By Ray T. Bohacz
Here are the 750,000-gallon bulk chemical storage towers on Power Service's 26-acre manufacturing facility.
Diesel fuel is a commodity we spend a lot of money on, know little about, and hardly ever see. Yet it is the lifeblood of our passion. If we are what we eat, then a diesel engine is the fuel it consumes. Just as we can fill our bodies with junk food, the same holds true for the fuel tanks in our vehicles. For a healthy, running compression- ignition engine, you need to know a little about the fuel you're buying.
Understanding Fuel
The diesel fuel you purchase at the pump is a commercial-grade product that's often traded on the commodity market, much in the same manner that base gasoline, precious metals, and grains are. The federal government imposes minimum acceptable standards for all motor fuels used on public highways, but these requirements are only bare minimums. Diesel has not yet gained the popularity of gasoline as a motor fuel, so the oil companies and retailers traditionally haven't looked at it as a competitive market, but rather as a consumable that will sell itself. Unlike gasoline brands that are advertised to include additives that will clean fuel injectors and carbon deposits, nothing is ever mentioned about diesel fuel.
A Lesson In Gasoline
To begin to understand diesel fuel quality, gasoline needs to be referenced. Gasoline is wholesaled as a base product that's not always refined by the company whose name is on the pump you're buying it from. The base fuel must meet a federal requirement called the lowest acceptable concentration (LAC) standard for fuel-injector cleanliness and minimizing deposit formation. Some expensive gasoline brands advertise their fuels as having more additives that will function above the LAC level. Thus, discount fuel has the federally mandated LAC level while others may have enhanced chemical properties. Gasoline is made into brand-specific products (Chevron, Exxon, Shell, and so on) at the distribution depot as the additives are mixed into the tanker truck as it's filled with base product.
Diesel Fuel Basics
Diesel fuel that's purchased at the pump can be considered very similar to unbranded gasoline when examined for additives that will improve its performance. In other words, there really is no upscale or designer diesel fuel that you can buy. That means the task to make the fuel into what you want it to be for your engine is completely up to you.
Diesel fuel is a complex mixture of hydrocarbon molecules derived from crude oil by distillation. It's normally blended from several refinery streams with procedures that are often used to increase the production rate. As much as 20 percent of diesel fuel can consist of relatively heavy paraffinic hydrocarbons, which have limited solubility in the fuel. When cooled sufficiently, these particles tend to come out of the solution as wax. This is often referred to as fuel gelling that takes place in cold weather.
In addition, water can't be completely eliminated from diesel fuel. It can get into the fuel at various stages as it progresses through the distribution network from an oil refinery to your vehicle's fuel tank. Microbes may also grow in diesel fuel if it stays stagnant for a long period of time and is not treated.
When discussing diesel fuel, many technical terms are often used, but as a consumer, you don't need to understand all of them. The important ones are cetane, combustion improvers, detergent, lubricity, injector cleanliness, and cold-filter plugging point.
Cetane
Diesel fuel's cetane number, which will usually be between 40 and 45, indicates the readiness of a diesel fuel to ignite spontaneously under temperature and pressure conditions in the engine's combustion chambers. The higher the cetane number, the shorter the delay between the fuel's injection and ignition.
Cetane improvers (ignition improvers) are used to increase the cetane number of diesel fuel by reducing the delay between injection and ignition when the fuel is sprayed into the combustion chambers. Diesel's cetane number is the most widely accepted measure of ignition quality and is determined by using a special cetane test engine. The cetane number of a fuel is determined by comparing its ignition delay, under standard operating conditions, with those of blends of two reference fuels (cetane and heptamethylnonane) having cetane numbers of 100 and 15.
Several types of chemicals-alkyl nitrates, ether nitrates, nitroso compounds, and certain peroxides-have been identified as effective cetane improvers. Studies by the petroleum industry have proved that engines care little if the improved cetane rating came from the refined fuel or an end-user-supplied additive.
The cold-filter plugging test confirms the effect of additives on the fuel's ability to stay liquid.
Why Cetane Is Important
Your diesel engine's combustion sound is affected by the fuel's cetane rating. Low ignition-quality fuel is directly responsible for higher combustion noise levels. The rapid burning of the larger amount of fuel injected before ignition results in higher cylinder peak pressure, producing the characteristic diesel engine knock. The noise tends to be most evident during cold starting or cool running, as when accelerating after a period of idling or light-load operation. Cetane is not the only criterion of fuel-related combustion noise, however, as other characteristics such as volatility and composition can have a direct influence on the combustion event and the noise generated.
Most engines experience an increase in ignition delay of about 2 degrees crank angle when the cetane number is reduced from 53 to 41. In one test, direct-injection diesel engines were found to have a larger difference in the levels of ignition delay. Older indirect-injection diesels had similar, if relatively short, ignition delay periods when the fuel's cetane level was reduced.