Comparison of trigger types

[Slowie]

Hi

Whilst I understand the various trigger methods available in the Syvecs, I was wondering if anyone could explain some pros and cons of the various patterns, e.g. Is keyed by single cam less accurate than single gap single cam, theoretical rpm limits on each, etc.

I am interested in the reasons behind why, given a free choice, you would pick one over the other.

If my question is too vague or large to answer, links to suitable reading material would be greatly appreciated.

Thanks.

[pat]

Slowie,

Syvecs ECUs run angle clock hardware, so where others try to "time" events to happen at some point in the future, we schedule things to happen at some future angle rather than time. In order to maintain an accurate correllation between the angle clock and the true crank angle, information from the crank is needed. If it only had one tooth every revolution, we only get one update per revolution which we can use to keep the angle clock and the crank in lock - needless to say a lot can happen in 360 degrees! If we have two teeth then we can update twice per revolution, and so on.

Consider for a moment the crank speed during starting. As a piston comes up on compression, the starter is loaded up, the crank slows down etc. If we didn't know this was happening (ie the angle clock was to continue at the original pace) then it would get to the point of intended ignition before the crank did and we'de have too much advance. It pays to have additional trigger edges available to allow the angle clock to realise the crank is slowing down (or speeding up). You might be forgiven for concluding that the more teeth the better - Nissan took that to the ridiculous extreme of one tooth every degree (but then completely missed the point by putting that sensor on the wrong end of a stretchy cam belt!).

Having too many teeth is just as bad as not enough. In digital electronics, we still operate in the time domain, so there must be some conversion between time ticks and angle ticks. At 6000 RPM there are 100 revolutions per second, which is 10 milliseconds per revolution, or 27.777 microseconds per degree. If our reference clock ticked at 1MHz, we could increment the angle clock every 27 or every 28 ticks, but we cannot do so every 27.777 ticks (we can on average but not every time). So if we had a tooth every degree on the crank, then sometimes the time domain clock would count 27 ticks, and sometime 28 - counts of 27 would give an RPM of 6173, counts of 28 would give 5952 RPM. That's not too clever. If we space the teeth out a bit more then we gain resolution - take the 36-1 pattern for example, this has teeth every 10 degrees. That would give 277.777 ticks between teeth referenced against a 1 MHz time domain clock. Of course we still cannot count 277.777 ticks, we would see 277 or 278, but now 277 ticks equates to 6019 RPM and 278 gives 5995 RPM - altogether much better accuracy. [NB, you can of course count time for X teeth rather than 1 tooth, but then why bother having the extra teeth to start with ?]

So, many teeth is good, but not too many, and ideally they want to be spread evenly round the crank. One missing tooth in a "non-critical" area (eg 50 ATDC) will work well as a datum so the angle clock "knows" where TDC No.1 is. If we don't have any unique features on the crank then we cannot figure out where TDC No. 1 is and we must rely on other means. This is the difference between single gap single cam and keyed by single cam. The single gap can sync up the crank without a cam signal at all, albeit without phase information (no problem on most symmetric engines, not ideal on 5 potters though ). Add a cam pulse to the equation and you get phase (is No. 1 coming up to TDC compression or TDC exhaust ?). On the other hand if we don't have a unique feature (such an 2JZ-GTE 12 tooth pattern) then we only know the crank is turning (also how fast) but we don't know where the pistons are - for this we use a cam pulse as a datum (ie "when you see a cam pulse then the next crank tooth will be at 90 BTDC No.1 compression).

Is a 36-1 any more "accurate" than a 36-0 running as keyed by single cam - the inter-tooth gap is the same, so therefore the angle clock lock on the crank will be equal. Only in the gap could this change, which is why I suggest placing the gap at a point we aren't interested in doing anything. If we aren't scheduling anything to happen then it doesn't matter if we are out slightly because we will get back on track with the following teeth and it will be in plenty time before we actually need it.

RPM limits are notionally not affected by pattern type, only if the sensor cannot keep up with the teeth on the crank would you run into issues. Remember that we have a true angle clock hardware. Tracking a complex pattern places no more load on the CPU than a simple pattern does. It's done in hardware, it doesn't need software CPU cycles. The choice of pattern should be governed by what allows the best lock between the crank and the angle clock, and as I have explained above, a sensible number of teeth evenly spaced with one or more missing will work well. Official upper RPM limit is in the region of 20,000 RPM.

Hope this helps,

Pat.

[Slowie]

Wow! Thank you Pat. That is exactly what I was looking for.

I find it most informative.

[AGsTony]

Nice Good reading Pat

AGsTony

[Andrearally]

I missed this in the past, very nice and deep informations!