Straight Vegetable Oil Fuel Systems

by John

Straight Vegetable Oil as a Fuel for Diesel Engines
Feedstock Selection and Iodine Value
Used Cooking Oil as Fuel
Two-tank SVO Systems
SVO links and online resources

Straight Vegetable Oil as a Fuel for Diesel Engines.
It is possible to use unmodified vegetable oils as a neat fuel for diesel engines within certain limitations. Straight Vegetable Oil (SVO) has been shown to be an effective fuel for many applications and has been used in different parts of the world for many years. However, practical results vary, depending on the type and condition of the vegetable oil, engine, and fuel system. For this reason, SVO should still be considered as an experimental fuel, and may be found unsuitable for some applications. Better results are generally found with indirect injection engines, and with inline injection pumps. However, this is not to exclude direct injection engines or rotary injection pumps, particularly if the engine and injection system are in good working condition.

Diesel engines do not use carburetors and rely on spontaneous combustion for fuel ignition, so the fuel must be atomized upon injection into a very fine mist to ensure a complete burn. Although the fuel injection system is designed for petroleum diesel, similar results can be found if the viscosity of the oil is lowered to nearly that of diesel fuel. This is typically accomplished by heating the SVO to a relatively high temperature (about 70-80 degrees Celsius) primarily by utilizing waste heat from the engine. In this type of system, two separate fuel systems are used. One involves petroleum diesel or biodiesel as a "start/stop" fuel, the other involves SVO as an "operating" fuel. So, for example, if biodiesel is used as a "start/stop" fuel and SVO is the "operating" fuel, then the engine would initially be started cold on biodiesel. Once sufficient temperature is reached, it is switched to SVO, which is then used for the majority of engine operating time. Prior to engine shutdown, the engine is switched back to the biodiesel for just enough time to purge the fuel injection pump and injectors of the SVO fuel so that it can be started again cold on biodiesel. The engine operating systems, including injection timing, injection pressure, engine/valve timing, and compression pressure, can remain unchanged. However, due to the higher viscosity nature of the vegetable oil, worn injectors and/or injection pump may present problems if injector pressure is lower than normal.

Feedstock Selection and Iodine Value
Some oils are inherently better to use than others, related mostly to their respective melting points and oxidative stability. Oils with a higher melting point can require more heat in colder climates to become useful, and can present handling difficulties. If the melting point is higher than the ambient temperature for a particular climate, then the oil will need to be kept warm for transferring it. Fuels that readily become oxidized can present operating problems, as peroxides and polymers can form in the oil. Polymerization occurs faster in conditions of high heat and pressure such as found in the fuel system of a diesel engine. The polymers formed are plastic-like solids and can gum up the fuel filter or other parts of the injection system, and do not thin out with heat as readily as the oil will. Engine failure can result. A good indicator of the oxidative stability is the iodine value, which is available for most vegetable oils. Oils with a low iodine value are more stable than those with a high value. The higher the level of saturated fatty acids present in the oil, the lower the iodine value and the more resistant to oxidation the oil will be, as the oxygen reacts with the carbon-carbon double bonds present in unsaturated fatty acids. However, saturated fats have a higher melting point than unsaturated fats.

Therefore, tropical climates are in many ways ideal for SVO as a fuel for diesel engines. Ambient temperatures generally higher, so vegetable oils with lower iodine values can be used without the higher melting points of those oils presenting problems. Palm oil and coconut oil both present greater oxidative stability and higher cetane numbers than canola or sunflower oil, which are commonly used in colder climates for SVO fuel systems. In addition, these oils are much higher yielding crops than those found in colder climates because they can be harvested many times throughout the year.

Used Cooking Oil as Fuel
Used cooking oil is commonly used as a fuel for SVO systems. Canola, sunflower, safflower, palm, and many others are commonly found in used cooking oil, along with beef tallow, lard, or other fats depending on what was cooked in the oil. Most of these oils are suitable for use in SVO systems. Canola is generally the best of the oils commonly used for cooking. Any type of hydrogenated oil should be avoided in most climates due to handling difficulties, as hydrogenated oils have high melting points. Two of most important factors other than the type of oil are how long the oil was used and how old it is. The high heat of cooking creates free fatty acids in the oil, and accelerates the oxidation process. Neither of these are good for the oil if it is to be used as a fuel. Generally, problems can be avoided by using oil that was not kept in a fryer for an extremely long time, and using the oil soon after it is collected. Oil that was used for cooking and then left to sit for more than a few months should be avoided. However, since the oil needs to be filtered it is a good idea to let the oil settle for some amount of time if possible to ease the filtration process and preserve filter life. Let the oil sit for a few weeks undisturbed. Often the oil will form distinct layers as sediments settle out to the bottom, forming a clear layer on top and thick deposits on the bottom. Siphon off the very top layer for use, and discard the rest.

Two-tank SVO Systems
This is the most common and most widely applicable approach to straight vegetable oil fuel systems. In addition to the stock diesel fuel system, another fuel system including tank, filter, and selector valve should be added to the vehicle. The best way to transfer engine heat to the vegetable oil tank is with the cooling system. A heat exchanger can be added to the tank and hot coolant from the engine can be routed through it for heat. The liquid coolant has a high heat capacity that is very consistent and can transfer a large amount of heat energy to the vegetable oil. The heat exchanger can be as simple as a few coils of copper, steel, or aluminum tubing, but should be placed in the tank so it directly contacts the oil for maximum heat transfer. Copper is most commonly used due to its low cost and because it is easy to work with. However, copper is more corrosive in the presence of vegetable oil than steel or aluminum and it would be wise to periodically check the copper and replace it if necessary. The tank itself can be insulated for greater effectiveness, particularly if it is mounted under the vehicle and exposed to cold air. The lines carrying vegetable oil to the filter can also be heated by bundling them together with coolant lines traveling back to the tank, or by using special "hose-in-hose' lines that carry fuel in an inner hose surrounded by an outer hose that carries the coolant. The filter itself should be heated as well. Filters designed for large diesel engines in very cold climates work best, as they have greater filtration and water separation capacity and usually include built-in coolant heat capability. The flow of the hot engine coolant should be designed counter-current with the fuel flow, so the hottest coolant coming right from the engine should contact the fuel closest to the engine. Be sure to consider the return line as well. Do not let the SVO be returned through the diesel filter or back to the diesel tank, as this will contaminate the cold startup fuel system as well as creating the possibility of overflowing the tank. In some cases one of the fuel systems can be looped, but some fuel return is necessary for proper pump operation and to allow air to be purged from the system.

In many cases it is wise to include electric supplementary heat as well, especially if coolant heat alone is found to be insufficient. Some products, such as the Veg-Therm, are designed to rapidly heat the vegetable oil to sufficient temperature as it passes through the unit so long as it is warmed slightly beforehand. A system such as this it allows for less elaborate coolant-heated systems since the electric heat does most of the work. Some in-tank heating as well as filter heat will still be recommended however, especially in colder climates or if more saturated fats are used as feedstock.

The selector valve can be a solenoid, but motor driven valves are more reliable with vegetable oil due to its higher viscosity. A selector switch can be mounted on the dash for quick and easy operation. For complete fuel feed and return systems for both tanks, a 6-port valve can be used or a combination of 2 3-ports.

There are several commercially available kits that include all the different components to convert most diesel vehicles to SVO, and there are many different ways to design and build a custom system using salvaged or aftermarket parts. Cost and design of custom systems vary greatly and are limited only by your imagination, as long as the oil is heated to a minimum of 70 degrees Celsius and is filtered as well as the original equipment or better.

SVO links and online resources

Complete 2-Tank Kits and Other SVO-Specific Products

www.biofuels.ca - Neoteric Biofuels (also has a single-tank kit available)

www.greasecar.com - Greasecar

www.greasel.com - Greasel

www.diesel-therm.com - ATG

Single Tank Systems

www.elsbett.com

Diesel Fuel Heating Systems

www.davcotec.com - heated fuel filters, model 234 is designed for SVO/Biodiesel

www.webb-sales.com - Vormax filter and HotStk standpipe heater

www.arctic-fox.com - heated fuel lines, in-tank heaters and standpipes, etc

www.espar.com - fuel fired coolant heaters

www.parker.com/racor/ - heated fuel filters

General Info and Discussion

www.journeytoforever.org/biodiesel_svo.html - Journey to Forever's SVO page

www.biodiesel.infopop.cc/eve - follow the links to the SVO forum