Converting the Bus
The Big Green Bus is currently being converted to run on waste vegetable oil (WVO) instead of diesel fuel.
Waste vegetable oil would be a suitable substitute for diesel fuel in a standard diesel engine except for the fact that it has a much higher viscosity than diesel at room temperature (the viscosity of diesel fuel is 4.3 mm^2/s at 27° C, but the viscosity of pure soybean oil is 65 mm^2/s at the same temperature). Luckily, at 70° C, which is about the operating temperature of a diesel engine, the viscosity of vegetable oil (9 mm^2/s) is much closer to that of diesel, enabling it to be used by an engine in place of diesel fuel.
The engineers on The Big Green Bus team will add another tank to the bus to hold waste vegetable oil. The bus will be started running on normal diesel fuel and driven until the engine warms up. The bus’s coolant lines (which are warm, since they take heat away from the engine) will be rerouted through copper tubing in the oil tank, dispersing heat and warming the oil until it is liquid enough to flow through the engine. Then the bus engine will be switched to vegetable oil, and will run without diesel fuel.
Here is a diagram summarizing the conversion:
When the bus is stopped, the engine must be run on diesel again in order to flush out the vegetable oil and avoid buildups and clogging. As vegetable oil cools, the viscosity increases, which can clog the engine and prevent it from starting again. Since diesel engines are not designed to work with vegetable oil, the larger droplets that WVO forms when injected into the engine tend to not burn up as completely as diesel fuel, so carbon wastes build up more frequently. These can be burnt off by running the engine hotter or using the diesel fuel rinse to dissolve them.