Diesel density and stability - In this blog, the last in our five-part Myth Buster series, we explore the myth that diesel density is too close to petrol to justify claims that it is superior. We also look at the perceived need to add chemical stabilisers to ensure fuel stability.

Myth: Diesel density is too close to petrol to make a real difference 

The energy density of fossil fuels is particularly important in the transportation sector. The most significant benefit of fuel density is fuel economy, thereby inferring that the higher the density, the greater the range of a vehicle. The fuel density of conventional diesel is around 836kg/m³ compared to an average of 744kg/m³ for conventional petrol. While this equates to a density difference of ‘only’ 11 percent, the real difference is in fuel economy. 

Comparing the fuel efficiency of a diesel-powered vehicle with a petrol equivalent of a similar size clearly illustrates this point, with a diesel engine getting 25 to 35 percent better fuel economy than a petrol engine. On top of that, diesel engines have far more low-end torque than their petrol counterparts, presenting them with around 10 to 15 percent more tractive effort. These figures enable diesel vehicles to cover a longer distance than petrol vehicles of comparable size.

The density matter applies equally to other types of fossil fuels for road transport, most of whom have considerably lower fuel densities than diesel. Diesel is exceptionally high in energy in relation to most other fossil fuels. It is made of large, long-chain hydrocarbons that give diesel a high energy density because of their size and length. Hence, diesel’s density and heating values provide the ideal balance for fuel economy and tractive effort.

It’s a near certainty that the new generation of diesel fuels, engines, and equipment will, therefore, continue to play a dominant role in powering essential services and key sectors of the economy.

Myth: Chemical additives are necessary to improve fuel stability

Since diesel fuel is an organic product, it can degrade, oxidise, and break down from the time it is refined until the time it is consumed. By determining its oxidation rate (stability) through laboratory testing, it is possible to define the extent of degradation between the time of manufacture and time of use. This rating is significant because as diesel fuel oxidises, it generates fine, free carbon particulates that can congest fuel injectors or filters.

The need for a stability additive varies widely from one fuel to another – it all depends on the crude oil source and how the refinery processes and blends the fuel. Stability additives involve some complicated chemistry that typically blocks one step in a multi-step reaction pathway. Because of this complexity, the best results are obtained when the additive is added immediately after fuel manufacturing. 

Acid-base reactions are another type of fuel instability. The industry typically uses stabilisers comprising strongly basic amines with a concentration range of 50 ppm to 150 ppm to counter these reactions. These stabilisers react with weak acidic compounds to form products that remain dissolved in the fuel but without further reactions.  

Local refineries produce fuel according to the South African National Standards (SANS) of which the SANS 342 standard applies to diesel. These companies spend vast amounts of money and engineering resources on developing a specific additive-fuel package. Therefore, buying fuel with an unknown origin or brand can cause severe damage to your fleet. The same applies to using non-approved aftermarket additives, some of which causes corrosion, injector deposits and so on, all of which reduces asset life. 

While there are some decent aftermarket stability additives available, an additive that is effective in one fuel may not be effective in another. Diesel users should also note that other aftermarket performance additives can negatively affect the fuel’s stability. Lastly, bear in mind that fuel stability was much more of an issue in the high-sulphur diesel fuels of yesteryear. In contrast, today’s low-sulphur diesel is much more stable and reliable.