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Old 03-10-2010, 08:48 AM   #51 (permalink)
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can you define what you mean by "ideal clutch"? This driver is scratching his head on that stipulation.

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Old 03-10-2010, 08:56 AM   #52 (permalink)
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Just that you don't have any losses in the clutch. You can cycle it an infinite number of times and it will never wear out, never heat up, never slip, etc. In other words, you can ignore its existence beyond the fact that it switches the engine on and off like a light.
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Old 03-10-2010, 09:01 AM   #53 (permalink)
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ok, so when you engage this magic clutch in top gear, you and your vehicle are instantaneously going the speed determined by the gear ratio and rpm?
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Old 03-10-2010, 09:08 AM   #54 (permalink)
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or are you saying that the engine will run from 0 RPM and up? Or was there an initial speed stipulated?
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Old 03-10-2010, 09:16 AM   #55 (permalink)
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ok, so you add a magic energy storage device that has zero weight and is 100% efficient at capturing and returning energy so you can keep the load at peak bsfc and shut down sooner.

Did I win yet?
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Old 03-10-2010, 09:42 AM   #56 (permalink)
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Quote:
Originally Posted by dcb View Post
or are you saying that the engine will run from 0 RPM and up? Or was there an initial speed stipulated?
Yup. I know it looks like a rather silly example right now, but on Friday I'll explain what the point is. Suffice to say for the moment that what is interesting is the class of solutions to this problem, and the critical parameter values where the system switches from one subclass to another. More on that later.

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ok, so you add a magic energy storage device that has zero weight and is 100% efficient at capturing and returning energy so you can keep the load at peak bsfc and shut down sooner.

Did I win yet?
You've already got that magic energy storage device. It's the car! Since everything is frictionless, any energy you put into the car will not be lost.
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Old 03-10-2010, 11:03 AM   #57 (permalink)
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Building on Robert's idea, of letting it start backwards with the clutch disengaged. You now have two scenarios. 1- The car reachs a speed at the bottom of the wave at which the engine would rev past its efficiency island. 2- The car never makes it to the most efficient speed just rocking back and forth from it's innitial potential energy.

Scenario 1: You let it rock backwards up the left hand side of the wave. From there going down it will cross the most efficient rpm somewhere before the bottom of the wave. It will also cross it back while slowing down somewhere on its way back up the wave to the car's original starting point. Assuming your new "starting position" is now on the left hand side of the wave pointing down, you then calculate and subsequently execute two pulses, both as close to the efficiency island load and rpm as they can be, one on the way down and one on the way up, that will give you the energy needed to get over the first wave.

Scenario 2: You let it rock backwards so your new starting position is on the left hand side of the wave pointing down and you do one single pulse starting at the very bottom of the wave, where you're speed will be the greatest.

Edit: Thinking about it, for scenario 1 the car should be rocking back and forth in the starting wave while applying infinitesimal bursts each time the sweet spot speed is crossed when the car is going forward, the amplitude of the movement increasing all the way untill the car can go over the first wave.

I have long been suspecting the definitive answer is lying in an unpractical particularity of the excercise, mostly irrelevant to the real world.
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Old 03-10-2010, 12:30 PM   #58 (permalink)
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I have recently recieved a CAN OBDuino which can log directly to an SD card.

I'm currently logging all my trips gathering the following data, each PID being sampled 3-4 times every second:

Time since engine start
Mass air flow
RPM
VSS
Intake air temp
Coolant temp
Equivalence ratio
Calculated load value

Calculated load value = airflow / peak theorical airflow. Mapping this against the maximum torque curve of the engine I should be able to compute an estimation of the actual torque the engine is producing without having to consider a whole lot of variables.

So I should be able to produce a bsfc map of unknown accuracy this way, but it would still be interesting to see. What I'm really curious to see are the differences between the two bsfc maps I could produce this way. One with a low IAT range and one with a high IAT range.
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Old 03-11-2010, 12:47 AM   #59 (permalink)
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Ah, yes, I think Martin has it with the well-timed infinitesimal pulses. The only way to improve upon it without cheating too blatantly would be to use the above method, but stop a few hundred Joules short of being able to crest the hill. Then have the driver get on the roof of the car, and run off the back of the car as fast as he can.

There are a few variations on this theme that involve the driver removing noncritical components (the interior, bodywork, exhaust, whatever) and vigorously ejecting them rearward while the car is at rest or traveling forward. But these are all examples of human power, and if that's allowed, then you may as well just push the car faster little by little, as you would push a child on a swingset.




While commuting today, I was thinking about what the data would look like in MATLAB. If you look at my TPS readings (or using fuel injector pulsewidth as a surrogate), you'd think I'm an indecisive driver. Maybe I am, but traffic can be unpredictable.

Do you automatically discard unusable data by running the observer only across timescales where the fuel injectors are active? Perhaps you could record the position of the neutral safety switch (or manually select timescales where you were coasting) to allow you to include data for neutral coasting. I'm sure engine efficiency is one of the most unpredictable parameters, so being able to set engine torque to zero some of the time would improve your ability to estimate all of the parameters, right?

Your project is as interesting to me for its ability to estimate CdA despite gravitational and rolling resistance effects, as for its ability to measure BSFC.
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Old 03-15-2010, 02:05 PM   #60 (permalink)
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This is only 5 days worth of driving for my wife in her Elantra. 17000 unfiltered raw data points and 150000 individual values logged on my CAN OBDuino. Things are starting to look interesting. I just need to take the car and spin it in RPM/loads regions it's not usually used to get a better picture.

XYZ you have RPM, BMEP in kPa and BSFC in g/kWh

Fuel flow was calculated from the MAF sensor reading corrected with the equivalence ratio from the stock wideband sensor.
Brake torque was estimated from the Calculated engine load % (airflow/peak theorical airflow. I need to figure out if peak airflow is adjusted from baro pressure or not in the Elantra.) mapped against the max engine torque curve from the manufacturer.

What I like about this method is that fuel flow does not depend on the repeatablity of fillups and calibration of external sensors, but it assumes the MAF and WB02 sensors are accurate. Even though torque is grossly estimated, I kind of like the fact this method does not care about the car's mass, rolling resistance or air drag, all of which can greatly vary in time.

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