Can a diesel engine really run on vegetable oil? Yes! When Rudolf Diesel first build the diesel engine, one of the fuels he experimented with was peanut oil. Today's diesel engines still rely on the same thermodynamic principles as the first diesel engine did more than 100 years ago, and can run on vegetable oil with little modification.

Our bus burns waste vegetable oil (WVO), which is the leftover oil used in deep fryers to cook food. At room temperature, WVO is much thicker than diesel fuel, so it must be heated to reduce its viscosity to that of diesel fuel. We have a fully automated Frybid system which we've modified to better suit the needs of our bus. The WVO system is heated by coolant before it goes to the radiator. We start the bus on diesel (or biodiesel, in our case) and then drive it for a few miles to allow the engine to warm up. As the engine warms, it also heats the vegetable oil to the point where it can flow through the filters and pumps (about 180ºF), and be properly injected into the cylinders. Once the engine is warm enough, the system switches over, and runs on 100% WVO. At the end of the day, we switch back to diesel fuel to purge the injectors and other unheated parts of the fuel system, so that we can start up again the next morning.

When we first collect waste vegetable oil, it is pumped through a series of Rosedale filters which strain out any particles down to a half micron, and then into the wet tank. In the wet tank, it undergoes a heating and cooling cycle, which allows and suspended water to drop out. After draining the water off, the vegetable oil is transfered to either the dry tank or to our chase car, a vegetable oil powered VW Jetta. From the dry tank, the VO is by an in-tank heat exchanger, aluminum tube-in-hose fuel lines, and a 16 plate flat-plate heat exchanger. Next, it hits the coolant heated FASS fuel-air separation system, which removes any air bubbles and debris the VO might have picked up in the tanks. After passing through the supply valve, the VO (or diesel) passes through the stock fuel pump and into each of the fuel racks.

Our bus has a Detroit Diesel 8V92TA engine, which is a 12 liter 400hp turbocharged V8. It is unique in that it is a 2 cycle engine, very similar to locomotive engines. Unlike a gasoline 2 cycle engine, our engine still has its own engine oil (no mixing like in a gasoline two cycle). It also doesn't have an injection pump, and instead has a "fuel rack" on each of the two banks of cylinder heads, a sort of primitive common rail fuel system.

The fuel returning from the fuel racks goes back through the return fuel valve. While on diesel mode, the fuel is simply returned to the diesel tank. When we're burning VO, the valve diverts the fuel back to the FASS filter unit, which loops it back into the fuel supply. When we hit the purge button, the VO pump shuts down and allows the system to backflush with diesel fuel. With the pumps, fuel racks, and injectors filled with diesel, we can then shut the engine down and allow it to cool off.