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The road to pure plant oil in diesel engines?

Advantages and disadvantages of running on PPO

Practical experience of running on PPO produces mixed results. On the one hand users are generally enthusiastic because engines make less noise (due to the better lubrication of PPO), via the glycerol present in the fuel. This better lubrication has a positive effect on the lifespan of the engine. Users are also pleased with the lower noise level and optimum gear changing.
For PPO this is around 1,000 revs, while for conventional diesel fuel this is around 1,200 revs. This is due to the oxygen levels in the fuel, but there is no further clarity on this [Aberson, 2004; Togashi, 1998].

On the other hand, using PPO in cars has a number of disadvantages, i.e. the limited availability and the high conversion costs. The smell is another disadvantage for some people, although with modern vehicles this plays very little role due to the better engine combustion. Various studies have also shown that pyrolised particles and carbon flakes can build up in the combustion chamber and on the valves and injectors, which can damage the engine. Recent practical tests in Germany have confirmed this [BLT Wieselburg, 2003]. When the PPO is sufficiently refined this type of problem will no longer occur [Togashi, 2003]. For users this is an important point. Problems with the stability of the lubrication oil are also mentioned in the literature, when the PPO lubrication oil is contaminated [Jensen, 2003] and [BLT Wieselburg, 2003]. This is caused by PPO particles that (due to incomplete combustion) remain in the engine oil on the pistol wall and springs. Fossil-based diesel evaporates at normal engine operating temperature from the engine oil when this occurs, but PPO’s higher boiling point means that it remains in the oil [Thuneke, 2004].

Experiences

Several dozen vehicles have now been converted in the Netherlands. After an initial test period, McDonalds has now converted all its 23 trucks to run on PPO. Germany currently has 10,000-20,000 vehicles running on PPO. Germany and Austria have run extensive tests on 100 and 35 agricultural tractors respectively. There are also examples of ships and trains running on PPO.
There are also signs from Germany that the car industry is not interested in PPO as a fuel alongside biodiesel. This is due to the insufficient quality of the fuel. Car manufacturers also give no guarantee with respect to their vehicles running on this fuel [Bockey, 2004]. This guarantee – or reimbursement of the cost of technical problems that are not caused by the user – is therefore given by the conversion companies.

Energy

The energy usage (in energy terms) is similar to that of standard diesel vehicles. Usage in litres is slightly higher (up to 10%) than a standard diesel engine, due to the lower energy content [Ricardo, 2003], [IEA, 1996].

Vehicle emissions

With respect to the regulated exhaust emissions, literature currently available gives no clear picture. Some sources quote increased emissions, while others show lower emission levels.
The information is often not detailed enough to compare these sources and draw conclusions. Naturally the engine technology and fuel quality play an important role and emissions can only be compared in identical (the same) vehicles before and after conversion.
Studies on this are very limited. Therefore, in order to provide some indication of PPO emissions, the following brief results are given for a number of tests.
Converted vehicles In Switzerland the EMPA [Folkecenter, 2000b] carried out a series of measurements in 2000 on a VW Golf (Euro-1 test) and a VW Lupo (Euro-3 test).
The tests on the Euro-1 diesel car with pre-chamber mixing appear to show that PPO can be used without increased emissions in indirect-injected (IDI) diesel engines. However, for modern direct-injected (DI) engines this is slightly more complex. These engines are so optimised to produce minimum emissions that changing a number of parameters and exchanging some components can negatively influence emissions performance.
The results do not clearly show the difference between a converted modern vehicle running on PPO and a standard modern diesel vehicle, because the roller resistance was not the same for the ‘ standard tests’ and the tests after conversion. However, a number of general comments can be made:

• Emissions from modern PPO vehicles are probably not lower than those of standard vehicles. However, the number of vehicles tested is insufficient to allow conclusions to be drawn ;
• Performance partly depends on the way in which a vehicle is converted. The cheapest solution is often not the one producing the lowest emissions;
• A fairly small vehicle was used in the aforementioned test, which generally has lower emissions and thus no problem meeting emission criteria. This means that a VW Lupo can also run on PPO and can still meet these criteria (at least for a number of components), while this is not necessarily the case with a larger car. The sulphur content of 430 ppm is no longer the standard (which is 50 ppm). This produces a higher PM10 emission for the standard vehicle than shown in practice;
• The high NOX emissions can only partially be explained by the higher engine temperatures at increased roller resistance.
  Emissions for unconverted vehicles

In 2003 Ricardo Consulting [Ricardo, 2003] carried out tests on a VW Passat and a Peugeot 106. These vehicles were not converted, but tested with low-sulphur diesel and PPO. The testers simply added a heat exchanger and did not change any of the internal engine settings.

With respect to the aforementioned test, the following comments are relevant:

• Both vehicles show greatly increased CO and HC emissions. This is primarily caused through reduced atomising of the fuel in the cylinder, which creates a less than optimal combustion. This leads to partially burnt particles and unburned residues being found in the exhaust;
• The increased PM emissions for the VW Passat can be explained by the increased volatile organic particles in the PM, because no additional amounts of elementary carbons were measured;
• The Peugeot 106 generally performed better than the VW Passat. This is due to the high exhaust gas temperature, which ensures that the fuel is better atomised and combusted;
• Measured over one practical cycle, it is possible that there is less difference between these two vehicles, because the test cycle is not representative of actual use of the vehicle. In particular, acceleration is very low, whereby the engine temperature remains low;
• It is not possible to confirm with certainty the effects between the various types of injection. A DI vehicle has higher injection pressures, which leads to better atomisation, while an IDI vehicle has more possibilities for evaporation and spreading in the cylinder [Ricardo, 2003].

Other experiences

The literature is also not clear regarding emissions from PPO vehicles. One test, by MAN B&W on a ship’s engine showed that the NOX emissions were higher, while a Finnish study with mustard oil showed increased emissions of ultra-fine particles (<100 nm) that are particularly damaging to our health [Jensen, 2003]. Experience with the PPO converted vehicles from SITA/McDonalds also show a significant reduction in fine particle emissions due to PPO use.

An Austrian report concerning a German test with agricultural tractors – the ‘100 Tractors project’ shows increased NOx emissions [BLT Wieselburg, 2003] and [Hassel, 2004]. The CO emissions generally remained lower than for diesel cars [Hassel]. Optimising the engine management systems ensures that the increased NOx emissions remain limited and around the same levels as those for diesel.
Based on the oxygen level23 of the fuel and the lower cetane number, an increased NOx emission and lower PM emissions can be expected, provided that the fuel is well atomised in the cylinder [EPA, 2003] and [Ricardo, 2003].

Influence of the fuel quality

The fuel quality has a considerable influence on emissions. If the fuel is not refined then the chance of high emissions is greater. An excessive slime level in the fuel causes contamination of the injectors and combustion chamber. This disturbs the correct atomisation of fuel in the cylinder, whereby particles emission increases. Free fatty acids also damage the engine as they are corrosive. High concentrations of solid particles (ash) in the fuel cause particle emissions and blockage of any filters present.

Cautious initial conclusions

Based on the limited information available it is not possible to draw definite conclusions concerning PPO influence on vehicle emissions. However, the information available provides the following picture:

• If the vehicle is converted correctly and good-quality PPO is used, emissions of CO, hydrocarbons PM10 will probably be lower than when the same vehicle runs on diesel;
• The NOx emission will probably be higher.

The extent to which changes to the engine management system will effect the emissions is difficult to determine.

Emissions of Euro-4/5 vehicles: a preview

In future, vehicles will have to meet stricter environmental norms, as per EU legislation from Brussels. The implementation of Euro-4 and Euro-5 norms (in 2005 and 2008 respectively) will also ensure that new technologies, such as selective catalytic reduction (SCR) and diesel particle filters (DPF) will be introduced.
At the moment, phosphor (that is present in PPO as it is added during the growth of the plant) is seen as a possible problem. Phosphor contaminates the catalyst in the SCR system, whereby its activity and the effectiveness of the emission reduction is lessened (DAF). PPO is almost sulphur-free, which is an advantage when using an SCR catalyst. Sulphur also contaminates the catalyst, just like phosphor. However, small-scale oil presses (cold pressing) produce an oil with a very low phosphor content. See also [Folkecenter, 2000a], [Widmann, 1998] . During large-scale production, the phosphor can be removed during the refining process. This is therefore not expected to cause insurmountable problems.
Calcium too is a problem, as it does not combust in the engine and leaves the engine as ash particles. The ash particles collect in the filter, which reduces its effectiveness. For this reason calcium has recently been added to the quality standard for the DIN norm that is currently being developed [Thuneke, 2004]. It is assumed that calcium can be removed during refining.
It will cost the car and engine manufacturers more time and money to meet the ever-stricter emission criteria. It is possible that large amounts of money will need to be invested in order to meet the environmental criteria in the future, which is why not all manufacturers are enthusiastic about PPO [Bockey, 2004].
In October 2004 a European organisation was set up for PPO, in which various European countries will work together. According to Aberson (Solaroilsystems), researching the emissions behaviour of cars and lorries of various Euro-categories and conversion systems (single-tank or two-tank systems) will be one of the first activities of this new organisation.

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