Vegetable Oil Conversion Design - Heating The Veg Oil

Vegetable oil can be used in a diesel engine if it is heated to reduce its viscosity so that it will perform in a similar fashion to diesel fuel.

A number of studies and ever growing empirical experiences have shown that the use of such systems with many engines can give operating characteristics and reliability similar to when running diesel fuel. Emissions and power have been shown to be comparable or improved.

When environmental, production, social and economic considerations of the use of biofuels for transportation fuels has been considered, studies have found straight vegetable oil to be the most suitable energy source.

When designing a fuel heating system a number of factors need to be considered to ensure reliable operation:

- direct / indirect fuel injection
- rotary / inline injector pump 
- ambient temperatures encountered
- type of oils to be used
- computer / mechanical controlled injection

The time when problems are most likely to occur is when the engine is being started from cold. The engine does not have the latent heat that would aid combustion and the engine may not fire.

A growing number of companies offer conversion kits and conversion services that allow vegetable oil to be poured into the tank instead of diesel. These kits use electric heating and often replacement injectors to ensure that the right conditions for combustion can be obtained.

The Twin Tank or Duel Fuel System

A simple and effective way to safely run on vegetable oil is to run a duel fuel system. The engine is started on petro-diesel (or bio-diesel etc) and the waste heat created by the engine is used to heat the vegetable oil. This avoids running vegetable oil through a cold engine at start up. A temperature gauge in the fuel line allows you to see when the oil is hot enough for the fuel supplies to be switched. 

The fuel supply is switched by a three way valve. Before shutting down for extended periods the fuel supply is switched back and the engine is allowed to run for a few minutes (approx 3-5mins depending on set-up) so that the vegetable oil is flushed from the fuel system. The vehicle is then ready to be restarted on diesel fuel. Placing the fuel selection valve as close to the injector pump as possible reduces the flushing time.

Twin Tank Fuel System

Fuel Return

In most engines the injector pump is supplied by the transfer pump with more fuel than is required. The excess fuel is generally returned to the fuel tank although some engines recycle the excess back to the injector pump.

When running a duel fuel system the commonly utilised ‘return to tank’ system will require modification. Getting vegetable oil returned into the diesel tank can be undesirable as it will accumulate and would cause problems associated with running unheated SVO. Getting diesel in the vegetable oil is not a problem mixing diesel into vegetable oil improves its performance.

Having a three way valve in the return line allows fuels to be sent to the appropriate tank. 

Especially with thick oils, care should be taken to feed back to the diesel tank only after vegetable oil has been purged from the system avoiding diesel tank contamination. When using oils with high melt points this has proven to be less of an issue.

Having a single return to the vegetable oil tank is another possibility but diesel would be pumped into the vegetable oil tank when the vehicle was running on diesel, in the wrong circumstances this could lead to an overflowing tank.

Another option is to run this fuel, by using a T connector, into the injector pump intake line. There is the possibility of a build up of air due to slight leaks and the closed circuit giving the air nowhere to vent. Excessive air would lead to the engine being starved of fuel, as well as the possibility of the injector pump which uses the fuel oil for lubrication being damaged. A device that allowed trapped air to escape introduced to this closed loop would offer a solution. Many examples of this fuel system configuration have been utilised without problems. Extra care is required to ensure all pipe connections, filter seals etc. are air tight.

This method offers the advantage of mixing the fuel returned from the injectors, which has been heated through compression in the injector pump and contact with the engine, with the fresh fuel supply. Upon switching the vegetable oil is gradually mixed with the hot diesel, the vegetable oil percentage increasing as the remaining diesel fuel is used. A mixture of fuels is more prone to combust completely at lower temperatures allowing the SVO to be introduced sooner. Also the volume of fuel being pulled from the fuel tank is reduced to the amount of fuel consumed by the engine. This reduces the amount of work the transfer pump has to perform which also reduces the level of suction ‘up stream’ of the pump making air ingress less likely.

The time required to run on diesel before the fuel supply can be safely switched to SVO is dependant on engine design, fuel characteristics, heating equipment, ambient temperature, engine temperature and fuel system design. Assessing exactly when to switch the fuel supply can be achieved by a process of gradual reduction, monitoring for undesirable engine run characteristics and if possible fuel temperature and vacuum/pressure. A vacuum gauge fitted before a transfer pump will show if the pump is performing excessive work pulling cold oil.

Heating Methods

Cold weather is a problem with diesel fuel as it can begin to solidify below -7°C and will block the fuel filter [11]. Products that use heat to aid diesel fuel flow in cold weather are available and many are suitable for application in a SVO system. Sometimes it may be appropriate to ‘tune’ these products as they are generally designed to heat to a temperature below that which is desirable with an SVO system. Heating systems specifically designed to be used in SVO systems are available or can be fabricated.

Engine Coolant Heat

These units are utilised on liquid cooled engines. The hot engine coolant fluid is used to heat the vegetable oil. The coolant in an engine generally runs between about 75 and 90°C (158-203°F) so it is at a suitable temperature to heat the SVO via a heat exchanger.

Engine Oil Heat

Hot engine oil is used to heat the veg oil. Engine oil will heat to above 100° so may be a better option than coolant in some applications. Leaks in the oil system are much more likely to be terminal for the engine. Great care should be taken to avoid engine oil leaks.

Exhaust Heat

The heat from gasses in the engine exhaust system can be used to heat the fuel. A reported system used metal fuel line wrapped around the exhaust system of a genset. As a generator engine runs at a steady rate using a steady fuel flow heat was regulated by the number of turns around the exhaust. The unit was not dual fuel and there were some problems with cold starting. Using this heat source in a dual fuel system could lead to problems overheating the oil before switching.

Systems have been proposed using a valve to allow a regulated exhaust gas flow away from the existing exhaust system and into other piping where the hot gasses are used to heat the vegetable oil. The valve opening could be regulated to give the required amount of heating. An idea with potential considering the very quick heating times that could be achieved.

Returned Fuel Heat

As discussed above the fuel returned from the injectors has been heated through compression in the injector pump and high pressure lines and from contact with the hot injector. The heat of the fuel can be used by T-ing the return into the fuel supply as described, by sending it to the SVO tank or through a heat exchanger with the incoming fuel.

Electrical Heat

An electrical heating element can be used to heat up the oil to suitable temperatures. In a vehicle application care has to be taken not to have a power demand that would be excessive for a given electrical system. Rapid heating is possible with an electric heater allowing a dual fuel system fuel supply to be switched sooner. A suitably modified engine equipped with electrical heaters can be started and run on SVO alone.

Injector Pump Heating

Assessment of the first vehicle converted using a dual fuel system showed the possibility of insufficiently heated oil running through the engine and causing crud build ups in the cylinders and on the injectors. The perceived problem was when the fuel supply was switched from petro-diesel to vegetable oil. The hot vegetable oil would rush into the fuel injection pump, which was considerably cooler than the oil, cool down and then get injected into the engine at a temperature below that which would be desirable.

The engine was equipped with a CAV rotary pump, known to be less durable when pumping SVO, and there was the possibility of damage from the increased heat generated pumping the thicker oil.

A method of heating the injector pump was devised. Both fuel supplies were heated with coolant before the injector pump. This way the injector pump would be gradually warmed by the heated diesel fuel before switching to vegetable oil. To avoid the possibility of fire care should be taken to ensure a fuel heated above its flash point can not leak from the fuel system. Some engines (some Mercedes etc.) use engine oil to lubricate the injector pump which provides heat to the pump.

Vegetable Oil Tank and Lines

Depending on the type of oil used and at what ambient temperature, the fuel tank, lines and filter may need heating to allow free flowing of fuel. At low temperatures SVO will be thick or solid and hard or impossible to pump. The filter is a definite bottle neck. SVO will flow more freely through thicker fuel pipe.

Having a filter system for each of the fuel supplies will reduce the amount of fuel that needs to be flushed when changing fuel supplies. If a filter becomes blocked due to waxing or impurities the vehicle can still be run on the second fuel system.

Some proponents advocate using a more course filter in the vegetable oil line to allow thick oils to flow at the desired rate and help to reduce waxing. This has to be balanced against the increased size of particles that may pass through the filter and cause wear to the injector pump. Heating the vegetable oil in, or up stream, of the filter will help the fuel flow and reduce the possibility of waxing the filter.

Diesel Fuel Return

Having a valve which allows the diesel fuel to be returned back down the vegetable oil feed through the filter will act to clean the filter, pushing any particles off the filter media and into the filter base or back to the veg oil tank. A drain plug at the bottom of the fuel tank or filter base allows settled particles/water to be evacuated.

Pre Heating the Engine

Electric, diesel, biodiesel, propane and petrol fired engine pre-heaters are available that are switched on for approximately half an hour before starting the engine. They heat the engine coolant and pump it around the system, sometimes using a thermo siphon technique providing heat for the engine cabin heater and any coolant/fuel heat exchanger fitted. 

An engine thus heated will be easier to start under adverse conditions due to the extra heat in the combustion chamber. Fuel inside a heat exchanger would be up to temperature before the engine was started. These units also reduce emissions and engine wear connected with cold engine running.