Collaboration Produces Faster Oil Engines
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" 'FHE changes in design and increase in power obtained by pressure charging and the use of higher speeds make the modern high-speed oil engine one of the most efficient prime movers for its size," said Mr. L. D. E. Brodie, in concluding his paper, " Modern Trends in High-speed Diesel Engines," which was presented to the Diesel Engineers and Users Association. in London yesterday.
He recalled that until fairly recently the high-speed oil engine was governed to a cut-off speed of about 1,850 r.p.m. under no-load conditions—a limit imposed by the designers' inability to cater for increased bearing pressures and rubbing speeds, whilst ensuring efficient fuel injection and scavenging above that speed. It was through the metallurgist, chemist and oil specialist working with the designer that engine speeds had been increased.
Higher Piston Temperatures
Pistons, which up to a few years ago had been restricted to a safe maximum temperature of 1,400° F., were now operating at up to 2,000 F. This had been achieved by alterations in design to improve heat-transfer rates • and advances in the materials used.
White-metal bearings had given good service incomparatively slow-speed engines in which cylinder pressures did not exceed 850 p.s.i. at 6' after top dead centre—the approximate point of maximum pressure. However, pressures in the high-speed engine could easily be above 900 psi.
Improved surface finish of crankshaft journals achieved, by modern production methods had made it possible to use lead-bronze and copper-lead bearing materials. These alloys, aided by induction hardening or nitriding of the crankshaft, made it possible to accept such loadings.
Improved materials and alloys used in • the construction of cylinder heads had largely eliminated cracking through thermal overstrain, whilst hardened valveseat inserts had increased life between overhauls. Although valve bounce with high-speed engines had usually been oyescome by using double valve springs, it was thought that more research into cam profile design might be of assistance.
Cooling systems were often designed somewhat haphazardly and although they might fulfil design requirements on the test bench, they would not repeat this performance on site, particularly overseas where altitude and increased ambient temperatures could affect them. Fully controlled cooling systems which used hydraulic pressure to permit a variable fan speed and thus gave constant engine temperatures throughout the speed range might be the eventual solution of this problem.
Jerk-type fuel injection pumps were reaching the limit of their practical or economic development. The American Bosch single-plunger pump worked well at high speeds, whilst the Cummins P.T. system, employing a low-pressure pump to feed a common gallery to the injectors in which the nozzle needles themselves were raised by cams to control injection, had few moving parts and gave good atomization.
However, the rotary distributor pump exemplified by the C.A.V. DPA. unit was a great advance on practically every other type in use at present. Apart from being cheap and simple, it could be used with conventional injectors.
Under extremely dusty conditions air filtration still required close attention. Here, again, paper-element filters had been found to be of great value. They were so effective that they could quickly become choked, however. To avoid this a large oil-bath-type air cleaner might be used to supplement the paper element.
Pressure Charging An obvious way of increasing the power-to-weight ratio of an engine was by pressure charging. Mechanically driven blowers were most effective, as they delivered air in direct proportion to engine speed throughout the :range of the engine. They did, however, absorb engine power which reduced the gross power gain and usually increased the specific fuel consumption.
The turbocharger was still open to much development. It had at present the disadvantage of being almost ineffective at reduced engine speed, due to the low exhaust-gas velocity. At high speeds, however, it became very efficient, requiring no engine power to drive it, whilst producing a power increase of up to 50 per . cent. Unlike the mechanical blower it tended to reduce, rather than to increase, fuel consumption.