The TDI-SVO controversy

We've said there are doubts about running DI (Direct Injection) diesels on straight vegetable oil using simple two-tank SVO kits that pre-heat the oil and start up and shut down on diesel fuel (or biodiesel). Others say so too.

Yet others say it's not an issue, it works just fine.

Which is right?


"There is no issue with DIs and SVO. I'm convinced that DI engines on a heated 2-tank system are reliable. At 170 deg F, different types of oil don't display large differences in operation. Unlike cold oil."
-- Steve Spence, who sells Greasel two-tank kits in the US

Greasel in the US says of their two-tank kit: "Ready-to-install kit that will allow you to run any diesel on waste vegetable oil. The key to running a diesel on vegetable oil is heat. This is done by a special tank and fuel line, heated with the hot coolant your engine is already producing." Greasel denies there is any problem running SVO in "Direct injection, Indirect injection, Common rail, VE, Rotary inline, unit injectors, Computer controlled" diesels with their kit.

"For dual tank vegetable oil fuelled operation, with heated tank & if COLD, fuel line, I would recommend ANY diesel engine providing that the oil is hot before it hits the injector pump and is purged before shutdown. The problems with DI diesel engines seems to be mostly associated with carbon build-up on injectors and that can be reduced significantly if not eliminated by use of heated veg-oil."
-- Tony Clark, President of the Western Australian Renewable Fuels Association, Inc.


Today's DI diesels are efficient, clean-burning engines. The fuel injection technology is highly sophisticated:

"... Siemens, Delphi and Bosch all are ramping production of sophisticated new injectors that can handle today's sky-high fuel pressure, and inject infinitesimal fuel droplets so quickly that upcoming diesels may at times employ as many as five distinct injections for each cylinder's combustion 'event.'

"... the most promising advantage of common-rail technology is the ability to deliver extremely high fuel pressures on the order of 23,000 psi (1,600 bar) or more. This type of pressure means that in microseconds, astonishingly tiny fuel droplets can be injected; these droplets more effectively mix with the induction air, boosting power production and reducing emissions.

"Only Volkswagen AG, Europe's diesel volume-sales leader, has resisted the common-rail development path, instead relying on its unique 'unit injector' system that combines each injector with an integral fuel pump, energized by its own lobe on the camshaft. This system develops pressures of up to 29,300 psi (2,020 bar)..."
-- From "
Super Diesels!", Ward's Auto World, September 1, 2001

"The latest multi-jet direct injection system with fuel supply by 'common rail' at a pressure of 1600 bar is another noteworthy feature. Fuel supply to the five-hole nozzles is controlled via an ultramodern solenoid valve. The fuel injection system's response time is less than 20 millionths of a second, ensuring extremely fine metering of the fuel and up to five injections per working cycle." --
The Opel Eco-Speedster

Five distinct injections for each cylinder's combustion "event"... fuel pressures on the order of 23,000 psi... astonishingly tiny fuel droplets... five-hole nozzles... millionths of a second...

With such technology, fuel viscosity and combustion characteristics would have to be critical. "At 170 deg F (76.6 deg C), different types of oil don't display large differences in operation." True?

This is what one study concludes:

The properties of canola oil and diesel are very similar, except a significant difference in viscosity, with canola oil having 12 times the viscosity of diesel. Even after heating to around 80 deg C it is still six times as viscous as diesel. This leads to problems with flow of oils from the fuel tank to the engine, blockages in filters and subsequent engine power losses. Even if preheating is used to lower the viscosity, difficulties may still be encountered with starting due to the temperatures required for oils to give off ignitable vapours. Further, engines can suffer coking and gumming which leads to sticking of piston rings due to multi-bonded compounds undergoing pyrolyses. Polyunsaturated fatty acids also undergo oxidation in storage causing gum formation and at high temperatures where complex oxidative and thermal polymerisation can occur.

To date there have been many problems found with using vegetable oils directly in diesel engines (especially in direct injection engines).

1. Coking and trumpet formation on the injectors to such an extent that fuel atomisation does not occur properly or is even prevented as a result of plugged orifices,
2. Carbon deposits,
3. Oil ring sticking,
4. Thickening and gelling of the lubricating oil as a result of contamination by vegetable oils, and
5. Lubricating problems.

Other disadvantages to the use of vegetable oils and especially animal fats, are the high viscosity (about 11 to 17 times higher than diesel fuel), lower volatilities content which causes the formation of deposits in engines due to incomplete combustion and incorrect vaporisation characteristics... At high temperatures there can be some problems with polymerisation of unsaturated fatty acids, this is where cross-linking starts to occur between other molecules, causing very large agglomerations to be formed and consequently gumming occurs.

Although some diesel engines can run pure vegetable oils, turbocharged direct injection engines such as trucks are prone to many problems.

-- From "
Research into Biodiesel Kinetics and Catalyst Development", by Adam Karl Khan, Department of Chemical Engineering, University of Queensland, 17 May 2002 -- Acrobat file, 432Kb:

With the current injector technology, what would "large differences in operation" mean, exactly?

A well-known French study found large differences, using a DI engine:

"Fuel structure and characteristics have been shown to have great influence on engine performance and emission behaviour. One of the most important parameters is the spectrum of fatty acids. Length of carbon chains and number of double bonds in the fuel molecules affect low temperature suitability, spray formation and carbon residue. Net calorific value and density also affect the energy content of cylinder charge.

"It is possible to preheat the oil up to 150 deg C where it attains the same viscosity as the diesel oil. Atomisation tests showed that at 150 deg C the performance of the rapeseed oil are comparable with that of the diesel oil."

That's double the temperature the two-tank systems use.

Advanced Combustion Research for Energy from Vegetable Oils (ACREVO)":

In 1979 the German company Elsbett developed an advanced all-new DI car diesel engine which had true multi-fuel capabilities -- it could run on vegetable oil as well as petrodiesel. This engine was the forefather of the DI car diesels in production today. It was fitted with PDI unit injectors, now being used in the current VW PDI Pumpe Düse models.

Elsbett Technologie provides single-tank SVO systems for diesels including DI diesels, with special injectors, special glow plugs and fuel heaters, with full warranty. They run on SVO, petro-diesel, biodiesel or any blend of the three, without dual tanks or switching fuels for start-up and shut-down -- just start up and go.

Another German company,
VWP, Vereinigte Werkstätten für Pflanzenöltechnologie, supplies professional single-tank SVO systems, for DI and PDI diesels, also with special injectors, special glow plugs and fuel heating.

A third German company,
WOLF Pflanzenöltechnik (WOLF Vegetable oil technology), also supplies professional single-tank SVO systems, using a similar technological approach, with systems available for both Common-Rail and PDI diesels. The company expressly dissociates itself from using two-tank systems with these engines. See: WOLF Pflanzenöltechnik -- Technische Neuheiten (technical innovation)

Niels Ansø of the
Folkecenter in Denmark reports on using the Elsbett and VWP kits with DI diesels:

Please let me contribute to the SVO PDI/TDI discussion.

Using SVO in TDIs and PDIs it not an issue when using proper conversion technology and proper SVO fuel quality, meeting the limits specified e.g. in the German RK standard. Proper conversion includes injectors, glow plugs, timing and other fuel settings.

See some single-tank SVO cars here. We have made 65 so far.

We have converted several TDIs and one PDI with SVO single-tank systems plus heater (boiler) for winter starts. It works absolutely fine. The PDI is a Lupo 3L 1.2, and has been running on Faroe Island for more than a year now. They have about 5-10 deg C ambient temperature around the year. Some of the TDIs have passed two years and about 100,000 km. We have imported one TDI from Germany with more than 330,000km on SVO with a single-tank system.

Two weeks ago I tried the new VW Touran 2.0 PDI (4 valves/cyl) with a single-tank SVO system. It was very convincing, both the start and driving. The German company who converted it (VWP) claims that they make the type emission approval for all their conversions, which for this car is EURO4.

SVO professionals claim that the high injection pressure with PDIs is not an issue. If you study the German '100 tractor programme' (by VWP) you will see that some of the most successful conversions use PDI technology.

The 'original' 3-cyl 1.5 liter Elsbett Multi-Fuel engine had a PDI system 30 years ago, so it is not new.

-- Niels Ansø, Biofuel mailing list, 27 Sep 2004

VWP provides single-tank SVO conversions for the German government-supported "100 tractors" program, working with Deutz Agrotron tractors with advanced 6-cylinder diesel engines with electronic engine control and PDI Pumpe Düse high-pressure unit injectors working at up to 1,400 bar.

Deutz says all the Agrotron tractor engines are "suitable for the use of RME", rapeseed methyl esters biodiesel, which should be good news for US TDI owners concerned that the new-model VW PDI diesels are not suitable even for biodiesel, let alone SVO. TDI, CRD-CDI and PDI diesels are all fine with biodiesel -- just as long as it's well-made biodiesel, well-completed and well-washed, with no free methanol or glycerides and it hasn't been oxidised by bubble-washing, which is not always the case with either homebrewed or commercially produced biodiesel. See fuel quality.

But can DI diesels run safely on SVO using two-tank heated systems? Elsbett also sells two-tank systems, but not for DI diesels. "Two-tank systems definitely do not work for DI diesels," Klaus Elsbett told us. "The main engine breakdown reason is related to the lube oil, especially on turbocharged engines."

So far this has mainly been about using new oil, but Greasel and others say "any diesel on waste vegetable oil", which is a different matter.

This is what Ed Beggs of two-tank SVO system vendor Neoteric Biofuels in Canada said about it:

Re TDI: I get a lot of people, especially in the USA, determined to run brand new TDIs and PowerStrokes on WVO!

I tell them not the best idea. They insist. Fine, free country, I guess, but at least go two-tank, and get the best WVO you can find, and then also thin it out 10% or so with a suitable solvent blending agent. That's the best advice I can give to somebody who insists on doing this.

So, would I myself run one of our kits on a coveted TDI of my own, one of these days, when I can afford one?

Yes I likely will, but it will be when the engine is out of warranty, the car has 100,000 km on it already and is depreciated, and it will be very good used Canola oil only, two tank, heated, and thinned, and I'll use the 10-micron filter. That much of a chance I'll take, but personally, not more than that.

In conclusion, there IS an "issue" with using two-tank SVO systems with DI diesels, and especially with WVO. We don't recommend it. Currently the German professional single-tank SVO systems are the only SVO systems suitable for "any diesel".

Biofuels at Journey to Forever
En español -- Biocombustibles, biodiesel
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Make your own biodiesel
Mike Pelly's recipe
Two-stage biodiesel process
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Nitrogen Oxide emissions
Biodiesel resources on the Web
Do diesels have a future?
Vegetable oil yields and characteristics
Biodiesel and your vehicle
Food or fuel?
Straight vegetable oil as diesel fuel

Ethanol resources on the Web
Is ethanol energy-efficient?

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