Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Beam-break sensors for ignition delay measurement

  1. Feb 2, 2008 #1
    Hi all..

    I have been working on a project which involves the measurement of ignition delay of fuel droplets using optical sensors of the beam-break type. What my project supervisor has asked me to do is to fabricate an apparatus in which such measurements could be made. The basic principle would be to trigger a stop watch as soon as the beam of the sensor is intercepted by a fuel droplet and stop the timer as soon as ignition takes place.

    Being a student of mechanical engineering I do not have much knowledge of sensors and other electronic circuit components. I have been browsing the internet for a while now looking for sensors and the suitable circuit that would be required in this case. During my search I also found out that photodiodes in reverse bias have very high resistance and their resistance decreases when light falls upon them. I was wondering how could this change in resistance be utilized for triggering a stop-watch.

    Can anyone suggest a possible circuit design for measuring the ignition delay?
    The operating temperatures and pressures would be quite high and so the sensor should be able to work in those conditions as well.
    It would be nice if anyone could help. Thanks
    Last edited: Feb 2, 2008
  2. jcsd
  3. Feb 4, 2008 #2


    User Avatar
    Gold Member

    Wecome to PF.
    I hate to see you sitting here all by yourself, but I can't help. There are, however, people here who can. I'm sure that one of them will be along shortly.
  4. Feb 4, 2008 #3


    User Avatar

    Staff: Mentor

    Here is a selection of Photointerrupter circuits from Digikey. Select some simplifying terms in the menus, and click on "Apply Filters":


    But, your temperature requirements are going to likely be too high for simple production photointerrupter components, so you will likely need to make your own from LED and a PIN Photodiode, both somehow housed outside of the fuel droplet path (both to isolate them from the high temperatures, and to make sure that they don't interfere with the combustion process (standard photointerrupters are not intrinsically safe devices).

    This wikipedia.org article will start to give you some background on the operation of a photodiode:


    You will likely want to have a fast responding circuit, so that means using a photodiode (not a phototransistor or other photosensor), in reverse bias in a fast current-to-voltage circuit, like figure 13 here:

  5. Feb 5, 2008 #4
    best way is to use a laser can be cheap (HeNe) and have a photo detector on the other side. Then you put the input of that detector into a fast data acquisition system (GageScope is great) LabView is also good. You can also use a stand alone digital oscilloscope although they are a little less user friendly and sometimes a little more expensive. Anyway, this is the way we measure spray and droplet durations in shock-tubes, also, onset of ignition can easily be measured using a photo detector (PMT) in combination with the right filter (OH* at 310 nm and CH* at 430 nm). Please let me know if you have any specific questions because I am pretty familiar with these kind of setup.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook