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Pure plant oils as fuel
- 1 Adoption
- 2 Characteristics
- 3 PPO in compression ignition (Diesel) engines
Due to environmental and fuel security concerns along with the increasing price of diesel fuel the use of PPO has grown considerably in recent years. Supportive tax regimes and the low production costs led to the rapid growth of the rapeseed oil fuel market in some European countries, particularly Germany.
The properties of plant oils can vary considerably. An oils property will be influenced by
- The type of plant
- The verity of the plant
- The method of oil extraction
- Refining processes performed on the oil
- The storage and handling of the oil
A number of characteristics of vegetable oils have major effects on their use in engines.
- The surface tension of an oil influences its behaviour upon injection. Wikipedia surface tension page
- The viscosity of an oil effects the flow through the fuel system and how the oil sprays from the injector.
- The properties of an oil effect the reactivity with other materials - see material compatibility
In order to provide fuel with suitable properties for use in a diesel engine and limited variability work began in the 1990s in Germany to define a fuel standard for rapeseed oil fuel. This work has culminated in the establishment of a DIN standard for rapeseed oil fuel
|DIN 51 605 - German Rapeseed Oil Fuel Standard|
|Density (15°C)||DIN EN ISO 121185||
|Flash point||DIN EN ISO 2719||
|Kin.viscosity(40°C)||DIN EN ISO 3104||
|Calorific value, lower||DIN 51 900-2||
|Cetane number||IP 498||
|Carbon residue||DIN EN ISO 10370||
|Iodine value||DIN EN 14111||
|Sulfur content||DIN EN ISO 20884||
|Total contaminates||DIN EN 12662||
|Acid value||DIN EN 14104||
|Oxidation stability 110°C||DIN EN 14112||
|Phosphorus content||DIN EN 14107||
|Earth alkali (Ca+Mg)||E DIN EN 14538||
|Ash content||DIN EN ISO 6245||
|Water content||DIN EN ISO 12937||
Accompanying research for the standardisation of rapeseed oil as fuel for vegetable oil suited diesel engines in vehicles and combined heat and power plants - German document examining work towards a fuel standard.
Vegetable oils are commonly extracted from oil bearing plants by crushing or pressing oil rich nuts and seeds.
- Solvent extraction - on a large commercial scale oil seeds are crushed and then mixed with a solvent to strip as much oil from the solid matter. The solvent is then recovered leaving the oil and seed cake.
- Cold pressing - on a smaller scale oil is extracted by simply pressing the seed. Oil yields are not as high as with solvent extraction. The process is simpler and more suitable for decentralised production.
Used cooking oils
Often used cooking oils and fats are cleaned and utilised as diesel engine fuel.
PPO in compression ignition (Diesel) engines
It is often necessary to modify an engine to allow it to run reliably fuelled with plant oils. Such modifications are commonly classified in two categories -
Plant oils have a number of characteristics that effect their performance in fuel injection equipment. Their bulk modulus gives a different performance when under the compression of an injection pump. Viscosity and surface tension effect spray formation.
Increased injection pressure has been shown to improve the otherwise degraded performance of fuel injectors when spraying vegetable oil. This is not as significant in indirect injection engines where the fuel is further atomised in the pre chamber before being drawn into the engine cylinder.
Papers examining effects of injection pressure with vegetable oils-
Injector opening pressure
Where applicable increasing the injector opening pressure provides a higher injection pressure at injector opening and a higher peak injection pressure. Injection timing will become more retarded due to the extra time required for the pump to reach the higher opening pressure. The injection rate, duration and quantity of fuel injected is also likely to be effected.
Some more modern engines such as the Volkswagen group Tdi engine with VP 37 injection pump and the BMW M51 use two stage injectors with opening pressures set for each stage, pilot and main and a needle lift sensor that monitors the timing of start of injection so that the ECU can maintain the correct start of injection. Special equipment is required to set the main stage opening pressure and there are a limited number of injection specialists able to complete this work. Modifying only the pilot opening pressure will in effect advance the main injection event due to the ECU correction.
The condition of the fuel spray and the timing of the injection event make a considerable difference to the quality of combustion. Ensuring that the injection equipment is in good condition is of increased importance due to the spray degradation caused by fuel with a higher viscosity and surface tension. Heating the fuel will also act to improve the quality of the fuel spray by reducing viscosity and surface tension.
Plant oils and diesel fuel burn at different rates. A number of studies have examined the cylinder pressures and burn rates when fuelling with plant oils and how modifying the diesel engine fuel delivery timing effects the running of engines.
Rabé, E.L.M. (2006) : Jatropha oil in compression ignition engines - Study examines injection timing and cylinder pressure
Narayana Reddy, J.; Ramesh, A. (2005) : Parametric studies for improving the performance of a Jatropha oil-fuelled compression ignition engine - summary - Study examines Injection pressure, injector opening pressure and timing when fuelling with Jatropha oil
Nwafor, O. M. I. (2002): The effect of elevated fuel inlet temperature on performance of diesel engine running on neat vegetable oil at constant speed conditions - summary
Czerwinski1, Jan; Zimmerli1, Yan; Neubert1, Tobias; Heitzer, Armin; and Kasper, Markus : Influences of Alternative Fuels GTL, RME & ROR on Combustion and Emissions of a Modern HD-Diesel Engine - summary
Combustion after cold starting
Engines will not burn fuel as efficiently until the whole block is heat soaked which can take some time (20 mins+?). Operating strategies can be altered or devices added to improve combustion during this period:
- combustion chamber glow plugs that are kept energised after cold starting
- inlet air heaters
- cold running timing advance modules such as [KSB modules fitted to Bosch VE injection pumps]
- increasing the cold engine idle speed
- pre heating the engine block / coolant
- the use of an exhaust brake
Combustion during periods of low engine load
Due to their differing chemical composition plant oils behave differently to conventional diesel fuel in a diesel engine. The University of Rostock presentation - Rapeseed as an Agricultural Fuel shows results from tests of rapeseed oil and diesel fuel performed as part of a Tractor fleet project
- page 5 - Fuel standards for diesel, biodiesel and rapeseed oil.
- page 6 - Surface tension of diesel, rapeseed methyl ester and rapeseed oil at different temperatures.
- page 7 - Injection pressure and volume of diesel and rapeseed oil at 90° Celsius and different injector pump speeds.
- page 8 - Relative difference of fuel spray drop diameter at different distances from the injector nozzle.
- page 9 - Beginning of injection at different engine loads - with rapeseed oil the beginning of injection is 1 degree advanced.
- page 10 - The burn rate of diesel, rapeseed oil and adapted rapeseed oil at different degrees of crank rotation - the burn rate of rapeseed oil peaks 9 degrees of crankshaft rotation later than that of diesel.