How to make an engine last hitting the rev limiter? 1

[ELS122]

What rev limiter type is the easiest on the engine internals? Obviously first thing is to choose the right rpm. but other than that?

I was thinking that a spark cut limiter could wash out the cylinder walls and/or valve seats, while a soft limiter would overheat most valve seats.. so a fuel cut could be the most ideal in that sense
But what else is there to consider? maybe bottom end loading depending on how the rev limiter is set up?

Also I've read that Honda in the late 80s had a failsafe limiter, where the spark was cut at the limiter and then the fuel too 200 rpm above that in case of overheating leading to pre-ignition.

 

[BrianPetersen]

Simple. Don't bounce it off the rev limiter!

What application do you envision in which this is a consideration ...

In a modern fly-by-wire implementation, the sensible thing to do is wind the throttle back as the engine approaches the rev limit. At the actual rev limit, the commanded throttle position can be whatever throttle position achieves that RPM in neutral. Driver can have the pedal to the metal all they want, they're not getting full throttle. I don't see why this would have anything to do with valve seat temperature. Essentially, at rev limit, the engine will be running at no load. High revs - but no load. (Even if they are in gear.)

Ignition-cut in a modern, production, emission-controlled application will overheat the catalyst in no time flat. Fuel-cut followed by winding back the (fly-by-wire) throttle regardless of driver command is the sensible thing to do.

Most modern cars have a fair bit of idiot-proofing. Rev limit may be lower in neutral and/or interlocked with transmission engagement logic to prevent people from doing stupid stuff with the transmission. Rev limit may be a function of coolant temperature. ECU probably keeps a log of driver stupidity.

 

[TugboatEng]

If you want to run continuously at a certain maximum speed you don't want a rev limiter, you want a speed limiting governor. These are standard on diesel engines. With modern fly by wire engines it should be simple enough to program one by intercepting the throttle signal.

 

[ELS122]

That's like saying "Well simply don't improve things, because it takes effort to fix the resulting problems"

I'm not worried about emissions or cat's overheating since it'll be a race engine. But I've seen far too many engines blow things like wrist pins when being on the rev limiter for too long so I'm wondering if there's some reason for this that could be improved upon? Especially when there's no load, for example reaching max gearing and on the limiter engines just sound like they're dying.
Or is that just from a limiter that is set too high?

Oh the soft limiter, I meant ignition retarded limiter not a throttle by wire.
Although nowadays most fuel cut limiters are "soft"...

 

[BrianPetersen]

You haven't told us what the application is. "Race engine" is pretty vague.

I know that with my own "race engine" - which is that of a Yamaha R3 motorcycle - I avoid hitting the rev limiter. There's no point in doing so, it's above the optimum-gearchange RPM anyhow, and hitting the rev limiter means no longer accelerating (it would have been better to do the gearchange sooner and thus maintain acceleration). The rev limiter is only there in the event of me making a mistake, and is not for habitually running up against it. Thus, what happens when sitting constantly against the rev limiter is not a consideration. No drive-by-wire on that engine. Pretty sure the rev limiter cuts fuel delivery only. (Stock Yamaha ECU, though remapped)

A fraction of a second of ignition-cut in the absence of fuel-cut won't do any damage, although it may lead to a loud backfire. There's no backfire upon hitting rev-limiter, which suggests fuel-cut.

My quick-shifter - which I use constantly - is set up to cut both fuel and ignition. But it only cuts those for about 70 milliseconds.

Another way to do a "soft limiter" is to cut fuel to one cylinder at a time in the last couple hundred RPM before rev limit.

There is nothing happening to the stresses on the con-rod and wrist-pins that isn't already happening, or worse, on TDC through valve-overlap (no compression pressure) or TDC through a normal firing sequence (much higher compression pressure). If someone is breaking wrist-pins it's because they're spinning the engine too high.

Downshifting too early and thus mechanically back-driving the engine beyond the rev-limit - or "money-shifting" - is gonna break stuff no matter how you set up the rev limiter.

 

[TugboatEng]

You should first consider your own speed limiting governor to prolong your own life before worrying about how long your engines will last. Speed kills

 

[ELS122]

> There is nothing happening to the stresses on the con-rod and wrist-pins that isn't already happening, or worse, on TDC through valve-overlap (no compression pressure) or TDC through a normal firing sequence (much higher compression pressure). If someone is breaking wrist-pins it's because they're spinning the engine too high.

That's obviously false, ask any racer and they'll tell you that a limiter adds additional stresses on the engine than no limiter.

 

[gruntguru]

. . . and of course racers are the font of all technical wisdom.

je suis charlie

 

[BrianPetersen]

Not in my world. Mechanical failures of roadracing motorcycle engines due to normal occasional hitting the rev limiter doesn't happen.

Idiots sitting in the parking lot bouncing their engine off the limiter to attract attention are on their own.

You still haven't told us your application.