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A Diesel Primer: What to Ask for When Ordering Premium Fuel

Over 80 years after Caterpillar introduced the first diesel-powered tractor, the fuel is the essential fluid of any farm operation. Buying good fuel and understanding why it needs to be high quality is crucial now more than ever, due to advanced engine technologies with tighter tolerances. This fuel primer should help you negotiate buying high-quality diesel. To start with, there are three stages of diesel combustion that include:

  • Delay: This is at the end of compression and after the start of injection, when the fuel does not ignite immediately.

  • Rapid combustion: A sudden rise in pressure (diesel knock) is caused when the first fuel is burned.

  • Controlled combustion: Once the first fuel is burned, injection continues with the additional fuel combusting in the chamber as it mixes with intake air.

With this knowledge in mind, here are answers to common questions about diesel fuel.

Q: Why does the fuel consumption of my engine seem to vary tank to tank doing the same work under the same conditions?

A: Since diesel fuel is a commodity, each load you buy will vary in energy density depending on the refinery it came from, the crude oil stock used, and the weather during refining. A gallon of diesel fuel can contain as little as 132,000 Btu of energy or as much as 152,000 Btu, a 15% difference. Higher Btu equals more work per gallon. Fuel density can be measured easily with a dedicated hydrometer test kit. Though it will not divulge the Btu, it will be a good indicator of the fuel’s composition, ignition quality, power, economy, low-temperature properties, and smoke tendency.

Q: What does cetane mean?

A: Cetane is a rating of the fuel’s readiness to ignite during the first stage of combustion. The higher the cetane, the easier the fuel is to ignite, and the shorter the delay between initial injection and ignition.

In North America, the average cetane rating is 40; in Europe, it is 45. Industry spot checks have found many suppliers selling fuel with cetane ratings in the mid-30s or lower. This is a very poor fuel.

Cetane can be natural (created during the refining process) or it can be added chemically to the fuel after it is made.

A cetane rating that is derived naturally will provide less smoke on cold start than the same number achieved with an additive. Once the engine is warm, the cetane value performs the same regardless of how it is created.

A good cetane rating (45 or above) is essential for the best engine performance and combustion quality.

Q: I use additives to stop gelling in the winter. Is there anything else I should be concerned with in colder weather?

A: Gelling is only one concern. The additive you use should include wax crystal modifiers, pour-point depressants, flow improvers, cold filter plugging point modifiers, and cloud-point depressants. These should all be listed on the product label. If it only states that it is an anti-gel, then it probably does not address the other concerns created in cold weather.

Q: Why should I use fuel additives?

A: Not using an additive to enhance the fuel’s characteristics for the three stages of combustion would be like planting a seed without any fertilizer. The plant will grow, but it won’t have a good yield.

The fuel the refining industry produces is at odds with the true requirements of the engine and your need for performance and reliability. This can only be met with the use of a quality additive that addresses all of the deficiencies of the base fuel and includes a detergent and lubricity enhancement (for older engines).

The good thing is, you can greatly improve combustibility and the way the fuel interacts with the engine by using additives. Unadditized diesel is considered base fuel, and that is what you are buying, but it’s not what you should be using.

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