John Speake

There's quite a wide contact area on the rotor arm... so there is some tolerance in the system for spark timing

It's been pointed out that I should not have said" banks " of cylinders, this implies the LH or RH heads.

The cylinders are connected in two groups 1,7,6,4 and 5,8,3,2. for both ignition and fuel injection. That is "Two and Two" from each block.

All available info that I've seen says that the two "groups" are controlled seperately for the fuel injection,(just a simple inhibit function) but not for the ignition.

Z

Yes.

Since the cams are turning at half the speed of the crank, the 9 degrees of ignition timing change would actually only be 4.5 degrees of rotor movement. There is no actual "connection" by the rotor to the cap. When the spark is triggered there's a jump from the rotor tip to whichever cap terminal happens to be closest and offers the path of least resistance.

Tony

Its does say that, but the 1st sentence on ithe image shown?
"concerened cylinder by 3 degress"

The subject actaully got pretty interesting!

Jim bailey - 928 International

I agree Tony ,what I recalled was that the engine size/ bore size changed the harmonics of the ping and had read also that extra noise vibration from a blower could trigger retard. But this did get very interesting and indicates that maybe it is hard to improve upon the stock S-4 Ignition system which does appear to be smart enough to retard ONLY the pinging cylinder. I had wondered exactly what the Hall signal generator was doing checking ,I knew it defaulted to retarded timing when it failed.

Z

I think that using the term "banks" has probably led to some misunderstanding and mixing up of information about both the LH and EZK systems, and I don't only mean here in this thread.

There's one EZK that sends out ignition trigger signals from two different pins (pin 15 and pin 32) to the two separate ignition modules. These ignition trigger signals are sent out by the EZK by whatever "thinking" it does based on the maps, and various inputs it recieves (load, RPM, temperature, knock, coding plug, etc.) Since it knows what position the engine is in, and what cylinder is firing next, based on what the hall sensor and TDC signal are telling it, it can change the timing of the signal that it sends to the appropriate one of the two ignition modules. The ignition module has no idea which cylinder is going to fire next, what position the engine is in, or anything much at all for that matter. It just recieves the signal to fire, and sends it to the coil that it's connected to. It's job is done. The coil that received that signal from the ignition module doesn't know what's going on either, and just sends the high voltage out to the distributor that it's connected to when it gets that signal from the ignition module. The rotor then just ends up passing off the high voltage to which ever connection on the cap that happens to be closest when the high voltage arrives, and the plug wire that's connected to that cap terminal takes it from there to the spark plug. Except for the EZK, none of the various components need to "know" anything about what cylinder is firing or when. All they do is pass on electricity when they get it. The EZK is knows when to send the signal for each individual cylinder, which ignition module to send that signal to for the appropriate cylinder to get it, and based on that manual page above, can vary when it sends out the individual cylinder signals.

The fuel injectors are kind of separated into two "banks" by the way that they're connected together by the wiring harness. Those two wiring harness "banks" are then both connected together though, and both go to pin 18 of the LH. The injectors may be considered two different banks by some descriptions, but both of those banks have to fire at the same time because they're connected together, and both get the same signal from the LH. There's only one injector output connection from the LH (pin 18). The electrons coming out of that pin don't know to only take a right turn, or to only take a left turn at the junction in the road ahead.

BC

I should get some damn good gas mileage going to sequential if the OEM setup is *8* injectors firing for each intake cycle. Jesus. I thought it was at least 2 banks of 4. Even my megasquirt will be doing that.

macreel

right on, Z... good info.

Thnx.

Lorenfb

"The EZK is knows when to send the signal for each individual cylinder, which ignition module to send that signal to for the appropriate cylinder to get it, and based on that manual page above, can vary when it sends out the individual cylinder signals." - Z -

He's right on! Good writeup, also.

Z

Don't count on too much of an improvement. It's going to take roughly the same amount of gas for an engine to run at the same power level or speed, whether it's batch fire or sequential ignition. With the stock batch fire system all eight injectors fire once each crankshaft revolution. With the sequential the injectors fire individually every two crankshaft revolutions, but for roughly twice as long of a period each time that they do fire. At a given load and RPM, you might have the batch fire injector for a cylinder opening for 3ms every crankshaft revolution. With the sequential injection you'd have the injectors open for 6ms every other crankshaft revolution.

John Speake

OK Tony, I agree that the WS manual extract implies that single cylinder ignition retard might be used. But it is rather ambiguous.

Theoretically the hardware appears to be there to give single cylinder advance/retard in real time, but the real unknown is if the software is there in the 928 EZK.

I think if the 928 had featured individual cylinder igntion control, Porsche would have mentioned it, as it would have been pretty leading edge technology at that time. My favourite "theory" would be that the ignition can be retarded in the groups of 4.

It's interesting to note that the diagnosis software does not store fault codes other than those for knock sensors and Hall sensor. Nor will the tester tell you where the knocks are from, as it counts them in real time. If the EZK was capable if identifying which cylinder or group of cylinders were knocking, then there would have been no reason not to output a specific fault code to help fault finding.

As EZKs so rarely go wrong, I haven't built a dedicated test jig for them. That would be a good way to be able to simualte various operating conditions, and see how the ECU responds. Otherwise a full analysis of the software would reveal all, but that also is a significant task to undertake.

"Z" - I agree about the 2 "groups" of fuel injectors , but there is the facility in the later cars for the "ignition protection relay" to switch one of them off, to protect the cats. The injectors are connected in the same groups as the two ignition groups - 1,7,6,4 and 5,8,3,2

Garth S

Could we further clarify the 'ignition protection relay' function - as these are all interrelated. My limited understanding is that the red/green diode trip indicators were activated by a RTD type of sensor on feedback of a too low temperature in the exhaust port of one cylinder per bank (1-4, and 5-8 respectively)- ie., raw fuel passing on for whatever reason towards the cats, which quenches the sensor in its path.
Does this control act to shut injectors down in the asymmetric 1-7-6-4 and 5-8-3-2 groupings as do the ignition modules/distributors - or in the true 'bank' 1-4 and 5-8 fashion? - or do something entirely else ...

John Speake

Garth,
This system works on a temperature differential betwen the exhaust ports of a cylinder in each head. But these cylinders are each in different groups of the 1-7-6-4 and 5-8-3-2 groupings .

I understand that the exact cylinders that are being monitored were changed at some point, but I don't have that info. But yes, it does act to shut injectors down in the asymmetric 1-7-6-4 and 5-8-3-2 groupings That is all the protection relay can do.

Regards

docmirror

I have two cents, can I try some ideas?

The knock sensors of the older variety are piezio-electric crystal, mated to a hard phenolic plate that transmits vibration well(I guess a mechanical microphone is a good simile). The output of the sensor is a sine wave, of quite low amplitude, and that's why there is a shield, or coaxial type cable employed to transmit it from the head, to the electronics assembly. This sine wave output is sampled by a circuit called a 'fast fourier transform algorithm', which I won't bore you with, it's proprietary anyway. The algorithm is basically a filter to take out the natural 2nd, 3rd, and 4th harmonics of the primary signal, thus reducing the likelyhood of a false positive knock sensed.

John tells us that these samples are collected in real time, so by definition, we can time capture the power pulse from each cylinder within a 45 degree window (360/8). So, if we sample the output of the knock sensor every 2ms at 3600RPM (3600RPM = 60 RPS = 1 REV@ .016S DIV 45DEG = .0020S = 2ms), and keep it in time with the typical knock time of +10DEG ATC on each cylinder, that will tell us not only that a knock has or has not occured, but on which cylinder. At that point, the output of the signal will be sent to the ignition control module to begin retarding that individual cylinder by 3DEG first, and then 3 more, and then 3 more if the knock continues. A safety operation mode is selected when the output of the hall, or knock sensors is deemed unreliable, and all the output to the ignition module is retarded by 6DEG. This allows the car to run, even if one of the sensors is inop.

There will be an aggregation and an integration algorithm that goes with the knock sensor signal to keep track of the count in a time window, say maybe every 6 seconds(guess). Also, will be a advance back check at the end of this window to move the ignition forward if the knock has gone away.

Now, we have the following terms for similar, but not exact artifacts;

Knock, ping, detonation, dieseling, preignition.

The way I understanding it, ping and detonation are the same artifact, and are influenced by C/R, amount, stratification and type of fuel. There is a fascinating machine, called the "motor Octane Machine", that certifies fuels for use in motor vehicles, based on the rated octane and cetane numbers(aircraft use a different test). Dieseling was prevalent in the early 80s when manufacturers allowed low C/R engines, and iffy emmision controls to build up enough carbon in the combustion chamber to 'glow' and ignite fuel even without spark. Cars could run on and on for minutes without the key on if there was a nice hot spark source in the chamber! Cool.

Preignition is a different set of circumstances, and as the name implies, the charge is being ignited before it is ready for the power stroke. In this case, flame front propagtion is corrupted, and the mixture is 'blown out' causing a shock wave near TDC. Both detonation and preigniton cause serious mechanical trouble for the non-diesel IC engine.

I don't know which, or maybe both artifacts the Porsche 'knock' sensor is intended to capture. They are different artifacts, but share similar signatures, and cause similar damage. My guess is that they are capturing 'preignition' which is the most destructive, and the easiest to fix through ignition retarding. Usually, detecting and curing detonation is much more difficult, and also much more costly to implement.

Well, maybe a bit more than two cents......

klatinn

Hi,

Just adding my small change as well :-)
I doubt very much that the 928 ECU uses 'fast fourier transforms' to extract the knock signal. The processing power to execute a FFT in realtime with engine RPM is quite significant and just not available in the 928 ECU. Earlier knock sensing systems used 'tuned' knock sensors. They are mechanically tuned to the knock frequency of the engine like a tuning fork. This way a simple amplitude detection could be used. More recent ECUs use analog bandpass filters and 'broadband' knock sensors. This allowed the manufacturers to use the same knock sensors for different engines. Only very recently have special DSPs been used to do FFT based knock sensing. BTW, the knock signal frequency can be calculated:

fk = m * c / pi * B

where fk is knock frequency, m is a mode constant, c is sound velocity and B is bore size.
Sound velocity c = sqrt ( 401.8 * T)

where T is average gas temperature in the cylinder during combustion (in Kelvin)

The mode constant refers to the vibration mode during knock and has typically the following values (they can be mixed for mixtures of vibration modes):

Mode1 m = 1.841
Mode2 m = 3.054
Mode3 m = 3.832
Mode4 m = 4.201
Mode5 m = 5.332

Usually the filters are designed for the most prevalent mode constant.

Regards,
Klaus

Lorenfb

The two knock sensors in the 928 basically "hear" the same noise, but are used to provide
greater sensitivity to the knock area. They are not used, nor can they be used, to determine
a bank or cylinder which produces the knock. Their placement is on the same metal
plane between two cylinders. The major differences between the sensors' outputs
would be their amplitudes.

"I think if the 928 had featured individual cylinder igntion control, Porsche would have mentioned it, as it would have been pretty leading edge technology at that time."

Technology such as this is not what marketing would consider as consumer useable/meaningful hype.