
Studying my car's sensor data with Open Torque Viewer - rcpt
http://ryancompton.net/2015/02/28/mpg/
======
leoedin
I wouldn't be so certain that the error in speed measurements is due to the
speedometer. Car speedometers work by measuring wheel rotations, using
knowledge of the wheel diameter and calculating the speed. You can get errors
when the wheel slips (very unlikely except at very low speeds), and when the
wheel diameter is less or more than the assumption (level of tread wear, tyre
inflation). For a given journey (and over a period of days) this error will be
pretty constant and linear in nature - the faster you go, the greater the
error by some factor. Regulations mean that it's much more likely to over-
report than under-report the speed, but I'd assume that this would be by a
fairly constant amount.

GPS speed is calculated by distance travelled between 2 positions in a given
time. The problem is that GPS positions aren't completely accurate. Rounding
errors, atmospheric effects and signal reflection can all affect the accuracy
of the position. At low speeds, these errors can be as big as the distance
travelled between measurements. The GPS speed may not take into account
altitude (which is considerably less accurate on consumer GPS receivers), and
so would under-report speed going up or down hills. It will also assume
perfectly straight lines between points, while your car may in fact be
travelling a less straight path. There's also likely to be a degree of
averaging involved in the calculation, which would filter out small speed
changes reflected in your speedometer data.

Your plot shows times when the GPS speed was as high as 20 mph when the car
speedometer was reporting 0 mph. No car with a functioning speedometer will
report 0 mph when it's actually moving at over 20 mph.

The convergence at higher speeds is probably more to do with the accuracy of
GPS speed measurements at high speed than it is to do with the accuracy of a
car speedometer at high speeds.

~~~
smileysteve
Car speedometers also tend to read fast [possibly for safety]. By spec, I
believe that they are allowed to read 10% faster than actual speed but more
severely penalized if the read slow.

Best / Quickest source : [http://www.popularmechanics.com/cars/how-
to/a3127/4260708/](http://www.popularmechanics.com/cars/how-to/a3127/4260708/)

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rcpt
I’ve been using Open Torque Viewer combined with the Torque App and a basic
OBDII bluetooth sensor to log my car’s sensor data over the past month and a
half. Charts are in the link.

Plotting was done with seaborn - the code is available as an ipython notebook
:
[http://ryancompton.net/assets/torque/mpg_plots.py.html](http://ryancompton.net/assets/torque/mpg_plots.py.html)

~~~
huuu
Aren't these Bluetooth sensors a great access point for hackers?

I can't find the article but I think this was in the news lately.

~~~
klez
I used to develop OBD diagnostic software[0], the ones used by mechanics.

For the cars I developed for, usually actuators had some preconditions that
had to be met before they could go off. For example you shouldn't be able to
tell the car to brake via OBD if the car is not still.

At the moment, I can't think of something 'dangerous' that could be done via
ODB.

[0] [http://www.texa.com/](http://www.texa.com/)

~~~
VLM
One thing you can do without connecting is sniff -n- track. Regardless if you
connect, you can sniff things like tire pressure monitor transmitters or
bluetooth addresses.

At least officially there is no publicly known database of license plate pix
and sniffed MAC addresses. Obviously being blindingly obvious idea, there
probably is a secret one. So you only need enough optical data gathering to
match MACs to plates with excellent lines of sight and OCR and so forth, then
you can get away with deploying simple RF sniffers all over the place that
don't need LOS and can be much cheaper.

Note that you can track cops this way, not just victims ^H^H err consumers I
mean. And obviously the "smarter" the car the easier this tracking works,
you're not going to track a restored 1972 Gran Torino off its bluetooth.

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gambiting
Yep, reading out data from your car through an OBD2 port is super interesting.
I used to drive a 4.4L V8 Land Rover and it was very interesting to find out
that it would use ~$0.50 worth of petrol within 30 seconds of starting, just
to warm the engine up. Then the consumption fell down significantly - quite
high in city driving,then lowest I have ever seen while cruising at
120-140km/h(80-90mph), and peaking at its highest at top speed of
200km/h(120mph). It was very interesting to see.

~~~
phkahler
>> I used to drive a 4.4L V8 Land Rover and it was very interesting to find
out that it would use ~$0.50 worth of petrol within 30 seconds of starting,
just to warm the engine up

The reason is to reduce emissions. An ex-GM guy once told me that you'll
produce more NOx and CO starting your car than the entire trip. The catalyst
need to be hot to clean the stuff up. They probably run it rich to get more
fuel into the catalytic converter to heat it.

~~~
auxym
This is likely right, it's also the reason when you start up most car you'll
see the rpm stay higher than normal idle for a couple seconds. Heating up the
catalyzer so it can do its job.

~~~
refurb
Not only heating up the catalytic converter, but heating up the engine in
general. Efficiency is pretty low when the engine is cold.

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madhurjain
Thanks for putting that together! Charts and the statistics are quite
interesting and its good to know about driving habits. I am gonna give Open
Torque Viewer a try soon.

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VLM
Author came very close to demonstrating some other cool Torque feature such as
coupling the GPS and car position data to the mass of the car (which you
manually enter) and torque outputs a fairly accurate measure of instant or
peak measured horsepower. Oh look my ancient '97 Saturn really does accelerate
at 120 horsepower as calculated based off 2nd derivative of position and input
mass. And, likewise, by continuous monitor of the stats and doing some very
simple timing, torque can output 0-60, 0-100, quarter mile, etc, you hit
reset, it waits until the throttle position sensor is slammed or it sees over
0 mph then times until it sees 60 or 100 or whatever. I think it can do
braking distance the same way although its been awhile since I tried.

Its interesting to watch the other readouts like coolant temperature. How long
does it REALLY take a car to warm up in the winter vs the summer and stuff
like that.

I believe I have the same no-name adapter the author has, and it works well.
One common discussion topic for torque users is finding a BT adapter that auto
powers off when the car shuts off, so you can leave the device plugged in all
the time if you want without killing the car battery.

And a final additional commentary is the community is always discussing /
trading map files. So on my wife's prius if you download the special prius map
file you can do deeper diagnosis and see the voltage across battery #11 or
whatever which is moderately interesting, but without the map file it doesn't
know how to map the additional telemetry packets into individual cell voltages
or whatever. Google will help. Not all cars have enhanced mapping data of
course.

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IgorPartola
Just FYI, AutoZone and places like that will read a check engine light for
free. Did this once and found out I needed a new gas tank cap.

~~~
unwind
Wow, that's a pretty epic sensor coverage on that car.

How does that work? I had to Google it, and came up with this example status:
[http://www.obd-codes.com/p0452](http://www.obd-codes.com/p0452). It seems to
be based off a fuel-tank pressure sensor. Neat.

~~~
IgorPartola
It was the OBD code + expertise from the AutoZone employee. He read the code
that corresponded to a fuel leak detected somewhere and said the most likely
cause is that the gas cap was not on tight enough. He said I can either try to
re-tighten the cap, or just get a new one for $6 or so.

~~~
a3n
The only time my 1990 Honda Civic (original owner, looks like shit (both of
us)) failed emissions test was a worn gas cap. When to $AUTO_PARTS_STORE,
bought a cheap cap, passed test.

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euroclydon
Anything like this for iPhone or USB to a laptop? [Edit] Obviously the USB OBD
II connection exists: Any recommendations for hardware/software?

~~~
camera_guy
I purchased this hardware from Amazon that is iOS compatible: iKKEGOL WiFi
OBD-II ODB2

And I downloaded EngineLink from the appstore. You connect to the sensor via
Wifi (dont need to be on a wifi network, the sensor creates one that you
connect to), then start up the app and you are good to go. There are many apps
that will work with the sensor though, so check out a few and see which works
best for your needs.

~~~
semerda
Possible to export the data out of the EngineLink app?

Make more sense to create your own and shoot that data to a box in the cloud
for analysis? Or this already exists in some open source form?

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zissou
Glad you're enjoying Open Torque Viewer.

It's my creation. :)

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hackbinary
Thanks for this. This is awesome! I have been wanting to monitor my car, and
was wondering this in the last couple of months.

