WVO vs Diesel

I have been thinking, or rather fascinating about how the diesel engine works.  If you are like me; you can't help but visualize the inner workings of mechanical devices.  While driving down the road, I can literally "see" the injectors pumping veggie oil into the compression chamber a little before the piston reaches the top of its stroke...I did a two day road trip a couple weeks ago and the mind wonders...

Anyway, it got me to thinking about some of the workings of diesel engines and the differences between diesel fuel and WVO.  It is a beautiful think how well vegetable oil works in the diesel engine and it still puts a smile on my face every time my truck switches over from diesel to veggie.  However, I never really looked into the properties of the fuel other than the energy content and viscosity.  It is common knowledge that the primary issue with WVO is the difference in viscosity and that pre-heating the oil to 160F is necessary to match the viscosity of diesel fuel.  This will create an appropriate spray pattern from the injectors and thus produce complete combustion avoiding the dreaded residue and piston ring coking.   I have talked to people and seen kits advertizing that pre-heating the WVO with electric line heaters will reduce switch over time.  I believe that this is totally wrong and even measuring fuel temperature is unnecessary and could be misleading.

It's all about the engine temperature.  Yes, I have wrote about my feeling that the engine is going to control the temperature of the oil, regardless of the temperature it is pre-heated to.  So, hot engine, hot oil, cold engine, cold oil.  But that isn't a complete picture of what is happening in the engine at startup. (This is where I get geeky)  We know that it is the "heat of compression" that produces the heat to ignite the fuel in a "compression ignition engine".  But how much heat is produced?  Well, I could break out the ideal gas law and go from there but I found an online calculator HERE to do it for me. 
Here are a couple scenarios:

My 7.3L has a 17.5:1 compression (50% Efficient)
@ 0C (32F) - 470C (878F)
@ 15C (59F) - 511C (951F)
@ 30C (86F) - 552C (1025F)
@90C (194) - 660C (1220F)

Now, lets take a look at the fuel:

Clearing up a couple terms:

• The flash point of a volatile liquid is the lowest temperature at which it can vaporize to form an ignitable mixture in air.
• The autoignition temperature or kindling point of a substance is the lowest temperature at which it will spontaneously ignite in a normal atmosphere without an external source of ignition, such as a flame or spark

OK, so now that we have some ideal numbers for what should be happening in your engine, I will make my thoughts and conclusions:

Additional Thoughts
- I know I would be hard pressed to start my truck below 0C without glow-plugs on diesel.
- I can start my truck on Veggie on a warm summer day though it is not easy or pretty.
- I can start my truck on Veggie easily if it is at running temperature
- Idling an engine for extended periods on diesel will produce coking form un-spent fuel.

Conclusions
1) Based on these numbers you need about 2X the autoignition temperature to properly ignite the fuel.   
2) Running Veggie on anything but a hot engine will probably leave un-burnt fuel around to cause problems
3) High End Veggie systems should have a load sensor or exhaust temp switch to ensure that veggie is only being used when there is sufficient engine temperature and load.