Efficiency
Although typically less than 40% of the energy in diesel fuel is converted to electricity in diesel generator sets (or gensets), there is limited room for improvement in diesel engine efficiency that can be achieved by modifying the fuel. This is due to the thermodynamic limit to the efficiency of the diesel cycle, losses due to friction in the engine, and inefficiency of the electric generator. Estimating the amount of energy that is left for the engine to produce electricity based on these limiting factors provides some context for the potential impact of fuel additives.
Even with the outstanding improvements in diesel engine technology afforded by electronic control systems and by adapting fuel injection to changing engine conditions, the overall efficiency of diesel combustion cycles is limited by thermodynamics. This much-studied subject of compression combustion shows that the theoretical maximum ability to convert the chemical energy of a fuel into work in an ideal compression combustion cycle is limited to around 60% [67]. The exact limit depends upon properties of the engine itself (e.g., compression ratio of the engine and when in the cycle the exhaust is expelled in relation to the completion of combustion) rather than the fuel. For non-ideal compression combustion in a relatively small diesel engine, the efficiency is limited to less than 40% primarily due to irreversible processes in the combustion [68].
Today in Alaska's utilities, efficiencies on the order of 15 KWh/gallon have been reported for utilities using relatively large engines running on available commercial diesel fuels. This represents an overall thermal efficiency of about 37%. Since the electrical generator itself is nominally 90% efficient (depending upon load) due to its mechanical losses, these diesel engines appear to be run very near their maximum possible efficiency. Smaller engines tend to be less efficient: in Alaska utilities smaller diesel engines are generally reported to be in the range of 12 to 14 kwh/gallon, corresponding to 30 to 35% overall thermal efficiency if they are running on No. 2 diesel.
Lower efficiencies are realized for all engines running at low loads and for smaller versus larger engines. These factors are important considerations in electrical utility powerhouse design and operation.
It seems obvious that much more energy could be used by extracting heat energy from the exhaust, where about 40% of the diesel fuel energy ends up. An additional 20% of the diesel fuel energy ends up heating the engine cooling water. Utilizing diesel gensets as combined heat and power plants has the potential to improve the system efficiency much more than fuel additives could. In the early 1980s, many plants incorporated recovered heat to schools or other facilities when relative fuel prices increased. As fuel prices decreased in the mid-1980s, several of those systems fell into disrepair. With fuel price increases of nearly 300% in the past ten years, such systems are now being repaired or newly designed. Partnerships between power plant owners and heat users such as water and sewer systems, schools and other facilities are increasing. Worth noting also is that the new low sulfur fuels emit less particulates and less sulfur-containing compounds (high sulfur fuels result in a high sulfuric acid dew point, resulting in rapid corrosion of exhaust stacks—new fuels don’t have this problem), allowing for better heat recovery systems.

FUEL ECONOMY
Here again, engine design is more important than fuel properties. However, for a given engine used for a particular duty, fuel economy is related to the heating value of the fuel. In North America fuel economy is customarily expressed as output per unit volume, e.g., miles per gallon. The fuel economy standard in other parts of the world is expressed as volume used per unit distance – liters per 100 kilometers. Therefore, the relevant units for heating value are heat per volume (British thermal unit [Btu] per gallon or kilojoules per liter/cubic meter). Heating value per volume is directly proportional to density when other fuel properties are unchanged. Each degree increase in American Petroleum Industry (API) gravity (0.0054 specific gravity decrease) equates to approximately two percent decrease in fuel energy content.
ASTM International specifications limit how much the heating value of a particular fuel can be increased. Increasing density involves changing the fuel’s chemistry – by increasing aromatics content – or changing its distillation profile by raising the initial boiling point, end point, or both. Increasing aromatics is limited by the cetane number requirement
4
3 Khair, Magdi: “Combustion in Diesel Engines,” ECOpoint Consultants, http://www.DieselNet.com
(aromatics have lower cetane numbers [see page 36]), and changing the distillation profile is limited by the 90 percent distillation temperature requirement. The API gravity at 60°F (15.6°C) for No. 2 diesel fuel is between 30 and 42. The specific gravity, at 60/60°F, and the density, at 15.6°C, are between 0.88 and 0.82. (See Chapter 4 – Diesel Fuel Refining and Chemistry for an explanation of fuel blending, density, and API gravity.)
Combustion catalysts may be the most vigorously promoted diesel fuel aftermarket additive (see page 81). However, the Southwest Research Institute, under the auspices of the U.S. Transportation Research Board, ran back-to-back tests of fuels with and without a variety of combustion catalysts. These tests showed that a catalyst usually made “almost no change in either fuel economy or exhaust soot levels.”4
While some combustion catalysts can reduce emissions, it is not surprising that they
do not have a measurable impact on fuel economy. To be effective in improving fuel economy, a catalyst must cause the engine to burn fuel more completely. However, there is not much room for improvement. With unadditized5 fuel, diesel engine combustion efficiency is typically greater than 98 percent. Many ongoing design improvements to reduce emissions may have some potential for improving fuel economy. However, several modern emissions control strategies clearly reduce fuel economy, sometimes
up to several percent.

In short, the higher the Cetane number the more easily the fuel will combust in a compression setting (such as a diesel engine). The characteristic diesel "knock" occurs when fuel that has been injected into the cylinder ignites after a delay causing a late shock wave. Minimizing this delay results in less unburned fuel in the cylinder and less intense knock. Therefore higher-cetane fuel usually causes an engine to run more smoothly and quietly. This does not necessarily translate into greater efficiency, although it may in certain engines.

Xl.4 Celane Number
X1.4.1 Cetane number is a measure of the ignition quality of the fuel and influences combustion roughness. The cetane number requirements depend on engine design, size, nature of speed and load variations, and on starting and atmospheric conditions. Increase in cetane number over values actually required does not materially improve engine performance. Accordingly, the cetane number specified should be as low as possible to assure maximum fuel availability.