Engine technology

GASOLINE – Spark-ignition, gasoline fuelled engines are the leading power source of passenger cars. Spark-ignition engines are simple and cheap when compared to compression-ignition diesel engines. In addition, stoichiometric air-to-fuel ratio allows usage of three-way catalyst (TWC), which is capable of reducing carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NO_x) emissions simultaneously and efficiently. A drawback of spark-ignition engines is their lower efficiency when compared to compression-ignition engines. Therefore fuel consumption of spark-ignition engines is higher than that of diesel fuelled engines both in energy and in volumetric terms.

Gasoline cars equipped with carburetor engines were available until the late 1980s. Today, spark-ignition engines are port-injection engines, mostly equipped with multi-point fuel injection (MPFI, fuel injected into the intake port). In the 1990s, direct-injection spark-ignition engines with higher efficiency and lower fuel consumption appeared on the market. Models using lean combustion with excess air were also introduced in the 1990s, but they soon disappeared from the market. Spark-ignition engines, whether in-direct- or direct-injection, are now based on a stoichiometric air/fuel ratio, and are equipped with TWC catalyst.

The exhaust emissions from spark-ignition engines using a stoichiometric air/fuel ratio can be efficiently controlled with a three-way catalyst (TWC). In TWC which carbon monoxide and unburnt hydrocarbons are oxidized simultaneously with the reduction of nitrogen oxides. With TWC even more than 90% reduction in engine-out CO, HC and NO_x emissions is achieved, and emissions occur mainly at cold start or heavy acceleration. However, in some conditions TWC catalyst may generate ammonia and nitrous oxide emissions. TWCs operate efficiently only in a very narrow lambda window close to the stoichiometric air/fuel ratio and therefore TWCs cannot be used in engines running with a lean mixture, such as diesel engines. The benefit of a lean mixture would be improved fuel consumption, but at the cost of increased NO_x emissions. Exhaust gas recirculation (EGR) is one of the common technologies used for reducing the NO_x emissions of diesel engines, and it is also used in spark-ignition engines. For direct-injection spark-ignition cars, particulate matter emissions are high, and therefore particulate filters may become necessary.

Spark-ignition engines today are less sensitive toward fuel than older engine generations, and absolute mass emissions are low. However, at cold starts, heavy driving conditions, and at low temperatures, there may be large differences, absolute and relative, between fuels for all cars. In the past, carburetor engines were especially sensitive toward fuel, for example, drivability and vapor lock problems were experienced. Most of the gasoline-fuelled cars today can tolerate at least up to 10 vol-% ethanol in Europe and the U.S.

DIESEL – Due to their high efficiency compression-ignition diesel engines are the leading power source in heavy-duty vehicles, because of their high efficiency. Today diesel engines are becoming more popular also in light-duty cars. Emission control devices and internal engine solutions have crucial effects on the exhaust emissions. Diesel engines are running on a lean mixture, which improves fuel consumption, but at the cost of increased nitrogen oxide emissions (NO_x). NO_x emissions are formed from nitrogen in the air at high temperatures. High particulate matter (PM) emissions are another problem of diesel engines.

Selective catalytic reduction (SCR) and exhaust gas recirculation (EGR) are the common technologies used for reducing the NO_x emissions of diesel engines. EGR is an internal engine technology, whereas SCR is an exhaust after-treatment device using a reducing agent, such as ammonia or urea. With EGR some of the exhaust gas is returned to the engine cylinders, which lowers the combustion temperature and consequently NO_x emissions. High EGR ratio may lead to problems with engine cleanliness, and particulate matter emissions may increase. Oxidation catalyst reduces volatile organic emissions. Particulate filters reduce efficiently particulate matter emissions.

Composition of gasoline and diesel

Both gasoline and diesel fuel consist of hundreds of different hydrocarbon molecules. In addition, several bio-origin components, such as ethanol in gasoline blending, are common.

Gasoline contains mainly alkanes (paraffins), alkenes (olefins), and aromatics. Diesel fuel consists mainly of paraffins, aromatics and naphthenes. The hydrocarbons of gasoline contain typically 4-12 carbon atoms with boiling range between 30 and 210 °C, whereas diesel fuel contains hydrocarbons with approximately 12–20 carbon atoms and the boiling range is between 170 and 360 °C. Gasoline and diesel fuel contain approximately 86 wt-% of carbon and 14 wt-% of hydrogen but the hydrogen to carbon ratio changes somewhat depending on composition.

Paraffinic hydrocarbons, especially normal paraffins, improve ignition quality of diesel fuel, but low-temperature properties of these paraffins tend to be poor. Aromatics in gasoline have high octane numbers. However, aromatics and olefins may worsen engine cleanliness, and also increase engine deposits, which is an important factor for new sophisticated engines and after-treatment devices. Aromatics may lead to carcinogenic compounds in exhaust gases, such as benzene and polyaromatic compounds. Olefins in gasoline may lead to an increase in the concentration of reactive olefins in exhaust gases, some of which are carcinogenic, toxic or may increase ozone forming potential. Additives may be needed to ensure adequate properties of gasoline and diesel fuel.

Traditional gasoline and diesel fuel are not covered extensively in the "AMF Fuel Information System". Instead, focus is given to alternative blending or replacement options of gasoline and diesel. However, engine technology together with legislation and standards for gasoline and diesel are discussed briefly.

Gasoline – legislation and standards

The engine and after-treatment technology impose requirements on fuel quality. Basic fuel analyses were developed to screen general performance and operability of fuels in internal combustion engines. Fuel properties important in environmental contexts, such as compatibility of fuel with emission control devices, were defined subsequently. The functionality and general performance of gasoline can be defined, for example, in terms of octane rating, volatility, olefin content, and additives. Environmental performance can be defined, for example, in terms of aromatics, olefins, benzene content, oxygenates, volatility, and sulfur (lead is not allowed in most countries). Fuel properties are controlled by legislation and by fuel standards. There are also a number of other regional and national standards on fuels.

In Europe the Fuel Quality Directive, 2009/30/EC, defines the requirements for basic fuel properties for gasoline. The European standard EN 228 includes more extensive requirements than Fuel Quality Directive to ensure proper functionality of gasoline on market. CEN (European Committee for Standardization) develops standards in Europe.

In the US, ASTM D 4814 is a specification for gasoline. The ASTM standard includes a number of classes, waivers, and exceptions taking into account climate, region and, for example, ethanol content of gasoline. In 2011 US EPA accepted waiver for usage of 15 vol-% ethanol blend for 2001 and newer cars. In the US gasoline-oxygenate blends are considered “substantially similar” if they contain hydrocarbons, aliphatic ethers, aliphatic alcohols other than methanol, up to 0.3 vol-% methanol, up to 2.75 vol-% methanol with an equal volume of butanol, or higher molecular weight alcohol. The fuel must contain no more than 2.0 wt-% oxygen except the fuels containing aliphatic ethers and/or alcohols (excluding methanol) that must not contain more than 2.7 wt-% oxygen. In the USA, so called P-Series fuel consisting of butane, pentanes, ethanol, and the biomass-derived co-solvent methyltetrahydrofuran (MTHF) is allowed for FFV cars.

Automobile and engine manufacturers have defined recommendations for fuels in “World Wide Fuel Charter” (WWFC). Category 6 is the most stringent WWFC category for “markets with further advanced requirements for emission control to enable sophisticated NOx and particulate matter after-treatment technologies”.

Selected requirements and fuel properties are shown in Tables 1 and 2 below.

Table 1. Selected requirements for gasoline properties in Europe and in the U.S., together with automanufacturer's recommendations (WWFC). Complete requirements and standards are available from respective organizations.

^a Several classes. Vapor pressure limits depend on season.  ^b Mono-alcohols and ethers with final boiling point max. 210 °C. ^c May include other requirements than presented here. ^d In 2011, US EPA accepted 15 vol-% ethanol blend for 2001 and newer cars. "Substantially similar" rule allows e.g. up to 2.75 vol-% methanol with an equal volume of butanol, or higher molecular weight alcohol; fuels containing aliphatic ethers and/or alcohols (excluding methanol) must contain no more than 2.7 wt-% oxygen. ^e The vapor pressure of gasoline is measured at 100 °F (37.8 °C). ^f Stabilizing agents shall be added for methanol, and may be added for ethanol. Ethanol for blending shall conform to EN 15376. ^g RON 102, MON 88 quality also. Requirements for e.g. fuel injector cleanliness, combustion chamber, Particulate Matter Index (PMI).

Table 2. Examples of some non-limited gasoline properties.

* Electrical conductivity depends on the concentration of metallic ions. ^a Murphy 1998  ^b CRC handbook  ^c Degaldo 2007

Diesel – legislation and standards

Engine and after-treatment technology impose requirements for fuel quality. Basic fuel analyses were developed to screen general performance and operability of fuels in internal combustion engines. Fuel properties important in environmental contexts, such as compatibility of fuel with emission control devices, were defined subsequently. The functionality and general performance of diesel fuel can be defined, for example, in terms of ignition quality, distillation, viscosity, and additives. Environmental performance can be defined in terms of aromatics and sulfur content.

Fuel properties are controlled by legislation and by fuel standards. In Europe, Fuel Quality Directive 2009/30/EC defines the requirements for basic fuel properties for diesel fuel. European standard EN 590 includes more extensive requirements than Fuel Quality Directive to ensure proper functionality of diesel fuel on market. In Europe, CEN (European Committee for Standardization) develops standards.

In the US, ASTM D 975 is a specification for diesel fuel. ASTM standard includes several classes. There are also a number of other regional and national standards on fuels.

Automobile and engine manufacturers have defined recommendations for fuels in “World Wide Fuel Charter” (WWFC). Category 5 is the most stringent WWFC category for “markets with further advanced requirements for emission control to enable sophisticated NOx and particulate matter after-treatment technologies”.

Selected requirements and fuel properties are shown in Tables 3 and 4 below.

Table 3. Selected requirements as examples for diesel fuel properties in Europe and in the U.S., together with automanufacturer's recommendations (WWFC). Complete requirements and standards are available from respective organizations.

^a Only selected requirements presented here. ^b Several classes. Climate-dependent properties vary. ^c Limits depend on test methods. For diesel fuel containing above 2 vol-% FAME.
 

Table 4. Examples of some non-limited diesel fuel properties. ^a,b

* Electrical conductivity depends on the concentration of metallic ions. Degaldo (2007) reported 200 µS/m for hydrous ethanol and 10 µS/m for Brazilian gasoline.      ^a Murphy 1998   ^b Murtonen et al. 2009

References

Chiba, F., Ichinose, H., Morita, K., Yoshioka, M., Noguchi, Y. and Tsugagoshi, T. High Concentration Ethanol Effect on SI Engine

Degaldo, R., Araujo, A. and Fernandes, V. (2007) Properties of Brazilian gasoline mixed with hydrated ethanol for flex-fuel technology. Fuel Processing Technology 88 (2007) 365-368.

Emissions (2010) SAE Technical Paper 2010-01-1268.

EMA Statement. (2010) Technical Statement on the Use of Oxygenated Gasoline Blends in Spark Ignition Engines. Engine Manufacturers Association. January 2010. http://www.enginemanufacturers.org/.

Kabasin, D. et al. (2009) Heated injectors for ethanol cold starts. SAE Technical Paper 2009-01-0615.

Lupescu, J., Chanko, T., Richert, J. and DeVries, J. (2009) Treatment of vehicle emissions from the combustion of E85 and gasoline with catalyzed hydrocarbon traps. Society of Au-tomotive engineers. Technical Paper 2009-01-1080.

Murphy, M. (1998) Motor fuel Options for Heavy Vehicle Diesel Engines: Fuel Properties and Specifications. Battelle.

Murtonen, T., Aakko-Saksa, P., Kuronen, M., Mikkonen, S. & Lehtoranta, K., Emissions with Heavy-duty Diesel Engines and Vehicles using FAME, HVO and GTL Fuels with and without DOC+POC Aftertreatment. SAE International Journal of Fuels and Lubricants, 2010: 2, page 147-166. Also as SAE Technical Paper 2009-01-2693. 20 p.

Owen, K. and Coley, T. (1995) Automotive Fuels Reference Book. Society of Automotive Engineers. Warrendale. ISBN 1-56091-589-7.

West, B., López, A., Theiss, T., Graves, R., Storey, J. and Lewis, S. (2007) Fuel economy and emissions of the ethanol-optimized Saab 9-5 biopower. SAE Technical Paper 2007-01-3994.