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I am currently an undergraduate student leading a discussion section for an introductory electricity and magnetism course. I was asked this question that I was unable to answer and I am hoping for some help in understanding what is going on.

I was told by this student that he and his girlfriend were driving along a highway when lightning struck the roof of their car. Although they were unharmed by the lightning (Faraday cage), my student claimed that his car stalled and was unable to turn it back on right away. However, after about 5 to 10 minutes, the car started up immediately. So my question is how does lightning striking the roof of a car stall a moving car?

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  • $\begingroup$ Dude, this isn't funny. Pls try to be serious $\endgroup$
    – user34304
    Apr 18, 2014 at 9:29
  • $\begingroup$ A colleague of mine claims to have seen this happen to a car, though in that case it restarted immediately. My guess is that the EM pulse reset the ECU. $\endgroup$ Apr 18, 2014 at 9:33
  • $\begingroup$ @JohnRennie: Hi John. My first guess was the lightning strike raised the potential of the car so high that the spark plug was not able to create a spark across its gap, therefore, stalling the car. $\endgroup$
    – Carlos
    Apr 18, 2014 at 10:50
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    $\begingroup$ @carlos: raising the potential of the car relative to the ground would not change the potential difference across the spark plug. $\endgroup$ Apr 18, 2014 at 11:31
  • $\begingroup$ @JohnRennie: I see how my first guess was not correct. Thanks. $\endgroup$
    – Carlos
    Apr 19, 2014 at 0:05

2 Answers 2

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I suspect that there are must be two different effects: one that causes the car to stop, which would be more or less immediate in effect, and another that dissipates after some minutes.

For the immediate effect that causes the car to stop, this would most likely be electrical, such as an induced surge in the automobile's Engine Control Unit (ECU). According to a NOAA site about lightning safety, "damage to the antenna, electrical system, rear windshield, and tires is common" in automotive lightning strikes.

For the longer term effect that might prevent the car from immediately restarting, the effect is probably either:

  1. thermal - an obvious effect of a lightning strike is the rise in temperature of objects and atmosphere in the immediate vicinity of the current path of the lightning. If the incoming air into the engine is superheated, the effect would be that the fuel/air mixture would be off far enough to prevent the engine from starting. Other possibilities could include the temporary vaporizing of fuel or electrical heating of the starter motor causing clearances to temporarily change, making it require more energy to turn.
  2. residual electrical - the vehicle's ECU has memory which stores parameters about the engine. In the event that these parameters are damaged or erased, the computer needs to effectively "re-learn" these parameters (such as ignition timing fine-tuning, and fuel injection timing), possibly restarting from factory defaults if the engine fails to start after multiple attempts. See this in depth article about the details of ECU operation.
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A lightning striking a car creates huge transients in all kinds of electro-magnetic fields as you go from 0 to some thousands to 0 amperes in a very short time span. My first guess would be that the fields created by the lightning interacted with some components of the car. In modern cars a of lot of electronics is used. What caused the problem exactly, may turn out impossible to tell.

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  • $\begingroup$ I agree with everything you stated, I was just hoping for more detail. See John Rennie's and my comment above. Thanks. $\endgroup$
    – Carlos
    Apr 18, 2014 at 10:52

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