Getting the switches under control

Posted by Lerner on Sep 15, 2010 at 05:44 PM
Tools: Print | | (0) Comments

In August, we received the higher voltage DC power supply for the trigger.  By increasing the voltage of the HV power supply that charges the trigger generator from 20 to 40 kV, we were able to increase the maximum voltage on the trigger from 17 to 20 kV.  The limit on the voltage was that the trigger pin of the spark plug was still shorting out to the adjacent electrode of the switch.  However, by increasing the charging voltage of the capacitors from 27 kV to 31 kV, we were able to get the total voltage of the switch (the sum of the charging and trigger voltages) above the 50 kV needed to fire the switches together.

We found the switches would still stop firing even after we had adjusted the spark gap carefully with its micrometer screw top.  Measurements showed this was due to the impact of the switch firing causing the spark plugs to move slightly, and a motion of more than one or two thousandths of an inch was enough to cause misfiring.  Dr. Subramanian came up with the simple idea of using an Anti-Backlash Stabilization Mechanism (better known as a nut) to stop the screw from moving.  This has worked well.

Unfortunately, during the testing of the HV power supply, we inadvertently overstressed one of the trigger heads and it will have to be replaced.  We expect to do that in September or October, depending on delivery of the parts.  To maintain symmetrical firing, we took an oppositely placed capacitor out of service, so we are firing with 10 switches.  On September 1, we were able to fire all 10 switches together.  Figure 1 shows the rate of increase of the current on shot 9-1-10-13.  We see a sudden jump within 100 ns to the maximum rate of increase.  In contrast, with the old automotive spark plugs, the peak dI/dt (rate of current increase) was achieved after only 300 ns (see Figure 2).  Such an improvement in simultaneity is essential to the optimal functioning of the DPF.

Figure 2. shot 04-26-10-02

We can and will continue to function with the current spark plug design, but we will also improve it.  We need a more 21st-century method of keeping the spark plugs in adjustment, perhaps with actuators (small motors).  One of LPP’s investors, Rudy Fritsch, has volunteered to use his mechanical engineering expertise to work with us in designing a second generation spark plug.  Thanks, Rudy!

Our next immediate step is to re-adjust the trigger pins uniformly upwards so that we can prevent pre-firing at higher charging voltages, all the way up to the machine’s maximum of 45 kV.