MoTeC Global Forum

I was recently provoked into thought by someone claiming that they were having problems with their Motec pit speed limiting strategy (similar to the rev limiter strategy) breaking crankshafts. Presumably an uneven firing was twisting the cranks and causing bearing damage and resulting in crank failure.

Without doing a lot of testing (and potential crank breaking) is there any accumulated knowledge about setting up limiters with an eye toward minimizing damage?

For example

spark versus fuel cut?

Randomizers set at 75 (near random) or 189 (more Bosch like consistent pattern of cylinders)?

Methods of preempting cuts with ignition timing or coil dwell? Easy for a revlimiter but tougher for something like TC or pit lane speed limiting.

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What are the big knobs in this game?

thanks for the thoughts,

Eric Schieb

What type of engine / car was this? I would be dubious about a pit limiter causing a crank failure. Having said that it is logical that a smoother cut is going to be better for the engine.

As an alternative to a Randomiser number "0" can be selected which uses a seamless prime fractional cut sequence instead of a randomiser. This will work better at lower RPM. I would also work with a control range of at least 100. If you are using the Ground Speed Limiting function you can apply a gain to the Control Range to increase the Control Range as a function of Throttle Position.

Setting a target Throttle Position for the driver displayed on the dash or SLM is also a good idea. It is common to see drivers using too much throttle under pit limit conditions. This could also be done using the Translation Table on a car equipped with DBW.

The limiter type may vary depending on the engine type (manifold, injector location etc). In Australian V8 Supercars the pit limiter is fixed (control ECU software) and uses Limiter type 2 (Ignition Cut with Fuel Cut 100 RPM above). Type 3 (Ignition and Fuel) did not allow the car to pull away as cleanly when released from the Pit Limit. Type 5 seemed to cause some over-fuelling problems. These cars run on E85 with a single injector per cylinder located well away from the inlet valve so drivability, flat spots and air box fires are all potential problems.

How is Randomiser 189 like a consistant patern of cylinders?

RB:>The limiter type may vary depending on the engine . . .

Thanks for this whole discussion. Generally I am worried about short stroke, low inertia engines. I have customers with a variety of injector locations (and customers with hi/lo injection that effectively changes location as a function of RPM and throttle). I noticed that you did not list the fuel cut only or fuel followed by ignition. However, you said

RB:>Type 3 (Ignition and Fuel) did not allow the car to pull away as cleanly when released from the Pit Limit.

And I was wondering if this implies that a fuel cut only would lead to softer firing events (due to poor mixture preparation). Fuel cuts seem to be preferred by some of the OEMs and their suppliers (and their race derivatives). Why?

Does someone have significant experience (good or bad) with the fuel first cuts?

When you do a overrun based fuel cut you have the option to add an additional amount of fuel to compensate for the dry port walls. Is this/should this be part of torque reducing cuts? Would this address the problems you saw?

RB:>It is common to see drivers using too much throttle under pit limit conditions.

I agree. Thankfully I have the more critical application drivers trained to use part throttle. However, this tactic is a bit more challenging for things like TC. Or, when they are looking for that last second and banging the limiter to avoid a shift. Or,

RB:>How is Randomiser 189 like a consistant patern of cylinders?

For example: 30% cut on a 4 cylinder will cut one of the cylinders each time it comes around for 6 cycles and then it will change to another cylinder for 6 cycles then . . .

thanks,

Eric Schieb

To add another question to this...

How does the trigger pattern affect the limiting?

If I use ref sync mode 2 I can get a limiter so smooth the riders don't know they're on it until it hits the hard limit.
I also get near perfect launch and pit lane limits.

If I use ref sync mode 7 I can't achieve the same results, the limit is not anywhere near as smooth.
I remember there being some changes made to the software to address this but it is still an issue.
Using a large control range gain makes the pit limit acceptable but the launch limit is still not great.

I regularly see other riders using pit limiters that look so harsh it's no suprise things break.

RossB wrote:As an alternative to a Randomiser number "0" can be selected which uses a seamless prime fractional cut sequence instead of a randomiser. This will work better at lower RPM.

Ross,
Could you explain randomiser "0" please?

ive got your spreadsheet visualising what the change in randomiser actually does. Maybe you should put it up here


Cheers
M

Martin,
I have put the spread sheet up here: http://www.motec.com.au/forum/viewtopic.php?f=11&t=175&hilit=randomiser . The "0" option works differently, it avoids consecutive cutting to the same cylinder at low cut levels but It does not avoid consecutive cuts to the same cylinder at cuts above 50%. You end up with only one cylinder firing occasional and most cyl cutting most of the time so you get consecutive cutting. Most people would not spot the difference between 0 and the other modes during RPM Limiting but it is a lot smoother at low RPM levels such as Pit Limiting.

Eric
Fuel cut is not realy favored in a race engine, particularly when the injectors are located a long way from the intake valve or where there are high levels of stand off. This is partly due to the fact that when you cut the fuel this only stops the fuel being injected and not the fuel that is still about in the intake so there is a risk of igniting a lean mixture and damaging the engine. The other problem is a lean mixture causing a backfire and igniting the fuel vapor in the air box. Having a Recovery Fuel parameter would only fix part of the problem.

Using an ignition cut only can sometimes cause a lot of unburnt fuel to accumulate in the exhaust which can explode and damage the mufflers. In some cases using the Fuel Cut with Ignition Cut 100 RPM above can help this problem.

Fuel cuts are often used by OEMs because they are smoother and less likely to cause catalytic converter damage but a production road car engine has injector location and fuel targeting that allows them to have greater control of the fuel compared to that of a typical racing engine.

When it comes to randomiser numbers values anywhere between 64 and 182 are fine - just choose something that gives a nice smooth cut. For a more random effect the randomiser should not be too close to a multiple of 255 / #cylinders. Numbers too close to 0 or 255 result in a long run of cylinders firing then cutting which probably isn't what you want.