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Aviation History
1939
1939 - 0223.PDF
JANUARY 26, 1039 Supplement to 0@0t • 1 AIRCRAFT ^ ENGINEER No. 156. (V°'^e ,XV") 14th Year January 26, 1939 LOW-COMPRESSION DIESELS Some Notes on the Possibility of Improving the Performance of Four-stroke Compression-ignition Engines for Aircraft By VSEVOLOD V. SMIRNOFF 1HE author of this article maintains that the disparity between the four-stroke diesel and the petrol engine will probably never be sufficiently narrowed by the increase •of boosting, unless this is accompanied by a decrease oj C.R. Means are suggested for making possible the operation of four-stroke C.I. engines at low C.R.s. On the basis of calculated performance of the C.I. aero-engine with low C.R. it is shown that this engine, even without any cooling of the air supply, could develop considerably more power than a comparable petrol engine of the same swept volume. The ratios of frontal area and of weight to power are less in the (ase of the C.I. engine, and the fuel consumption at normal maximum output is about 0.425 lb. per b.h.p. • . The high combustion pressures and the necessity of having excess, air in the cylinder of a compression-ignition engine in order to ensure sufficiently complete combustion of fuel result in a greater weight and a lower power output of the four-stroke C.I. engine when compared with a contemporary petrol engine of the same swept volume, it the engine speed and the induction conditions are equal in both cases. In the case of a .C.I. engine, the boost pressure is not limited to the same extent by detonation problems as it is in a petrol engine. This makes it possible to improve its performance by heavy boosting. Yet it seems that this method alone is not sufficient for narrowing the marked disparity between the modern petrol aero-engine and the four-stroke C.I. aero-engine. On the one hand, induction pressures, in the case of petrol engines, continually increase along with the improvement of fuel quality ; and on the other hand, in the C.I. engine it is neither useful nor permissible to carry boosting beyond certain limits. Indeed, induction pressures in a four- stroke C.I. engine must be very much higher than in a petrol engine if B.M.E.P.s are to be equal, and higher still it the weight/power ratio is to be comparable with that 01 the petrol engine. The compression pressure increases in proportion to the induction pressure, approximately, and with regard to the ignition lag it is impossible to prevent further increase of pressure during the combustion. Iherefore the maximum combustion pressure inevitably rises with induction pressure and, due to the high com pression ratio, it would ultimately attain extremely high values. It is true that the ignition lag decreases with boost increase. This makes it possible to reduce somewhat, •at the cost of fuel economy, the rate of the maximum pressure increase, but not to prevent it. Gas temperatures "] the cylinder of a heavily-boosted C.I. engine would also e ver y high, due to the increase of the induction tem perature, assuming that an air cooler is omitted in order to avoid additional weight, drag and installation difficulties. Thus with increase of boost the working conditions of the C.I. engine would ultimately become too severe, due to excessive pressures and temperatures in the cylinder. As yet there are no means known to reduce substantially the minimum amount of excess air required for the normal operation of high-speed C.I. engines. Therefore a high- duty four-stroke C.I. aero-engine must necessarily be heavily boosted, but means have to be found for avoiding excessive pressures and temperatures in the cylinders of the engine if it is to become a serious competitor of the petrol aero-engine. Theoretically there exists a very simple solution of the problem : heavy boosting coupled with the use of a correspondingly low compression ratio. Indeed, the higher the induction pressure and temperature in the cylinders, the lower is the necessary C.R. for raising the cylinder charge to the minimum required compression- pressure and temperature. In practice things are more complicated, due to variations of engine operating con ditions. It is necessary either to have an engine with variable C.R. (in case of the supercharger pressure ratio being variable with engine speed) or the induction con ditions must be kept within sufficiently narrow limits at all operating conditions of the engine, including starting. So far engines with variable C.R. are used in the labora tory only. Development of an appropriate engine for aircraft would certainly be accompanied by serious difficulties, loss of time and considerable expense. Indeed, it appears easier to ensure sufficient constancy of the induction conditions. It is true that at present, with the exception of the reciprocating-piston type of compressor, there is no supercharger which could deliver air at a sufficiently constant pressure ratio throughout the whole working range of a high-speed engine. Unfortunately, the use of this type, as well as of the Roots and the vane types of supercharger, is prohibitive due to their excessive weight and dimensions, especially with regard to the considerable mass flow of air through the superchacger in a heavily-boosted engine. There are in existence, however, quite reasonable means for ensuring the necessary con stancy of induction conditions in an engine provided with the usual centrifugal-type supercharger. The required degree of constancy of the induction conditions at normal speeds and loads of an engine with low C.R. could be realised with an exhaust turbine-driven supercharger by providing the turbine with an appropriate regulating device controlling the inlet to the turbine nozzles. The regulating device, which could be operated by some automatic boost control, should be arranged to maintain the pressure head through the turbine necessary
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