The use of pure or straight plant oil as fuel in diesel engines is an old idea. In fact it was the fuel of choice when the diesel engine was invented and first demonstrated. The only reason straight plant oil (more commonly referred to in literature as straight vegetable oil or SVO) cannot be used to run a diesel engine is if the oil is too thick to flow at the required rate through the fuel lines and filters of the engine. This, of course, is the case for most plant oils at ambient temperature. The diesel engine was named after Dr. Rudolph Diesel, a German engineer who filed the patent for a compression ignition (CI) engine in 1894. He then successfully operated a prototype engine in 1897. Then in 1900 the diesel engine was first demonstrated to run on peanut oil during the world fair in Paris by the Otto Company at the request of the French Government.
At that time the French Government was interested in using peanut oil as an energy source for its colonies in Africa. Thus the current interest in running a car or truck using straight or pure plant oil could be viewed as merely going back to what the inventor of the CI engine had originally intended. Unfortunately, the design and operating characteristics of modern CI engines are nowhere near those of the first CI engine that ran on peanut oil. Mainly as a result of the economics favoring fossil fuels, the developments in the design and engineering of the CI engine have been geared towards the use of a distillate of crude oil commonly known as diesel fuel instead of peanut oil or other plant oils. For this reason, most modern CI or diesel engines, including the combustion chamber and fuel supply and injection systems, are engineered to optimize the burning of diesel fuel, rather than plant oils.
As a consequence of the manner the design and engineering of the compression ignition engine evolved, plant oils that are generally much more viscous than diesel fuel cannot be injected into the cylinders as effectively and efficiently as diesel fuel. And even if the plant oil can be successfully injected, the injector, which was carefully engineered to the specifications of the diesel fuel, cannot provide a fuel mist as fine as when plant oils are used. When larger fuel droplets are formed upon injection, the fuel does not burn efficiently causing difficulty with starting and incomplete combustion, which leads to build-up of carbon residue.
As deposits build up inside the engine, they can cause poor engine performance resulting from low compression due to sticking piston rings or coked valves. In addition, deposits on the injector nozzle produce poor injection spray pattern resulting in even more incomplete combustion and possible piston damage due to increased heat on piston surfaces. Figure 4.1 shows a typical spray pattern inside the cylinder. When injected, diesel fuel normally produces fine mist ranging in size from 5 to 20 microns. The finer the mist and the more uniform the mist size, the better is the combustion inside the cylinder.