Engine knock sensor?

Archiver

Hello everyone,

Does anyone have any information on knock sensors for engines? I'm
going to be putting a turbo on one of my cars shortly, and it does not
come with a stock knock sensor...

All the info I have so far:
1) A knock sensor is usually a piezo element acting as a mic.
2) The output is fed into a band-pass filter which is tuned
around the knock frequencies
3) "The ECU then analyzes the signal for the signs of knock"

Well, thats all fine and good -- but I cant find two bits of
info... 1) What is the approx frequency of engine knock (which might
vary from engine design to engine design) and 2) what are the "signs
of knock". heh...

Anyone ever utilize a stamp in an application such as this?

Thanks,
brad

Archiver

At 10/30/2000 +0000 08:30 PM, you wrote:
>Hello everyone,
>
> Does anyone have any information on knock sensors for engines? I'm
>going to be putting a turbo on one of my cars shortly, and it does not
>come with a stock knock sensor...
>
> All the info I have so far:
> 1) A knock sensor is usually a piezo element acting as a mic.
> 2) The output is fed into a band-pass filter which is tuned
>around the knock frequencies
> 3) "The ECU then analyzes the signal for the signs of knock"
>
> Well, thats all fine and good -- but I cant find two bits of
>info... 1) What is the approx frequency of engine knock (which might
>vary from engine design to engine design) and 2) what are the "signs
>of knock". heh...

From "The Gasoline FAQ":

"The knock sensor can be either a nonresonant type installed in the engine
block and capable of measuring a wide range of knock vibrations ( 5-15 kHz
) with minimal change in frequency, or a resonant type that has excellent
signal-to-noise ratio between 1000 and 5000 rpm [noparse][[/noparse]72]."

and

" 7.2 What is the effect of changing the air/fuel ratio ?

Traditionally, the greatest tendency to knock was near 13.5:1 air/fuel
ratio, but was very engine specific. Modern engines, with engine management
systems, now have their maximum octane requirement near to 14.5:1. For a
given engine using gasoline, the relationship between thermal efficiency,
air/fuel ratio, and power is complex. Stoichiometric combustion ( Air/Fuel
Ratio = 14.7:1 for a typical non-oxygenated gasoline ) is neither maximum
power - which occurs around A/F 12-13:1 (Rich), nor maximum thermal
efficiency - which occurs around A/F 16-18:1 (Lean). The air-fuel ratio is
controlled at part throttle by a closed loop system using the oxygen sensor
in the exhaust. Conventionally, enrichment for maximum power air/fuel ratio
is used during full throttle operation to reduce knocking while providing
better driveability [noparse][[/noparse]24]. If the mixture is weakened, the flame speed is
reduced, consequently less heat is converted to mechanical energy, leaving
heat in the cylinder walls and head, potentially inducing knock. It is
possible to weaken the mixture sufficiently that the flame is still present
when the inlet valve opens again, resulting in backfiring "

> Anyone ever utilize a stamp in an application such as this?

No, but I suspect there in more valuable information in that FAQ.

The entire FAQ can be found at:
[noparse][[/noparse] http://www.faqs.org/faqs/autos/gasoline-faq/part1/ ]
and it's complete in 4 parts, of which this is number one.

>Thanks,
>brad

Sure

Hope that helps

Regards,

Bruce Bates