TF_CHNITUP108 Diesel design breakthrough
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A spark-assisted diesel would have a comparable fuel consumption but consume less energy in the refining process
WAY BACK in 1965 experiments in the Weslake laboratories had shown that piston inertia was the main limitation in raising the speed and output of a diesel. However, the development of a spark-assisted (SA). low-compression diesel gave promise that a unit could be produced to operate at the rpm of a petrol engine and produce comparable power by virtue of the low weight of the pistons..
An experimental engine had a compression ratio of 6.3 to 1 and was mentioned in a previous Technitopics (September 24, 1965).
Piston weight and friction could be reduced by virtue of the low compression ratio.
To provide for the maximum reduction in piston weight the combustion chamber was located in the cylinder head instead of the piston' to enable the crown height to be reduced. Weslake noted that the lower stresses involved also, reduced deflections of the piston/connecting rod assembly, and that this favoured uniformity of combustion characteristics.
In the past, 12 years, the low-compression diesel has been the subject of extensive research by a number of manufacturers. The Volvo TD1008 has shown that compression ratio can profitably be reduced to 12.5 to 1, given that exhaust back pressure is used to aid low-load running, and induction preheating is employed for starting.
Since the advent of the fuel crisis, spark-assistance has been reassessed as a fuel saver, notably as a unit which would reduce the -overallenergy .required.
This includes the energy used for refining as well as the fuel consumed by the engine.
The potential of the (SA) 'diesel engine and its advantages over the stratified-charge (SC) petrol engine were reviewed in detail in a paper by W. Tipler of Perkins at the recent I Mech E conference on SC power units.
It is claimed that development of the SA diesel would enable coal-derived liquid fuel to be consumed at double the normal thermal efficiency of coal utilisation for automotive engine applications. • Mr .Tipler contended that development of the SC carburetted engine should be abandoned in favour of the SA type While the advantages of the SA engine outlined by Mr Tipler indicate a different appraisal of its merits than that made by Weslake engineers, it may be reasonable to assume that at some future date, versions of this type of power unit will be produced which will satisfy the demand for a fuel-from-coalburning unit, and one which operated at petrol-engine rpm, producing comparable power.
Although this would not be as economical as a conventional diesel because of its higher friction losses and reduced thermal efficiency, it would serve a useful purpose for many types of application, and could be an important fuel saver.
In a table in Mr Tipler's paper showing the arbitrary units of energy of petroleum fuels expended in the refinery and in the engine, a petrol engine consumes 6.7 units, and 8 units are used in the refinery process, giving a total of 14.7 units.
A carburetted SC petrol. engine running on low octane fuel consumes 6.1 units, and refining accounts for 7.1 units, the total being 13.2 units. A conventional diesel engine and an SA diesel each consume 5.8 units, and refining requires 6.2 units, a total of 12 units.
In favour of the SA engine however is its higher fuel tolerance; it can burn a wider range of fuels and can therefore be expected to offer greater overall economy.
The miles per barrel of an SA diesel is given as 440, while the figure for a conventional diesel is 358.
As mentioned in the paper, use was made of liquid fuels derived from coal during World War ll in Germany and in the UK. Fuel of this type is currently produced in South Africa.
Mr Tipler points out that application of the fuel to petrol engines would be grossly inefficient, partly because of low thermal efficiency and partly on account of energy consumed in the refining process.
Comparing the consumptions of a conventional diesel and an SA engine running on a coal-derived fuel, it is claimed that coal refinery consumption of fuel for the former would be 14.5 units, and that this would be reduced to 8.3 units in the case of the SA diesel.
Although each would use '5.8 units of energy in operation the unit totals of 20.3 and 14.1 indicate that the latter has an overall advantage in excess of 30 per cent.
The respective consumptions of a petrol engine operating on coal-derived fuel are 6.7 units and 16.1 units (a total of 23.4) while those of a carburetted SC petrol engine are 6.1 and'12.2 units, a total of 18.3 units.
Recent recommendations prepared for the Transport Group of the Advisory Council on Energy Conservation, Include one that the SA engine should be fully developed because it can burn a wide range of fuels economically, and would be acceptable for car application on account of its relative lightness. The report is available at the Stationery Office.
As shown by Volvo, the main problem of operating a high speed automotive diesel is a 12.5 to 1 compression ratio is to overcome misfiring during low-load running.
At what compression ratio will the successful SA engine of the future operate? Could it be as low as 8 to 1? Will the type of spark assistance employed eliminate the problem in some way? And how will piston weight compare with the weight of a conventional diesel piston?
As mentioned earlier, Weslake pointed out 12 years ago that, a combustion chamber in the cylinder head instead of the piston crown enables piston weight to be reduced. But changing over could create design difficulties.
Never before, maybe, has there been so much interesting promise of a breakthrough in engine design. What are you waiting for, Mr Tipler? Not Government aid, surely?
iPaul Brockington