Fuel Temperature & Timing

Written by TsTDI

The next friendly adversary on the TDI engine is the fact that engine timing is mapped with fuel temperature. The reason why the TDI engine is so efficient is due to the intricacies of the fuel system. The one component of interest is the fuel temperature sensor. For explanation reasons, let’s assume we are talking strictly of diesel in the next two paragraphs.

Timing is adjusted with hotter and cooler fuels. Upon startup, which should be 100% diesel fuel, the fuel is cold and therefore quite dense. Density will change for liquids under two scenarios, temperature and pressure. The ECU senses the cold fuel, injects less fuel and advances the timing to compensate (you can actually hear this upon startup with slightly higher pitched ticking initially until it goes away). The reason why the ECU injects less fuel is there is a preset injected quantity amount the ECU abides by. To achieve the same mass of fuel to meet actual vs. requested injected quantity, the ECU must inject less. The reason for the advanced timing is because the fuel is denser (cold) and therefore has more mass. A denser fuel is less prone to combust. To combat this lower cetane, (not readily wanting to combust) advancing timing will help combust an inherently denser fuel by allowing more time for the fuel to atomize in the combustion chamber before it is compressed.

The exact opposite is the case for hotter fuels. When the fuel is hot (operating temperature) the fuel is less dense (hot) and therefore the ECU will compensate for a less dense fuel by injecting more. The ECU injects more fuel to match the actual vs. requested injected quantity. The reason for retarded timing is hotter diesel fuel has a higher cetane value. Higher cetane means the fuel is easily ready to combust. Therefore, the fuel does not require as long in the combustion chamber to combust fully; therefore, advancing timing is unnecessary strain. A less dense fuel is more prone to combust and therefore will combust and atomize much easier.

Understand that even though more or less fuel is injected depending on temperature, the mass of the fuel injected is the same in all circumstances.

What does that mean to the VO community?

It means we have a problem!!! Not only do we have a fuel (WVO) that is not readily to combust (low cetane, high dense fuel) we have a HOT fuel that is still WAY TOO DENSE!!!

http://www.frybrid.com/svo.htm

According to the graph that Frybrid developed that charts fuel viscosity vs. temperature, even at 160 deg, the WVO is anywhere from 10-15 centistokes more viscous than diesel at that of 0 deg C.

So what does the ECU do? The ECU senses the hotter fuel and retards timing because it thinks we are burning diesel fuel; however, we are burning WVO. So the ECU sees the hotter fuel (WVO) and retards timing even though the WVO is still denser then diesel at that temperature (185 deg). Remember the ECU’s only means of measuring mass is that of temperature of the fuel, it has no way of knowing that even at 160-180 deg, WVO is still thicker than diesel fuel because it has no way of knowing we are burning WVO. Logical sense tells us that engine timing can not be retarded with WVO because the thicker fuel needs more time in the combustion chamber to combust fully due to its lower cetane.

This originally was the one reason why I wouldn’t convert my vehicle because I thought there was no way around this. Until I found the fuel temp resistor modification that another TDIclub member introduced to me. This modification calls for the use of a resistor to be placed inline of the fuel temperature sender signal wire to fool the ECU’s reading of fuel temperature being sent from the sensor to the ECU. Fuel Resistor Modification

This modification causes your ECU to read fuel temperatures on WVO to read no higher than 20 deg C. On a cold engine, my fuel temps read 15 deg and on an engine that is at operating temperature, it read 20 deg C. These readings were taken during the summer time. This caused my fuel temp readings to drop from the true 85 deg C (185 deg F) to 20 deg C (68 deg F). Observations on WVO through VAG COM (see thread)

What have we accomplished??

We have fooled the ECU by eliminating the process of retarding timing for a higher temperature fuel (less dense but in the case of WVO untrue) and at the same time, we are injecting 185 degree WVO at a timing degree amount of diesel fuel at 68 degrees. We accomplished the best case scenario for burning vegetable oil in a TDI. Without this modification, your ECU views the fuel your burning (WVO) as a fuel with very little density (thin) and therefore to compensate, retards timing. WVO even heated is still more dense then diesel fuel and would require advanced timing to achieve adequate combustion efficiency.

Retarded timing causes less time for the WVO (an inherently thicker fuel of lower cetane value which is not prone to combust easily) to combust. Combustion efficiency would be sacrificed due to this and therefore, a complete burn would not be possible. Incomplete burn = tiny molecules of WVO that love to polymerize and coke on relative cool spots in the combustion chamber. You guessed it, a major problem. Relative cool spots in the combustion chamber include tips of the injector nozzle’s and glow plugs. This is why people say to pull your glow plugs and observe the tips for any WVO residue. If there is WVO residue present, chances are you got a problem due to incomplete combustion of WVO. If polymerization is occurring on injector nozzle tips, this is a major problem. Coking of nozzle orifices (the holes in the nozzle) results in poor atomization. Remember, WVO has a low cetane value and therefore, not readily to combust. Injector nozzle orifices that are compromised would result in poorer atomization and therefore the opportunity for even less combustion efficiency. The worst case scenario would be coking to the point of injector streaming. Basically the injector would not be doing its job of atomizing the fuel and it would be dribbling fuel on a constant basis. This would cause the fuel to not combust fully and eventually would accumulate enough into burning a hole in the top of a piston.

And what if you were running an EGR while this was happening? The poor atomization would cause more WVO molecules being re-circulated through the intake tract and therefore more intake clogging. This would lead to less air available for combustion but fueling would stay the same. This means even less combustion efficiency and eventually, blowing up your engine. Understand what I mean when I say, the WVO burning process can be a very slippery slope; however, we can prevent this from happening.

This whole scenario above is strictly unique to the VW rotary TDI. I know for absolute certain is applies to the 99.5-03 TDI, any early ones or the PD engine, I am unsure of. My guess it would be different for the PD because it’s an entirely new fuel system and that the earlier TDI would have a similar process of injecting fuel beings it uses similar nozzles.

The resistor caused a CEL on my car that went away when I deleted it with a scanner. Much like the EGR argument made above, the benefits of the fuel resistor modification exceed the drawbacks of a CEL. I have deleted the CEL and it has never come back so it is not a reoccurring code like the EGR code is.

How this was discovered?

I figured this out using probably the best indicators of combustion efficiency out there, an EGT gauge (exhaust gas temperature). For those that don’t know, an EGT gauge measures the temperature of exhaust gases. Basically this is a measurement of the temperature of the byproducts of combustion. Incomplete combustion results in higher EGTs because more molecules of fuel are not being used as energy for combustion. More complete combustion results in lower EGT’s because all molecules of the fuel are being used strictly as energy for combustion. This can be applied to the power enhancement section of any diesel. The goal of a diesel tuner is to achieve a "grey haze" when a diesel is at WOT (wide open throttle). This means that the ratio of fuel to air is near perfect and thus, the most amount of power your car will make without further air modifications. A "black cloud" means over fueling, high EGT’s and poor combustion efficiency.

So lets apply this theory to that of burning WVO. Prior to the fuel temp resistor mod, I noticed that my EGT’s were running 50-100 deg hotter in situations where I would expect them to be much lower. I also noticed my car to not have as much "pep" when I got into the throttle (designation of retarded timing and not as complete of combustion). After the fuel temp resistor mod, my power came back and my EGT’s are the same.

I cannot stress the importance of the fuel temperature resistor modification enough. Burning WVO comes down to two things, atomization and combustion. These are two easy words to say, but hard to achieve when there are tons of variables that affect it. Timing is a direct related factor of combustion. Bottom line, retarded timing = a bad thing. Low cetane fuels (WVO) are less prone to combust and therefore need advanced timing to compensate, we just learned this above. The fuel temp resistor modification prevents the unwanted affects of retarded timing and returns timing to what a car would operate at 68 degree diesel fuel temps. Timing changes on a continual basis; however, my highway log of timing (requested vs. actual) never registered anything below 0 deg (ATDC after top dead center). The typical range was anywhere from .5 deg – 2.5 deg (BTDC before top dead center) through use of cruise control. This is what we want to achieve.

FALLACY #2

It is worthy to note that the quieter engine while running on WVO is NOT due to vegetable oil being a more lubricating fuel. It is due to retarded timing or combustion not being as volatile. In the case of the TDI engine, both of these factors go hand-in-hand. Advanced timing makes combustion noises louder, retarded timing make them quieter. The noises you hear from a diesel engine are combustion noises. Quieter engine on WVO is not a good thing.