The future for fuel

For new engine and emission control technologies to fulfil their potential, petrol and diesel fuel must be improved and standardised on a global basis.

By Dr Klaus-Peter Schindler, Volkswagen AG

At the start of the twenty-first century people have higher expectations of individual mobility then ever. Society now demands that this is also environmentally sustainable.

Hydrocarbon-based fuels (most importantly diesel and petrol) are still the main transport energy sources. Their many advantages, including high energy density, low cost and wide availability, guarantee that this is likely to be the case for the next 40 years.

To meet customer, environmental and energy efficiency challenges, the automotive industry is exploring advanced propulsion technologies and ways of improving existing technologies. However, fuels are also engineering elements, and can contribute decisively to an engine's operation and emission control characteristics. The engine features most affected by fuel properties are:


Power output, torque, acceleration, speed and engine durability
Emission behaviour and environmental performance

To use energy in the most efficient manner, while maintaining engine performance, it is essential that engine and fuel developments are interlinked. In December 1998, in an effort to harmonise fuel quality globally, the automotive manufacturers' organisations of Europe, USA and Japan developed fuel specifications to be applied worldwide. These were published in the Worldwide Fuel Charter (WWFC), which also sought to take into consideration customer requirements and vehicle emission technologies. The objective was to develop worldwide recommendations for quality fuels that would have the following benefits:


Reduce the impact of motor vehicles on the environment, through reduced vehicle emissions
Consistently satisfy customer performance expectations
Minimise vehicle equipment complexities with optimised fuels for each emission control category, thus incurring less customer cost and increasing satisfaction

ACEA recommends fuel properties that are in line with the most stringent fuel category in the WWFC, so that they comply with the strict emissions regulations in force in most industrialised countries.

The future for petrol

With petrol, the octane number and sulphur content have the most significant effect on vehicle emissions and performance. The octane number is the measure of petrol's ability to resist auto-ignition, which can cause engines to knock. There are two numbers that are relevant in discussions on octane numbers: the research octane number (RON), which corresponds to low-speed, mild-knocking conditions, and the motor octane number (MON), which corresponds to high-temperature knocking conditions and part-throttle operation. The numerical difference between RON and MON is generally around ten, and a minimum for MON of 82.5 (DIN EN228, unleaded fuel) is recommended. If a customer uses petrol with an octane number lower than that for which the vehicle is designed and calibrated, knocking may occur, which can cause severe engine damage. Engines equipped with knock sensors can handle lower octane levels by retarding the spark timing, but this decreases fuel consumption and power, and at very low octane levels knocking will still occur.

Stringent omission requirements, combined with the need for life compliance, demand extremely efficient and durable after-treatment systems. It is generally recognised that catalyst hydrocarbon efficiency at 100,000 miles must be at least 97 per cent for a vehicle meeting ULEV/Euro 4 standards. However, sulphur deposits in the catalytic converters and on the EGO sensor reduce longevity. The manufacturers' goal of decreased fuel consumption also requires lean-burn technologies with the potential to reduce fuel consumption by 15 to 20 per cent. These technologies demand new exhaust-gas after-treatment systems with NOx control devices, but these are damaged by the sulphur in petrol. A reduction of sulphur in petrol and diesel to 10ppm is also necessary. In Europe the sulphur content of fuels will be reduced to 50ppm by legislation in 2005, but this is not enough to encourage the continued development of advanced exhaust-gas after-treatment systems.

Other important properties of petrol are its oxidation stability and the levels of lead, phosphor, alcohol, olefins, aromatics, benzene and oxygen. The stability of the oxidation of petrol should not be less than 480 minutes, to prevent the formation of components that could lead to corrosion and deposits.

Lead alkyl additives were used historically as inexpensive octane enhancers. Many markets have eliminated the use of lead in petrol, due to its negative health effects and its damaging effect on vehicle emission control technologies. In 'leaded' markets a maximum of 0.05g per l is tolerated, although Europe recommends a limit of 0.005g per l, to protect modern catalytic converters.

Like lead, phosphor damages catalysts and therefore should be eliminated from petrol. The amount of alcohols contained in petrol should not exceed 1Vol.-per cent as they Iead to the formation of aldehydes - they are only tolerated as a by-product of ether. Olefins are thermally unstable and can lead to deposits of hydrocarbons in the intake system. The emission of reactive, ozone-forming and toxic compounds can also occur. The level of olefins in petrol should not exceed a maximum of 10Vol.-per cent.

Aromatics are organic compounds that contain at least one full benzene ring. In general, they are high energy density fuel molecules and are good octane components of petrol. However, they can increase C02 emissions and engine deposits. Aromatics tend to produce benzene in vehicles with catalytic converters (pro-benzenes). Therefore, their content should not be higher then 35Vol.-per cent.

Benzene is a carcinogenic compound and its presence in emissions or as a component of petrol is unacceptable. The control of benzene levels in petrol is the most effective and direct way to reduce human exposure to benzene. A limit of 1Vol.-per cent is recommended.

Oxygenated compounds, in the form of ethers, are often added to petrol to increase the octane, to extend petrol supplies or to induce a lean shift in engine stoichiometry to reduce carbon monoxide emissions. The oxygen content in petrol resulting from these additives should be a maximum of 2.8 Vol.-per cent. It should consist of compounds with more than five C-atoms and a boiling-point below 215癈, but not methanol. If the oxygen content is too high, unwanted discontinuities (steps) can result in the boiling curve and lead to a corresponding increase in fuel consumption.

The last important parameter with petrol is its boiling point. A reduction of the final boiling-point leads to a reduction of CO and HC emissions as well as a reduction in deposits at the intake port (a high portion of the matter boiling above 200癈 is deposited on the inlet valves). Therefore, the final boiling-point should be a maximum of 195癈. The progress of the boiling curve should be well defined, to enable optimum tuning to the fuel type.

The future for diesel

Diesel technologies are becoming ever more important in the drive to decrease world energy consumption. Their high efficiency favours their employment in a wide range of vehicles. The main pollutants formed by diesel combustion are particles and NOx. Diesel vehicles, as with all modern vehicles, must comply with increasingly strict emission standards. The emission limits of the future will require advanced after-treatment systems, such as continuous regeneration, trap or particle filters and NOx storage catalysts. Diesel fuel quality plays an important role in the development of these systems and in the performance of the vehicle.

With diesel fuel the cetane number is a key indicator of fuel quality. The cetane number is the measure of the compression ignition behaviour of a fuel. This influences engine startability, exhaust emissions and combustion noise. The cetane number is measured on a test engine and reflects the effect of cetane improver additives. Increasing the cetane number decreases the engine crank time at a given engine speed. Cetane number has also been shown to reduce NOx emissions by up to 9 per cent. Natural cetanes, called cetane index, are calculated on measured fuel properties and have also been shown to reduce fuel consumption. The automotive industry worldwide has called for the cetane number to be set at a minimum of 58 and the cetane index at 54.

The level of sulphur in fuel is critical to the development of after-treatment technologies for lean-burn combustion systems. Diesel combustion is the most fundamental lean-burn system and hence also requires the desulfurisation of its fuel.

Viscosity and density are physical parameters in diesel that affect engine power and, consequently, engine emissions and fuel consumption. Emissions testing has shown that lower-density diesel reduces emission of hydrocarbons, carbon monoxide, NOx and particulates. However, due to the volumetric fuel injection in diesel engines, lower-density fuel also decreases power output and increases fuel consumption. Minimum and maximum density values for diesel are therefore required for the tuning of engines. A range of 820-840kg per m3 has been calculated to be the most suitable.

The formation of particulates by diesel engines is greatly influenced by the aromatic content and final boiling point of diesel fuel. The polycyclic aromatics present in the fuel directly correlate with polyaromatic hydrocarbons (PAH) found in the exhaust, which contribute considerably to the formation of particulates. Hence, maximum values of 10wt.-per cent for total aromatic content and 1wt.-per cent for polyaromatics are recommended in diesel fuels. The final boiling-point should be reduced to 350癈, as above this PAHs are formed.

Conclusions

Mobility in modern society depends on technologies based on petrol and diesel fuels, and this will continue, in all probability, for the next 25 years. To help motor vehicles achieve their maximum potential in terms of emissions and power performance, fuels must be designed and developed to suit specific engines. The globalisation of economic activities forces customer-driven harmonisation of technical products.

The worldwide availability of standard petrol and diesel fuel of the required quality is, therefore, a prerequisite for the market.

Author

Dr Klaus-Peter Schindler received his doctorate in physics from the Technical University of Braunschweig in 1979. He began his career with Volkswagen in 1980, researching diesel engines. He then became manager of European research projects for Volkswagen, before moving to his present position as manager of authorities and regulations in powertrain development
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