Diesel, in relation to other fossil fuels, is an exceptionally dense fuel. There are a variety of properties that are inherent in dense fuels. Because of the inherent properties of diesel, diesel engines are extremely energy efficient both respect to combustion efficiency and thermal efficiency. 

However, while diesel is extremely energy dense and diesel engines are more thermally efficient and have higher combustion efficiency rates than lighter fuels with low energy density, capturing the full potential of diesel can be difficult. 

Diesel engines — while considerably more efficient and less polluting than a gasoline engine and alternative fuel engines — still have limitations. But, the same inherent traits of diesel that make diesel engines efficient give it the potential  — as technologies advance — to generate even higher efficiency rates and lower emissions. 

The properties of diesel — with respect to fuel efficiency, combustion efficiency, and thermal efficiency rates — govern the energy and emissions outputs of diesel engines according to several laws of physics. Ideal Gas Law and the first and second laws of thermodynamics determine the environmental and economic value of diesel engines. As technologies improve efficiency and output rates should increase according to the laws of physics.

While we can develop technologies to increase the efficiency in which we use diesel, the laws of physics also tell us that gasoline and alternative fuels have limitations with respect to how much more we can improve the efficiency of the engines that combust them. 

The principal reason gasoline has greater potential to improve is because of its energy density and stability. Gasoline and alternative fuels are much lighter and less energy dense fuels and for that reason, improving gasoline and alternative fuel engines is extremely difficult. 

In other words, diesel is the ideal fossil fuel, potentially. 

The energy density and stability of a fuel determine potential fuel efficiency and potential energy output. 

Efficiency Potential of Not Just Diesel, but Diesel Engines  

Of all liquid fossil fuel combustion engines, diesel engines are the most thermally efficient and have the highest rates of combustion efficiency. Furthermore, diesel engines produce the least amount of emissions, save nitrous oxides. There are two reasons diesel engines are the best fossil fuel combustion engines with respect to efficiency and emissions: the energy density and compression resistance of diesel fuel. Because of energy density and the compression resistance of fuel, diesel engines have higher rates of both combustion efficiency and thermal efficiency.

Diesel has a High Energy Density

Diesel engines highly fuel efficient, for one, because on a volume scale — gallon, liter, square foot or meter — diesel has a much higher energy density than most other solid, liquid, and gas-state fossil fuels. Diesel certainly has a higher energy density than gasoline, natural gas (methane), and propane. 

Higher energy density means there is more energy per volume unit of measure — more energy per gallon — to push a vehicle down the road. The reason diesel has a higher energy density than other fossil fuels is that the hydrocarbons in diesel — the valuable components in every fossil fuel that ignites/burns/combusts — are made of long and complex molecules, molecules with very high carbon-to-hydrogen ratios.

“The calorific value of diesel fuel is roughly 45.5 MJ/kg (megajoules per kilogram), slightly lower than petrol which is 45.8 MJ/kg. However, diesel fuel is denser than petrol and contains about 15% more energy by volume (roughly 36.9 MJ/liter compared to 33.7 MJ/liter). Accounting for the difference in energy density, the overall efficiency of the diesel engine is still some 20% greater than the petrol engine, despite the diesel engine also being heavier.”

The greater the number of carbon atoms in a hydrocarbon, in relation to hydrogen atoms, the higher the fuel density of that fuel. On a volume scale, the energy density of diesel is between 15% and 25% greater than gasoline. The difference is dependent on whether or not the gasoline is low or high octane. The higher the octane of gasoline, the lower it’s energy density. The reason being, the additives used to increase gasoline octane have lower energy densities than gasoline. 

In other words, octane additives dilute gasoline. 

Diesel naturally has exceptional compressive resistance because it is a heavy fuel, stable fuel made of large, long hydrocarbon molecules. 

Diesel has High Compression Resistance 

The second reason diesel is highly efficient with respect to fuel efficiency is because diesel is a very heavy fossil fuel. As such, diesel is a very stable fuel. The stability of diesel is the reason diesel engines with high compression ratios are possible. Compression ratio plays into both fuel efficiency and emissions. Compression ratio is particularly important with respect to reducing emissions. The higher the compression ratio, the lower the emissions.

Diesel Engines are More Thermal Efficient than Other Fossil Fuel Engines

The third reason diesel engines are more efficient than any other liquid fossil fuel engine is that of thermal efficiency. Thermal efficiency is the total amount of energy generated by an engine’s combustion of fuel that becomes mechanical energy, an energy that pushes a vehicle down the road. The thermal efficiency of diesel engines is far greater than that of any other type of liquid fossil fuel engine. 

The thermal efficiency of diesel engines is, in part, due to the energy density and compression resistance of diesel fuel. 

Technologies that Increase Diesel Engine Compression Ratio

There are a number of means by which to increase the fuel efficiency and combustion efficiency — one of the most important variables with respect to emissions — of diesel engines. One of the simplest means of increasing fuel and combustion efficiency is to increase the compression ratio of an engine. 

What Compression Ratio Is

Compression ratio is a measure of how much an engine compresses a gas or vapor-state fossil fuel once the fuel is in the combustion chamber. In piston/cylinder engines, the compression ratio is a measure of the difference between two piston positions, top and bottom center. The ratio difference between when a piston is at bottom center — when there is the most volume in an engine’s cylinder — and when the piston is at top dead center — when the cylinder has the least volume — is compression ratio.

The higher the compression ratio of an engine, the more it compresses a vaporized or gas-state fuel. The more a fuel is compressed, the hotter it gets prior to combustion. The hotter a fuel is before compressions, the more efficiently it combusts.

The weight and stability of diesel fuel mean it is highly resistant to compression. The compression resistance of diesel means that when it does ignite — being highly compressed, thus extremely hot, — diesel burns far more completely than other fuels with less compression resistance. 

Technologies that Increase the Compression Resistance of Diesel  

Already highly resistant to compression, there are technologies available that can further increase the compression resistance of diesel. By increasing the compression resistance of diesel, again, it is possible to increase both the fuel and combustion efficiency of diesel. There are a variety of technologies that increase the compression ratio of a diesel engine. 

Some technologies that increase the compression resistance of diesel are old concepts with new advances. Others are new concepts that have yet to take hold in the diesel engine sector. Even others are old concepts that have potential, theoretically but have yet to be fully developed. And there are also new, proven technologies that work well with respect to increasing a diesel engine’s compression ratio. 

Cetane Additives

The easiest way to increase the compression ratio of a diesel engine is with cetane additives. The same way additives can increase the octane of gasoline, cetane additives increase the compressive resistance of diesel fuel. The difference is gasoline engines are set at a particular compression ratio while diesel engines have the potential for variable compression ratios. 

Once gasoline engine piston reaches a preset compression ratio, the engine’s sparks fire and drives the piston down. But, diesel engines are not spark fired. Diesel engines are compression engines. That means the diesel does not combust in the combustion chamber until it — the diesel — reaches a certain temperature. The temperature rise in diesel is a product of compression. A diesel engine compresses — therefore heats — diesel until it finally combusts. 

Increasing the flash point temperature of diesel — the temperature at which it combusts — increases the compression ratio. 

Drawbacks of Cetane Additives

While increasing the compression ratio increases the combustion efficiency of a fuel, which increases total energy output and the total amount of the hydrocarbons that combust, it is a measure of the total fuel available. Combustion efficiency is a percentage of the total hydrocarbons in a combustion chamber that burn up. The problem is, cetane additives actually decrease the total hydrocarbons in a fuel. 

In other words, while the compression ratio increases the combustion efficiency of the total energy available, cetane additives reduce the total amount of energy available. So while a greater percentage of the fuel available burns up during combustion, cetane additives mean a lower amount of total fuel in a combustion chamber at any given time.  

Common Rail Fuel Injection

Though the Common Rail Fuel Injection System has been used in diesel trucks and pickups since the mid-2000s — and since 1997 in diesel cars and marine engines, — it is a new technology relative to the age of diesel engines. The Common Rail Fuel Injection System was a major breakthrough both with respect to fuel efficiency and emissions. 

From the 1950s until the new millennium, diesel fuel systems used direct injection. “Direct injection systems pump diesel through injectors mounted directly in the cylinder and feeds fuel to a combustion chamber machined into the top of the cylinder itself.”  Direct injection systems have a low-pressure fuel pump that feeds each injector. The problem with traditional direct injection is that large droplets of diesel feed into the combustion chamber. 

The larger the individual droplets of fuel, the less completely they burn. By reducing the size of the droplets fed into a diesel combustion chamber, the total surface area of each droplet increases which promotes oxygenation. The greater the rates of oxygenation of each droplet, the more complete each droplet ignites/combusts/burns. 

And that is what a Common Rail Fuel Injection System does, dramatically reduces the size of the droplets of diesel that are fed into the cylinders of the engine. By vaporizing the air/fuel mixture, a Common Rail Fuel Injection System increases combustion efficiency dramatically, but nothing compared to Supercritical Fuel Injection

Supercritical Fuel Injection

The combustion efficiency generated by a Supercritical Fuel Injection System is greater than Common Rail Fuel Injection System. Combustion efficiency means greater fuel efficiency and lower emissions, much lower emissions. “Researchers in New York have demonstrated a supercritical diesel fuel-injection system that can reduce engine emissions by 80 percent and increase overall efficiency by 10 percent.”

While diesel engines are almost always more fuel efficient than gasoline engines, they have a tendency to produce more oxides of sulfur and oxides of nitrogen. Additionally, because diesel is a heavy fuel with extremely complex hydrocarbon molecules, the combustion efficiency is less than perfect. It is the combustion efficiency of diesel engines that a Supercritical Fuel Injection system improves. 

According to George Anitescu, a research associate at the Department of Biomedical and Chemical Engineering at Syracuse University in New York state, a Supercritical Fuel Injection System puts diesel in a state that is between liquid and gas. 

“The high molecular diffusion of supercritical fluids means that the fuel and air mix together almost instantaneously. So instead of trying to burn relatively large droplets of fuel surrounded by air, the vaporized fuel mixes more evenly with air, which makes it burn more quickly, cleanly, and completely. In a sense, it is like an intermediate between diesel and gasoline, but with the benefits of both.” 

Unfortunately, Supercritical Fuel Injection is still in the design and development state. There are several issues engineers must contend with prior to Supercritical Fuel Injection becoming commercially viable. For one, in order for diesel to reach a supercritical state, its temperature must be around 450 degrees Celsius. Such temperatures can cause diesel to solidify and choke. “Coking occurs when hydrocarbons in the fuel react, producing sticky deposits that can lead to fuel-system failures.” While there are a number of solutions to the problems presented when attempting to move diesel into a supercritical state, Supercritical Fuel Injection is not yet a complete practical application. 

Fuel Catalysts

While they have been around for a while, fuel catalysts are still an aftermarket product. Scientists and engineers — even laymen — have been aware of the problem of oxygenizing diesel fuels. As far back as 1936, Chinese fishermen tapped magnets around fuel lines in order to break apart the hydrocarbon clusters inherent in diesel fuel. Breaking up the hydrocarbon clusters increases the oxygenation potential of diesel. In the 1980s, ranchers in Wyoming made national news doing the same, though on gasoline engines. The Washington Post reported, 

“Although cow magnets have been sold in farming areas for half a century, the market has been limited until recently. Now it’s booming as never before since word got around that somehow the magnetic field can boost a car’s gas mileage by improving combustion if a pair of cow magnets is fastened to a car’s gas line close to the carburetor.”

Whether the magnets truly helped the Chinese fishermen and Wyoming’s cowboys achieve higher rates of fuel efficiency is debatable. What is not is the fact that — in principle — they were onto something. They knew that by increasing the oxygen content in a diesel fuel mixture, they could increase fuel efficiency. 

Modern fuel catalysts — made of similar precious metals as those found in catalytic converters — are also engineered to increase diesel fuel oxygenation. But unlike injectors that oxygenate diesel by increasing the surface area of diesel fuel droplets, fuel catalysts change the physical state of diesel on a molecular level. 

By depolarizing the charges that bind diesel fuel hydrocarbons into clusters, fuel catalysts expose the individual hydrocarbon molecules and molecule chains to oxygen. Again, the greater the oxygenation of a fossil fuel, the more completely it burns. The Rentar Fuel Catalyst, for example, increases fuel efficiency by between 3% and 8% on over the road vehicles and to a greater extent on heavy equipment, machinery, and marine engines.

Without question, the latest advances in diesel technology are making the world’s most efficient fossil fuel produce even more energy and, at the same time, reducing the emissions associated with diesel.