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iction losses and the high speed diesel
Page 79
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A detailed comparison between small automotive diesels and comparable petrol engines throws light on the penalties associated with high r.p.m.
['HOUGH many car engines are high efficiency units with an output suffito propel heavy commercial vehicles, )-to-date petrol engine in this category leen built or modified for commercial y British manufacturers and no realisomparison can, therefore, be made :en the performances of comparable s and petrol engines in the higher .ity range. Diesels are used fairly exrely for private cars in various European tries, however, and in many countries xis, including the United Kingdom, and las led to a demand for high efficiency with an output comparable to the power aced by petrol-engine counterparts.
a paper presented at a meeting of the Engineers and Users Association in :mber last year by Mr. C. J. W,alder, in discussions on the paper, the author various contributors to the discussion wed many detailed aspects of the design operation of smaller diesels, as employed rs and taxis, in comparison with equivapetrol engines. Some of their comments a wider significance applicable to !I' diesels.
I consumption curves
f particular significance, a series of fuelumption curves is included of consump
■ plotted against b.m.e.p.s. for a 1.8petrol engine and a 2.1 -litre diesel operatit 2,000 r.p.m., 3,000 r.p.m., 4,000 r.p.m. 5,000 r.p.m. and these show that the -ovement in fuel economy provided by diesel is reduced with increasing r.p.m. he reduction is very substantial and was ely attributed by the author and by other titers to relatively high friction and ming losses associated with higher ipression ratios and the use of heavier ons with a larger number of rings. These es would account for a reduction in the
offered by the diesel from about 30 cent at an easy touring speed, to less than per cent at a near-maximum m.p.h. or ble this speed. It is notable that the diesel ws little or no gain in economy at any ed when developing its maximum b.m.e.p.
irked superiority
t is noted by the author that in city service many stops and starts, the diesel can w to advantage by as much as 80 per cent virtue of its marked superiority when ning under light load and idling in traffic. kfter stating his view that friction was the iting factor in the operation of a diesel at
higher speeds, Mr. P. Howes of the CAV company, stated that its total friction at 4,000 r.p.m. was some 70 per cent greater than the friction of a petrol engine. Mr. Walder said that piston friction accounted for 30 per cent of the total losses.
Technically valid
Whilst these observations are obviously valid technically, it is notable that the only mention made of lubrication in Mr. Walder's paper is a reference to the possible value of oil cooling the pistons as a means of obviating the need to provide improved heat dissipation from the crown by increasing the weight of metal in the area and, therefore, the total weight of the piston.
Reducing the weight of the piston reduces friction, and oil cooling will undoubtedly be applied more extensively to all types of high-output engine with increases in b.m.e.p.s., but undercrown cooling can be only indirectly associated with the allimportant relationship between piston lubrication, temperature and friction. No mention was made of thiS in the paper or in the discussion.
Big gains
Without being dogmatic, it is reasonable to assume that 10 per cent is an underestimate of the gains in output and fuel economy provided by optimum viscosity of the oil on the bore surface (which is partly a function of bore-surface temperature) compared with viscosity-temperature conditions applicable to the engines of most commercial vehicles engaged on a typical service.
It is pertinent that drivers frequently report an outstanding, although temporary gain in performance when travelling at a higher speed after a particularly arduous climb with a full load when cooler air is drawn into the intake (which raises the volumetric efficiency) and the block has not had time to cool down. Overheating the intake air often offsets the advantages of operating at a high block temperature which tend to increase with engine speed. Changes in performance often appear to be independent of gauge readings of water and oil temperatures and undoubtedly bore temperatures can vary widely, if transiently,
without variation of the gauge readings particularly if the block is subject to direct cooling.
Unfortunately, the optimum block temperature with regard to performance may be dangerously near the oxidation temperature of the oil and this has acted as a deterrent in the development of "high-temperature" engines. In practice, an engine designer is often apprehensive that hot spots might develop in the cylinder head or round the upper ends of the bores, and consequently he "plays safe" by overcooling the unit.
Achieving an optimum bore temperature is a block-design problem rather than a cooling-system problem, and the practice of uprating existing engines by enlarging the bores or by increasing the r.p.m. tends to encourage the play-safe approach to improving temperature conditions round the block.
Piston friction
A reference is appropriate to the piston friction of high-speed short-stroke diesel engines, the piston speed of which is relatively low. This is conducive to low friction but the angularity of the connecting rod is relatively large and it is debatable whether this offsets the advantage of a low-rubbing velocity.
In the discussion of Mr. Walder's paper, Mr. V. H. F. Hopkins quoted reports that spark-started diesel engines were being developed that would operate at compression ratios and speeds equivalent to those of an efficient petrol engine. Harry Weslake technicians know a lot about this subject (as mentioned in Technitopics of the September 24, 1965 issue of COMMERCIAL MOTOR) and would surely agree that Mr. Walder's consumption figures encourage the conclusion that the low-compression diesel has a big potential.
The comment by a well-known engineer, quoted by Mr. Hopkins, that the worst thing Mr. Diesel did in his career was to eliminate spark ignition may yet prove to be valid. Possibly it will depend on the oil technologist and piston designer_ Or the production engineer?