Polymerization & ERG

EGR- Exhaust Gas Recirculation- EGR has been a popular emissions device on gasoline engines for a long time but arguably has been a disaster on the diesel engine. Problems caused with EGR equipped on diesels are intake clogging with soot and clogged turbochargers. This results in poor performance and poor fuel economy. The reason why intake clogging occurs is because diesel engines were manufactured from the factory with EGR’s before the introduction of ULSD (ultra low sulfur diesel). Sulfur laden diesel fuel that for the longest time was considered to be a lubricant for a diesels fuel system, causes particulate matter emissions. The easiest way to eliminate this was to take the sulfur out of diesel. Even with the reduction of sulfur in diesel fuel, it still seems that intake clogging continues to be a problem with diesel engines due to particulate matter accumulating over time. Even with ULSD, diesels equipped with EGR’s tend to still have the above listed problems.

So why does this apply to us?

Running a fully functional EGR system on a car that runs on WVO is asking for clogged intakes. The problem with WVO is polymerization. Polymerization of WVO basically turns into a hard carbon. If you are familiar with deep fryers or own one currently, the hard carbon deposits on the rim and the outside of a fryer is the result of polymerization. Polymerization is a result of hot oil coming into contact with cool surface temperatures.

In-depth look into Polymerization

The easiest way to explain this is the hot pan analogy. If you were to take a pan with a small amount of oil in it and set the stove to high you will be able to observe this process. The oil heats up with the pan but there are cold spots present around the lip of the frying pan. Once the pan reaches 300-350 degrees Fahrenheit, the oil itself will start to smoke (designation of burning). Applying more heat and allowing this process to continue, the oil will eventually burn to the pan in the form of carbon or splatter to relative cold spots and polymerize. This is easy to see at any fast food restaurant. In fact, I have cleaned my fair share of clumpy, moldy, oil laden fryer baskets and hard carbonized fryer filters. It is a result of hot oil coming into contact with relative cold spots, or oil that is continually heated to the object in which it eventually will burn to. Have you ever observed the underneath of a fryer station where they filter the oil on a daily basis? Have you observed the waste oil barrel at fast food restaurants? The moldy clumps and hard carbon deposits are a result of polymerization. This is what occurs in your intake manifold with a fully functional EGR while burning WVO.

Now, take that same hot pan, do not introduce oil to the pan, and heat the pan surface to 600-650 deg. (who knows if you can actually do this, I don’t expect this experiment to be done its just for explanation purposes). There are two points of interest here, the flash point and fire point. "Oil reaches its flash point at about 600°F when tiny wisps of fire begin to leap from its surface. If the oil is heated to its fire point 700° F, for most oils, its surface will start vaporizing and spontaneously ignite, surging up and out almost instantly."

http://missvickie.com/howto/spices/oils.html

Reaching these temperatures results in no opportunity for polymerization because the oil reaches its fire point, or for our purposes, the combustion point.

The Basic Vegetable Oil Burning Theory

Reaching this temperature results in no polymerization, just ignition of the oil (fuel) itself. This is the basic theory on why burning vegetable oils is possible. Although these fuel temperatures can’t be achieved through the use of a WVO kit, it can be achieved in the combustion event thanks to diesel compression ratio, boost pressures and cylinder pressures. All of these factors combine to raise the temperature beyond the ignition temperature of vegetable oil.

The only scientific data with thorough analysis of vegetable oil burning properties is the ACREVO study (advanced combustion research for energy from vegetable oils). In this study, pure rapeseed oil (virgin VO) was used, unlike WVO. The objective of the study was to observe the burning characteristics of vegetable oil droplets under high temperature and pressures. The complete study can be found here:

http://www.biomatnet.org/secure/Fair/F484.htm

According to this study, "Atomization tests showed that at 150°C the performance of the rapeseed oil are comparable with that of the diesel oil."

150°C equates to 302°F which is impossible to achieve through heat derived from a WVO kit alone. However, there are two parts to this equation and further reading results in the following information.

"The overall combustion performance of the rapeseed oil is very satisfactory in comparison with the diesel fuel while the rapeseed oil produces almost 40 % less soot than diesel fuel. The different volatility of this fuel respect to the diesel fuel is responsible of the different behavior of the sampled gas concentrations in the base of the flames while at the end of the flames, both attain almost the same values. It has been established that an addition of 9 % of ethyl alcohol (95 %) bring a great benefit regarding the pre-heating oil temperature. In fact, the presence of alcohol allows a reduction in the inlet oil temperature from 150 °C to 80 °C. Moreover, the combustion of the emulsion produces less soot and, at the exhaust, the amount is almost one half less than that produced by the combustion of rapeseed oil."

Amazingly a small amount of ethyl alcohol (ethanol) of 9% by volume of vegetable oil resulted in a required pre- inlet combustion temperature from 150°C to 80°C or 176°F. 176°F represents the typical fuel temperature achieved through heat of the WVO kit. Using a Greasecar kit, a 9% volume amount would equate to 1.17 gallons of ethanol to 11.83 gallons of WVO for a total of 13 gallons.

I DO NOT recommend adding ethanol to your VO mix but understand the chemistry of what is occurring. Ethanol is measured by octane, not cetane. Octane and diesel engines DO NOT MIX and should never be attempted. If you have ever mixed gasoline in your diesel tank, you would know this… Quite frankly, I doubt anyone who "splash mixes" fuels is doing much more than hoping for the best. However, understand the chemistry that caused the tremendous drop in pre-inlet temperature of the fuel before combustion. An inherently thin fuel that is much more volatile than vegetable was to be mixed with a thicker fuel oil. When introduced to the combustion chamber, the ethanol in small particle amounts is the first to ignite which becomes a catalyst for the entire mixture of vegetable oil to ignite more thoroughly. This causes the pre-inlet temperature to be much lower than if we were running 100% rapeseed oil.

Two things of interest here:

Pure virgin vegetable oil is thicker than waste vegetable oil and therefore will require more heat to achieve the adequate viscosity for combustion than waste vegetable oil.

In a two tank setup, small quantities of diesel fuel are sent back to the WVO tank in the purge cycle which is achieving the above scenario (thinner fuel mixed with a thicker fuel).

Diesel fuel is not as volatile as ethanol is; however, you’re achieving the same principle behind introducing other fuels to thin the WVO mixture out. Therefore, there is a less need for extreme amounts of heat to achieve the same viscosity of diesel fuel.

I do not blend fuels with my two tank setup, although I have greatly considered it. My recommendation if you feel you’re not getting your oil up to temperature (even with the FPHE and heated injection lines) would be to splash mix a gallon of kerosene or diesel in your WVO tank. Kerosene is known for its extreme thinning properties and would be an effective way of lowering the viscosity requirements of WVO for combustion.

My experience has shown this is not required as I have always had plenty of power and similar fuel economy compared to diesel fuel. However, the science behind it proves me wrong. Let it be known, there have been diligent people who have operated their WVO kit in the correct fashion and have had plenty of long term success.

Your worst enemy, EGR

So the most important part to take away from this lesson is hot oil that has not reached its flash/fire point that comes into contact with relatively cool surfaces will start to form carbon (polymerization). EGR is your worst enemy!!! Exhaust gas recirculation involves combustion gases that did not combust (un-combusted tiny WVO particle matter) to be re-circulated through the intake tract (relatively cool surface of the engine) and then to be combusted again. So in the process of this emissions device, the tiny molecules of WVO are being subject to the cooler surface temperatures of the intake manifold and intake valves. Intake temperatures on an engine that has reached operating temperature range anywhere from 120 to 200 deg. None of these temperatures are close to the flash point of vegetable oil and thus, creates the perfect environment for coking, polymerization, clogged up intake manifolds and intake valves. Go back to the example, hot oil on cool surfaces… recipe for disaster.

An interesting theory to note, older IDI engines are more prone to success. None of these are equipped with EGR systems.

EGR is the culprit to many WVO engine failures. It becomes a slippery slope. The more clogged your intake becomes, the less power you have available and the less fresh air you have coming into the combustion chamber. Air flow and boost pressures operate primarily on MAF input and load only. Intake restrictions cannot be compensated for with less fueling. It will continue to burn the same amount of fuel in which the fuel map specifies for (requested/actual injected quantity). So look at the scenario we have. We have a clogging intake which is more susceptible to more clogging due to the process already being started and we have continued decreases in combustion efficiency. The engine is receiving less and less air (being choked) and is still injecting the same amount of fuel. This will eventually cause decreases in performance, fuel economy and excessive smoking. All of which will exacerbate the intake clogging problem.

Primary problem #1 Coking of intake manifolds, and intake valves with polymerized vegetable oil.

Primary Solve- Reduce the EGR duty cycle through use of VAG COM or eliminate it all together (my recommendation). EGR causes way too many problems on a diesel engine that burns vegetable oil. Elimination of the EGR will cause a check engine light; however, I think it is very evident to see that the benefits of eliminating the EGR exceed the drawbacks of a yellow light. A CEL can also be eliminated through tuning.

Primary considerations- If you delete your EGR, your check engine light will be on all the time and will not go away unless you get your car chipped. Also if your state requires emission testing for diesels, there may be a problem in eliminating the system all together. Your best bet would be to reduce the EGR duty cycle to its minimum and then change it back to stock before testing. I do not have to worry about this as Maryland does not require emission testing for diesels. If you reduce the duty cycle and do not eliminate it all together, you probably should check your intake every so often, maybe every year to year and a half. This is why eliminating it all together makes everything much simpler and easier.

Here is an example of EGR at its best while running WVO. This is what will happen if combustion efficiency is poor and EGR is fully functional.

http://forums.tdiclub.com/showpost.p...27&;postcount=1

Scrolling down to picture 4, 5, and 6, you see the signs of a car with poor combustion efficiency while running on WVO and having EGR fully functional. Observe the buildup behind the intake valves (valve stem). Remember the thread about polymerization and relatively cool surfaces?

Observe the polymerization on the bottom of the exhaust valve (the flat surface, not the stem). The exhaust valve is the one without the polymerization buildup on the stem of the valve. Un-combusted WVO gases were coming into contact with relative cool surface temperatures in the combustion chamber. Also note, any type of carbon deposit in the form of polymerization on an exhaust valve is proof that combustion efficiency was quite poor to say the least. If that isn’t enough proof, the intake valve stems show exactly what happens when polymerization occurs in a diesel engine due to burning WVO with EGR.

So if you want to run your EGR while burning WVO, you better be absolutely certain that combustion efficiency is at its peak in all instances. If you’re cars air to fuel ratio while burning WVO is leaning towards rich, this is what very well can occur. I did myself the favor and avoided this in its entirety.