Why are revlimiters uneven?

[Commander Kinsey]

Is it to warn you? Even my toilet cistern can slow down smoothly.

[Rod Speed]

Commander Kinsey [e-mail] wrote

No idea what you are on about this time.

Rather different technology. Many dimmers cant, they cut off abruptly at the lower light levels.

[rbowman]

On cars? Yes. Cutting the engine abruptly when you hit the red line wouldn't be good so they simulate ignition breakup to give you a hint.

[Brian Gaff (Sofa]

Never saw the original post. There is a holding current in a triac, and once the current drops below this a kind of Hysteresis occurs so you need to turn it up to get it conducting again. Brian

[Brian Gaff (Sofa]

OK what do they use then, All I remember from my brief experience trying to dim leds was that the most successful way of doing that was by duty cycle, ie on to off times with them driven by some kind of oscillator with variable mark space ratios. However it is obvious that even the briefest of ons and the longest offs tends to still be visible in most cases, and not terribly accurate if many leds are used as the load, there being a spread of linearity in any given number. Brian

[Clare Snyder]

Dimming LEDs in a FC circuit can be very linear right down to almost out. On an AC circuit running at 60HZ the resolution gets pretty rough. The switching rate is only 60 times per second so the on/off ratio is pretty limitted whether using rizing edge, falling edge or zero crossing switching -

With DC the switching rate can be whatever the engineer wants, and the mark space ratio (duty cycle) can also be manipulated quite successfully

As for "Rev Limiters" in the subject I ASS U ME the original question was aboput engine rev limiters and how the engine "hunts" on the limiter. This is due to Fuel Cut being used to limit the speed and it being an "all or nothing" process - fuel injection shuts off at a programmed RPM and comes back on when the RPM drops to a lower programmed RPM. It is theoretically possible to make a smooth cut-off but it is programattically complex - and for what advantage??? The "choppy" rev limitter lets the driver know he has hit it - a soft limit would tempt an agressive driver to stay "on the limit" constantly (like a diesel truck on the governor)

[Fredxx]

PWM is the cheapest way as it needs no other component.

For a small improvement in efficiency then a constant current source would be best but obviously costs more.

[Max Demian]

Yes, I made a PWM dimmer circuit (for fun) based on circuits I found on the Internet and it works well, and dims to nothingness. (12VDC working a white LED strip.)

[The Natural Philosopher]

do it at high enough frequency and there is no visible flicker, and you can feed the LEDS via a small ferrite inductor with a parallel capacitor and reduce HF flicker too.

[Commander Kinsey]

But at that point you've already gone over the peak of the power curve. I'd just make it drop the curve more quickly so it can never go too fast. The driver would still feel the lack of power.

[TMS320]

Engines that run stochiometric have to be all or nothing. Diesel are smoothly limited. It ought to be possible to smooth limit stratified petrol but I don't know from experience because mine just shifts up and keeps going.

[rbowman]

My car is an automatic so that is what it does. I never tried putting it in first or second to see what it does when it hits the limiter.

Two of my bikes have carburetors so manipulating injectors isn't an option. I haven't hit the limiter on the other bike and just shift at

10000.

[jon]

Had a rev limiter on my RS1600 with electronic ignition in 1975, it worked well at 6700 rpm.

[Commander Kinsey]

It must be possible, all it has to do is what you would do if you wanted to hold it there - adjust how far you press the accelerator. I guess that gets more expensive as it would have to have a bit of intelligence to monitor how much it had throttled it. But aren't most modern cars already throttle controlled through a circuit board?

[Commander Kinsey]

In my Golf, the selection was only advice from me to the car. If I said "1st" and that was a stupid idea, it would use 2nd. Fun to select 1st when going at highway speed. It dropped the gears precisely when sensible to do so and stopped very quickly.

[Commander Kinsey]

You're getting f*ck all performance if you've gone over the power curve anyway.

No, the guy that made the group f***ed it up. How did you notice?

[Commander Kinsey]

I did not have a rev limiter on my Renault Espace built in 1993. I broke the engine badly. I thought they all had them by then!

[The Natural Philosopher]

That depends entirely on how the engine is set up. A turbocharged angine for racing can easily be delivering nearly flat torque all the way to disitegration and will develop full power on the limiters edge

[TMS320]

It's a primitive technique for an engine under-endowed with torque. When you change up, there is an inevitable drop in power so the idea of revving beyond peak power is to get further up the curve in the next gear.

[The Natural Philosopher]

Kinsey simply has no idea. In the old days you strangle the breathing so customers wouldn't over rev the engine as it was obvious that you would get more power at lower revs.

But even my MG midgets of 1970s vintage could produce more power beyond the orange and up into the red, by dint of having more carburation

Turbo and supercharging makes it even easier to get good torque at te top of the rev band

These days you cant get 'over the power curve'