Rev limiter Fuel

[glens]

... Bob ... seems to think that one fuel and spark are removed, the motor can continue to accelerate..

So you feel the momentum the flywheels build during fast acceleration will dissipate to 0 at the split second the fuel is cut? ...

I think you're both right.  A sudden unloading of the crank under high throttle pressure will cause the crank to accelerate.  Once the limiter does its thing, the engine might achieve a yet higher speed due to momentum, but it will already be slowing in terms of acceleration rate.

Sounds similar to a discussion whether or how long/far a bullet accelerates outside the barrel.

[whittlebeast]

Glen, there is no such thing as momentum of acceleration.  When the force goes away, the acceleration goes away.

The issue with rev limiters is the next force event may already be scheduled and will happen.

[Coyote]

Glen, there is no such thing as momentum of acceleration.  When the force goes away, the acceleration goes away.

The issue with rev limiters is the next force event may already be scheduled and will happen.

Finally a comment that makes sense and doesn't violate the laws of physics. 

[hrdtail78]

I was looking at the data in post 13.  It looked like the data in that log was rather snappy base on the time stamp.

The data that comes off my PC5/wideband data logger setup is 66 samples per sec.

The Motec stuff I have been working on lately is more like 20 - 100 samples per sec depending what we are looking for.

Regarding a MAF, you may want to play with one on a few motors. I have already given you the keys to the kingdom.  All you need to do is open the door and look inside.

What a data logger can record at and what is being logged is different.  The recording in post 13 is logging at about 27-28 F/S but you can see that the controller hooked up to analog is not reporting anything at that speed.  It is down to about 10-12 F/S. 

The PC5 might be recording at the above speed, but the AT100's hook to it are not reporting at that speed.  Last I heard they are about a 10th of that speed.  This is the nature of the beast and the reason why you don't see a 4.2 sensor controlling a CL function at higher speeds and loads.

[Steve Cole]

Steve I don't know you but going by the things I have read on this forum I cant understand why your letting this irrelevant fool  pull your chain, I think most people here understand it for what it is. he is full of it an Aussi would call him a wanker . by the way I found a use for a MAF ,I glued it to my head light and connected it to my speedo.

I just hate to see this kind of crap getting spread around to people who do not know any better. I believe there are really people who want to learn and its just this kind of crap that scares them away. Worse yet is they believe this fool and then damage something by doing what he says, then it effects there whole outlook on tuning.

For those of you that care, if the sensor has an update rate at ~ 250 mS that means it can only measure changes 4 times per second. Anything faster and it is missed. So you have to know when to use the data and when not too or the picture you see from the data is completely wrong. Just imagine drawing a line between 4 data points such that you have a straight line as that is what you saw from the data. Now if you understand that the data is only capable of showing you those 4 samples per second but the real data is moving at 50 samples per second the picture can and does look completely different. The ECM has ALL 50 samples to work from but since it didn't have the time to send them all out, you cannot know for sure what is truely happening. This is just why it is important to understand where the limits are and when to use them and when not.

Most, if not all the aftermarket equipment uses a Bosch Broad Band O2 sensor for measuring and the sensor when new has a response time of ~200 - 250 mS. Now most advertise there electronics have a faster response time, and most of them do, but you are still limited by the sensor. So if you were to hold the engine in a steady state condition and then average the sensor output you can get a close assumption to what is really happening. Since no one rides in a steady state condition all the time one must be very careful of using what the data APPEARS to show you.

So when trying to look at a Rev Limiter function and the only thing you have is very slow data and a Broad Band O2 you are going to miss completely what is really happening.

[Admiral Akbar]

... Bob ... seems to think that one fuel and spark are removed, the motor can continue to accelerate..

So you feel the momentum the flywheels build during fast acceleration will dissipate to 0 at the split second the fuel is cut? ...

I think you're both right.  A sudden unloading of the crank under high throttle pressure will cause the crank to accelerate.  Once the limiter does its thing, the engine might achieve a yet higher speed due to momentum, but it will already be slowing in terms of acceleration rate.

Sounds similar to a discussion whether or how long/far a bullet accelerates outside the barrel.

As soon as a bullet leave the barrel and there is nothing pushing it faster, it starts to slow down due to the drag force..

Remember  Newtons second law..

[Admiral Akbar]

Nice simple description with a little history thrown in..

http://csep10.phys.utk.edu/astr161/lect/history/newton3laws.html

[redmtrckl]

My head hurts. Therefore I am going to set my rev limiter to 7200. That way I won't ever hit it.
There! Problem solved 

[whittlebeast]

My head hurts. Therefore I am going to set my rev limiter to 7200. That way I won't ever hit it.
There! Problem solved 

Yep, build the motor to spin 7300 and cam it to spin 6300 and the problem is a non issue.  The rev limiter is there just to save your butt when things go bad.

[Admiral Akbar]

My head hurts. Therefore I am going to set my rev limiter to 7200. That way I won't ever hit it.
There! Problem solved 

Yep, build the motor to spin 7300 and cam it to spin 6300 and the problem is a non issue.  The rev limiter is there just to save your butt when things go bad.

If the motor can spin 7300, should be able to set the limiter close to it.. Assuming that the response time on the limiter is good.. ECUs need to know the RPMs every time they calculate fuel... At least on sequential port injection.. Since they know exactly what the RPM is they can cut fuel when they see it reach the limit..

[whittlebeast]

The other workable solution is to play with the VE tables for the motor to go butt rich at the same time the rev limiter comes in so that the remaining (fueled) cylinder stays at least reasonable when the other injector goes off line.  Note that this is only an issue on motors with a Y intake manifold and center spray open intake manifolds like the late 80s GM EFI stuff.

[hrdtail78]

Set shift light at 6200, and power enrichment is your friend.

[Steve Cole]

The other workable solution is to play with the VE tables for the motor to go butt rich at the same time the rev limiter comes in so that the remaining (fueled) cylinder stays at least reasonable when the other injector goes off line.  Note that this is only an issue on motors with a Y intake manifold and center spray open intake manifolds like the late 80s GM EFI stuff.

So the ECM has turned the fuel off to the injector, so how is adjusting the VE tables going to do anything  . If you do not want the Rev Limiter to work as design I would suggest that you just turn it off as no amount of twisting the VE table is going to do a thing other than make it run poorly.

[whittlebeast]

The other injector should still be on it's VE table unless we just uncovered a new Harley Hidden Table.

[FLTRI]

Funny thing is...NOONE should be shifting at the rev limiter. There is a performance disadvantage when the engine hits the limiter.
IMO the ONLY value of a rev limiter is to prevent engine damage by keeping the engine from over-revving when something goes wrong...ie: missed shift or a broken chain/belt/gear.
Most sophisticated street rev limiters have lead up ignition and/or fuel modifications BEFORE the limit is hit in an attempt to keep from exceeding the limit.
Some of the earlier (carbed) HD limiter grossly retarded the timing as it got to the limiter.
Most racing EFI systems (ie: MoTec, EFI, Magneti Marelli, etc) offers a variety of limiter strategies.
Bob

[whittlebeast]

Yep, the good stuff typically has soft limiter logic and hard limiter logic.  All totally adjustable.

Way cool stuff

Andy

[Admiral Akbar]

Funny thing is...NOONE should be shifting at the rev limiter. There is a performance disadvantage when the engine hits the limiter.
IMO the ONLY value of a rev limiter is to prevent engine damage by keeping the engine from over-revving when something goes wrong...ie: missed shift or a broken chain/belt/gear.
Most sophisticated street rev limiters have lead up ignition and/or fuel modifications BEFORE the limit is hit in an attempt to keep from exceeding the limit.
Some of the earlier (carbed) HD limiter grossly retarded the timing as it got to the limiter.
Most racing EFI systems (ie: MoTec, EFI, Magneti Marelli, etc) offers a variety of limiter strategies.
Bob

What about electronic shifters that cut the ignition and shift?

Some limiters can be used if set up is right.  It can be your shift switch.. Bump the limiter and automagically shift to the next gear.. I seem to remember Some Beemer guys using it to race.. All they did was hold up on the shifter until the limiter was bumped then the bike went into the next gear.

Give me a date and years for when the limiters were used and what they were used on.. Also on the how the limiters cut fuel.

[Admiral Akbar]

Yep, the good stuff typically has soft limiter logic and hard limiter logic.  All totally adjustable.

Way cool stuff

Andy

Why is a soft limit needed? 

[whittlebeast]

The soft limiter is used to roll off the torque to let the driver know things are getting close (and past the usable power band) The hard cut is for failure protection.  It does very little for a real bad downshift.

I normally set up a race cars with smart shift buzzers.

See https://www.youtube.com/watch?v=cq2pMKXyOlE

The third beep is set at like like 8900 with a hard rev limit of like 9000. 

Andy

[Admiral Akbar]

The soft limiter is used to roll of the torque to let the driver know things are getting close (and past the usable power band.  The hard cut is for failure protection.  It does very little for a real bad downshift.

I normally set up a race cars with smart shift buzzers.

See https://www.youtube.com/watch?v=cq2pMKXyOlE

The third beep is set at like like 8900 with a hard rev limit of like 9000. 

Andy

Sounds logical.. Soft limit is nothing more than a warning..

[whittlebeast]

An effective soft limit is tough to program on a wobble fire 2 cylinder.  There are simply not enough ignition events to work with.  Normally you would resort to pulling timing to roll the torque off.

[whittlebeast]

These motors spend lots of their life within about 25 RPM of their soft rev limit.

https://www.youtube.com/watch?v=gSs9kgUtW8E

300 HP out of 1500 cc.  When they suddenly unload at WOT and full boost, they will over rev as much as 1500 RPM over the rev limit.

Andy

[FLTRI]

When they suddenly unload at WOT and full boost, they will over rev as much as 1500 RPM over the rev limit.

Andy

Max? How can that be?
Bob

[Coyote]

When they suddenly unload at WOT and full boost, they will over rev as much as 1500 RPM over the rev limit.

Andy

Max? How can that be?
Bob

Time for Max's remedial physics class. 

[Admiral Akbar]

When they suddenly unload at WOT and full boost, they will over rev as much as 1500 RPM over the rev limit.

Andy

Max? How can that be?
Bob

Time for Max's remedial physics class. 

The remedial class was up in reply 55 and 56.. 

Bob the answer is pretty simple..

Motor revs at 38000 rpm/sec  (message 44)

1500 RPM represents   

1500 / 38000   = 39 milliseconds  ..

That is the amount of time that the rev-limiter delays to cut spark and remove all fuel..   You want me to go further, I'll need schematics and wiring diagrams plus how the limiter is deployed.

BTW even Wittlebeast says there is no such thing..

Glen, there is no such thing as momentum of acceleration.  When the force goes away, the acceleration goes away.

The issue with rev limiters is the next force event may already be scheduled and will happen.

Newtons second is secure.. And the response from the limiter is too slow.. Nice thing about HD ecu is that since it controls everything, it can make the RPM limit issue decision and cut fuel in probably less than a micro second..   Only about 39000 times faster.