When you pull a car or truck into a gas station, no matter what kind of fuel the vehicle takes, you can't help but notice that diesel fuel is almost always an option. If your own vehicle runs on standard unleaded gasoline, you might wonder why others do not. What makes diesel fuel special? If it has "elite" properties, why don't all cars use it?
These questions lead to inquiries that are less about diesel fuel itself and more about the diesel engine, and why the development of the diesel injector pump in the late 1800s represented a technological leap forward. The main idea to keep in mind as you read is that diesel engines use physical compression instead of an actual ignition spark to make their fuel hot enough to burn.
How Are Diesel Engines Different?
Lighting something on fire, boiling it or "nuking" it in a microwave oven are all obvious ways to increase the heat content of that object. But it is not as intuitive that greatly increasing the pressure of a gas without allowing heat to enter or leave can dramatically drive up the temperature of the chamber.
In a diesel engine, air is compressed to roughly 1/15 to 1/20 of its usual volume just before the diesel fuel is injected, or pumped, into the engine. The fuel-air mixture becomes hot enough to burn, driving expansion of the cylinder (piston) in the engine. Just as during the air-compression phase, there is no heat transferred into or out of the engine; that occurs only during the exhaust phase.
The Diesel Fuel Pump
The fuel injection system in a diesel engine consists of an injection pump, a fuel line and a nozzle (also called an injector). When the air is compressed, the pressure inside the cylinder briefly rises to 400 to 600 pounds per square inch (normal atmospheric pressure is less than 15 psi), driving internal temperatures into the range of 800 degrees Fahrenheit to 1,200 F (430 degrees Celsius to 650 C).
A diesel engine features the same cycles and physical arrangement as a gasoline engine; it is the process of ignition, not the structure, that sets them apart. In general, they are more reliable, generate more power per kilogram of fuel and are more efficient overall; diesel fuel also poses less of a fire hazard.
Diesel engines do carry disadvantages compared to their conventional gasoline counterparts. They must be of a hardier construction because of the high pressures incurred during the air-compression phase, which presents both an engineering challenge and a costlier product. Also, the high pressures can make diesel engines difficult to start.
The Diesel Engine Cycle
The diesel engine undergoes a four-step cycle to complete one compression-expansion movement of a piston. The first of these is the air-compression step; because the same amount of heat is kept in a rapidly shrinking space, it drives up pressure and temperature. In the second (ignition) phase, pressure remains constant as volume begins to expand.
During the third phase, called the power stroke, the volume and pressure both decrease as the engine does work, ultimately powering the car. Finally, in the exhaust phase, the volume remains constant at its highest level, and then the cycle begins anew when air is drawn in for compression in the first phase.
Fuel for diesel engines is heavier than gasoline, because it is made from the residues of crude oil as opposed to the more volatile by-products that result in the formation of gasoline. Like regular gas, it comes in a number of grades that can be tailored to the needs of specific engines.
Using the wrong diesel fuel can cause operational problems from poor starting to "knocking and pinging" to overly smoky exhaust.