Measuring and testing – Engine detonation – Fuel rating
Reexamination Certificate
2000-08-02
2003-08-26
Williams, Hezron (Department: 2855)
Measuring and testing
Engine detonation
Fuel rating
Reexamination Certificate
active
06609413
ABSTRACT:
BACKGROUND TO THE INVENTION
The present invention relates to a method of and an apparatus for measuring the cetane number of a diesel fuel.
DESCRIPTION OF THE PRIOR ART
Diesel fuels (otherwise known as gasoils) are designated with a cetane number (CN) which is an indication of the readiness of the diesel fuel to ignite when injected into the compressed air in the combustion chamber of a diesel engine. The higher the cetane number, the quicker is the ignition which follows the injection. Thus the cetane number is a characteristic of the self-ignition quality of the diesel fuel.
The cetane number is currently determined internationally by the procedure specified in ASTM D-613. This test is required to be manually performed on a sample of a finished blend of diesel fuel using a single cylinder four stroke diesel engine, having a variable compression ratio. The test requires a manual and critical adjustment of the quantity of injected fuel, as well as the injection timing and the compression ratio of the engine, in order to produce a constant ignition delay corresponding to a crank angle of 13°, (i.e. a delay of from 13° before top dead centre to top dead centre). The test is performed on the finished blend sample, as well as on reference fuels having known cetane number, which are available from Phillips Petroleum of Bartlesville, Okla., USA. The cetane number of the finished blend sample is determined by a linear interpolation of the pre-chamber compression plug position found for the blended fuel, as compared to the corresponding positions for the reference fuels. This manual test lasts about one hour and is required to be performed on a sample of diesel fuel after blending. Thus the cetane number is only measured when the blending operation has been completed, thus preventing correction of the formulation of the diesel fuel blend on-line in the refinery.
Most importantly, the manual test also has poor precision. Although historically there has been much research worldwide to improve the inaccurate procedure of ASTM D-613, the repeatability of the test is as high as 0.9 cetane points and the reproducibility of the test is as high as 3.9 cetane points. No alternative test standard is accepted today, for example by the European Committee for Standardisation (CEN). Accordingly, the ASTM D-613 standard is likely to be in use for several years.
There are increasing commercial and technical requirements for cetane number determination to be accurately and reliably achievable. There is also an increasing demand for the cetane number to be determined on-line in the diesel fuel blending system. The cetane number has been included in the European diesel fuel specification EN590 specified by the European Committee for Standardisation. Since Oct. 1st, 1996 the cetane number has been required under EN590 to be at least 49 and the cetane number will need to be a minimum of 51 from Oct. 1st, 2000. It is likely that the diesel fuel standards will require increasingly higher cetane numbers in the future. The cetane number of the diesel fuel needs to be certified by the refiner. These increasingly higher requirements for the cetane number of diesel fuels will tend to force diesel fuel refiners to put commercially more valuable refinery streams and cetane improver additives into diesel fuel. This will make diesel fuel more expensive.
There is thus a need for refiners to be able to measure the cetane number of diesel fuels accurately and continuously on-line in the blending system so that a particular target threshold for the cetane value to meet the required standard can be achieved during the blending process without unnecessarily increasing the cetane number over the target threshold which constitutes a “give-away” of cetane number which is not commercially recoverable by the refiner. Currently, using the manual testing of finished blends, the blends can be found to be out of specification or can give-away cetane number, leading to significant loss of commercial value, and of time which is required for re-blending and re-testing of the fuel.
There have been prior proposals for the automatic on-line determination of cetane number in a CFR (Committee of Fuel Research) engine which is the engine specified in ASTM D-613.
Thus EP-A-0610118 discloses such a method which operates the CFR engine at constant compression ratio and compares the measured ignition delay of the blend sample with those of two reference fuels. Since this automatic process is an over-simplication of the process required by the ASTM D-613 standard, and in contrast to the standard does not have a constant ignition delay, this automatic process may not be used for product certification in accordance with that standard.
WO-A-97/39349 discloses an automated method which also requires the CFR engine to operate at constant compression ratio and compares the measured ignition delay of the blend sample with that of one reference fuel. The difference in ignition delay between the two fuels is employed to calculate a difference in cetane number, based on the calibration of the engine with two known reference fuels, once at the start of blending. Both the blend sample and the reference fuel have their own injection pump, which are each adjusted for flow and timing of the injection, the adjustment being at the start of blending. The cetane number determination is fed back to the adjustment of the components of the diesel fuel, and the mixture of cetane improver additives. Again, as this process is an over-simplication of the process specified in the ASTM D-613 standard, and does not have a constant ignition delay, it may not be used for product certification.
EP-A-0895080 also discloses a method of measuring cetane number of a diesel fuel in which the cetane number of a reference fuel is calculated using the difference between the average values of the ignition delay of the fuel to be measured and the reference fuel.
GB-A-2163878 discloses a process and device for adjusting the cetane rating of gasoil produced in a refinery. Again, during the measurement phase the compression ratio is maintained at a constant value.
U.S. Pat. No. 5,457,985 also discloses a process for measuring the cetane number of supply fuels for diesel engines which measures an autoignition delay of a diesel engine operating at constant speed with a constant volume compression ratio.
It has also been proposed to measure the cetane number of diesel fuels continuously by measuring the infra-red spectrum of the blended fuel and calculating the cetane number from algorithms based on a Fourier transform analysis. However, such calculation methods have a lower accuracy than the standard CFR methods discussed above.
There is a standard ASTM D-2885 for the on-line automatic measurement of the octane number of gasoline which is used by refineries during the gasoline blending process. The test procedure is based on an automated CFR octane engine, which compares the blend header gasoline with “prototype fuels”. Such prototype fuels are themselves periodically compared with “standard fuels”, which have been separately tested (by “round robin” testing by a number of different refiners in the petroleum industry) in at least sixteen manual CFR engines. Such a process has been considered to have a better accuracy than manual determination, due to the use of the average octane number of the standard fuels which have been tested in “round robin” testing, and equally due to the large number of repetitions of the comparative measurement of the blend header octane number with that of the “prototype” fuels. Installations embodying this procedure are currently used all over the world. However, there is no such corresponding automatic cetane number determination process using the CFR engines meeting the parallel ASTM D-613 standard for the cetane number of diesel fuel.
SUMMARY OF THE INVENTION
In summary, there is a need in the art for a method of and apparatus for determining cetane number of diesel fuels which enables the on-line measurement of cetane number permitting certification of the c
ATOFINA Research S.A.
Jackson William D.
Stevens Maurice
Williams Hezron
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