April 29 2008

A few words about units.

About units used on these pages and units in general. For one, I don't (and no one should!) use Imperial English units or non-decimal fractions for engineering purposes and calculations. Fractions are OK for, say a stock market to judge a value of something but not do math with. Imperial units may be confusing and using them might lead to silly errors at least and embarrassing and expensive ones at most. But I don't criticize those who do use Imperial units, it's just what people grew up with and got use to. Of course, there is nothing inherently wrong with inches, pounds or gallons; they are just foreign to me. They were meant for household "convenience", not engineering and I'm an engineer. But it's not about me. When you do calculations in non-metric units, you will have to remember to factor in all sorts of weird numbers in order to accommodate these non-standard (as defined in any physics book) units. To let you know, a power is torque*rotation speed, period. Not torque*RPM/5252 as you might think. This 5252 comes up only because you are more comfortable and prefer to use horse power instead of Watts, choose foot-pounds instead of Newton-meters, and use to minutes instead of seconds, not to mention revolutions per minute rather than radian per second. If your car gets 25 miles per gallon, don't forget to mention if it is American gallon or British gallon; the same goes for "mile". ( Turn out there is statue mile, nautical mile, even metric mile, not to mention Scots mile or Irish mile). In contrast, if you say your car gets 7 liters per 100 km, it is universally the same liter and kilometer everywhere. But hey, fine by me, use anything you like, knowing that anyone outside USA or perhaps Britain, isn't going to understand you very well. You may not care, but that doesn't change the fact you're alone in this world. Ignoring accepted (for good reason!) engineering practice is not to your advantage. People outside USA who use metric system are not stupid - if it would be such a great idea to ignore ISO standards and adopt ANSI ones, America wouldn't be all alone on this, but the fact remains, so far it is.  I hope metric system will get implemented one day here.  Anyway, this page is not about debating old habits and trying to convince diehards. Again, I'd refer them to read this. And it's not my place to try to change your opinion on the matter (it probably won't change anyway). Do as you pleased. But it is really to your advantage to get use to very slick and logical system the rest of the world is using very successfully. To make long story short, you won't see non-standard units on these pages. So please get use to meters, kilograms and liters if you want to understand information here.

Notable exception to this is travel distance and pressure. Even though I use meters and kilometers in simulation, being in USA, the speedometer, navigation and few other subsystems sold here are calibrated in miles. I'm not about change overlay of the Audi speedometer. However mechanical parts dimensions on drawings are only expressed in millimeters. Another exception is tire pressure. TPM sensors are calibrated in PSI and so are tire ratings. So whenever it comes to purchase and use of such existing hardware, kgf/m2 (kg force per square meter which is Pascal) as a unit of pressure will be substituted for lb/in2 (Pound per Square Inch, or PSI). But again, not in calculations.

Now, please mind your electrical units, even universally accepted ones! Don't express vehicle energy consumption in W/km or battery energy density in Ah/kg. It's important to get this straight if you're about to do an EV conversion and intelligently share your results with others. "My car gets only 200 W/mile" is not a spelling error. It's just as much nonsense as saying "my car is capable of 25 hp/mile - no one would understand what you're talking about! And asking how powerful (how many kW) electric motor do you need if you want 50 miles range electric car is sort of like asking if a 100 hp engine has enough power to get you from San Jose to San Francisco. You get the point.

On these pages I express volts with symbol V or VDC (Volts of Direct Current). V assumes VDC. In contrast, AC voltage is always noted like that (e.g. "VAC", not just "V"). The "~" symbol is often used to designate AC (Alternating Current), for instance "~120 V" but I avoid such customary improvisations. In math "~" means "approximately". Well,  ~12 V can mean either 12 VAC or "approximately 12V" (AC or DC) - not clear which one, and that's the problem!. To avoid double meaning and confusion in such cases I always explicitly specify units. "~12 VDC" means "approximately 12V of Direct Current voltage source" (for instance battery).

Finally, when writing units in expressions please pay respect to great people after whom these units are named, not to mention that this is worldwide ISO standardized notation. If a physical or electrical unit is named after a person, the unit expression is *always* in capital letter; "N" for Newtons and "A" for Amperes, "F" for Farads. Multiplier may be small or capital depending on a value. Exception to this is if a unit is spelled out with a multiplier. For instance pressure of 5 kPa would be written as 5 kilopascal, not 5 kiloPascal. 30 mV, but 30 millivolts, not 30 milliVolts.

"Milliamperes" is mA, not ma. 5mV is 0.005 Volts and 5MV is 5,000,000 Volts. But never 5mv. 20uF is 20 microfarads. No 20uf please. It's just a bad taste at least, and sloppy ignorance of accepted notation in most cases. "Kilo" is *1000 multiplier, and saying "20 kilo" meaning 20 kilogramm is just as sloppy as "temp sensor" instead of temperature sensor. Next you know you'll be saying 'temp value" and no one will know for sure if you mean "temporary value" or "temperature value". Don't rely on context, just define everything and articulate normally, you'll be fine and well respected by others (not only by engineers).

Well, enough lecturing about units, and forgive me for being more sensitive to the issue than most people are. You get the point. Let's focus on the conversion project now. I'm about to start taking the car apart.

There is one thing you will definitely need for any conversion work - genuine factory repair manual and electrical wiring diagram. Without it you won't know what you're unplugging and how to simulate no longer existing sensors so your dash computer don't go nuts trying to display engine oil pressure or fuel consumption. Your goal is to fool it into thinking that the engine and transmission is still in place and well. I'll get to this as the work progresses. For this very reason don't cut out wiring harness or connectors. What you want is to fit the maintain the same type of sensor output as for the sensor you're removing. For instance if you unplug oil pressure sensor and you see in the manual that it changes resistance from 200 Ohm to 40 Ohm within normal working range, you'd need to connect at least a fixed resistor of, say, 100 Ohm to the same harness. You can retain the plug, or remove it connecting resistor directly to the wiring. In reality not all car subsystems are so stupid, some sensors are verified during self-check start up and their parameters must be different from when the vehicle is driven; else "engine check" indicator will be on and complaining text messages appear on the display. The key is to find out how the sensor works and what is its expected output. Once you know, simulating it is easy part.

Before starting disconnecting things and modify car electronics I need to know a maximum current consumption from 12V systems to size a DC-DC converter appropriately. You wouldn't want to turn on high beam of head lights and wiper while air blower is running and radio plays music just to find out that your 12V net browns out to 10.5VDC Here is photo of a DC clamp meter measuring current drawn from 12V auxiliary battery. You'd do it turning on and off various loads. For exact measurements you'd clamp the meter around alternator output cable rather than battery because current consumed from ~14.3VDC alternator puts out is higher than from 12VDC battery, but precise measurement is not important here. I can safely assume the current for most resistive loads (lights)is about 14.3/12=~1.2 times higher than measured from the battery. For the loads like radio it's the same. For those that have internal switching power supplies it's actually lower at higher input voltages.

bottom Using clamp DC amp meter.

What I'm finding is summarized in the table here.

Condition Current out of 12V battery Comment
Stand by ~100mA DC "long term" stand by
ECU "hot" ~1.6A DC After open and shut driver side door; reverts to long term stand by in ~20 sec.
Ignition on ~11A DC  
Brake lights on ~3A DC  
Side marker lights on ~3A DC  
Low beam on ~12A DC  
High beam on ~18 DC  
Radio/CD exchanger ~2.5A DC Just on (stand by), no output to speakers

These values should add up for estimations. For instance, with ignition on, low beam on and holding down brake pedal (so brake lights are on) total current out of 12V battery is about 26 ADC

For the radio, around 20W per channel is plenty loud for me. 40W at 12V supply 3.33A, with efficiency, say, 75% it's about 4.4A. Radio can peak at higher currents but large filter capacitors take the surge. I can allow for all audio hardware (radio and CD exchanger) maximum 10A total continuous current. IT is difficult to determine exact power consumption since various controllers in the vehicle turn on and off in random intervals doing their work, but again, this is not necessary - as long as I have healthy margin to keep all the hardware happy, that's all. It appears like in worst conditions total consumption will not exceed around 50A. That's the minimum output of a DC-DC I will aim for.

In a word conversion consists of 2 major stages: a) removing unneeded ICE related stuff and b) installing EV related hardware. Before it's a good idea to empty the gas tank by driving the vehicle - draining the tank may prove to be challenging and detaching it while full of gasoline makes no sense. Also, it's a good idea to keep track of the weight of everything you remove and install, so you can do accurate estimation of the performance and sometimes decide where things will go. So here is the sequence of removal I followed.

First I drained all unneeded liquids from our vehicle - engine oil, antifreeze and washer fluid I'll refill later. I'll leave draining remaining gasoline for later when tube from the gas pump in the tank to the fuel injectors become better accessible. Since fuel is pressurized, there is special procedure for this described in the Audi manual. Removing ICE components is dirtiest part of the job and kind of satisfying to free the car from nasty chemicals for the rest of its new life.  Removing gasoline without ruining the gas tank (drilling holes in it) may be challenging as there is no drain plugs and you can't reach the bottom of the tank through the filler.

Overview of the engine compartment before anything is done to it.
This car is no longer addicted to oil...

Removing The interior carpets and reinforcing hardware from the rear cargo area. Because of intended setup which will not take advantage of the gear box and the drive shaft, these components will be removed. Stock rear differential will also be removed since the gear ratio I need is not available for it. It will be replaced with non-Audi diff. To remove engine and transmission assembly, of course the drive shaft has to be unlinked and front half-shafts removed from the transaxle. Well, to remove the drive shaft, exhaust system has to go first, and to remove front half-shafts, front suspension bottom ball joints have to be disassembled, so that's what I did next.

This is what bottom of Audi A6 Quattro looks like.
Simultion Front part of the car.
Rear part of the car.
Rear cargo space cleared of carpets, reinforcement elements and spare tire.

I want to sell any part I remove to someone who may take advantage of it - besides obvious engine and transmission that would be mufflers, gas tank, exhaust insulation lining, etc. So whenever possible I'd avoid cutting drilling and other destructive methods of removing things. Here is removing exhaust - one of dirtiest operations you will perform. For 7 years old car exhaust pipes joints rusted together well enough so removing is difficult. However a shots of WD40 does its magic well, especially if you have patience to wait 10-15 minutes for it to penetrate the rust and loosen up parts. Rock and twist them while pulling apart.

Removing exhaust pipes clamp
Front and middle mufflers are disengaged
Before removing middle and rear muffler assembly, suspend rear one with soft wire
Now you can lower mid part of exhaust system without any troubles.
After muffler is gone, I removed liner protecting the body from hot exhaust components
Removed exhaust parts as well as engine cover are on the scales.
35.2kg off. Not bad.

Next part to be removed is the drive shaft. It has middle support bearing, so it is unbolted first. The rest of procedure was trivial. Again, few squirts of WD40 onto the bolts holding drive shaft flanges ease their disengagement.

Front flange of the drive shaft has catch guard. It is bolted t the transmission with Torx bolts
Middle bearing support removed.
Unbolting rear flange from rear differential pinion shaft.
Rear flange of the shaft disengaged.
Drive shaft removed.
Shaft on the scales - 11 kg off.