Easy starting of low-compression diesels
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A pre-war Hungarian device could provide a means operating a turbocharged compression ignition engine on i lowest useful compression ratio without an external startil
aid
AS LONG ago as 1928 or thereabouts railcars were running on the then Royal Hungarian State Railways which were powered by Ganz indir,ect-injection diesel engines having a compression ratio of 12.4 to 1, the normal ratio of pre-combustion diesels being well over 20 to I. The engines incorporated device which raised the temperature of the instake air without external aid by delaying the opening of the inlet valves, and this was sufficient to provide easy starting in the coldest weather.
Although the device has been mentioned from time to time over the past 40 years in various technical jpurnals (including CM) no other engine manufacturer has apparently appreciated its practical potential. And mindful that the Volvo TD 100B direct-injection turbocharged low-compression diesel is of very recent origin and that the gains obtainable by operating on a lowcompression ratio were not taken seriously by Volvo or by the many UK makers experimenting with the system until the turbocharger had proved its worth, this is not surprising. Who would now produce a turbocharged diesel with a compression ratio of more than 12 to 1, or possibly a maximum of more than 10 to 1, if this did not create starting difficulties and impair low-load running? Achieving the lowest useful compression ratio that caters for service requirements is virtually a universal target.
Phenomenal
The Ganz. system was described by the inventor, George Jendrassik of Budapest in a paper published in 1929 and, according to Mr Jendrassik, starting would have been impossible without the device unless the compression ratio had been raised to 26.1 to 1. As mentioned by the inventor, the system is based on the principle that "the temperature of gases rises if they are streaming into a container at increasing pressure" and he goes on to show with thermodynamic formulae that the device provided an increase in temperature of the aspirated air from 320 deg C (608 deg F) to 480 deg C (896 deg F). To have started and to have continuously operated an anti-chamber type of diesel having a compression ratio of 12.4 to 1 must have been phenomenal by any designer's yardstick in those days, and it would be today. The engines had electric starters.
Having six cylinders and developing 82kW (110 bhp) at 1000 rpm the State Railways diesel was the most powerful unit of a range of engines equipped with the device, the smallest of which had an output of 5.2kW (7 bhp) at 6000 rpm. A railcar unit developing 54kW (72 bhp) was operated by another Hungarian railway and it was claimed that all the engines gave complete satisfaction in service.
It is perhaps significant that a copy of Mr Jendrassik's paper was given to the author over eight years ago by a member of the research department of a wellknown equipment maker following a discussion on the experimental opera tion of a Rolls-Royce opposed-p two-stroke diesel, the compression of which had been reduced to 10 tc assess its performance had it been with variable-compression-r pistons. The low compression em more than double its normal outp be produced with no penalty in fuel sumption or increase in peak pres the only snag being that starting impossible unless the intake air wa! heated. A variable-compressionpiston would have automaticall: creased the ratio for starting and load running but it is a very plicated and costly device. Whit Volvo system involves the use of E preheater to facilitate starting frorr and an exhaust back-pressure regt. to eliminate misfiring at low load relatively simple and cheap to ma
Great lengths
The question is whether a versi the Ganz system could readil applied to standard types of dies( whether it could be used to improv. load running, no mention of the being made in Mr Jendrassik's tgOlni.ANDics continued from page 21 But before an attempt is made to answer this question it is pertinent to cite a recent French system to show the lengths to which a designer will go to enable a diesel to run on very low compression.
At the 19th Annual International Gas Turbine Conference and Products Show held in Zurich in April last, 'mention was made of the French Microturbo TCS14 turbocharger and its application to the Poyaud Hyperbar-process 75kW (100 bhp) military diesel. The pressure ratio of the turbocharger is 5 to 1 and the engine operates on the remarkably low compression ratio of 8 to I. An electric motor spins the turbocharger rotor for starting and air is supplied to a combustor to heat the, intake air; during normal operations the combustor is kept burning, on a low flame so that it can be cut-in as required to give full output for acceleration. If it is accepted that the Ganz system enabled the compression ratio of an indirect-injection diesel to be reduced from over 25 to 1 to around 12 to I it should be capable of promoting combustion of a diect-injection diesel with a compression ratio of well below 10 to I when starting from cold and (hopefully) of assisting combustion at reduced loads.
Second camshaft
The Ganz diesels had a second camshaft for operation of the inlet valves, and axial movement of the camshaft provided for engagement of an auxiliary cam with each follower to delay opening of the valve until late on the intake stroke. While it is not stated in the article that the opening period was reduced, this would have been necessary to give full compression. It is stated that the change-over was automatic and that the engine was normally run for some minutes with the starting cam engaged.
Modification of a single-camshaft engine to a twin-camshaft unit would obviously be too costly, but a single sliding camshaft might suitably be employed in some applications and there could be other ways of varying inlset-valve opening appropriately with minimum complication. These could include the use of a rocker type follower with an eccentrically mounted fulcrum or a modified form of hydraulic valve lifter. The latter could be incorporated. in the follower in the normal way and a bleed could be arranged automatically to delay opening and reduce the opening period, of the valve at low speeds. And it should be possible to vary opening to match engine load.