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The Circuit Cellar BBS
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We’re going to cover a few new topics in this installment of
ConnecTime. The first thread deals with sensing engine knock in a
racing motorcycle. Next, we move into high-speed data collection.
Finally, we’// make some noise-white noise, that is.
Msg#:58203
From: KELLY DRESSER To: ALL USERS
Has anyone out there worked with pressure sensing in
internal combustion engines? What I’m looking for is
essentially a poor man’s Kistler. I don’t need absolute
accuracy as much as low cost and ease of use. I want to
sense the pressure peak and have enough frequency response to detect uncontrolled combustion [“knock”) in a
single-cylinder four-stroke 6OOcc racing motorcycle engine.
It’s air cooled, which adds even more of a heat problem to
an already messy sensing environment. Has anyone used
any unconventional or clever techniques to measure [or
infer] the pressure profile [with respect to crank angle]?
Msg#:58227
From: MIKE RAPP To: KELLY DRESSER
If you -really_ need to get cylinder pressure data then I
think there are no cheap and dirty solutions. In your
application, you might be able to avoid the instrumented
spark plug by drilling the head and directly mounting a
pressure sensor [no need to worry about water jackets]. Your
data sampling and storage will be nontrivial (1” rotational
resolution at 6000 RPM will need 36,000 samples/set.).
On the other hand, if what you actually need to do is
detect detonation (knock), then there is a much simpler and
cheaper approach. All you need to do is to detect the audio
frequency sound (pinging) produced by the detonation. It’s
much more commonly heard in a car than on a bike, but
will be produced by any engine that is driven into detonation.
The standard approach is to mount a vibration sensor
on or near the head and use the filtered output to indicate
detonation. This is already being done in production of
certain cars and trucks (particularly with turbo charging).
The frequency of interest is somewhere around 6 kHz in the
automotive application. Your aluminum air-cooled engine
might be different. You can find out by mounting up a
sensor and smacking the head area with a hammer. The
predominant frequency produced by the sensor should be
usable for detecting detonation. (Hammering on the engine
is how the automotive sensors are tested!)
Best source for a sensor might be the parts department
at a GM dealer. This technique is even used in laboratory
(dyno) testing since the vibration sensor will detect detonation well before even the best operator can hear it.
Msg#:58492
From: KELLY DRESSER To: MIKE RAPP
Thank you, Mike, for your reply. You confirm my
general hunches about how I’m going to get the info I want.
I’m still hoping for some unorthodox manner in which to
skin this cat. Already, with a water-cooled two-stroke I’ve
had much better than beginner’s luck at sensing the
cylinder pressure by epoxying the sounding disc from a
piezo squeaker on a flat spot next to the spark plug. Got an
incredibly clean and strong signal. The piezo even survived
the temperature, but did display a whopping DC signal that
varied with the rise and fall of the head temperature.
However, an air-cooled four-stroke is a lot hotter, a lot
noisier (mechanically), and there’s no place to stick the
thingumy.
Mr. Kistler makes good stuff, but it’s out of reach in
cost. Maybe someone has found another rugged sensor that
will work in this application. Your suggestion about using a
resonating piezo sensor from an automobile engine is a
technique I want to try last, since I really do want a pressure trace (for some combustion phasing fiddles) plus be
able to detect knock, and doing both with the same sensor
still attracts me.
I’ve been though all the recent SAE papers, but without
anything looking really good, except maybe an under-thespark plug washer (piezo again) that Nissan (I think] has
used in the past. Any experience by anyone out there with
such a sensor?
As far as having a fire hydrant of data pointed at me,
I’m not yet worried-some combination of digital and
analog techniques can minimize that. In any case, these
bikes are running on an inertial dyno that a friend and I
constructed (it’s just like Dynojet’s) and there’s a PC
already in the vicinity ready to swallow more data.
I’ll keep on plugging, looking for an elegant and/or
cheap (especially) solution to this problem.
The Computer Applications Journal
Issue #29 October/November, 1992
95