Omega Owners Forum
Chat Area => General Discussion Area => Topic started by: Golfbuddy on 15 January 2008, 00:13:44
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I've been to my evening class tonight and we got to strip down an engine. Removed cylinder head, manifolds, camshaft and related gubbins. The engine was an 8 valve, normally aspirated Ford 1600.
When looking at how the 'toothed belt' connected the camshaft and crankshaft, it got me thinking about some of the threads I have read on here about sensors.
On this engine, the camshaft drove the distributor which set the timing and order of firing. This I understand is replaced by the electronic ignition on a modern car and I guess must take it's signal to fire from the camshaft sensor? Thus, if the camshaft sensor fails, the car wouldn't 'know' what position the pistons were in and would result in the spark being fired at completely the wrong time or not at all.
If the engine 'knows' what position the camshaft is in, and this is connected directly to the crankshaft by the toothed belt, what is the point of the crankshaft sensor? And what are the consequences of it's failure.
:)
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I've been to my evening class tonight and we got to strip down an engine. Removed cylinder head, manifolds, camshaft and related gubbins. The engine was an 8 valve, normally aspirated Ford 1600.
When looking at how the 'toothed belt' connected the camshaft and crankshaft, it got me thinking about some of the threads I have read on here about sensors.
On this engine, the camshaft drove the distributor which set the timing and order of firing. This I understand is replaced by the electronic ignition on a modern car and I guess must take it's signal to fire from the camshaft sensor? Thus, if the camshaft sensor fails, the car wouldn't 'know' what position the pistons were in and would result in the spark being fired at completely the wrong time or not at all.
If the engine 'knows' what position the camshaft is in, and this is connected directly to the crankshaft by the toothed belt, what is the point of the crankshaft sensor? And what are the consequences of it's failure.
:)
Not bad reasoning but, not how it truely works.....
The ECU uses crank position to determine when to fire the ignition because the most important thing about spark is to fire at the correct piston position (and the timing varies with load and revs). Hence its actualy the crank shaft sensor which gives the position (hence the term CPS - Crank Position Sensor).
So, why the need for a cam sensor?
Well, its used simply to determine which cylinder is next to fire and on earlier wasted spark systems this is purely to allow sequential injection (firing the fuel injector as the inlet valve opens) and on later coil per plug cars it also allows the correct coil to fired as well. But, the key thing to remember is that the Cam sensor plays no part in the timing.
SO, what happens when it fails.....
Two key things......the injectors can now no longer fire sequentialy as you dont know which cylinder is on the inlet stroke and which is on the power stroke so the ECU adopts a limp mode and fires both of the injectors at the same time whilst limiting the revs to 4000rpm.
Similarly on a coil per plug setup, you dont know which cylinder is on the compression stroke, this is no big deal to a wasted spark setup but, on a coil per plug arrangement you now fire two coils at the same time and again limit the revs.
Of course there are some minor variations to operation in limp mode dependent on who wrote the software.
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Very interesting info.
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Thanks Mark. I guess that wasted spark means that all spark plugs fire at the same time?
Secondly, if the crankshaft position sensor tells the ecu when to fire the spark plugs, is this why positioning of the sensor is so critical? Presumably if the sensor is fitted slightly off square or out of alignment with the crankshaft it will send an incorrect signal to the ecu.
It's always good to have a bit of theory when trying to understand a problem. :y
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As I understand it, a wasted spark ignition system fires two pairs of plugs (cylinders) (not all of them) together. One of these cylinders will be on the power stroke and the spark from the plug is, obviously, required. The other cylinder will be on the exhaust stroke and the plug will still be fired as they are a pair. however, this spark is not needed hence the term "wasted spark". I believe that this system was first implemented on the Citroen 2CV!
The CPS is, I think, a magnetic "hall effect" sensor. If it positioned too far away, it will not be able to "see" the part of the crank shaft which has the opposite end of the sensor. The quality of the signal is vital for this component as it is for many other sensors.
I am sure that other, more knowledgeable people (Marks DTM) will be able to enhance what I have said .............. :)
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Forgot to mention above tat a while ago Mark wrote a series of excellent posts about the various sensors. These can be found in either the FAQ or Maintenance Guide sections. They are well worth a read IMO.
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CPS's are normally variable reluctance, in that a coil of wire is wound round a magnet and then timing marks cut into the flywheel pass by the tip of the magnet.
As ferrous metal comes close to the magnet it causes the magnetic flux around the magnet to temporarily collapse, inducing a current in the coil.
As the metal moves away again, the magnet begins to re-establish the field and the current reverses. The fact that the current reversal happens at a very precise point on the flywheel's rotation, and that it can be detected easily using a zero crossing detector makes this a very accurate way of establishing the engine's crank angle.
The sensor is also cheap and robust - in 99% of vehicles. Perhaps not in V6 Omegas, although that's more of a wiring issue. ::)
Kevin
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As I understand it, a wasted spark ignition system fires two pairs of plugs (cylinders) (not all of them) together. One of these cylinders will be on the power stroke and the spark from the plug is, obviously, required. The other cylinder will be on the exhaust stroke and the plug will still be fired as they are a pair. however, this spark is not needed hence the term "wasted spark". I believe that this system was first implemented on the Citroen 2CV!
The CPS is, I think, a magnetic "hall effect" sensor. If it positioned too far away, it will not be able to "see" the part of the crank shaft which has the opposite end of the sensor. The quality of the signal is vital for this component as it is for many other sensors.
I am sure that other, more knowledgeable people (Marks DTM) will be able to enhance what I have said .............. :)
Hmmm, my logic on this one says that, if two cylinders fire together on a four cylinder engine, then that would mean that two cylinders are firing at the same time, not one one after the other. I guess that this is fairly basic stuff but I though that the cylinders fired one after the other, e.g. 1,3,2,4 ??
Please forgive the assinine questions but I have quite a thirst for knowledge at the moment. :y
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Two spark plugs fire at once, but only one of the cylinders they're in contains any fuel.
So, the cylinders do indeed still fire one after the other. It's just that pistons 1 and 4 and 2 and 3 on a 4 pot are always at the same point in the bore, so their top dead centre timing is the same, and required spark timing is at the same crank angle. Each one only fires every 2nd rotation of the crank though so they take it in turns to actually "use" the spark.
Kevin
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Two spark plugs fire at once, but only one of the cylinders they're in contains any fuel.
So, the cylinders do indeed still fire one after the other. It's just that pistons 1 and 4 and 2 and 3 on a 4 pot are always at the same point in the bore, so their top dead centre timing is the same, and required spark timing is at the same crank angle. Each one only fires every 2nd rotation of the crank though so they take it in turns to actually "use" the spark.
Kevin
Thanks for that Kevin. I did notice on the engine last night that pairs of pistons appeared to be at the same postion and was wondering why that should be the case.
Therefore, I assume that, when pistons 1 and 4 are at the top, one of them is compressing the fuel air mixture for combustion and one of them is releasing the exaust gasses?
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Correct. 1 and 4 at the top of stroke, one of which is on compression, the other on exhaust. 2 and 3 at bottom of stroke, one at the end of its power stroke, the other at the end of its induction.
4 cylinders, each with one of the 'suck, squeeze, bang and blow' components..... :y
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Eureka!!!
So, the flywheel keeps up the momentum of this cycle then?? :-/
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Basically, yes. Also supplies the means to start same by the toothed ring on the outside which is turned by the starter motor. It also supplies the interface between engine and gearbox, either as a clutch or as an autobox....
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Therefore, I assume that, when pistons 1 and 4 are at the top, one of them is compressing the fuel air mixture for combustion and one of them is releasing the exaust gasses?
Yes, and to determine which is which, you need to look at the position of the camshaft.
On one cylinder, in this position, both valves will be shut as it prepares to fire. On the other cylinder, the exhaust valve will be closing and the inlet valve will be opening as you're at the end of the exhaust stroke and the beginning of the next induction stroke.
If you think about the distributor, it's driven by the camshaft, so it "knows" which cylinder will fire next. If the ECU doesn't have a cam sensor, it doesn't, and this is how wasted spark systems work. They don't need to know.
Put a cam sensor on, and the ECU has all the information that is available to a distributor, so it can inject fuel to whichever cylinder has its' inlet valve open, instead of injecting onto an closed inlet valve on some of the cylinders. It can also control an individual spark to that cylinder instead of a spark to both cylinders in the pair.
The main timing reference is always taken from the crankshaft, however, because the cam shaft is connected to it through a belt or chain which has a little bit (a few degrees) of slop. This is a big advantage over having the timing reference from a distributor. Timing can be controlled much more accurately with a crank sensor.
Kevin
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Please forgive the assinine questions but I have quite a thirst for knowledge at the moment. :y
Nothing wrong with that - it is how we all learn.
Congratulations on doing your engine course BTW and I wish you well with it.
I don't know a very great amount about engines - just the basics. Most of my knowledge I picked up by reading Car Mechanics magazine - which I thoroughly recommend and on here of course!
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Notice how (old style) diesels dont have cam sensors for same reason - the ecu doesn't control the firing sequence for injectors (and obviously no ignition)
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Most old injection systems dont have cam sensors i.e. 3.5 and 3.9 Rover V8s, early, L-jetronic, most SPi setups....
As for the wasted spark, here is some more info for the melting pot.
You have two spark plugs firing at the same time but, they will not have an equal voltage across them......but why...
Well again its simple, the voltage required to create a spark across a given gap (i.e. a spark plug) is related to a number of factors and one of the important ones is pressure....
So, if you were to measure the voltage across the sapr plug you would find about 40KV across the one on the compression stroke and around 4KV across the one on the exhaust stroke.......and as some of you may ahev noticed this i a close correlation to the compression ratio and is indeed due to the fact that teh cylidner pressure at the top of the compression stroke is 10 times that of teh cylinder pressure on the exhaust stroke.
This works in your favour as you now only need a coil that is insulated slightly better thn a standard one rather than what some expect would need to be twice that of a standard one.
As for the type of sensor used for cam and crank.....it does vary from system to system with reluctance being a common one but, hall sensors not being un-heard of.
The cam sensor on the 4 pot is actualy unusual as it has a sine wave of some few hundred KHz applied to it!
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The cam sensor on the 4 pot is actualy unusual as it has a sine wave of some few hundred KHz applied to it!
I've noticed this. Is it still a variable reluctance sensor or some other type?
I'm guessing this is done to improve the low frequency reponse of the sensor over one with a "DC" magnet.
Kevin
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and this would explain the wiring mod in the Cavs that were the first vehicle to receive the Simtec 56.1 system with the 2.0 16V engine.
What was happening was that the cam sensor part of the loom was routed round by the alternator and this was not shielded correctly. There were two mods as I recall, number 1 being to move the loom out of the way of the alternator and, mod 2 was the fit additional shielding. My cav had option 2.
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Thanks for all the great answers guys. I now better understand what is going on under the bonnet, and a bit deeper, of my car.
Martin
:y :y :y
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Thanks for all the great answers guys. I now better understand what is going on under the bonnet, and a bit deeper, of my car.
Martin
:y :y :y
What are you turning into :-/
I have only been away for a few days ;D ;D
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Thanks for all the great answers guys. I now better understand what is going on under the bonnet, and a bit deeper, of my car.
Martin
:y :y :y
What are you turning into :-/
I have only been away for a few days ;D ;D
Sorry, who are you????
:P