Why Starting A Diesel Engine Can Overpower A Force Created By The Earth Itself

Hey Autopians! It’s me, Emily, whom you might remember from a few things I’ve written
here, like about how I 3D printed new rubber gaskets for my truck, or how I built an e-
bike out of junk that I scavenged.
Today I want to talk to you about a compass that lives in my friend Fredy’s 1992 Chevy
G20 diesel — nicknamed “Poopandus” — which just exhibited a remarkable
phenomenon for us last weekend.

The compass is one of those ball ones of the sort that I distinctly remember living on the
dashboard of the neighbor’s RV when I was growing up. I always thought these things
were neat, so when I found one for a couple of bucks at a thrift store last year, I grabbed
it.

I thought it would look better in Fredy’s van than in my Honda Fit, so that’s where it
lives now.

The compass in question, and subject of this article.


Last weekend, Fredy and I were hanging out after visiting a museum as part of an
assignment I had been procrastinating on for my ceramics class. We had switched from
my Fit to the Chevy van in an attempt to pick up a cool (and extremely heavy) steel
cabinet with sliding shelves from the side of the road. We failed to get the cabinet
because someone else picked it up while we were swapping vehicles, so we went on
with our day and picked up some really excellent fried chicken to eat in the park.

Poopandus, the van, parked elsewhere besides the park where we ate our chicken.


After devouring the chicken and spending some time sitting around and digesting, we
climbed back into the van to leave. Just before Fredy started the van, something
strange caught my eye. The dashboard compass had spun completely around.
A moment before, it showed us facing due south, it now said we were facing
north. Then, it flipped back around and showed us facing South again.

I wasn’t sure what I had just witnessed, and remarked on it. Fredy looked at the
compass, thought for a second, and said, “Glow plugs?”

Fredy turned the van off, and pressed the glow plug button on the dashboard. Sure
enough, the compass flipped from south to north again. As soon as Fredy let go of the
button, it spun back to south.

So what’s going on here? It doesn’t take a rocket science to figure it out, but, for the
record, I did consult a friend of mine who is a Caltech graduate to make sure I correctly
did the math I’m about to explain.

Clearly the van’s electrical system is creating a magnetic field that’s pulling the compass
away from true north, but how?

Sorry. It’s Math Time

Since the effect only appears when the glow plug button is depressed, it’s pretty safe to
assume that it’s the glow plugs that are responsible. A glow plug is a thin little metal rod that reaches into an engine’s combustion chamber to pre-heat it to facilitate immediate compression-ignition in a diesel during a cold-start.

I did a little bit of research and
found that individual glow plugs can draw between 10 and 20 amps each when first
powered on. The diesel engine in a G20 has a part that GM calls a “glow plug
controller,” so all the current for the glow plugs must pass through it. If we’re
conservative and guess each plug is drawing 12 amps, and there are 8 plugs, then…

12 amps * 8 plugs = 96 amps

… 96 amps are passing through the glow plug controller when the button on the
dashboard is pressed. That’s a considerable amount of current.

Now we need to figure out how much of a magnetic field 96 amps would generate.
Helpfully, physicists have already figured out a formula for this:

Let’s define a few of those variable real quick.

• I: This is the current flowing through the wire, i.e. the amps we calculated above

• π: This is pi, the ratio of a circle’s diameter to its circumference. It’s roughly
3.14159

• µ0: This is a mathematical constant representing the “permeability of free space.” This just describes how well a magnetic field spreads through empty space. In this case, there’s air and a plastic engine cover in the way, but those aren’t going
to make much of a difference in this case. The constant’s value is 4 π x10 -7 Henrys per meter. (Yes, there is a scientific unit called Henry; it’s a unit of inductance)

• r: This is distance from the wire carrying the current in meters

• B: This is magnetic flux density, essentially how strong the magnetic field is. This
is measured in teslas, which, I have to point out, since you are car people, are
not the same as Teslas.

We already know our current, and we know our permeability constant, so all we need to
solve this equation is determining how far away the compass is from the wire carrying
the current to the glow plugs. This is one of those vans with a short nose and much of
the engine in the passenger compartment underneath a doghouse cover, and the glow
plug controller is on top of the engine, somewhat towards the end that protrudes into the passenger compartment, as seen here:

Always read the manual before attempting math.


The compass is sitting in a tray on that doghouse cover, so I’m going to take a wild
guess that the compass is roughly 6 inches from where the 96 amps is flowing. Six
inches is 15.24 centimeters, or 0.1524 meters. I’m also assuming that the wire the
current was flowing through was more or less straight, because if it was coiled up, this
would get more complicated.

If we plug everything we know and we’ve assumed into the equation, we get this:

I’ll spare everyone, including myself, having to go through all the steps of solving that
right here, but if you do solve it, you get a value for B that equals 0.000126 tesla.

“It checks out to me as long as those units are right,” said my friend, the Caltech
graduate, about my math, while also declining to be named for this piece (clearly they’re confident).

According to Wikipedia, Earth’s magnetic field varies between 0.000022 tesla and
0.000067 tesla. Generally speaking, Earth’s magnetic field is strongest at the poles and
weakest at the equator.

Fredy and I live in LA, which is closer to the equator than it is to the North Pole, so it’s
safe to assume the Earth’s magnetic field isn’t at its strongest here. If we did our
magnetic field calculations correctly, then the magnetic field from the glow plugs was
probably much stronger than the Earth’s magnetic field was in the vicinity of where we
were eating our fried chicken on a pleasant Sunday afternoon. So, it makes a lot of
sense why the compass spun around when the glow plugs were powered on.

So, what to make of all this? I dunno. Science and math can explain a lot about what we
experience in the world? Fried chicken is good? Even when you procrastinate in school,
you might still learn something?

Bonus fried chicken photo.

(All pics Emily Velasco; Top Images also includes images from Wikimedia Commons, Chilton’s)