lever, a lever mechanism at the opposite end of the clutch cable forces a shaft to release the spring pressure on the clutch pressure plate, freeing the connection between the drive (friction) and driven (plain) clutch plates; this disengages the connection between the engine’s “power unit” and transmission. As the clutch lever is released, the pushrod retracts, allowing the spring pressure to be fed through the pressure plate into the clutch plate stack, resulting in the power unit once again driving the transmission mainshaft.
Note that since the kickstarter engagement is through the transmission mainshaft, the clutch must be ENGAGED (static, lever out) for the kickstarter to rotate the engine. If the clutch is actuated, this disengages the link between the power unit and transmission; thus, the bike cannot be kickstarted with the clutch disengaged (lever pulled in).
Oiling system
Driven by a worm gear from the timing end of the crankshaft, the dual gear oil pump first draws oil from the oil tank through the main circuit, to the crankshaft at the timing (right) side end which is drilled though to both connecting rod journals; it is pumped into the mating surface between the crank’s journals and plain bearings of the connecting rod big ends, where it simply squirts out, splashing all of the rotating and reciprocating parts including the underside of the pistons, to keep the cylinder bores lubricated. Oil control rings in the pistons keep excess oil from entering the combustion chamber; thus, if the rings are excessively worn, it becomes apparent from oil-fouled smoky exhaust and excessively oil-fouled spark plugs. This same circuit feeds a smaller capillary tube that runs up to the head where it feeds the four hollow rocker arm pivot shafts. This oil lubricates the rockers at their pivot points, then drains down through the pushrod tube tunnel, lubricating the cam followers (“lifters” or “tappets”) and the cam.
The scavenge (secondary) circuit draws the oil out of the crankcase bottom through a sump screen, and back to the oil tank; there is also a “telltale” scavenge return orifice which allows inspection and verification that oil is indeed returning to the tank.
The last circuit in the oiling system is the oil pressure relief, which is in common with the crankshaft main end feed passageway, and incorporates a spring-loaded plunger, which (on late models) allows excess oil pressure buildup to be “dumped” into the crankcase sump to avoid over-oiling of the rod bearings; on earlier models, excess pressure is re-routed back to the oil pump inlet.
Crankcase Breather system
The pressure buildup within the crankcase as the pistons travel downwards would force oil to leak from every possible point on the engine’s many mating surfaces and assembly hardware unless some form of relief was present.
In the early model twins, the method employed is a tiny disc keyed to the primary side (left) end of the camshaft. The two tiny openings in the disc align at the proper time with companion openings cut into a chamber in the left side crankcase half which has it’s exit through a pipe that runs to the breather inlet pipe at the top of the oil tank.
Later models incorporated a breather trap behind the crankshaft, with a mesh filter to deter excess oil from escaping into the breather system; the vent hose was routed to the same location as in earlier systems.
The final breathing arrangement was via a common opening between the crankcase and the timing chest; the pressure was vented to the upper rear (inside) corner of the timing chest to the same oil tank breather inlet as previous models.
The outlet breather of the oil tank on early models was to a pipe routed into the airbox; on later models it is routed out a hose to the rear of the bike.
Transmission (4-speed)
Power from the engine is fed through the clutch center hub to the transmission mainshaft which has spline-driven gears that can be shifted to various positions resulting in different ratios between the input (mainshaft) and layshaft, to the sleeve gear (output shaft). Power is transmitted from the mainshaft, through whichever gear pair is selected, to the layshaft, which turns the final drive sprocket, which drives the final drive chain, which drives the rear sprocket, turning the rear wheel. (In top gear, power is fed from the mainshaft directly to the sleeve gear).
The gear selector apparatus is a somewhat intricate assembly of pins, springs, pawls, cam and forks that operate thusly:
When the gearshift lever is depressed, a pin in the mechanism, that slips into a roller, that sits captive in a semi-circular “cup” in a gear-like apparatus that pivots on another pin, rotates slightly. The gear-like apparatus’ opposite end causes a camplate gear to rotate on it’s shaft. The front side of each of the two shifter forks has a pin with a roller slipped over it; these pins with their rollers wend their way through two curvaceous grooves cut in the face of the camplate as it rotates.
The two gear selector forks slide left-to-right on a shaft, with the tines of one fork riding captive in a groove of one gear on the mainshaft, and the tines of the other fork riding captive in a groove of another gear on the layshaft. So, as the gearshift camplate rotates, it moves the position of the appropriate gears one way or another, via the shifting forks, to align each matched pair in succession as the gears are selected. Regardless of the gear selected, power comes in through the mainshaft, feeds the selected drive gear, which feeds it’s paired driven gear, which feeds the layshaft, which feeds the drive sprocket.
The kickstarter’s ratchet gear engages another gear on the mainshaft, which turns the clutch, which turns the crankshaft via the primary chain. As the pedal is released, or as the engine spins, the ratchet's teeth release, allowing the pedal to remain free from counter-rotation except as allowed by the return spring, to the point where an internal stop limits it's travel just beyond the point where the dog tooth in it's ratchet gear disengages the starter gear.
Ignition systems
Driven by the timing end of the camshaft, the ignition system is timed to provide spark to the combustion chamber at the exact point in the 4-stroke sequence for proper combustion and operation of the engine.
In a contact breaker points based system, the camshaft turns an automatic advancer plate upon which pivots a points cam; this points cam is advanced as engine speed increases, by means of centrifugal flyweights which vary the points cam’s relative position to the exhaust cam by a pair of springs. The faster the cam spins, the more centrifugal force is exerted on the flyweights, which rotate the points cam via a tooth-in-groove arrangement, effectively advancing the timing to provide the spark earlier in the compression stroke, allowing the mixture in the combustion chamber to ignite and burn appropriately. 12 volts are fed (through the system voltage sources, more later) to the contact breaker points, which when actuated, send a primary voltage pulse to on of the ignition coils, inducing a high secondary voltage pulse through the ignition lead to the spark plug associated with that coil & points set; the same thing happens again 180 degrees of CAMSHAFT rotation (360 degrees crankshaft rotation) later, and the other set of points fire for the other cylinder.
In a magneto ignition system, the cam drives the magneto’s armature and breaker points cam simultaneously; the magneto’s armature produces the primary voltage for the points, which, when triggered, induce a high voltage in an encapsulated coil which feeds both of the ignition leads to the spark plugs. This results in one cylinder firing, and a “wasted spark” once every cycle of 360 degrees of crankshaft (180 degrees of camshaft) rotation.
In an typical modern electronic ignition, the engine’s cam drives a rotor assembly with two magnetic triggers affixed to it; as these magnets pass in proximity to two pickup coils mounted in the stator plate, an electromagnetic pulse is triggered in the primary circuitry. This primary pulse travels into the ignition’s “black box” which in turn magically (well, through the wonders of modern electronics), sends a single12 volt primary pulse through a pair of 6 volt coils, inducing a secondary pulse in both coils, causing a spark at both spark plugs simultaneously, resulting in one cylinder firing, and a “wasted spark” as with the magneto ignition.
Alternator and charging system (single phase)
On the left end (primary side) of the crankshaft, a magnetically polarized alternator rotor is keyed to the shaft; the rotor spins in close proximity to the fixed alternator stator which has multiple copper-wound coils encapsulated in it. These coils are wired in such a manner that an alternating electromagnetic pulse is triggered as the rotor’s magnets spin past the coils. The stator’s output leads are wired such that two wires carrying alternating current (AC) feed into a diode array called a “rectifier”. The rectifier’s diodes are composed so that the alternating current pulses that are sent through the array, exit in a flow of direct current (DC). This voltage is sent to the storage battery, and all of the electrical systems, as well as a Zener diode which regulates the maximum voltage allowed into the electrical system at just above 12 volts; any additional voltage above that (from the alternator spinning at high speeds), is “bled off” in the form of heat. That is why Zener diodes must always be mounted to some form of heat sink, or they will rapidly fail due to excessive heat buildup.
Carburetion
The various makes and models of carburetors operate in essentially the same manner:
Fuel enters the carb and encounters an orifice that is controlled by a float needle; as the bowl fills with fuel, the float rises to a fixed point at which the paired needle rises in it’s bore until it seats at the orifice, block flow of fuel into that carb so that it doesn’t “flood”.
At idle, a fixed system consisting of a restricted passage or a replaceable “low speed” or “idle” jet allows a tiny amount of fuel to bypass the carb’s main circuits, allowing the engine to idle without intervention; the ratio of air to fuel at idle and low engine speeds is controlled by the low speed mixture screw.
When the throttle twistgrip is actuated, the cables pull on the carb’s slides, lifting them out of the airstream, allowing more air into the carb; as the opening increases, a vacuum is formed by relatively low pressure in the “venturi” of the carb, relative to the higher atmospheric pressure in the float bowl. The greater the air pressure differential, the more fuel flows though the carb’s main jet; this flow is restricted, moderated and adjusted by the slide needle affixed to the carb’s slide. The general running condition fuel-to-air ratio (mixture) is controlled by an intricate combination of different sized and shaped slide, main jet and needle jet, and different shaped and positioned needles.
Exhaust
The spent products of combustion cannot simply be dumped out of the combustion chamber into open air without drastically negative effects on the engine’s performance. The exhaust headers exit the head and minimally channel the exhaust to the rear of the bike; a secondary effect of the bends in the exhaust pipes causes a pressure buildup that interacts with the flow in such a way that a return pulse moderates the flow to a point at which nominal performance is gained. Customizing the diameter of these pipes, and the complexity or simplicity of the circuit, can result in increased effectiveness of the system, resulting in increased performance of the engine. The mufflers may or may not contribute to (or detract from) performance, but are mandatory for road use.
Any questions?
Thanx to L.A.B. for several fine-point corrections.
