Engine starting
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I'm really going to stick my neck out here, and challenge BEagle's statement.
A normal magneto , if turned by hand , can be felt to break the magnetic"grip"
That instant when it "lets go" is when the lines of flux are broken and the current is induced in the windings,,,hence the points-gap is critical, as this IS the deciding factor in getting the timing spot-on for a fat spark, which will only occur if you have the maximum generated current at the instant the points open and collapse the field....that's what induces the HT voltage in the secondary winding.
At cranking-speed, the rotation can be too slow to generate a sufficient primary current....so we have an IMPULSE START.
This is basically a clock-spring in the couplung between the drive and the magneto"innards"....a Pawl engages the spring-mechanism and continued rotation winds the spring until the pawl hits a trip-mechanism which releases the magneto to be "flicked" round by the spring unwinding. The rapid twirlthrough the break-point gives a full spark. the trip-mechanism is set to operate at a point well retarded from the normal "run" timing. The Pawl has a centrifugal weight, such that, should the engine fire, the Pawl retracts and the spring , uncoiled, acts as a solid drive in it's normal "run" advance timing.
If you crank with a "normal" Mag engaged, It will fire aroung the same time as the impulse starts winding up on the other mag...It could fire a "good" mixture and stop the piston dead then reverse it's direction (a "Kick-back" ) OTOH, the impulse timing is late-enough that by the time the cylinder charge is burning and expanding, the piston and crank have gone over TDC and thus the expanding gases push the piston on the right side of TDC.
Note the impulse -coupling is NOT SPEED DEPENDENT (except to disengage) This means it will ALWAYS (unless broken!) be engaged on a stationary engine....no matter how slowly you turn the prop by hand, the coupling will flick the mag round at it's release -point.
IT THEREFORE STANDS THAT IF THE MAG IS "LIVE" IT WILL SPARK.
A broken ground-wire will give this situation, that's why a running "drop-test" is important, it shows the mag is working AND the cut-out is also working!
Summary...Start on impulse and switch to "both" as soon as the engine runs.
Always be aware that a slight movement of the prop , could trip a spark and start an engine.
Sorry Mr. Beags...If I'm wrong, please tell us!
A normal magneto , if turned by hand , can be felt to break the magnetic"grip"
That instant when it "lets go" is when the lines of flux are broken and the current is induced in the windings,,,hence the points-gap is critical, as this IS the deciding factor in getting the timing spot-on for a fat spark, which will only occur if you have the maximum generated current at the instant the points open and collapse the field....that's what induces the HT voltage in the secondary winding.
At cranking-speed, the rotation can be too slow to generate a sufficient primary current....so we have an IMPULSE START.
This is basically a clock-spring in the couplung between the drive and the magneto"innards"....a Pawl engages the spring-mechanism and continued rotation winds the spring until the pawl hits a trip-mechanism which releases the magneto to be "flicked" round by the spring unwinding. The rapid twirlthrough the break-point gives a full spark. the trip-mechanism is set to operate at a point well retarded from the normal "run" timing. The Pawl has a centrifugal weight, such that, should the engine fire, the Pawl retracts and the spring , uncoiled, acts as a solid drive in it's normal "run" advance timing.
If you crank with a "normal" Mag engaged, It will fire aroung the same time as the impulse starts winding up on the other mag...It could fire a "good" mixture and stop the piston dead then reverse it's direction (a "Kick-back" ) OTOH, the impulse timing is late-enough that by the time the cylinder charge is burning and expanding, the piston and crank have gone over TDC and thus the expanding gases push the piston on the right side of TDC.
Note the impulse -coupling is NOT SPEED DEPENDENT (except to disengage) This means it will ALWAYS (unless broken!) be engaged on a stationary engine....no matter how slowly you turn the prop by hand, the coupling will flick the mag round at it's release -point.
IT THEREFORE STANDS THAT IF THE MAG IS "LIVE" IT WILL SPARK.
A broken ground-wire will give this situation, that's why a running "drop-test" is important, it shows the mag is working AND the cut-out is also working!
Summary...Start on impulse and switch to "both" as soon as the engine runs.
Always be aware that a slight movement of the prop , could trip a spark and start an engine.
Sorry Mr. Beags...If I'm wrong, please tell us!
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Hi, we once had a Petter stationary engine, on a water pump, that had an Impulse Magneto. After it had run for a bit to get it warm, if we needed to re-start it, all we had to do was crank the flywheel over slowly (even taking 10 seconds from BDC to TDC.) and when it reached TDC the Impulse would occur, and the engine would thump into life.
The voltage produced by a coil with a contact breaker is proportional to the rate of change of current (di/dt.). So if the points open instantaneously, there is a very large voltage or back-emf produced.
(I have forgot the actual formula but suspect it is something like
dv = L di/dt where L is the Inductance.)
The voltage produced by a coil with a contact breaker is proportional to the rate of change of current (di/dt.). So if the points open instantaneously, there is a very large voltage or back-emf produced.
(I have forgot the actual formula but suspect it is something like
dv = L di/dt where L is the Inductance.)
Cockneysteve, thank you for that very clear explanation. Now can you tell me how shower of sparks magnetos work? The ones with a buzz box that feeds off the battery while the starter is engaged.
Many thanks if you can!
Many thanks if you can!
Thread Starter
Hi Beags,
The FRCs (RAF) say use left (impulse) as does the BAe manual. But I have been told by a Lycoming chap to use both. I cannot think what the disadvantages are for using both; if we get a good spark from the impulse mag, then surely the No 2 will have no effect, good or bad.
Mind you, my engine turns very slowly as I need a new starter motor. I'm going for the lightweight SkyTec which has good reviews. That said, it can overheat if used for too long.
Now, having read Cockney Steve's explanation, I think I'll try starting with just the impulse mag, though I have not had an examples of kick-back using both.
The FRCs (RAF) say use left (impulse) as does the BAe manual. But I have been told by a Lycoming chap to use both. I cannot think what the disadvantages are for using both; if we get a good spark from the impulse mag, then surely the No 2 will have no effect, good or bad.
Mind you, my engine turns very slowly as I need a new starter motor. I'm going for the lightweight SkyTec which has good reviews. That said, it can overheat if used for too long.
Now, having read Cockney Steve's explanation, I think I'll try starting with just the impulse mag, though I have not had an examples of kick-back using both.
Last edited by sharpend; 17th Aug 2013 at 11:53.
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I think it is ambiguity or a missing " - I think it means 1/4 inch - and as has been said 1/4 inch cold start no problem.
Still think one should follow the procedure in the manual and will have no problem.
Any need to depart from the manual's procedure does suggest something in the airplane is non-standard.
Still think one should follow the procedure in the manual and will have no problem.
Any need to depart from the manual's procedure does suggest something in the airplane is non-standard.
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@ Piper Classique.... I am not familiar with the technical side of "shower of sparks" magneto, but it's very simple to do with coil ignition. The points could operate a transistor to hold an astable oscillator "off"...the points open, transistor switches oscillator "on" oscillator , in turn, feeds it's output pulses through the primary of an ignition-coil thus you get a shower of sparks which switches off when the points close again.
As a kid, I had a "shock-coil" -basically it's a transformer, (as is an ignition coil)
the primary connected in series with a buzzer/electric bell....connect to a battery and it operates as follows...
solenoid coil of buzzer/bell and primary of HT coil (series!) energise...solenoid pulls spring-loaded armature (a bit of soft iron) towards it...the primary circuit is arranced so a break in the wire is connected one side to a fixed contact point and the wire continues from a contact -point attached to the armature... thus, when the armature moves towards the coil, it breaks the circuit, the magnetic field collapses and the armature springs back to complete the circuit again...
Distance from coil to armature, strength of spring (and one or both contacts can be sprung as well!) and supply voltage all affect the rate of oscillation....
Meanwhile, the pulsing on-off current is passing through the shock/spark coil primary winding.... every time the coil is energised, it magnetises it's core and every time the current stops, the magnetic flux breaks down and induces a voltage into the secondary.
this is necessarily a shortened explanation, i've deliberately left out the stuff Phiggs alluded to, such as quenching capacitor across the points.
I suppose a magneto could be arranged with a modified form of impulse-coupling, such that when the coupling "flicks" the mag, the rotating part of the Mag has a one-way clutch with a single engagement-point and sufficient mass that the impulse is able to spin the armature multiple times until it slows sufficiently for the drive to "lock " again that would give a brief shower of sparks, but remember the Mag has a permanent magnet and the armature is "braked2 every time it's poles pass the poles of the field magnet.
Physics....the energy for the spark has to come from somewhere.
I must start going out!
Forgot to say, an electronic ignition system can trigger a shower of sparks and , to prolong plug-life, revert to single-spark under benign running conditions.
@Phiggs...yep, I had an air-cooled single "Tuff-Tuff " with a Wipac impulse mag....hand cranked it would "clink....clink.....clink as the impulse fired....if you cranked hard enough, you could exceed the impulse cutout speed
just pulling the flywheel to the "Clink" would often set it going.
we used to drag it out and set it going on the workshop floor...as it had a speed-govenor, it would scuttle about as you revved it, and judicious jiggling of the throttle would produce backfires of vivid blue flame from the exhaust stub....a length of pipe could be held with one end near the stub and be "tuned" like a trombone
Amazingly, I found time to work as well! Think it was a couple of quid, non-running at an auction. best value entertainment ever.
As a kid, I had a "shock-coil" -basically it's a transformer, (as is an ignition coil)
the primary connected in series with a buzzer/electric bell....connect to a battery and it operates as follows...
solenoid coil of buzzer/bell and primary of HT coil (series!) energise...solenoid pulls spring-loaded armature (a bit of soft iron) towards it...the primary circuit is arranced so a break in the wire is connected one side to a fixed contact point and the wire continues from a contact -point attached to the armature... thus, when the armature moves towards the coil, it breaks the circuit, the magnetic field collapses and the armature springs back to complete the circuit again...
Distance from coil to armature, strength of spring (and one or both contacts can be sprung as well!) and supply voltage all affect the rate of oscillation....
Meanwhile, the pulsing on-off current is passing through the shock/spark coil primary winding.... every time the coil is energised, it magnetises it's core and every time the current stops, the magnetic flux breaks down and induces a voltage into the secondary.
this is necessarily a shortened explanation, i've deliberately left out the stuff Phiggs alluded to, such as quenching capacitor across the points.
I suppose a magneto could be arranged with a modified form of impulse-coupling, such that when the coupling "flicks" the mag, the rotating part of the Mag has a one-way clutch with a single engagement-point and sufficient mass that the impulse is able to spin the armature multiple times until it slows sufficiently for the drive to "lock " again that would give a brief shower of sparks, but remember the Mag has a permanent magnet and the armature is "braked2 every time it's poles pass the poles of the field magnet.
Physics....the energy for the spark has to come from somewhere.
I must start going out!
Forgot to say, an electronic ignition system can trigger a shower of sparks and , to prolong plug-life, revert to single-spark under benign running conditions.
@Phiggs...yep, I had an air-cooled single "Tuff-Tuff " with a Wipac impulse mag....hand cranked it would "clink....clink.....clink as the impulse fired....if you cranked hard enough, you could exceed the impulse cutout speed
just pulling the flywheel to the "Clink" would often set it going.we used to drag it out and set it going on the workshop floor...as it had a speed-govenor, it would scuttle about as you revved it, and judicious jiggling of the throttle would produce backfires of vivid blue flame from the exhaust stub....a length of pipe could be held with one end near the stub and be "tuned" like a trombone
Amazingly, I found time to work as well! Think it was a couple of quid, non-running at an auction. best value entertainment ever.
cockney steve was correct. The "buzzer" or "shower of sparks" system uses switched LT from the aircraft battery in the primary winding of the magneto to generate a series of sparks in the starting period when the magneto is turning slowly and so generating little LT itself. Cheers
OAP
OAP
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Sharpend
I would advise only using the left mag, part of the job that the impulse unit does is retard the spark to just after TDC thus all the effort of the first power pulse is to send the piston in the correct direction.
The right mag will always fire at (depending on the O360 variant ) 20 or 25 degrees BTDC and try to send the piston in the reverse direction if it fires at the slow cranking speed. So at best having the right mag on during start will do nothing at best and at worst it will appose the efforts of the starter motor.
The right mag will always fire at (depending on the O360 variant ) 20 or 25 degrees BTDC and try to send the piston in the reverse direction if it fires at the slow cranking speed. So at best having the right mag on during start will do nothing at best and at worst it will appose the efforts of the starter motor.
It's many years since I had a starter separate from the mag switch - both on before key works starter. I can see the point when handswinging, although I've done it with both on.
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Maoraigh1
both on before key works starter
cockney steve & A and C, you are both correct and I was wrong. I didn't realise that the right magneto fired quite so early - so particularly with sharpend's low cranking speed, left magneto only is indeed the correct selection.
Last edited by BEagle; 19th Aug 2013 at 08:29.
Thread Starter
Hi Guys, thanks for all the advice. I tried starting as per the RAF FRCs yesterday (left mag only) and it appeared to start better. Only trouble is, problems do come in twos or threes! When I press the starter button for the first time, the engine will not crank, but there is a whirring noise from the bulkhead. Second push activates the starter, but the engine cranks very slowly. Battery voltage is fine
Luckily the engine normally fires after a couple of turns of the prop.
So my diagnosis:
1. Starter relay knackered.
2. Starter knackered.
ps recently replaced the bendix.
Luckily the engine normally fires after a couple of turns of the prop.
So my diagnosis:
1. Starter relay knackered.
2. Starter knackered.
ps recently replaced the bendix.
Or a bad connection. First push heats it up - expands - better connection, but still poor, turns starter slowly. Very cheap to check the connections, clean, retighten, and try again.
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Maoraigh1 Or a bad connection
Check battery cranking-volts! A £5 multimeter is OK....12V battery should be 13-14V dropping to~10V cranking....24V battery, ~26 resting, ~20 cranking....any lower and suspect a dying battery or see Maoraigh's post.
If it's pre-engaged....check it's the correct pinion, right number of teeth (YES, REALLY !) and properly engaged in the operating fork. you can check on the ground with a pair of jump-leads and battery....connect to the bug terminal and the casing...bridge the small terminal to the big battery terminal and the pinion should slide out with a smart CLUNK. the other big terminal is connected to a short-stout lead that goes into the carcase....connect the other big terminal (or the jump-lead direct to this and the starter should spin merrily......if you can blow out with an airline, it's good to get rid of all the carbon-copper from brush-wear.
Inertia starters DO use a Bendix...this is a quick-thread sleeve on the armature-shaft that engages a female component fixed in the pinion....a bit like a conjourer pulling out the table-cloth, stsrter spins and pinion slides along the quick-thread until it reaches the end, they then both rotate as a whole.
Cracked sleeve or "star" (female part) or dirt/oil/grease can stop it working. again count the teeth! easy to fit wrong one and it will bind on the ring-gear.
DID THE PROBLEM ONLY START AFTER REPLACING THE PINION?BENDIX?
Due to computer crashing, above was somewhat shortened.
having re-read some of the thread, it seems the starter has been sluggish a while. Is it Pre Engaged?.....that is, it has an integrated solenoid with a direct feed from the battery...there is a small terminal, which , when energised, pulls in the solenoid....this moves a forked-arm to push the pinion into mesh with the engine's ring-gear (flywheel) It also has a heavy contact which joins the 2 big contacts on the solenoid end-cap one is the battery, the other is the starter windings. these internal contacts wear but can be replaced usual symptoms, erratic starting...sometimes nothing, sometimes normal, sometimes slow.
If you are on permit, AIUI it's OK to DIY...check the brushes are long-enough....too worn and the pressure springs or tail-wires hit stops and the brush cannot press firmly against the rotating Commutator (copper segments on the armature)...broken or jammed brushes, same effect...worn bearing bushes will allow pole-shoes and armature to foul each other....very rare! excessive carbon-dust from brush-wear can track down a lot of current to earth..always worth blasting innards with an airline. A field-winding that's tracking to earth...tricky to find, high current consumption is a giveaway but you'll need a meter for ~200 amps! (excess battery-drop whilst cranking is another giveaway and the motor will get hot.
Thrown solder on the commutator segments....caused by using a starter too long, especially with a "low" battery...bad or open windings on the armature + low speed and overheating.
All can cause low cranking speed.
many modern starters have permanent-magnet motors, they're smaller, lighter and run at high speeds,so incorporate (usually) epicyclic reduction gears.
Overheating these motors ,weakens the magnets...starter gets sluggish AND overheats....self destructing,if abused.....the old field-coil starters are much more robust in this sense, but see "thrown solder" , "partially -shorted field windings".....yer don't get owt for nowt!
Last edited by cockney steve; 20th Aug 2013 at 17:19. Reason: added permanent magnet starters
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all we had to do was crank the flywheel over slowly (even taking 10 seconds from BDC to TDC.) and when it reached TDC the Impulse would occur, and the engine would thump into life.
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"in the old days"....some cars were equipped with "shower of sparks"ignition....I'm sure a search for "trembler-coil ignition" will throw up something!
Cars into the 1930's routinely incorporated a toothed ring in the centre of the steering -wheel protruding from the hub, radially, were three levers that had ratchet-engagement with the toothed ring....they were hand-throttle, mixture and advance-retard.
a popular trick was to leave the car fully-retarded before switching-off...upon return, switch on ignition and advance timing....invariably a 6-cylinder would start..a 4-cylinder a bit more of a gamble, but it saved the hand-cranking.
Quality cars of the period often had both coil-ignition and Magneto.
not much new in the basic internal-combustion engine, just refinement of old ideas Modern materials and techniques help, too.
Cars into the 1930's routinely incorporated a toothed ring in the centre of the steering -wheel protruding from the hub, radially, were three levers that had ratchet-engagement with the toothed ring....they were hand-throttle, mixture and advance-retard.
a popular trick was to leave the car fully-retarded before switching-off...upon return, switch on ignition and advance timing....invariably a 6-cylinder would start..a 4-cylinder a bit more of a gamble, but it saved the hand-cranking.
Quality cars of the period often had both coil-ignition and Magneto.
not much new in the basic internal-combustion engine, just refinement of old ideas Modern materials and techniques help, too.
Well, that was all very interesting. Thanks, all.
BTW, when we replaced our starter on the Cub the manufacturer's docs recommended a dry silicone spray on the bendix every 25 hours. Comes from caravan shops if you want to get some.
BTW, when we replaced our starter on the Cub the manufacturer's docs recommended a dry silicone spray on the bendix every 25 hours. Comes from caravan shops if you want to get some.
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@Piper Classique -Are you saying that your starter is of the inertia-type? IE the pinion slides along a helix (quick-thread) and not pre-engaged? In that case, you'd have a separate solenoid (just a bloody great relay, really) in the lead from battery to starter.......I did have a car where you pulled a knob to start....the knob connected to the solenoid via bowden cable.....main contacts have to take a couple of hundred amps,plus , when you actuate it, you're switching an inductive load...so the solenoid contacts have a hard life.
Another "gotcha" for the inertia starter.....the engine firing gives a flick which reverses the pinion and throws it back along the Bendix, where it's arrested by a big cush-spring.......but what stops it vibrating back along the shaft?... there's a light control spring that sometimes breaks.....when it does, the pinion can keep hitting the revolving ring-gear on the engine.........so, if you hear an intermittent metallic tinkling, check the starter!
Another "gotcha" for the inertia starter.....the engine firing gives a flick which reverses the pinion and throws it back along the Bendix, where it's arrested by a big cush-spring.......but what stops it vibrating back along the shaft?... there's a light control spring that sometimes breaks.....when it does, the pinion can keep hitting the revolving ring-gear on the engine.........so, if you hear an intermittent metallic tinkling, check the starter!
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Tim
Selecting the start position on the starter key earths the right mag.......... Sorry being an American aircraft I should say it grounds the right mag.
Last edited by A and C; 23rd Aug 2013 at 12:19.