Knock detection without a knock sensor?

[keithu]

The US owner's manual says the FJ-09 and Tracer engine, "Was designed to run on 91 octane ([R+M]/2) or higher." Some of us have, on occasion, burned lower-octane fuel (87 octane in the US) because we were stuck in some rural area and that was the only thing available. Some of you, I understand, even run low octane regular all the time. I've done it on a few occasions, and I never noticed any pinging, higher running temps, or other ill effects. I've never heard anyone else report ill effects from running low octane fuel, either. 

The CP3 motor does not have a knock sensor. My question is this: Is there some other method in a modern engine management system to detect knock, and then maybe adjust for it? I'm curious because I never noticed knock when running lower octane fuel, and apparently nobody else has either. So I wonder if there's something else going on in my bike's brain to protect it from damage caused by pre-ignition.

 

IMPORTANT: I am NOT interested in debating about the best kind of fuel! I'm glad you only use high octane ethanol-free dolphin safe USDA organic ribbed Super Mega Ultra Premium with Techron (tm). We've all fought in that holy war too many times and nobody wants to do it again. Just don't. This is a purely technical question about the engine management system in our bikes. 

[CRFan250]

Nope...you will hear it if it happens.  The truth is 91 octane is a fail safe for all conditions.  87 will work if you don't ride hard, it isn't 90plus degrees and you are not putting a high load on the motor.

[keithu]

27 minutes ago, CRFan250 said:

87 will work if you don't ride hard, it isn't 90plus degrees and you are not putting a high load on the motor.

See, the problem with that is that when I've had to buy 87 octane fuel I'm usually in a place that looks like this:

One has a tendency to put a high load on one's motor in circumstances such as these. And at such times it's usually pretty warm out. 

[peteinpa]

Yep, all you have is your ears to detect knock. It can be subtle, and hard to hear.

Avoid high throttle demand at low RPM, let it rev with 87 and you'll avoid most problems.

[skipperT]

No it doesn’t, and you’ll be hard pressed to hear it at any speed IMHO. It’s much more subtle than on a car. 

-Skip

[pilninggas]

I've bought low octane (RON not MON) when ive been in Eastern Europe (but on other bikes not the Niken) and not had pinging  - the timing is set very conservatively to suit, even at large loads and large throttle delta conditions they wont ping-up.

Some modern car engines can detect knock by analysing the signal from the CPK - in the same way they can detect misfires, through looking at small changes in angular velocity against the expected values. I think they look for changes in the 3rd order or higher [angular jerk] and if certain patterns emerge [quickly] then it is calculated by the algorithm as knock/pink/ping and mitigation enabled (retard timing) - i had a few 90s cars where you could trigger knock and hear the system kick in when you gave it a large load.

I did remember a motorcycle engine designer saying on another forum, that the high revs means that the valve clatter can be mis-detected as knock by knock sensors (basically a very clever piezo microphone/accelerometer). Although I would have though signal conditioning/signal processing would be able to get around that now - which makes me think that the conventions of bike design are the main factor (like the clutch running in oil or the use of springs for timing chain tension - relatively antiquated, but acceptable and cheap).

Given as much timing advance as is safe is good for emissions, and knock is bad for emissions (NOx goes through the roof) that EURO4 or 5 would have been drivers to add the extra sense and adapt function, but they do know what they are doing and slightly retarding is cheap [see above].

 

[2and3cylinders]

My 98 VTR1000F relatively large V-twin has CV carbs, a rudimentary ignition system with 1 spark plug per 4 valve head at only 10.5:1 C.R., and obviously no knock detection / mitigation yet 87 octane is specified.  Wouldn't 20 years of engine development and only a 12.5:1 C.R. permit us to run 89 if not 87 octane occasionally if not routinely safely?

[skipperT]

Problem is, that gas isn’t as good as it was 20 years ago. 
You can run whatever octane you want, you just also need to accept the carbon build up that occurs on your valve stems, seats, and piston crowns. It’s not always about performance, it’s also about the byproducts of combustion and what that leaves behind and what the long term effect(s) are...

occasional use, ok. constant? The $0.50-$1.00 per tank you are saving isn’t worth it IMO.

Motorcycle engines are much higher performance than most car and truck engines. We demand a lot of performance from something thats engineered to be light, powerful, reliable, and meet emissions standards.

-Skip

Edited April 21, 2021 by skipperT

[bwringer]

It is possible to use something called "ion sensing" to detect detonation by monitoring the conductivity of the gasses in the combustion chamber via the spark plug electrodes.

AFAIK, it's only seen production in a few high-end cars, but it is a proven concept.

Pretty good article on the concept:

What is ion-sensing knock detection? PH Explains | PistonHeads UK

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I'm not sure how Yamaha gets by without knock sensors, but maybe it's rare enough that they're not really needed, or the engine has other ways of detecting problems.