Bio-diesel is a type of diesel being blended as an alternative fuel. Generally vegetable or animal oils can be used. Many different mixes of bio diesel are used. With no modifications to vehicles some people use up to 20 percent and from 50% to 70% on modified engines. First the bio diesel has to go through a chemical separation process where the glycerin has to “fall out” of it. This is called transesterification and is done with the use of methanol and lye. Neither chemicals are great to work with but it can be safe with pre cautioned use. Generally clean oils are used to avoid the acidic state of used cooking oil. Some people don’t mind the extra work to cleanse used oil, because the oil can be extremely cheap and many times is free. Fast food places use alot of oil and are glad to get rid of it. Water and acid will need to be removed from the used oils. A washing of the fuel will also need to be done to remove excess methanol, soaps and other contaminants.
When Rudolph diesel first set out to build a more efficient engine, he needed to start with compression ignition. A normal spark ignition engine compresses the fuel air mixture then when it’s fully compressed it fires a spark across the spark plug. The spark ignites the mixture and moves the piston turning the engine. The compression of a spark ignition engine is limited by the octane number. If the compression is too high the engine will pre-ignite which is a condition that will destroy the pistons and the rest of the engine. More power and higher efficiency is possible at higher compression levels. That is the reason racers use race gas which is simply higher octane fuel. The race gas doesn’t provide more power (unless oxygenated) the increased compression does.
Diesel engines can be 2 stroke or 4 stroke in design. The difference in the diesel comes in its ignition system. It uses compression ignition to ignite a burn in the combustion chamber. The efficiency of a diesel is generally better due to the high compression ratios used. Large diesels are generally of 2 stroke configuration for simplicity. Most road vehicles use the 4 stroke configuration.