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Pure plant oils as fuel

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Adoption[edit]

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.

Characteristics[edit]

Oil Properties[edit]

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

Study detailing oil properties

Fuel Standards[edit]

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
Parameter Method

Specification

Unit

Density (15°C) DIN EN ISO 121185
900-930
kg/m3
Flash point DIN EN ISO 2719
min. 220
°C
Kin.viscosity(40°C) DIN EN ISO 3104
max. 36
mm2/s
Calorific value, lower DIN 51 900-2
min. 36000
kJ/kg
Cetane number IP 498
min. 39
-
Carbon residue DIN EN ISO 10370
max. 0.40
%(m/m)
Iodine value DIN EN 14111
95-125
g Iodine/100g
Sulfur content DIN EN ISO 20884
max. 10
mg/kg
Variable Properties      
Total contaminates DIN EN 12662
max. 24
mg/kg
Acid value DIN EN 14104
max. 2
mg KOH/g
Oxidation stability 110°C DIN EN 14112
min. 6
h
Phosphorus content DIN EN 14107
max. 12
mg/kg
Earth alkali (Ca+Mg) E DIN EN 14538
max. 20
mg/kg
Ash content DIN EN ISO 6245
max. 0.01
%(m/m)
Water content DIN EN ISO 12937
max 750
mg/kg

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.

Oil Production[edit]

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[edit]

Often used cooking oils and fats are cleaned and utilised as diesel engine fuel.

PPO in compression ignition (Diesel) engines[edit]

Engine modifications[edit]

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 -

Fuel Injection[edit]

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.

Injection pressure[edit]

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-

Injection Characteristics of an In-line Fuel Injection System Using the Alternative Fuels

Study on rapeseed oil as alternative fuel for a single-cylinder diesel engine

EFFECT OF FUEL TEMPERATURE AND AMBIENT PRESSURE ON A COMMON RAIL RAPESEED OIL SPRAY

Stationary Applications of Liquid Biofuels

Influences of Alternative Fuels GTL, RME & ROR on Combustion and Emissions of a Modern HD-Diesel Engine

Phase Doppler anemometry measurements of a dense rapeseed oil spray

The analysis of spray parameters of fuels of different viscosity sprayed by a typical and rotary-swinging needle injectors

Study on noise of rapeseed oil blends in a single-cylinder diesel engine

Factorial analysis of diesel engine performance using different types of biofuels

Study on cottonseed oil as a partial substitute for diesel oil in fuel for single-cylinder diesel engine

Injector opening pressure[edit]

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.

Combustion[edit]

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.

Study examining coconut oil

Study examining sunflower oil

Experimentation into using glow plugs as combustion chamber temperature sensors

Experiments and studies using rapeseed oil

Study examining effects of changes to injection timing with rapeseed oil

Study examining changes to injection timing with used cooking oil

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[edit]

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[edit]

Rostock presentation[edit]

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.

Studies[edit]

Detailed study examining a wide range of methods to utilise a range of vegetable oils in different engines and problems encountered