EGT and O2 sensors time resolution

[Bernd Felsche]

Bruce tapped away at the keyboard with:
>From: "Bernd Felsche" <bernie at innovative.iinet.net.au>
>>Bruce tapped away at the keyboard with:
>>>From: "Bernd Felsche" <bernie at innovative.iinet.net.au>
>>>>Santi Udomkesmalee tapped away at the keyboard with:
>>>>>2 questions...
>>>>>1)  What sort of time resolution do EGT sensors, and O2 sensors  have?

>>>>O2 sensors can be faster than the stream of exhaust gas from the
>>>>valve. Depending on type, placement and temperature, you're looking
>>>>at response times in the region of 10 to 100 milliseconds.
>>>Let's try the math on a v8 at 7,000 rpm single exhaust and sensor.

>>Let's.
>>You deserve all you get choking a V8 through only one exhaust! :-)

>Swing by sometime, I'll take ya for a ride in a choked v8.
>10 msec is wayyyy to slow to identify individual cylinder firings,
>on the above example.  At over hig cruise speed your game is over.

You will find that most O2 sensors are slower than 10 ms. 100ms is
around the "age" limit for sensors. O2 sensors aren't typically used
to identify individual cylinder firings (it's useful if they can).
O2 sensors are used to maintain _average_ mixture to be compatible
with emission-control eqt and requirements.

Why is 10ms _wayyyy_ too slow?
Sure; 2ms would be nicer and _easier_. 10ms isn't _impossible_.

How long does the gas take to travel from the valve to the sensor?
Maybe 10ms, depending on geometry?

>>>>EGT depends a great deal on the type of sensor and the exhaust gas
>>>>temperature _gradients_. i.e. how quickly they rise and fall. The
>>>>sensor has a finite response time dependent on its thermal mass.
>>>>Response time to reach a "near enough" reading can be in the
>>>>seconds.
>>>>>2)  Would a closed loop system be benificial to a high performance
>>>>>application?  That is to say, ideally the engine should be at WOT
>>>>>or idle most of the time.  Would there be any gains to
>>>>>implementing some sort of closed loop system in this application?
>>>>What sort of high-performance application? Aircraft?
>>>>Just the usual benefits:
>>>>Engine life, mainly. Also fuel consumption and engine power
>>>>optimisation along with emission control.
>>>>Other main benefit is that it allows more aggressive control
>>>>strategies.

>>>How so?.

>>By eliminating the operational "safety" margins that are necessary
>>because you're not measuring those quantities under actual, ambient
>>conditions. i.e. fuelling and timing doesn't have to be as
>>conservative to avoid excessive temperatures and emissions which
>>might occur under worst-cse (or tuning session) conditions.
>>The engine is operated under best-case conditions as far as is
>>possible under ambient conditions given the physical engineering
>>constraints.

>So your comparison is for a none tuned to conditions engine to a
>closed loop system.  the limits are with the engine not the tuning.

I don't assume that the whole world will operate the same way as
during tuning.

You may be able to control
  1. fuel quality (density, temperature as well as chemistry)
  2. air quality (including density, moisture content, O2
     concentration, impurities)
  3. engine and ambient temperatures
  4. engine load
  ...

I can't. So I'd prefer closed-loop control to dynamically tune the
measurable quantities to the known physical properties inherent in
the engine design.

-- 
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