This article is a culmination of my extensive experience with 2-EHN and the many articles I have written over the years on its use. It is one of my go-to components when developing and formulating diesel fuel additives.
The overall quality of diesel fuel depends on many factors. But when it comes to how your engine actually runs, diesel fuel quality is heavily dependent on its cetane number. The cetane number is an index of the ignition point, or combustion quality of diesel fuel. The standard UK and European EN590 diesel from the pump has a minimum cetane number of 51, with premium pump diesel slightly higher. By comparison, the ASTM D975 standard in the United States requires a minimum cetane number of just 40. This massive gap explains why American diesel owners often experience more dramatic improvements from cetane additives than European drivers do.
Depending on engine design and driving conditions, the optimal cetane rating for most vehicles is in the mid to high 50s. Any value greater than 60 may not always yield additional benefits and may alter ignition timing to the point that power is lost and emissions increase.
Matching cetane to the engine is essential to maximise performance. A fuel with a cetane number too low for a particular engine will result in reduced cold-start performance, rough running, increased engine noise, and worse combustion quality. This leads to reduced performance, excess emissions, and potentially carbon buildup throughout the engine and emission system components (intake, EGR and DPF).
A higher-cetane fuel correctly matched to the engine will reduce ignition delay, improve overall combustion quality, release more energy from the fuel, and improve performance and MPG. It will also reduce engine noise, deposit buildup, and exhaust emissions.
What is 2-EHN and How Does It Work?
Alkyl nitrates still offer the most significant improvement in cetane number, with measured increases of up to eight points. When it comes to alkyl nitrates, 2-Ethylhexyl nitrate (2-EHN) is the worldwide standard and the most respected. It offers more consistent ignition quality while reducing unwanted combustion conditions.
Other chemistries do exist, such as organic peroxides like di-tertiary-butyl peroxide (DTBP). However, DTBP provides less reactive methyl radicals than the highly reactive hydroxyl (OH) radicals provided by 2-EHN, making it less effective at equivalent dosage rates. Peroxides also present storage stability and safety issues, which is why 2-EHN remains the undisputed global standard after more than eighty years of commercial use.
To understand why 2-EHN is so effective, we have to look at the chemistry. Diesel fuel requires only the right combination of pressure and temperature to ignite, with no external assistance from a spark plug. The auto-ignition temperature of diesel fuel is around 220 degrees Celsius.
The chemical 2-EHN is formed by reacting 2-ethylhexanol with concentrated nitric acid. The manufacturing process requires strict quality control, particularly with respect to pH and water content. 2-EHN is slightly soluble in water, and excess moisture can lead to hydrolysis, degrading the chemical and potentially causing corrosion in fuel systems. When purchasing 2-EHN or products containing it, you want assurance that the raw chemical is double-filtered to ensure the lowest possible water content before blending.
Due to its chemical composition, 2-EHN decomposes at a much lower temperature than diesel fuel, starting at just 130 degrees Celsius. This exothermic decomposition breaks chemical bonds and releases highly reactive free radicals, leading to successive chain-branching fuel reactions that initiate combustion earlier and more smoothly during the compression stroke.
By initiating combustion sooner, 2-EHN reduces the "ignition delay" (the period between the start of fuel injection and the onset of combustion). A shorter ignition delay means the fuel burns longer and more completely, allowing pressure to rise more smoothly in the chamber. This progressive rise in pressure is exactly what eliminates the harsh diesel "knock" and reduces engine vibration.
High Cetane HVO and GTL Fuels
It is crucial to understand that higher cetane from paraffinic fuels like Hydrotreated Vegetable Oil (HVO) or Gas-to-Liquids (GTL) does not always deliver the same effect as the cetane delivered by 2-EHN, but it still reduces the need for cetane improvers like 2-EHN, because their cetane numbers are naturally high (often 70 to 90).
I have reviewed combustion pressure trace tests, indicating that 2-EHN can still provide some improvement, but nowhere near the level of conventional pump diesel. The reason comes down to the chemistry of ignition delay. Because paraffinic fuels combust so quickly on their own, the fuel essentially "outruns" the additive. The fuel has already ignited before the 2-EHN can fully decompose at 130 degrees Celsius and release its free radicals. In these highly paraffinic fuels, adding 2-EHN provides rapidly diminishing returns.
The Reality Behind Popular Diesel Additives
Fuel additive manufacturers now recognize the benefits of boosting the cetane number and using 2-EHN, so most offer cetane improvers. The question in this case is, what are you getting for your money?
A few years ago I reviewed test results from arguably the most popular diesel additive in the UK. A household name with substantial brand history and recognition. This particular product, which comes in an attractive 500ml bottle with a novel integrated spout, has also gained significant popularity throughout Europe. This prompted a respected colleague to arrange a series of comprehensive tests, including FTIR analysis (to determine chemical composition) and titration (to measure detergent strength).
The results confirmed what we already suspected. The product is essentially 2-Ethylhexyl Nitrate (2-EHN) diluted in a solvent with a fatty acid lubricant and negligible detergent content. In other words, it is primarily a glorified cetane booster.
The market is flooded with brands selling thousands of bottles of diesel treatments daily that contain little more than diluted 2-EHN with a substandard lubricant. And yet, they often receive glowing reviews. Why? Because 2-EHN will make a noticeable difference to most diesel engines, and additive manufacturers know this.
These companies focus on the one function that customers are most likely to notice: a more responsive engine from the cetane increase. It is much harder for the average user to discern improved lubrication or effective cleaning. What is particularly concerning is that these products typically contain less than £1.50 worth of chemicals in a 500ml bottle, despite their premium pricing. The single-shot treatments usually contain less than £0.50 worth of chemicals.
The Cetane Response Curve and Additive Claims
There is only one industry-wide accepted Cetane Response Curve for predicting how much 2-EHN is needed to raise the cetane number of diesel fuel. This graphed data, developed over two decades ago, depicts three anticipated response levels: high, average, and low response fuels.
"Higher response" fuels show greater improvement in cetane numbers with the addition of 2-EHN than the equivalent percentage of 2-EHN in "average responding" and "low responding" fuels. The curve also demonstrates diminishing returns. Adding more 2-EHN yields progressively smaller increases in cetane number as the concentration rises.
Unfortunately, many diesel fuel additive suppliers intentionally use curve data to substantially overpredict the average, typically expected cetane engine number response of their products. They do this by using only the cetane improver dosage rates shown for "high response" fuels to predict increases across all fuels. When using only high-response fuel data, the required volume of the cetane-improver component will be dramatically reduced.
Since cetane improvers typically constitute the largest volume percentage in premium diesel fuel additive formulations, this overprediction drastically reduces the price of fuel additive products but yields less-than-desired results for the consumer. The associated costs for conducting cetane engine tests to determine the true cetane number can easily exceed £200 per sample. As a result, the buyer is often forced to rely solely on the integrity of the supplier.
Dosage and Application Guidelines
Commercial cetane improvers typically require dosage rates between 0.5 and 4 ml per litre of diesel fuel, with most applications falling in the lower ml/L range. The optimum amount of pure 2-EHN is around 20 to 100ml per tank of fuel, depending on the engine and base cetane level. The relationship between dosage and cetane improvement is approximately linear within normal operating ranges. Each ml per litre of a typical commercial product (containing 20 percent 2-EHN) provides roughly a 1 to 2 point cetane number increase.
| Treatment Level | Dosage Rate | Application Scenario |
|---|---|---|
| Light Treatment | 0.2 to 1 ml/L | Maintenance applications / high-quality base fuel |
| Standard Treatment | 1 to 3 ml/L | Significant performance improvement / average fuel quality |
| Heavy Treatment | 3 to 5 ml/L | Exceptional performance improvement / problem fuels |
Overdosing is a genuine risk. Excessive cetane improver use, typically defined as exceeding 5 ml/L, can cause several problems, including engine-running difficulties, injector deposit formation, and excessive exhaust emissions. Some of these can occur below 5 ml/L, depending on the base fuel and how the treated engine responds to additional cetane.
The Lubricity Problem
While 2-EHN is exceptional at improving combustion, it has a significant drawback: it can reduce fuel lubricity. Modern Ultra Low Sulfur Diesel (ULSD) already suffers from poor lubricity due to the desulfurization process. Adding neat 2-EHN to ULSD without a supporting lubricant package is a recipe for accelerated wear on fuel pumps and injectors.
If protecting your fuel system is important to you, verify what lubrication technology the product uses. Most additives on the market still use cheap mono fatty acid technology. I know this because I have seen the FTIR analysis for many of them. A recent YouTube video that tested many mainstream brands also confirmed this. Outdated diesel lubricity additives are mostly ineffective when combined with 2-EHN, as the cetane improver actively interferes with the fatty acid's ability to form a protective film, reducing the HFRR response.
Caveat: It is worth noting that many UK pump fuels now have a lower HFRR wear scar (better lubricity) out of the pump due to the mandatory inclusion of FAME biodiesel (up to 7% under EN590). Biodiesel is an excellent natural lubricant. In these specific fuels, the lubricity reduction from 2-EHN is not as severe an issue as it was a decade ago. However, relying on pump fuel consistency is a gamble.
For optimal protection, look for products that use modern ester lubricants that are not negated by 2-EHN.
Should I Use a Cetane Improver?
Active cetane improvers are essentially a form of combustion modification. However, when combined with the right detergent, fuel catalyst technology and lubricity additives, they can turn even mediocre pump fuels and biodiesels into super diesel that outperforms the best premium pump fuels.
The key is to look for products that deliver multiple functions. True value comes from formulations that provide effective cleaning, robust lubricity, fuel stabilisation, and combustion catalysts alongside the cetane boost. Do not be fooled by diesel additives marketed as premium products (and there are a lot of them) that are little more than diluted 2-EHN.
When we formulated AR6900-D for Archoil, our goal was to pack as much value as possible into the product, using only high-end, proven components. The result includes a pure 2-EHN base, a comprehensive detergent package, a fuel-borne catalyst (FBC), and an ester lubricant that demonstrably outperforms the cheaper variants. The ester costs approximately 3 times as much as traditional diesel lubes.
Oilsyn Diesel Power DNA is similar but takes it a step further, with even more lubricant based on its Orisyn technology, and an enhanced detergent package fortified with polyether-amine to improve ongoing carbon removal.
A 50ml dose of Oilsyn Diesel Power DNA from a 1L bottle can contain more detergent performance and overall value than some single-bottle treatments that sell for over £10 per bottle.
Summary
The best advice we can offer is to pay closer attention to what you put in your fuel tank and not be swayed by marketing claims alone. Look for products with transparent information about their formulation and test data to support their performance claims.