(Knock detection methods) Re: Maximum advance

[dom]

How would one go about hooking the knock sensor up to an amplifier?
-Dom


on 7/12/01 7:22 PM, Shannen Durphey at shannen at grolen.com wrote:

> Axel Rietschin wrote:
> 
>> 
>> With a copper pipe bolted on the head and connected to a "stethoscope" on
>> the other end, across a fully soundproof wall, you will hear even isolated,
>> spurious knock bursts and be able to back off timing very quickly. For this
>> process to work well you need to stabilize the engine at every load/rpm
>> points and adjust them individually, and you need silence and concentration.
> 
> As you mention further in the post, a knock sensor connected to an amplifier
> and
> a good set of headphones will reveal huge amounts of engine noise, knock
> included.  It's a good experience, IMO, for every average tuner to drive
> around
> and listen to one of these babies.  Even if you're not chasing knock, it's
> good
> to listen to what the knock sensor normally hears.  For the guys that like to
> set timing almost at knock, it's a great experience.
> 
> Shannen
> 
>> 
>> Knock is very random in nature and you may only have one burst of 10 or 15
>> out of 200 or 300 burns that pings, I'm not sure how you'll catch and
>> display that in real time with a PC, but your ears definitely will, every
>> time, provided you are in a silent environment.
> In a car and with good headphones you'll hear a fair amount of noise also.
> 
>> 
>> When advancing timing small steps at a time you will feel an ever so subtle
>> change in the absolutely chaotic internal engine noise that will tell you
>> knock is just about to show his ugly face and soon thereafter you'll hear an
>> erratic burst of noises somewhat similar to what you hear when you lit a
>> match, only much shorter, slightly standing out of the huge background
>> noise. When you hear that, quickly back off 2-3 degrees and you are done
>> with that map point. If you back off timing but still hear more matches
>> lighting up, reach that big red pushbutton (the one saying "emergency stop")
>> as fast as you can but chances are it is already too late.
>> 
>> When I mapped my engine we were up to three in the booth, the person driving
>> the dyno, myself on the laptop and the third person with his hand on the red
>> button. We had one expensive piezo sensor connected to an oscilloscope
>> (useless), one standard Bosch knock sensor connected to a Nagra tape
>> recorder with me on the headphones and the copper pipe for the dyno
>> operator; the only words allowed were "OK" to move to the next rpm site and
>> "Stop" to back off immediately.
>> 
>> We spent about 5 minutes on every row on the map including maximum load,
>> releasing the brake in small steps to stabilize the engine at every rpm
>> sites on the current row; it took me about 10-12 seconds per site to reach
>> the premise of knock and back off two degrees or so; we let the engine idle
>> at 3000 rpm for 2-3 minutes between rows to allow the turbo to cool down
>> from bright yellow to dark red; the ECU was taking care of the mixture
>> details using its own NTK sensor, precisely tracking the target lambda set
>> in the appropriate map (we did a rough fuel map before using a very
>> conservative ignition map).
>> 
>> The whole process took several hours and take my word for it, it takes a lot
>> of nerves to play with the knock limit at high load for 5 minutes flat at a
>> time with the whole exhaust header and turbo yellow-white, with up to 1100C
>> on the temp dial toward the end of the rpm range. The ECU was connected to
>> all the usual engine sensors and we had additional sensors for almost
>> everything, pre and post intercooler temp, per cylinder exhaust temp, oil
>> pressure, fuel pressure, oil and water temp in and out of the engine, oil,
>> water and fuel flow, manifold pressure, crankcase pressure and another WB
>> O2 - nearly 20 extra sensors  - with computer controlled normal range, alarm
>> and emergency stop values for every of them (the dyno would shut down
>> automatically if one of the value went out of the allowed range) - I
>> describe all this just to emphasize you can't really do a proper ignition
>> mapping job on a chassis dyno, much less on the road, and that you need more
>> than a plug sensor, no matter how good, to do it right.
>> 
>> Also I don't want to discourage anyone, but the only ECUs I know that are
>> capable of using those pressure sensors for closed loop ignition are using a
>> dedicated DSP chip just to process the sensor's signal, and they _are_ the
>> ECUs, I mean they already knows where the crank is and when they fired the
>> cylinder they are monitoring. I believe the signal is extremely noisy and
>> somewhat hard to interpret properly in real time. The software has so many
>> adjustable parameters in the knock strategy section it makes me wonder how
>> long it takes to adapt it effectively to a particular engine. Note that I'm
>> not against progress (I believe I was the first to post references to the
>> NTK/NGK spark plug sensors here a while ago) it is also well known that Saab
>> and others pioneered innovative ways of sensing knock but for most of us,
>> the old way is still by far the best IMHO.
>> 
>> --Axel
>> 
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