BB00.40-P-0125-00A Fuels For Gasoline Engines

BB00.40-P-0125-00A Fuels For Gasoline Engines
- sheet 125.0


Liquid fuels for gasoline engines are mixtures of chain and ring shaped hydrocarbons as well as certain oxygenic compounds (ether, alcohol), that have a boiling point between approximately 30 and 200°C and are almost exclusively extracted from mineral oil. In each country they are offered for sale in either one or several grades. Three gasoline grades are sold in the Federal Republic of Germany, all of which are specified in terms of both properties and quantity in DIN EN 228:







Requirements, properties, parameters
Fuel is subject to particular requirements aimed at enabling a gasoline engine to be operated reliably under all manner of climatic and driving conditions.
In order to comply with such requirements a variety of specifications must be adhered to which reveal information on the quality of the gasoline.

Although these specifications, which as a rule are based on standardized test procedures, are indeed useful, they do not always fully satisfy the need to define important quality criteria for handling gasoline and for its combustion in the engine.

A certain number of such specifications, for which limit values have been defined, are called on to compile standards for minimum requirements. The valid requirement standard within the EU is EN 228 (adopted in Germany as DIN EN 228, see sheet 122.2), which lays down the permissible limit values. According to this, gasoline may also contain additives for improvement, likewise certain alcohols and ethers are permitted in limited quantities. In our opinion the addition of additives to gasoline to improve quality is absolutely essential. This has to take place through the supplier as he has overall responsibility for his product (refer to the Section "Additives" on this).

Knock resistance
The knock resistance is one of the most important quality features in motor-vehicle gasoline. It is decisive in ensuring a normal combustion process in the engine and thus crucial in terms of efficiency and specific power output.

The knock resistance is measured by the octane rating which in turn is determined through comparison with isooctane-n-heptane mixtures in a C.F.R. engine as under ISO 5163 and ISO 5164. Essentially one differentiates between two methods of determination:

a) F 1 method, Research Octane Number (RON) ISO 5164
b) F 2 method, Motor Octane Number (MON) ISO 5163

The differences lie in the various testing conditions. In RON, one conducts the test with a constant engine speed of 600 rpm, a constant spark setting (13� before BTDC) and with air preheated to 52°C, in the MON method one uses a constant engine speed of 900 rpm, an automatically-adjustable spark setting depending on compressed ratio and a mixture preheated to 149°C.

Next to the RON and MON methods there is also the FON (Front Octane Number) and the SON (Street Octane Number). The FON is the research octane number of the constituents that dissolve into a vapor state during distillation up to 100°C. It has a specific significance with regard to the knocking during acceleration characteristic.

SON is measured in the vehicle's engine, because the fuel is assessed differently in every engine on account of its different design and operating conditions than would be prevalent in a C.F.R. test engine. The measurement draws on both primary reference fuels (compound of isooctane and n-heptane) as well as specified fuels with maximum boiling ranges. The measurement is conducted during straight-line non-knocking full-throttle acceleration in the highest gear from the lowest possible speed.

A vehicle's octane number requirement can be taken from the owner's manual.

The knock resistance of conventional gasoline differs greatly from country to country, a list is available on Sheets 124.0/.1/.2/.3/.4.

Boiling characteristics and vapor pressure (volatility)
The gasoline's boiling characteristics lie between approx. 30 and 215 °C. Important in the context of the boiling-point curve are its start with the so-called 10 % point, the 50 % point and the final boiling point. The following are dependent on the boiling characteristics:
^ Vapor lock
^ Starting characteristics
^ Evaporative losses
^ Transient response
^ Engine oil dilution
^ Combustion cleanliness

Therefore the boiling characteristics is different for summer fuels than for winter fuels. This is governed in DIN EN 228 by specifying the quantity evaporated at temperatures of 70°C, 100°C and 150°C.

The boiling characteristics alone are not enough to enable the fuel volatility to be determined, this is why the vapor pressure at 37.8°C is used as an additional criterion. This form of vapor pressure was determined for many years by the "Reid method", and was therefore frequently referred to as "Reid vapor pressure". Because the fuel comes into contact with water when determining the "Reid vapor pressure", this vapor pressure is often also referred to as "Wet vapor pressure". During the course of the past few years vapor-pressure measuring methods have been developed, which take account of the state-of-the-art opportunities presented by pressure sensors and electronic control mechanisms. These devices are easier to operate and provide more precise values, just as the presence of water (which in the "Reid vapor pressure" was the result of a thermostatization) has become irrelevant. As part of the European standardization process a modern vapor pressure method such as the EN 13016 T 1 has been standardized and incorporated into EN 228 as of issue 2/99 EN 13016 T. 1 delivers the "ASVP" ("Air Saturated Vapor Pressure") as a measured quantity, which can be converted by means of a correction formula to render the "DVPE" ("Dry Vapor Pressure Equivalent"). For fuels containing alcohol the "Reid vapor pressure" provides slightly lower values than the "DVPE", nonalcohol fuels are evaluated identically.

DIN EN 228 specifies the "DVPE" vapor pressure in summer to min. 45 and max. 60 kPa as well as for the winter to min. 60 and max. 90 kPa (100 kPa = 1 bar). In the transitional period during spring and fall the vapor pressure requirements for winter fuel are valid, but with an additional requirement in terms of the "VLI" ("Vapor Lock Index", a mathematical value: VLI = 10 * DVPE + 7 * E70) of max. 1150, when the vapor pressure is fully exploited (e.g. 90 kPa) this limits the permissible vaporized pressure at 70°C ("E 70") to max. 28.5 % by volume.
The European standard provides an opportunity, to also define the vapor pressure curve above 37.8°C (from 40°C up to 100°C) (EN 13016 T. 2). There are as yet hereto no limit values.

Density
Density is related to 15°C as under DIN 51757 and ISO 3675 because of the volume which changes with the temperature.
Density requirements as under DIN EN 228 for regular, premium. and super plus are 0.720 up to 0.775 kg/m 3

Calorific value
The calorific value of the fuel indicates which quantity of heat is liberated when it is combusted, i.e. how much energy is contained in a particular quantity of fuel. The measuring unit for the calorific value is MJ/kg. One differentiates between the specific gross calorific value (upper calorific value) and the specific calorific value (net calorific value), they represent the quantity of heat in relation to the mixture, which is released during combustion, once with and once without taking account of the condensation heat of the formed water. Because the exhaust gases constantly exhibit a temperature higher than 100°C, it is only the net calorific value that is of any interest in engine-related combustion.

In fuels that consist solely of carbon (C) and hydrogen (H), the calorific value is dependent on their C and H concentrations, this is because H has a calorific value of 119.8 MJ/kg, whereas C only has a calorific value of 33.9 MJ/kg. The C/H ratio (in % by weight C/H) of fuels is however only variable in close limits, namely theoretically from 86/14 to 88/12, the associated calorific values are around 43.8 and 42.5 MJ/kg, i.e. the maximum difference is around 3 %. As on the other hand fuels with a high C/H ratio inevitably have a higher density and fuels are sold by volume and not by weight, the calorific values stated should be multiplied by the associated densities in order to arrive at real ratios. For example, regular and premium-grade fuels with the following data:







In other words the energy contained in 1 l premium fuel is roughly 2.8 % greater than that for 1 l of regular-grade gasoline.

If the fuel contains oxygenic components, the oxygen will not contribute anything to its calorific value. The oxygen content should therefore be subtracted. This is to a certain extent compensated for by the higher H content in the oxygenic compounds, so that where the proportion is not too high (see section on "Oxygenic components") the calorific value is only insignificantly influenced.

Purity
Solid foreign matter and water may lead to problems arising with the fuel supply. Beyond this water can also cause corrosion; in turn corrosion products can also impair the fuel supply. The solid residue from evaporation of the fuel (50 ml of the fuel is evaporated by the air-jet method in a glass beaker at a temperature of 160°C) gives an indication of the degree of contamination to be expected in the intake system. Oily solid residue from evaporation is less harmful, but paint or resin residue from evaporation is less favorable.

The sulfur content in the gasoline should be as low as possible, we recommend the use of sulfur-free fuel in Mercedes-Benz vehicles. For additional information see Sheet 126.0 "Sulfur in gasoline".

Stability
The quality of the fuels should not deteriorate on the more or less long journey from the manufacturer to the consumer, i.e. the hydrocarbons in the fuel should not react with the oxygen in the air or with each other. This chemical instability results from the presence of unsaturated hydrocarbons in the fuel (e.g. diolefines) and is responsible for the so-called "gum" formation. This makes itself noticed through deposits throughout the entire fuel and intake systems and the intake valves.

The oxidation stability as per ISO 7536 and the residue from evaporation are used as assessment criteria.

Currently however, a proper quality assessment can only be made through a complete engine test.

Corrosion
Fuels are naturally practically anhydric, but they are known to dissolve small quantities of water when being transported. The dilution is dependent on the structure of the hydrocarbons as well as the temperature. When cooling down a portion of the diluted water is lost. Water and fuel are separated. As long as the water is dissolved, it does not have a corrosive effect. Free water causes rust and corrosion to effect both ferrous and non-ferrous metals.

Fuel agents, additives
In order to fulfill the specified requirements, additives are added to the fuel. Gasoline additives are split up into two categories:
^ Additives, that are intended to change or improve the fuel's characteristics.
^ Additives, that are intended to give the fuel new or additional characteristics.

Here one also has to differentiate between additives for transport and storage purposes, and those that are effective in the engine related combustion process.

Amongst the additives one counts dyes with which, for safety reasons, leaded fuels can be identified.
The color is not prescribed, but left to the discretion of the manufacturer, who can select specific colors to identify its own fuels by.

The sketched-out gasoline additive process, which is important, in particular, in terms of system purity and protection against corrosion, has enormous significance in terms of trouble-free and low emission operation in Mercedes-Benz vehicles. The fuel additive should be made available to the customer at the filling station; we reject the idea of allowing the customer to carry out the additive process on his/her own because this would rule out the possibility of guaranteeing that the additive was technically in order and appropriate for the fuel grade concerned.

Oxygenic components
Gasoline fuels have oxygenic components added to them for a number of reasons (in the main these are of economical nature, sometimes they have to do with increasing the knock resistance).

As a rule these are alcohols and ethers; most frequently the alcohols methanol and tertiary butane and as ether methyl.tertiary butyl ether (MTBE) are used.

Because oxygenic components can alter a fuel's characteristics, many industrial nations have made moves towards governing their usage. The regulations applicable within the EU will be gone into in detail shortly.

The maximum permissible concentrations of these components, which are also specified within DIN EN 228, are as follows:
^ Methanol 3 vol %
^ Ethanol 5 vol %
^ Isopropanol 10 vol %
^ Tert. butanol 7 vol %
^ Isobutanol 10 vol %
^ Ether (min. 5 C atoms) 15 vol %
^ Oxygen content 2.7 percent by weight

The oxygen content refers to the total mass share of oxygen, which is available in the oxygen-containing components in the fuel. As a consequence not all permissible maximum limits for the individual components can be utilized at once.

The fuel manufacturer is obliged to ensure that all gasoline fuels that contain oxygenic components do not exhibit any disadvantages when compared with fuels that do not contain oxygenic constituents.