Let me start by re-posing the questions to you that you failed to address which were based on your extremely wide sweeping and IMHO optimistic statement that:
PhillipM wrote:I haven't seen a rev limiter so crude it did that for about 60-70 years.
I simply asked:
aussiegman wrote:How were they actively limiting engine RPM to specific tables based on throttle position separate to driver input back in 1942? How were they cutting spark to individual cylinders in sequential or non-sequential order to limit RPM back in 1942 using point and distributor ignition systems??? I'd really like to know.
So is there an answer in our future to these questions or are they to be ignored? No?? OK moving on, so to address your other concerns:
PhillipM wrote:If your rev limiter control loop is so badly tuned that it oscillates the engine from 18000rpm to 17500rpm's, then I'm not surprised it damaged things,
Firstly, the arbitrary RPM values above were provided as an example only and are not an actual representation of RPM values for an F1 engine RPM limiter at work. Nor are they those which I have used in tuning said engines. These were an analogy and as such were only for exploratory discussion purposes. Perhaps, it seems, I should have used a disclaimer or perhaps it was that I (foolishly) expected a more adult discussion than has been forthcoming and perhaps even without the ad hominem personal attacks. But I digress.
Secondly, this whole discussion is centred on previous discussions surrounding "BOUNCING AN ENGINE OFF AN RPM LIMITER" and as you seem to feel the you can simply retard ignition to the point of limiting RPM, I'd like to see you try and "bounce" a racing engine off an ignition retardation based limiter. In my experience it doesn't work and only results in you overshooting your attempted RPM limit or a quickly overheating engine and engine components. So depending on the engine characteristics, one of two thing will happen depending on the engine configuration.
Where the engine had a natural RPM limit based on airflow (choke point) below its maximum mechanical RPM limit, it would simply stop at either the predetermined max air flow rate (under the mechanical RPM limit) regardless of said retardation limiter which had likely not been enforced OR stop accelerating where the torque of the engine was reduced to an equilibrium level whereby it equaled the force required to keep the engine moving at a constant velocity (zero acceleration). So there would be no "bouncing" as the engine would simply continue to hold a steady state RPM value based on either of these variables.
However, IMHO and experience, any engine used for racing (outside specific series restrictions) should have a natural RPM limit based on airflow (choke point) substantially above its theoretical maximum mechanical RPM limit. So where this is the case, retarding the ignition alone can only result in three basic outcomes:
1: The engine will stall and stop producing torque which defeats the purpose of using any system at all;
2: The use of maximum allowable (safe) ignition retard would be unable to prevent the engine reaching the mechanical maximum RPM limit at which it self destructs as it will continue to produce sufficient torque to exceed the theoretical mechanical maximum RPM limit regardless of the ignition retard available.
3: Where you keep pushing greater values of excessive ignition retard that has not caused sufficient reductions in torque production, severe overheating and subsequent damage to the engine will occur if implemented for extended periods. Excessive ignition retard can be as equally destructive as detonation.
PhillipM wrote: especially if it's cutting everything so fast that then engine is varying from 100% torque output to 0% and reversing the loads through the chassis, mounts and block.
OK lets dance this dance again.
I never advocated 0% torque production, only substantial torque reduction that results in the ceasing of engine acceleration as well as the acceleration of the engines components AND the deceleration or reversing of acceleration of the engine. Is that more sufficient for you to understand?? As the engine is still spinning due to inertia, it can still produce torque due to this motion regardless of acceleration or deceleration, it just has a very limited amount of potential energy to dissipate
You also don't seem to either understand, want to discuss or do not know the differentiation between:
1: a soft RPM restrictor that slows an engines acceleration due to a reduction in torque production by for example retarding ignition (which you seem to understand)
2: the separate and differentiated system of a HARD RPM LIMITER that decelerates an engine and usually takes the form of an ignition cut that drops spark to "individual cylinders in sequential or non-sequential order to limit RPM" (quoted from my previous posts) which determines an engines maximum RPM.
Such RPM limiters as that which F1 requires at 18,000rpm and is used in aftermarket ECU's are typically an "ignition cut" where ignition retardation either was not used for performance reasons or has failed to restrain the RPM's to whatever predetermined limit the tuner has set based on a theoretical mechanical maximum RPM for the engine and its components OR where the driver has simply continued to keep the loud pedal pushed firmly against the firewall for whatever reason.
Ignition retardation cannot in all/most instances stop an engine exceeding its theoretical maximum RPM and as such a secondary system must be employed if you wish to effectively govern the maximum RPM value the engine can reach, such as an ignition cut. There are also performance advantages to not using ignition retardation and instead implementing spark cut to individual cylinders in sequential or non-sequential order to limit RPM.
It should be noted that typically the effect of ignition retardation approaching the RPM limiter can be enough to warrant most drivers not proceeding much further into the RPM range due to torque (and therefore power) reduction. In simple terms they change gears and move the engine back into a lower RPM torque band. However, when a driver persists past the point where the ignition retard is effective, say during a burnout, a spark cut in some form OR throttle override control provided for by drive-by-wire systems must be used. The former is what has been the topic under discussion, "bouncing off the RPM limiter" or cut out and NOT an running through the RPM restrictor that ramps up to this provided for by ignition retardation.
Such an ignition cut may drop spark to individual cylinders in sequential, non-sequential or ramdomised order to limit RPM. Either way when this is hit it is a HARD limit that is reached and then continually "bounced off" as the engine acceleration is suddenly stopped as it is no longer accelerating and then the engine RPM's reduce by whatever amount is provided for.
Before you start bleating on about ignition cuts are not required, they are where an engine will keep producing enough torque to breach the theoretical maximum RPM determined by whomever as detrimental, undesirable or where damage maybe done, when ignition retardation is insufficient to stop this value being breached as discussed above. This could be (given that the natural RPM limit based on airflow is substantially above its theoretical maximum mechanical RPM limit) due to the speed at which the engine accelerates, momentum of the engine itself due to a higher mass flywheel/clutch assembly or a myriad of other reasons, you pick one. As such a HARD RPM LIMITER via ignition an cut (randomised, sequential, non-sequential or otherwise) can be used to stop the engine continuing to accelerate and exploding. Again, it is really that simple.
These are the reasons hard RPM limiters are required, which is primarily the simple fact that people will ignore the RPM restrictor and the reduced torque output and proceed straight on to the RPM cut and then proceed to BOUNCE THE ENGINE OFF THE RPM LIMITER, which is what this is all about. Not the most efficient and effective way to slow an engine down prior to reaching this maximum value.
So, again slowly, when a hard RPM limit via ignition cut is reached, the engine itself does not stop rotating from what ever arbitrary RPM value you decide (with the above caveats, this is an example only) to a zero RPM value (stationary), it just "stops accelerating", which is the rate at which the velocity of something changes over time, and then starts decelerating or accelerating in the opposite or reverse direction.
So simple physics to explain
When something is accelerating in one direction (increasing RPM's) then ceases this acceleration and starts decelerating, it has reversed this motion. Mathematically, deceleration it is acceleration in the opposite direction to that of the original motion. If something moves in an opposite direction of motion, it can be said to have reversed that direction. I hope you understand the difference.
PhillipM wrote:But with the amount of variable retard and random cut algorithms available on even the most basic ECU's you'd have to be pretty damned hamfisted in your setup to let it do that, it shouldn't be anywhere even close to that with a load on it, hell even with no load on it you should be taking account of the rate of rpm rise anyway
There is a huge difference between the hard RPM LIMITERS you can bounce off and the soft RPM RESTRICTORS that ramp up to an RPM limiter as described above. An imperfect but simpl-ish analogy (perhaps only to my mind) is the way a turbo restrictor works. These can also be considered as an engine LIMITER and RESTRICTOR as they limit maximum power and usable engine RPM (dependent on engine capacity).
These simple devices have a maximum air flow LIMIT defined by the size of the orifice over which it will not flow more air. Prior to this maximum limit, it acts as an airflow RESTRICTOR and as it approaches closer to its choke point where air speed goes super sonic, it will restrict or slow the rate
at which air can enter the engine. Once it reaches the maximum flow limit, it cuts off further air flow. Ignition cuts, while operating on very different principles, to my mind are somewhat similar in that they restrict air flow in steps similar to ignition retard by restricting but not stopping torque production, then it simply stops increasing air flow as an ignition cut simply cut spark in its many forms reducing torque production.
Aside from all these analogies, I am more than aware of the various algorithms used, as I am privileged enough know some of the people that wrote them for various ECU firms and even tested prototype systems some on my cars. Even these people agree that there are two (2) distinct stages to an RPM limit using ignition, the RAMPING stage and CUT stage. Both have VERY specific limitations as to effectiveness for certain engines. What they agree on is you CANNOT tune out the "stupidity" (sic) of some people who feel the need to hold an engine at the maximum RPM limiter and continue to bounce it off that limit for extended periods of time. They all agree this act WILL
eventually result in, and I quote "damage to an engine regardless of what methodology you use to implement said arbitrary limit due to the forces involved in reducing the acceleration of the engine at the maximum RPM limit of operation that will see those forces reverse due to deceleration."
As for my "hamfisted" setups, you have no idea what I have or have not done or used or implemented, so I would be VERY careful throwing stones.
What I will say is when I have used either Motec, Autronic, Pectel, DTA or Vipec (I have used all these) they all allow various ignition retard/ignition cut, drive-by-wire throttle (DBW) or boost limitation RPM restriction methodologies. However in 99% of instances the only 2 ways to effectively stop an engine continuing to accelerate is close the DBW throttle or cut the ignition, regardless of the ramping rate methodologies used leading up to these two events. Due to the downside in engine performance from using other methodologies, ignition cut is typically the preferred method used. However it does require a modicum of self restraint and understanding for the end user (driver) not to abuse it.
PhillipM wrote:and yes, you can even do that with a distributor!
Yes, I agree you can limit RPM with a distributor, but I am yet to see a distributor system from 1942 (your 60 to 70 year time frame) or that does not use a modern ECU intervention system somewhere (like an MSD box) that can replicate the action of cutting spark to individual cylinders in sequential, non-sequential or randomised order.
As an example of what a basic distributor system can do, look at the what distributor ignitions used in the form of solid state circuitry that interrupted the grounding of the ignition circuit, such as Ford used in the 70's with the Boss 302 V8 at 6250rpm via the Autolite RPM module. This effectively cut ignition after the mechanical/vacuum ignition retard systems failed to stop the engine accelerating past the maximum mechanical RPM limit. Sound familiar??? A two stage retard and cut system via ignition for RPM limitation similar to what I had said is used. Wow what do you know!!!
PhillipM wrote:You aren't trying to push something and then stop it, push it then stop it, you should be aiming to generate enough torque to hold the engine at the desired RPM's, and that's not even close to a zero value.
Again this was merely an example of the forces, not an exact representation of the movements in the engine. Last I checked, engines did not contain rotator cuff elements. Try reading it again more carefully. In trying to "stop" but not actually stopping the trolley, you are stopping the acceleration of the trolley and decelerating it OR stopping the acceleration in one direction and reversing the acceleration in the opposite direction. Then repeat at great frequency. This is similar to that which happens in an engine that "bounces off an RPM limiter".
All that being said, the effect and forces on certain components in the engine are exactly these forces of being accelerated in one direction, stopping (in any event where a force reverses there is a momentary halt in motion however brief) and then being asked to cope with forces of acceleration in the opposite direction that have reversed due or a "deceleration" event.
Primarily, it is the valve train and rod bearings that suffer most from this which is why failures in these areas are common for an engine that has been "bounced off the RPM limiter".