leChnitopicS SPARKS FOR DIESELS?
Page 57
Page 58
If you've noticed an error in this article please click here to report it so we can fix it.
Igniting the fuel of a diesel engine with a spark could enable a unit to be produced that had the combined advantages of a petrol and diesel engine
DURING a discussion on the potential of the variable-compression-ratio piston a few weeks ago, a top-level research engineer told me that he favoured the use of spark ignition as a means of initiating combustion of a high-output low-compression diesel operating under light load in preference to employing a VCR piston. Apparently the type of VCR system in which hydraulic fluid is displaced from one chamber to another with movement of the outer shell is not very satisfactory in operation because the fluid overheats, and providing for continuous replacement of the fluid is not practicable.
In a Technitopics some years ago it was suggested that there could be advantages in "sparking off a diesel", and now news from the MAN company reveals that a new combustion system, the FM, is based on spark ignition. The development of the system is described in detail in an SAE paper by two members of the MAN research staff, Mr. J. S. Meurer and Mr. A. C. Urlaub, read at the International Engineering Congress in Detroit last January.
It is noteworthy that the system has, in the main, been applied to diesel engines having a compression ratio sufficient to ignite the fuel by the heat of compression. While the engine was developed as a multifuel unit, capable of burning poor-quality gasoline (of the type that is a waste product of petro-chemical processes and could not be used in a conventional gasoline engine), the success of the experiments could be of far-reaching general importance to the road vehicle industry.
In many ways the system is similar to the stratified-charge type with fuel injection evolved by MAN to enable a petrol engine to run on "open throttle" throughout the load-speed range, which would give a valuable fuel saving at part-loads. But basically it represents a different concept.
Better combustion A spark is used to ignite the fuel before it is ignited by the heat of compression. And thereby the combustion process can be improved and the peak pressures reduced for a given output.
Whereas the stratified-charge system has a constant-volume combustion characteristic, the cycle of the FM system is more nearly constant pressure than the conventional constant-pressure diesel cycle. Of special significance, the authors of the paper
mention that the system could provide for operation at a compression ratio as low as 13 to 1.
As has been demonstrated by Prof. S. Timoney of Dublin University College (CM, January 24 1969), the VCR system that he has applied to the Rootes opposed-piston two-stroke engine having turbochargers in series enables the compression ratio of a diesel to be reduced to 9 to 1 to give a power output of at least double that of a standard unit operating on a ratio of 16 to 1 at the same stress level, the only penalty being an increase in fuel consumption.
Every type of diesel has a "break-even point" with regard to compression ratio and fuel consumption. The improvement in combustion efficiency that can be obtained by increasing the compression ratio obeys the law of decreasing return.
Above the break-even ratio, the improvement in thermal efficiency is more than offset by the higher friction and pumping losses that a further increase involves. In the case of a typical automative diesel, the break-even ratio is around 12/14 to I, higher ratios being required in practice to facilitate starting and in some cases to give even-running when the unit is operating at a relatively high speed under light load.
Igniting the fuel with a spark would eliminate combustion lag and resultant diesel knock. And the reduction in the peak operating pressure could be favourably ex ploited by a comparable reduction in the weights of the moving parts. In the view of Mr. Meurer and Mr. Urlaub it would be possible to use moving parts that were no heavier than those of a low-compression petrol engine. All this would provide a worthwhile reduction in friction losses. It would also enable the unit to run at higher speeds.
The FM system is a development of the well-known MAN M diesel-cycle system, which is noted for its quiet running. Fuel is sprayed on to the walls of the hemispherical combustion chamber and is swept off the walls by controlled swirl to give accurate control of mixing and combustion.
In one form of the FM system the sparking plug is located vertically and enters a slot in the piston as it approaches dead centre. The slot is open to the combustion chamber on its inner side and the fuel from the injector on the opposite side of the chamber is carried into an arcuate recess in the base of the slot by air swirl having a rotational movement horizontally around the centre of the chamber. The injector is inclined towards the centre of the chamber and its tip is close to the periphery of the opening when the piston is at top-deadcentre.
FM system advantages Mr. Meurer and Mr. Urlaub claim that the FM system brings together the advantages of the traditional diesel and gasoline engines. Practically any liquid fuel can be used in the engine without restrictions with regard to octane and cetane numbers. Fuel consumption is comparable to that of a diesel at all loads and speeds and in addition to the advantages mentioned of operating at low peak pressures, the emission of carbon dioxide and unburned hydrocarbons is minimal.
Reverting to the ability of the engine to run on poor quality (unleaded) ga.solines, I am reminded that little has been heard for some years of the possibility of the introduction of a tax differential favouring gasoline fuels because of the large quantities produced as a waste material of manufacturing processes. 1 understand that a large amount is derived from the plastics industry and obviously the total quantity will progressively increase. Perhaps the MAN company were considering the possibility of such a tax differential when they decided to develop the FM system. NOT ONLY is there a notorious dearth of statistics about road transport but those that are available are often unreliable and ambiguous. They are used with equal freedom to prove the opposite sides of a case.
In the 1969 edition of the invaluable Basic Road Statistics published by the British Road Federation the table to which most prominence was given shows the number of vehicles per road mile in different countries. The British figure of 59.2 vehicles is the least favourable of all. The USA and France both have fewer than 26 vehicles to the mile.
Most road users would draw the conclusion that more roads should be built in the UK. There is a growing body of opinion that this process cannot be allowed to continue indefinitely. From this point of view the tempting solution is a restriction on the use or even the number of vehicles.
Perhaps the statistics are being asked to bear too great a burden. The BRF take them as an indication that traffic congestion is now spreading more rapidly on roads in Britain than in any other major country in the world. Much more evidence would be needed before one could agree with this.