Filters Capture More Than Dirt
Cold weather operability can be a huge problem. Equipment won't start in the morning or even if it does, it might stop suddenly in the middle of a job. These issues are usually caused by solids created in the fuel as the temperature decreases.
Engines and filters do not differentiate between particles. Hard or soft, contaminant or pure hydrocarbon, solids in fuel will cause trouble. These problems are made worse by new-age fuels, the sensitivity of modern engines and the high efficiency required of filters designed to protect them.
Ice
When the weather turns cold, free water in fuel will freeze. Ice crystals will behave like any other hard particulate, loading in filters or causing abrasive wear to fuel systems. In large quantity, ice can completely block filters or pipes, preventing fuel flow. De-icers may be able to help get you running in an emergency situation, but adding alcohol to diesel is generally discouraged. Keeping free water out of the fuel is by far the best solution, learn more by reviewing solutions for water problems.
Gelling
Like water, hydrocarbons turn solid when they reach their "freezing" point. Unlike water, however, they do not turn into ice. Instead, they turn into a thick, waxy substance that cannot flow through filters. This is what is referred to as "gelling". It is a characteristic of both petro diesel and biodiesel. Diesel fuel is not one "thing". Instead, it is a complicated mixture of thousands of potential compounds, each with different chemical and physical properties. The particular formula is determined by the refinery at time of production. Typically about 250 different chemicals are included, mainly hydrocarbons. Precise freezing temperature varies widely from one hydrocarbon to another, which directly relates to winter operability issues. “Winter diesel” contains a blend of hydrocarbons with generally lower freeze points than “summer diesel”.
Freezing Point of Representative Hydrocarbons
| Compound |
Class | Freezing Point |
|---|---|---|
| Anthracene | Aromatic | 419°F / 215°C |
| Napthalene | Aromatic | 176°F / 80°C |
| Eicosnae | N-Paraffin | 97°F / 36°C |
| 2-Methylnonadecane | Isoparaffin | 64°F / 18°C |
| Decane | N-Paraffin | -22°F / -30°C |
| N-Pentylcyclopentane | Napthene | -117°F / -83°C |
| 1,3-Diethylbenzene | Aromatic | -119°F / -84°C |
In some countries there is a classification of fuel called "Arctic diesel" for extremely severe conditions down to -40°F/C and below. An easy way to think of fuel "freezing" is to compare vegetable shortening to vegetable oil. Both are essentially the same thing, but shortening is solid at room temperature whereas oil is liquid. The same is true of hydrocarbons. At a given temperature, some may be liquid while others assume the soft, waxy state which is the ”frozen” or solid phase of hydrocarbons. This is commonly referred to as gelling.
Winter Fuel
When cold weather threatens, refineries and distributors can and do improve diesel's cold weather operability properties in several different ways. They can:
- Select less waxy crude oils upstream of the refinery
- Extend the refining process to eliminate waxy elements with higher melt temperatures (i.e. they freeze at lower temperatures)
- Dilute fuel with #no.1-D diesel or kerosene, which has lower wax content
- Treat diesel with low-temperature operability additives (cold flow improvers)
Fuel suppliers manage hydrocarbon blends at the time and place of sale but cannot control unusual weather swings or fuel that is kept in storage or transported to colder climates. DO NOT add heating oil to your fuel in an attempt to lower the cloud point. This practice is strictly forbidden by most equipment manufacturers and may void your warranty.
Predicting Cold Weather Operability
There are a number of tests intended to predict the cold weather performance of a certain fuel. Their relative merits are the subject of some debate. No independent test data regarding their usefulness has been published since the advent of HPCR fuel systems, high efficiency fuel filters, ULSD and widespread biodiesel.
Cloud Point: When diesel cools, wax crystals begin to form and a noticeable white haze (or "cloud") appears. The wax drops out of solution and starts getting caught in fuel filters and lift pumps. Actual cloud temperature varies based on fuel characteristics. Some low quality fuels may have cloud points as high as 40°F/4°C, but most good quality fuels will have a cloud point around 32°F/0°C (untreated). As a rule, cold flow improvers do little to lower cloud point. There are some cloud point depressants that can significantly lower a fuel's cloud point, but their use is generally discouraged because they can actually work against the anti-gels that are intended to keep fuel flowing. The best way to lower cloud point is through the addition of a hydrocarbon with lower wax content such as #no.1-D diesel.
Cold Filter Plugging Point CFPP: This is the temperature at which wax crystals will rapidly plug fuel filters, starving the engine of fuel, preventing it from starting or stopping it cold (usually when least convenient). Cold flow improvers can depress CFPP by several degrees. They do not actually lower waxing temperature, but rather work on the wax crystal itself. They alter the size and shape of the crystals so that the fuel flows better and passes through filter pores at lower temperatures.
* A note of caution: Most cold flow improvers do not work as well in ULSD as they did in higher sulfur fuel. Be sure that performance claims are based on test results using ULSD. If not, they are irrelevant. The common test method for measuring CFPP is ASTM D6371. It was developed in 1965 and uses rapid cooling methods to determine the temperature at which 20cc of diesel will no longer flow through a 45 micron wire mesh in 60 seconds or less. A CRC (Coordinating Research Council) study in 1981 determined that CFPP is not an accurate predictor of real world performance. It tends to over-state minimum operating temperatures (i.e. cold weather performance in the real world is not as good as the test would make it appear).
Low Temperature Flow Test LTFT: This test (ASTM D4539) is considered to be somewhat more accurate at predicting the performance of additized fuels and is frequently recommended for North American heavy duty trucks. Instead of using an unrealistic rapid cooling method, this test method allows diesel to cool slowly (1°C per hour), which is much more representative of real world conditions. In this test 200cc samples are drawn through a 17 micron mesh screen using 20 kPa vacuum. The LTFT point is determined when 90% of the sample no longer passes through the screen in 60 seconds or less. Although deemed more accurate than the CFPP test at predicting cold weather performance in North America, the LTFT uses a 17 micron mesh screen in determining acceptable flow. This is finer than the 45 micron mesh used for CFPP, but one can still reasonably question its ability to predict fuel flow through the high efficiency 2 micron filters used to protect today's HPCR engines.
Pour Point: The temperature at which diesel freezes is called its pour point. At this temperature fuel will freeze solid in lines. Pour point is irrelevant in terms of predicting cold weather operability because it is lower than cold filter plugging point. If fuel cannot pass through filters to the engine, the vehicle will not run. In the absence of other complications, gelled or cloudy diesel should clear as it warms. The wax crystals will dissolve back into solution and the fuel will once again be perfectly liquid. If the fuel does not clear when warmed, then another factor is at work in addition to cold temperatures. Most likely additional chemistry is present and a reaction has taken place creating soft solids that do not melt at normal operating temperatures.
Glycerin
Gelled fuel and glycerin solids are frequently confused with one another. But while gelled fuel is a natural phenomenon caused by cold alone, glycerin is an entirely different chemistry that is only present in biodiesel. Glycerin and other related substances (glycerols) are byproducts of biodiesel production and are not found in petro diesel. Regulations require the removal of virtually all these materials, but even at very low levels they can immobilize a fleet. As long as glycerin remains warm and liquid, it generally causes no immediate problems. At low temperatures, however, glycerin assumes a solid waxy state. It drops to the bottom of tanks, gets caught in fuel filters, and forms sticky, corrosive engine deposits.
Glycerin can turn solid at relatively high temperatures, sometimes as high as 55°F/13°C or above. Unlike standard gelled fuel, glycerin typically does not re-liquefy when the temperature goes back up. Once solid, glycerin tends to stay solid, even at high ambient temperatures. This container of in-spec B100 was completely liquid until it was cooled off in a refrigerator at 40°F/4°C. At that temperature, a solid lump of glycerin formed and settled to the bottom. This solid did not re-liquefy, even when it was heated well beyond normal on-equipment fuel temperature. Although the origins are somewhat different, many of the consequences of glycerin and gelling are the same. Cold weather causes soft solids to form, small quantities of which clog fuel filters and prevent fuel flow. This prevents engines from starting or stops them due to fuel starvation. In cold climates, more and more indoor garages are being built so that fleets can be parked inside over night to be sure the vehicles start in the morning.
Consequences of Soft Solids
Soft waxy solids will rapidly disable filters, regardless of the age of a filter. Where will these solids form? If fuel is delivered cold, then solids may be pumped into your tank by the supplier. If the fuel cools off in the bulk tank, then solids can fall out at that point. If your diesel doesn't get cold until it is already in the onboard fuel tank, then that is where it might solidify. Wherever they form, soft solids will quickly clog the first filter they encounter.
The image to the right (below) is an extreme case of a filter clogged with glycerin. Usually, you will see nothing this dramatic. Instead, your filter will likely appear clean, with only a faint waxy sheen to the media or a small amount of deposit in the bottom of the filter can. Here you see images of cellulose, average efficiency filter media under a Scanning Electron Microscope.
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