Diesel engines actually require very little modification to run on waste vegetable oil (WVO). At room temperature, WVO is thicker than regular diesel, which means that it has to be heated before it can flow easily through an engine. WVO gathered from restaurants also contains tiny contaminants left over from cooking that can clog and damage engine parts. Therefore a successful WVO vehicle must have a means of both heating and filtering the WVO.

The Big Green Bus uses a combination of electric heaters and coolant routing to warm WVO up to the required 70°C. Most WVO vehicles start up on regular diesel fuel and allow the engine to warm up. Coolant lines, which carry heat away from the engine, are used to heat WVO that is stored in a second tank. When the WVO gets hot enough, the driver flips a switch, and the engine begins pumping hot WVO instead of diesel. For small cars in warm climates, the WVO can heat fast enough to allow switching after only a few minutes of driving on diesel. However, for a larger engine that operates in cooler climates (like The Big Green Bus), the engine may take a long time to heat up and may never even reach a full 70°C. Although the bus can run on slightly cooler WVO, the added viscosity puts more stress on the fuel pumps and injection system and therefore shortens the engine's life.

To speed up heating and therefore reduce the bus's diesel startup time, The Big Green Bus has four high-power electric heaters that run off of deep-cycle batteries. One heater warms WVO in the storage tank in the area just around the fuel pickup. It therefore concentrates energy right where it is needed, melting enough WVO to be easily pulled into the fuel lines. A second, flexible pad heater warms the filter element to minimize the WVO's resistance through the filter membrane. The filter is placed near the engine so that the oil has little time to cool before it reaches the fuel injectors. Nevertheless, two more in-line heaters warm the WVO along the final fuel path just prior to the engine. The concentrated electric heating has the potential to allow the bus to start without diesel, as long as the heaters are given a few minutes to warm the oil prior to startup. Once the engine is running, a 215 watt SunPower solar module tops of the heater batteries.

As the bus runs, coolant captures waste energy from the engine and transfers it to maintain the high WVO temperature. Coolant lines normally route excess heat from a car’s engine to the radiator, which dissipates the energy into the air and prevents the engine from overheating. That radiated heat is energy lost to the environment and a major source of engine inefficiency. The Big Green Bus recycles that energy by routing hot coolant through various components of the WVO fuel system before allowing what's left to escape through the radiator. For example, hot coolant warms the fuel pickup, which makes the oil flow from the tank more easily and extends the life of the bus's fuel pumps. Similarly, coolant warms the WVO filter and also passes through a heat exchanger that transfers energy from the coolant to the WVO before it reaches the engine. Finally, all WVO lines are bundled with coolant hoses to maximize the area of heat transfer.

The Big Green Bus still has the original (bio)diesel fuel system. It not only affords a backup, should the WVO system malfunction, but diesel is also crucial to purging the system before the engine is shut off. Over time WVO will gum up the small openings and moving parts inside an engine if it is allowed to cool. Therefore, the bus is run on diesel for a few minutes before shutdown to "purge" the system of WVO.

The second major consideration for running WVO is ensuring that it is properly filtered. During its former life as a cooking agent, WVO picks up particulates that can damage the finished surfaces inside an engine. When WVO is pumped onboard on the bus, it passes through a series of Rosedale filters that block anything larger than 5 microns or 0.0002 inches in size. The initial filtering extends the life of components in the fuel system, especially the primary oil filter, which removes water and anything larger than 10 microns from the oil. This onboard filter greatly extends the life of the engine and gets clogged/needs replacement less often because of the 5 micron filtering before it. Why does it ever get clogged if the oil is filtered to smaller than the primary filter? The size rating on most filters is "nominal," which basically represents an average-- some particles above the rating will get through. Moreover, the fuel tank itself has some contaminants from when it sat in a salvage yard. The onboard WVO filter is the last defense against these particles.