Why Diesel? A Brief Introduction to the Compression Ignition Engine and its Relationship with Vegetable Based Fuels

by John

A typical first response to the concept of operating an engine on vegetable oil is to question whether or not it works, why it is a better option than petroleum, and especially why it works in a diesel engine rather than a gasoline engine. Diesel engines are rather unpopular in the United States, often being thought of as loud and dirty. Worldwide, however, diesel engines are very well established in a wide variety of applications, as they are much more efficient than gasoline engines and generally longer lasting.

            So what makes these engines different? The key is in the ignition system. Although they are both internal combustion engines, they are very different in the way they ignite the fuel in the combustion chamber. Gasoline engines are spark ignition engines. They operate by first creating a mixture of fuel and air, and then using an electrical spark to ignite the mixture. Diesel engines are compression ignition engines. They operate by compressing air to a very high pressure in the combustion chamber and then injecting fuel into it. The heat and pressure in the combustion chamber causes the fuel to spontaneously combust, providing power to the engine. Some of the different fuels that work in this type of engine include diesel fuel, kerosene, heating oil, vegetable oils, and animal fats.

            Modern diesel engines are not well adapted to run on vegetable oil because their fuel injection systems are designed to run on diesel fuel, which is significantly less viscous than vegetable oil. This problem is easily overcome however, by either of two rather simple techniques.

One method is to heat the vegetable oil prior to its delivery to the engine, reducing its viscosity to levels suitable for the fuel injection system. This requires the addition of a separate fuel system for the vegetable oil, often heated by the waste heat from the engine. With this system the engine can be started and warmed up on petroleum diesel, then switched to vegetable oil when significant temperatures are reached, then switched back prior to turning off the engine to purge the vegetable oil from the fuel pump and injectors, preventing clogging as the oil cools down.

A second approach is to process the vegetable oil so that its viscosity at normal operating temperatures is suitable for the fuel injection system. This can be done by chemically removing the glycerin molecule from the vegetable oil. Glycerin is what gives vegetable oil its viscosity, so removing it creates a product that has a similar viscosity to petroleum diesel fuel and is known as biodiesel. Processing vegetable oil into biodiesel allows it to be used in a modern diesel engine without any modification to the engine or creating a separate fuel system. In fact, it can also be blended with petroleum diesel in any amount. Fuel blends that include biodiesel are becoming more popular worldwide as they generally burn more efficiently than 100% petroleum diesel and with fewer emissions.

            Each system has its advantages and disadvantages, but either will allow a modern diesel engine to be operated effectively on vegetable oil as fuel when implemented properly. Both methods have been in use for quite some time now, with people all over the world logging thousands of miles on vegetable oil. Anyone interested in experimenting with vegetable oil fuel is highly encouraged to pursue further education about diesel engines and biofuels technology.