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3616 无凸缘筒形件模具设计,3616,无凸缘筒形件模具设计,凸缘,筒形件,模具设计
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Toward Higher Speeds and Outputs From the Small Diesel Engine D.BroomeRicardo & Co.Engineers(1927) Ltd.(England)THE AUTHORS company has long been concerned with the development of the small, high-speed diesel engine, and is particularly associated with combustion systems for this type of engine. Although such engines are not common in the North American continent, production and use in Europe and Japan is considerable, totaling several million units. These are, typically, naturally aspirated 4-cyl engines of 25-35 in3 (400-600cm3) displacement per cylinder, operating up to speeds of 4000-5000 rpm, with a limiting piston speed of about 2400ft/min(12m/s). In discussion with the U.S.Army Tank-Automotive Command (USATAC) at , Mich.it was proposed that the military requirement of high power from a small lightweight package could be achieved by exploiting higher speeds than hitherto, rather than the application of increased levels of turbocharger alone, and this led to the formulation of a research program to study combustion and breathing problems under such conditions. This paper describes the work carried out to date, which has involved the design, manufacture, and preliminary test work on special single cylinder engine. THE PROJECT The project specifications finally laid down by USATAC can be summarized as follows:1. Design, procure, build, and test a single-cylinder engine of 3-1/2 in (88.9mm) bore and stroke, to operate at the highest possible speed, but certainly above 5000 rpm. Simulation of turbocharged conditions to be achieved using a separate air supply.2. To develop the single-cylinder test engine to achieve performance targets such that a 4-cyl version for military duties could produce 1 bhp/in3 (45.5kW/cm3) displacement with a target dry weight of about 3.5 lb/bhp (2.13kg/kW).3. The design not to be influenced by conventional practices, with the aim of minimizing mechanical and thermal stresses.4. Operation on CITE-R fuel (MIL-F-46005A (MR) to be the primary requirement. Initially, fuels down to aviation gasoline were to be investigated, but this latter requirement was subsequently relaxed.5. Lubricating oils to the MIL-L-2104B specification to be used if at all possible.6. The final phase of the project to include a design study for a 4-cyl military engine, embodying the lessons learned on the single-cylinder test unit.7. Starting, idling, and light-load operation of the multicylinder engine must not be compromised.PRELIMINARY DESIGN CONSIDERATIONS A simple examination of the cylinder size and power output target rapidly showed the limitations that the maximum engine speed would have on performance (Table 1). Starting from the minimum speed specified of 5000 rpm, it is clear that speeds of 6000 rpm and above entail piston speeds equal to those of racing gasoline engines. While the reductions in bmep through use of high speeds are significant, the increases in fmep (estimated from past results obtained at the authors company, much of which has been summarized in Ref.1) give very little return in reduced imep. Naturally aspirated automotive diesel engines working to the strict smoke limits of a few years hence can only operate up to about 145lb/in2 (1000kPa) imep; hence it was clear that some measure of turbocharging would be required. A further penalty of high speed and high engine friction is in fuel consumption, and Table 1 makes clear how the bsfc would worsen rapidly to levels no better than a gasoline engine, so losing one of the major advantages of the compression ignition cycle. In these circumstances,it was decided to limit the speed of the research engine to 6000 rpm. The major performance problems involved in the design of an engine to meet these requirements might be summarized as follows: ENGINE BREATHING-Previous experience on small high-speed diesels had shown that the major limitation on imep at high piston speeds is the breathing of the engine (2).Hence, valves of sufficient flow area had to be provided to allow efficient operation up to 3500 ft/min (17.8m/s) piston speed, some 50% higher than levels normally employed in diesel engines. This would certainly require departures from conventional cylinder head arrangements, involving inclined multiple-valve designs (Table 2);turbocharged operation brings a slight bonus in that the higher inlet air temperatures minimize pressure losses and reduce volumetric efficiency changes.In addition, possible turbocharger matching requirements had to be borne in mind. While, for automotive engines, torque backup requirements normally favor minimizing the available boost at the rated speed, so that a large exhaust valve area is not mandatory, in this case the very short absolute exhaust gas release periods suggested that the exhaust mean gas velocities should be kept low, and exhaust valve area about equal to that of the inlet. Also requiring consideration was the question of valve timings. For the inlet, high speeds are normally associated with a late closing point, yet in the case of a diesel, and with closing points later than about 45 deg abdc, there would be a progressive sacrifice in starting ability, as well as some loss of low-speed performance, which would further impair the natural torque backup characteristics of the engine. For the exhaust, the turbocharger matching requirement again dictates an early release of the gases on the expansion stroke, and timings later than about 60 deg abdc do not show to advantage at high speeds. While a long overlap period could contribute to reduction of exhaust gas and exhaust system component temperatures, such gains would be minimal at high speeds due to the very low quantity of scavenge air which might be passed relative to the trapped flow, and the mechanical problems of obtaining the piston/valve clearance would place a severe penalty on the combustion system. COMBUSTION PROBLEMS-A fundamental problem likely to affect the engine at the speeds contemplated was the likely duration of the ignition delay period. Ignition delay is a function of engine speed, compression conditions, and injection timing for a fuel of particular ignition delay is a function of engine speed, compression conditions, and injection timing for a fuel of particular ignition quality at normal running conditions (3),if factors related to the particular combustion chamber configuration in use are considered as being of second order. CITE-R fuel has a minimum specified cetane rating of 37,but the published data on engine delay using this fuel covered only low-speed conditions, and were not of direct use in predicting results at 6000 rpm. However, consideration of these available data, together with the known performance of small high-speed engines operating up to 5000 rpm on gas oil (55 cetane), led to the estimates shown in Fig.1,for the lowest compression ratio which would allow acceptable starting (higher ratios would give excessive heat losses and maximum cylinder pressures).These suggested that unaided or true compression ignition operation at 6000 rpm was feasible on CITE fuel, although the light-load condition would require inlet manifold air temperatures to be maintained significantly above ambient-not an impossible requirement for a turbocharged engine. Injection periods being controlled by the injection system will depend on the latters type, but for practical reasons there could be no possibility of developing new systems for the project, to replace the conventional jerk pump arrangement. With a fixed orifice area nozzle, there would be considerable problems in passing the required full load quantity of up to about 60 mm3/injection at 6000 rpm at the required rate, yet obtaining satisfactory characteristics for idling, the turndown ratio being about 11:1.The effect of an extended injection period on combustion at the high speeds required could be very severe on a direct injection (DI) combustion system, where use of a fixed orifice nozzle would be inevitable. In addition to this problem the major difficulty of the DI was seen as the high mechanical loading, accentuated by the higher smoke-limited fuel/air ratios (A/F) requiring higher boosts to achieve the target rating. While Ricardos earlier research work had shown that DI systems could be made to operate up to 4500 rpm naturally aspirated, on balance (see Table 3) the Comet swirl chamber system, developed over many years for the small high-speed commercial engine, was considered to offer greater potential for this particular application. The major problem foreseen was high thermal loading, although the unaided starting and the full multifuel capabilities were also less satisfactory than those of the DI: however, with built-in aids such as could be applied to the multicylinder engine, and with a restriction to CITE fuel, these latter were not considered to be too serious.At the start of the project, then, some consideration was given to the DI as an alternative, and in addition to test running under boosted conditions of an existing 4000 rpm single-cylinder research unit, designs were completed for a DI version of the test engine. Since that date, the increased pressure of noise, smoke, and particularly exhaust emissions legislation, has increasingly favored the divided chamber system, and test work on the DI version is not now likely to take place.ENGINE FRICTION-Comparing the proposed multicylinder high-speed turbocharged engine with a conventional commercial engine of the same cylinder size and number, it was clear that the former would have a significantly higher fmep through the use of higher rotational and mean piston speeds. As already made clear in Table 1,this would pose serious problems in relation to the attainment of both the target output and an acceptable fuel consumption.Fig.2 shows how using a typical commercial engine fmep/speed curve from Ref.1, the estimated fmep of the high-speed multicylinder engine was obtained. In this estimate, some increase in the mechanical friction of the basic engine structure was assumed, since the turbocharged condition would increase cylinder pressures and require larger bearings to give acceptable reliability. In addition, inlet and exhaust pumping losses could add materially to the high-speed fmep, unless acceptable valve sizes could be maintained. By gasoline standards,then,the mechanical efficiency of the unit would be poor,but experience had shown that although attention to detail throughout the design could yield gains,these low levels were implicit in the project specification.SINGLE-CYLINDER TEST ENGINE Based on the considerations outlined, the definitive single-cylinder test engine was designed, the boundary operating conditions for the engine being: bore and stroke,3-1/2 in 3-1/2 in (88.9mm88.9mm);normal full-load speed range,3000-6000 rpm; and maximum cylinder pressure,2500 lb/in2(17.3Mpa). While the cylinder pressure limit may seem high by conventional standards, past experience has shown the dangers of designing such engines to low limits, and thus inflicting unforeseen limitations on the test program. In fact, originally, with possible work on a DI version in mind, a limit of 3000 lb/in2(20.7Mpa) was set, but as noted, this limit was later reduced for the Comet version. The Comet swirl chamber engine layout is illustrated in Figs.3-5, and the complete engine shown in Fig.6.Of the major components, the following may be said: CRANKCASE-The crankcase and rear-mounted timing case and cover are in gray flake graphite iron to BS 1452:1961 Grade 14,spigoted or doweled and bolted together. The crankcase design was adopted from that of the Ricardo E/6 variable compression ratio gasoline engine, which results in the presence of the front chamber of the crankcase unit, where the E/6 timing drive was situated. Three main bearings are use, all of lead-bronze bushing type, the center bearing being the thrust bearing. The rear bearing acts only as a steady bearing to the otherwise long extension of the crankshaft, the clearance being adjusted so that it cannot take the firing load off the center bearing. CRANKSHAFT-The crankshaft is a one-piece forging in nitriding steel to BS 970:1955 En 40c.The balance weights are integral, and balance only the rotating loads, since primary and secondary balancer shafts are fitted to the engine. All journal and pin surfaces are nitrided, the diameters of the three journals being 3,3,and 2-3/8 in (76.2,76.2,and 60.4 mm),respectively, from front to rear, and of the pin 2-5/8 in (66.6mm). CONNECTING ROD-To obtain the better material properties associated with a forging without the expense of special dies, a search was made of commercial engines, and the connecting rod of the Ford 2700 series diesel engine finally selected as the most appropriate. While satisfactory big-end bearing loadings were achieved at the 3000 lb/in2 maximum cylinder pressures, the little-end design was considered inadequate, and the decision made to use this rod only for the Comet swirl chamber version of the engine, at a pressure limit of 2500 lb/in2. The bearings are as used on the 2700 series engine, that is ,15% reticular tin/aluminium half liners, the little-end bushing being a wrapped lead-bronze item. In addition to careful checking and polishing of the rod, the little end is reduced in width, the better to distribute the firing and inertia loads between the piston pin bosses and the little-end bushing. A higher torque than standard is used on the big-end setscrews, to prevent the cap lifting off due to the inertia forces at tdc exhaust, at 6000 rpm. Computer calculations carried out by the bearing suppliers, The Glacier Metal Co.Ltd., showed that the proposed bearing arrangements were acceptable, although the big end in particular has to accept very arduous conditions at high speeds due to the great inertia of the relatively massive connecting rod (Fig.7).The importance of correct form for both the pin and the bearing under these conditions cannot be overstressed; this apart, the only problem that occurred was rapid cavitation attack in the top (loaded) half liner with the original clearance. The cause of this is evident in Fig.7,and a reduction in clearance to 0.0022 in (56m) cured this trouble. PISTON AND WRIST PIN-The piston is a one-piece sand casting in 13% silicon aluminum alloy to BS 1490:1970 LM13WP,with the shallow trench and twin recesses of the Comet combustion system formed in one face of the angled (pent-roof) crown surface. Two compression rings are used, the top being a plain barrel-faced ring and the second a taper-faced internally stepped (twisted) ring; the slotted oil-control ring is of the conformable type. Rings are supplied copper-plated on the rubbing faces to assist in bedding in, but are not chrome-plated, since this facing is applied to the liner. The piston was designed deliberately of relatively great height, since it was feared that the very high (by diesel standards) piston speeds together with boosted operation would create difficulties in obtaining acceptable piston, ring, and liner conditions, and it was not thought desirable to accentuate problems more than was necessary. However, relatively little trouble has been experienced with the ring pack. Piston cooling and little-end lubrication is via an oil spray from a fixed jet located in the crankcase. This method was selected to avoid grooving the big-end bearing liner, which would have reduced its capacity. Two piston designs were developed, a tray-cooled arrangement and the soluble core design shown in the figures. To obtain acceptable cooling of the ring belt with the tray-cooled design, the struts transmitting gas loads from the crown to the wrist pin bosses were thinned as far as was thought practicable, but this arrangement was found to allow excessive distortion. The soluble core design has given excellent service to date. The wrist pin is of case-hardened steel,1-3/8 in (34.9mm) in diameter: CYLINDER LINER AND WATER JACKET-The high rates of local heat transfer associated with the use of a swirl chamber combustion system, together with the 2500 lb/in2 maximum cylinder pressure limit, led to design difficulties with the wet-type cylinder liner, since calculations showed that a conventional iron liner of thickness adequate to withstand the gas loads would give excessive surface temperatures for acceptable lubrication at the top ring reversal point. The solution adopted was to use a steel liner, with the bore given the necessary surface finish before being plated with hard chrome to a thickness of 0.0015 in (38m) by the Chromard process. Toward the top, the liner is thinned to provide the necessary temperature control, while the greater thickness lower down enhances rigidity to combat water-side attack. The liner is flanged at the top and seats on the cylindrical mild steel water jacket itself seating on top of the crankcase; radial location is provided by the liner spigoting in the crankcase, a water seal being obtained in the normal way by rubber O-rings. CYLINDER HEAD ASSEMBLY-The cylinder head, with its associated cambox, is the most complex single assembly of the engine, and presented considerable design problems. The most pressing of these centered on the provision of adequate valves and ports-the difficulties here may be appreciated when it is realized that ports suitable for a conventional engine of 5-1/2 in bore had to be provided on a 3-1/2 in bore-together with adequate cooling for the very high rating of 5.7 ihp/in2 (0.66 indicated kW/cm2) of piston area, this with a swirl chamber comber combustion system. The position of the Comet swirl chamber at the edge of the bore does not render the use of four valves very attractive, and although this and other possible layouts were examined, a 3-valve arrangement was finally adopted. A pent-roof head surface was necessary (Table 2), partly to obtain the necessary valve area but primarily to prevent excessive congestion higher up in the head. It is normally preferable to pair the exhaust valves to reduce their individual size and use a single large valve, but the opposite layout was finally chosen, as shown in Fig.8, since the paired valves had to lie in the center of the head and the intense heating of the head from the long port duct if this latter were the exhaust was considered unacceptable. Asymmetrical chamber/valye layouts were also investigated but rejected as offering no real advantages and being incompatible with multicylinder requirements. The so-called externally inserted form of the Comet chamber was adopted to minimize the space occupied by the hot plug forming the lower portion of the chamber. To cool the resulting four bridges ins the lower deck of the head, between the chamber and the inlet valves, and the inlet and exhaust valves, drillings were provided, giving an accurately controllable metal thickness between the hot gases and the coolant, and a clean surface on the coolant side. The swirl chamber hot plug carrying the throat, and made from Nimonic 80A alloy by precision casting, is a light fit on its sides as well as locating on a copper gasket on its upper flange, since experience showed that at these high ratings some direct cooling was necessary, unlike commercial engines where an air gap is used to improve warm up after starting. The upper part of the chamber carrying the injector is a spheroidal graphite iron casting. A transducer tapping into the cylinder is provided at the front end of the head. The light alloy head seats directly on the steel liner flange, no gasket being employed, and is clamped by eight suds rooted high in the head and passing vertically downward through the water jacket top flange. No difficulties with gas blow have been experienced, and it has been found possible to operate with a head stud load of only 1.4 times the full gas load. Separate overhead inlet and exhaust camshafts are carried in a cambox casting secured to the head by 11 setscrews. The camshafts operate the valves via inverted bucket tappets, lash adjustment being by means of pallets placed on the top of the valve stems. Camshafts with alternative inlet closing and exhaust opening points are available. The inlet valves are of BS 970:1955 En 59 “XB” steel: 21-4/n austenitic steel was specified for the single exhaust valve, but at the high exhaust temperatures experienced, some head cupping occurs, and two-part exhaust valves with Nimonic 80 heads are now available. High chromium iron seat inserts are fitted to the head. Primarily because it was realized that to meet the weight target for the multicylinder engine, aluminum alloy would have to be used wherever possible, the head and cambox castings are in this material. Heads were originally cast in BS 1490:1970 LM25WP Al-Mg-Si alloy, but these castings were found to be porous in the inlet port wall. Improvements were made in the method of casting, and the material was changed to the high-temperature RR 350 Al-Cu-Ni-Co-Sb-Zr alloy. No further troubles with large scale porosity were experienced, but rapid cracking of the inlet/exhaust valve bridges than occurred. Metallurgical examination indicated the presence of shrinkage microporosity and films formed during casting, but in view of the seriousness of these failures, temperature measurements with a pair of fixed thermocouples symmetrically located at different depths just inboard of the bridge were made-the preferred traversing type of thermocouple illustrated in Ref.4 could not be fitted in this instance due to physical limitations. These tests, given in Table 4,confirmed the original design calculations carried out according to the procedures also summarized in Ref.4, which indicated that while Lm25WP alloy should have been marginal for this duty, RR 350 was unlikely to fail simply by thermal fatigue alone. After making design modifications to improve the cooling of the bridges, as well as strengthening them by reducing the valve seat diameters to 1.050 in (26.7mm), and 1.505 in (38.2mm) for the inlets and the exhaust, respectively, mechanical loading tests were carried out at ambient temperature (that is ,cold), since it was known that some distortion of the head took place even during assembly. Miniature strain gages were fitted to the troublesome bridges prior to the fitting of the valve seat inserts, and the head completed, fitted, and torqued onto the engine, and the cylinder space pressurized to the design cylinder pressure, all while monitoring the gage readings. The results, compared with a similar set for a highly successful aluminum-headed 2-valve commercial diesel engine, are shown in Fig.9, and confirmed the existence of significant tensile strains in the bridges of both engines when cold, although these would be counteracted to some extent at least by the compressive thermal loads in the running engine. Analysis of the cold results on a Gerber diagram for the RR 350 material showed a significant safety margin even against fatigue data for elevated temperatures, and indicated no fundamental reason for the failures. The present head has, to date, completed over twice the number of hours needed to fail the earlier heads, and it would seem that these problems have been successfully overcome, and a viable head design developed. TIMING DRIVE AND BALANCER GEAR-The rear-mounted timing case houses an all-gear drive, downward to the two engine-speed primary balancers and the two twice-engine-speed secondary balancers, and upward to the half-speed drive. The balancer shafts, located beneath the crank throw, are formed from steel bar stock by milling away one side of the bar, over which is then pressed thin steel tube to provide a smooth external surface to reduce windage and oil churning. The half-speed output from the top of the timing case is forward via a coupling to the fuel injection pump and rearwards to a toothed belt sprocket. The two overhead camshafts are driven via this 1:1 toothed belt and a layshaft located on the rear of the head assembly; this layout gives flexibility to the head design and allows alternative chamber and valve layouts to be readily incorporated if required in the future. Ball and roller bearings are used throughout the timing drive system. FUEL INJECTION SYSTEM-As noted earlier, it was decided from the outset to use a conventional jerk pump fuel injection system, and a search was carried out for a 3000 rpm pump suitable for the application. The unit used is basically as developed for a 3-cyl, 3000 rpm, 2-stroke engine, the dynamics of the individual elements having been proved at the required speeds. To obtain the necessary higher injection rates and fuelings, a new camshaft was made up with all three cams in phase, so that individual elements could be coupled together hydraulically as necessary. The pump is driven via a manual advance/retard unit, to allow adjustment of injection timing without shutting down the engine. The location of the fuel pump adjacent to the water jacket allows a short high-pressure fuel pipe to be used, and a conventional S-size pintle nozzle is fitted to the injector. The design of nozzle heat shield developed for commercial engines is used to maintain low nozzle tip temperatures. COOLANT CIRCUIT-The necessary high coolant flow rates are provided on the test engine by an external, electrically driven pump, with a flowmeter in the main circuit. The engine circuit is a parallel system, however, with coolant fed to tow sets of entries. One set consists of two tangentially biased holes at the base of the cylinder water jacket, from where coolant flows spirally up past the liner and around the liner flange via a series of small cutouts in the flange support shoulder. Coolant then enters the head through 12 holes in the deck head inboard of the ring of securing studs-an O-ring provides the seal between the head and the water jacket. The second set of entries comprises the four drillings in the bridges in the cylinder head deck, the flows from which converge in the center of the head between the inlet valves before flowing outward around the inlet port duct to join the first coolant stream. Coolant exits from the head via a single take-off above the exhaust port. LUBRICATING CIRCUIT-Again, a separately driven pump is used to provide oil for lubrication and piston cooling; flowmeters allow the separate flows to be established. All lubricant feeds are via separate drillings and external pipe connections, experience having shown that on this class of engine such a layout achieves maximum reliability with low primary cost. Low-pressure feeds are provided for the valve mechanism. The sump is “dry” and is slightly pressurized to ensure that oil does not build up around the balancer shafts.ENGINE PERFORMANCE Development work on the engine combustion and injection systems is not complete at the present time, and no real work has been carried out to date to optimize the engine breathing. Hence, the result herein quoted are included essentially to show that the target performance requirements can be met, rather than to present the optimized performance of this advanced engine concept. Further gains are to be expected, particularly in the balance of performance over the speed range, and in addition it is hoped to obtain more data on combustion at ultra high speeds, and on the mechanical and thermal loadings involved. TEST INSTALLATION-The engine is coupled to a d-c electric swinging field dynamometer, which is also used for motoring the unit. Coolant outlet and lubricating oil inlet temperatures are automatically controlled via water-cooled heat exchangers. Boost air is supplied from a separately driven air compressor through an Alcock viscous flow airmeter and a heater/antisurge tank positioned close to the engine, to provide fine control of inlet air temperature and reduce ram effects. The exhaust is fed by a short pipe to an expansion chamber, the pressure therein being controlled at the outlet; provision is made at entry to the chamber or a range of orifices to be fitted, for use in simulating turbocharged operation during investigations of engine breathing. The test installation is shown if Fig.10. The normal method of test is to operate the engine over the load range at a fixed speed by varying the fueling, the inlet air (boost) and exhaust back pressure conditions being held constant and equal to one another. This does not simulate a turbocharged engine operating over the same load range, but from such results approximate predictions of turbocharged engine performance can be built up. Results are presented in terms of the indicated performance of the engine, derived from the brake results and the motored friction at the same boost and back pressure, to allow simple evaluation is terms of the multicylinder engine targets. Due to difficulties of supply of CITE fuel in the United Kingdom,USATAC approval for the use of Avtag (D.Eng R.D.2486) fuel of essentially similar composition and of cetane number 37 was obtained. Some tests were also carried out using Gas Oil C.I., essentially similar in specification to ASTM Grade 2-D diesel fuel, the cetane index being 55. TEST WORK-Apart from mechanical development of such a novel engine, most of the test work to date has been devoted to achieving the full target rating of 215 lb/in2 (1485 kPa) imep. Some data have also been taken over the normal full-load speed range, down to 3000 rpm. Initial tests at 6000 rpm indicated that the smoke-limited performance of the combustion system was acceptable, but that very high exhaust temperatures were occurring, up to 1650F (900C) as recorded by a thermocouple at the exhaust pipe flange.l This was considered to be due to the very long injection period and consequent late burning. Development of the fuel injection specification was therefore put in hand to increase the injection rate at the nozzle, but large increases in nominal (pump) rate, nozzle orifice area, and nozzle opening pressure,gave,overall,only a 20% reduction in injection period. Some simple calculations of the injection system performance confirmed that this disappointing result was fundamental in nature. It was therefore decided to accept for the moment the thermal problems inherent in the high exhaust temperatures and seek design solutions to these,as noted earlier. The performance over the load range at a boost pressure ratio of 1.9 and a speed of 6000 rpm is shown in Fig.11 for both fuels, and typical cylinder pressure and nozzle needle lift diagrams in Fig.12. From these it can be seen that this boost is just sufficient to enable the target rating to be reached on both fuels, the A/F at the “just visible” smoke point being about 0.055. This is a remarkable result, considering that the last fuel to be injected enters the chamber some 40 deg atdc,and can be attributed to the very efficient mixing pattern of the Comet combustion system, particularly that generated by the piston recesses following the outflow of gas from the swirl chamber. With gas oil fuel, combustion starts just atdc, about the normal optimum, for the Comet system, but with the lower ignition quality Avtag, it proved impossible to bring combustion as advanced as this with the inlet air conditions and compression ratio in use, with some inevitable loss of efficiency. A light load misfire problem is approached with Avtag at light loads, and some increase in the inlet air temperature may be necessary-on a multicylinder engine use of an air-to-coolant aftercooler with special control of the coolant flow could be used to give this result. As can be seen from the cylinder pressure diagrams, the maximum cylinder pressure is, in fact, the compression pressure at these high-speed conditions, and the measured value at full load of 1475 lb/in2 (10.2MPa) agrees well with calculations. The performance over the speed range at constant inlet air conditions and optimum injection timing at each speed on gas oil fuel is shown in Fig.13, the injection specification differing from that of Fig.11. Indicated is the extent to which the low-speed output of the combustion system is depreciated by high=speed requirements, although some improvements may be obtained with further development. Due to the rather small valves now employed, and the early inlet valve closing point of only 40 deg abdc, the volumetric efficiency is falling off above 5000 rpm, but torque back-up requirements of the multicylinder engine may render such a characteristic acceptable.MULTICYLINDER ENGINE POSSIBILITIES With test work on the single=cylinder engine far from complete, it has not been considered worthwhile to start the design study of the multicylinder version. However, to obtain some idea of what the latter might look like, some sketches were made of external views of a pollible 4-cyl engine, and front and side elevations are shown in Fig.14. The engine envisaged is an in-line,dry-sump unit, with the cylinder head arrangement essentially as on the single-cylinder test engine, except that a one-piece construction is used. The turbocharger is mounted above the flywheel and delivers to a combined inlet manifold/air-to coolant aftercooler unit mounted above the cambox. The engine,complete with all the usual auxiliaries, is 31-3/4 in (806 mm) long over the flywheel flange, 32-3/4 in (831) mm) high, and 24 in (609 mm) wide, giving a specific size of 9.3 bhp/ft3 (245 kW/m3) of box volume. To meet the specific weight target, a dry weight of 470 lb (213 kg) must be achieved. This is equivalent to 3.49 lb/in3 (0.096 kg/cm3) of swept volume,which should be realized, since values for conventional automotive diesel engines of this size lie in the range 3-5 lb/in3 (0.08-0.14 kg/cm3). Estimates of the brake performance of such an engine have also been made, based on the results achieved to date on the single-cylinder test engine (Fig.15). A torque back-up of 10% at 4000 rpm was postulated, so that, being a turbocharged engine, above this speed the torque requirement. Below 4000 rpm, the fueling must be cut back to maintain a smoke condition of about 5% opacity, and gains in the low-speed performance characteris
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