WHERE IS THE BRITISH 0-STROKE OIL ENGINE?
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IN this country-the design of oil engines for road-transport vehicles would appear to have reached a degree of crystal lization. Such units operate on the four-stroke-cycle principle and are either of the direct-injection or turbulentchamber type. In the years to come however, British manufacturers are likely to he seriously challenged by American competition' a direction from which we have, in the past, been in no danger. That country's vast industrial potential which _has been built up to meet the needs of war will now ue capable of producing, at a low price, a highly efficient oil-burning engine, backed by an unrivalled service organization.
Foremost among the transatlantic engines will undoubtedly be the interesting G.M. unit, which has been fully described-in this journal. The number now being. produced is said to exceed the total output of British engines.
In view of the advantages of the two-stroke-cycle for oil engines, and the practical success .of the Gallf. models, it seems strange that more interest has not been shown in this country towards the system. An authoritative paper was recently read by Mr. R. Wisner before the Institution of Automobile Engineers, whichmay instigate sufficient interest for British builders to begin experimental work...
Outstanding among the advantages of the two-stroke engine is the reduced cost for a given output over its fourstroke counterpart. Contributing largely to this .aving is the reduced number of-fizet pumps-arid injectors required. These represent a fair proportion -of the total cost of the engine and, even in mass production, it is difficult to make much improvement, as skilled labour is necessary to achieve the extremely close fits necessary for accurate performance.
The output of a good two-stroke unit is such that, theoretically, only half the number of pumps is required, although, in practice, this would necessitate a threecylindered unit to replace a six-cylindered four-stroke. In view of the need for special balancing gear in the case of the three-cylindered engine it is likely that the " four would be the least number of cylinders acceptable in this country, but smoother running would result from the greater number of firing strokes over the orthodox six-cylindered four-strcike.
No Unburnt Fuel Wastage
In Two-stroke Oil Engines
The layout of in-line cylinders with scavenge pump built on to the side, adopted in the G.M. engine, should be an ideal one for road-vehicle use, as the low overall_ length would permit -the maximum ratio of pay-load capacity to overall length.
Unlike the petrol-burning two-stroke, units using oil do not suffer from the great disadvantages of losing unburnt fuel through the exhaust port, as only air is contained in the cylinder during the period of possibld loss There are three recognized methods of cylinder scavenge,
and these at'e the crbss-flow, loop and uniflow. The first is the principle employed on some motorcycle engines. The descending piston compresses air in the crankcase until a transfer port is opened, when it rushes into the cylinder (Fig. I). A specially shaped piston crown deflects the air towards the head, the exhaust gases, meanwhile, Laing displaced.
As the scavenge air has a volume equal only to that of the cylinder it is not possible for all the exhaust gas to be swept away, because an appreciable quantity of unused air passes through the exhaust port. As the engine power is dependent on thequantity of fuel in the cylinder, and that, in turn, on the oxygen content of the air contained therein, the b.m.e.p. in engines of this type is low.
The loop system (Fig. 2) is a little better than the first. In this case, the inlet ports are arranged at 90 degrees to the exhaust, and the piton crown is almost fiat. Scavenge air rushes into the cylinder from the crankcase from-opposite directions and Meeting, moves up the cylinder. The exhaust gases are displaced downwards arid pass through the exhaust srstem to the atrersoshere. ,
The b.m:e.p. of the -.loopscavenge engines is usually about half that of a good four-stroke and, due to the air loss, a somewhat higher fuel consumption is incurred.
Best of 'thethree principles for road vehicle engines is, undoubtedly, the uniflow type (Fig. 3), in Which the inlet air enters at. one end of the cylinder and pushes the exhaust gas before it -through t h e exhaust porting. in an orthodox engine it is not considered good practice for the flow to be downwards, as the ports at thlower end of the cylinder, being piston-controlled, the piston would be subjected to the harmful effects of the exhaust flow over a part of the crown.
With the inlet porting in this position, the piston receives the beneficial effect of the cool incoming air, and adnquat e turbulence is readily achieved. • With engines of this type a high ban,e.p. is obtained, and the fuel-consumption figures are not materially worse than those of other engines built for similar duty and working under like conditions. This proviso is necessary, as the figures for the successful American uniflow-type engine are unfavourable as compared with good-quality British units, whereas when set against American four-strokes there is no marked difference.
From a study of available data, the inferenae is that, if produced by one of our manufacturers, a two-stroke unit could be marketed which would be as much in advance of American practice as are our four-stroke engines.
Turbulence, at already stated, is readily obtained in engines of this type, it merely being netessary to core the ports so that a tangential air entry is obtained. The rotational velocity is fairly critical. Too low a speed causes the incoming al: to assume a core form leaving a considerable quantity of exhaust gas in tbe neighbotirhoOd Of the cylinder wall. Too great a velocity, on the other hand, tends to leave a core of spent gas in the centre. A further disadvantage of high rotational Velocity is that more effort than is desirable must be used in setting the air iii motion, and 'the fuel dmsumPtion suffers,
The form of combustion chamber is influenced by the . necessity -of complete -scavenge. The deep pan shape, which is successful f o r direct-injection, four-strokes, is not satisfactory in twostrokes, as the scavenge air would be difficult to direct into the chamber.
In the G.M. engine there is a shallow, conical piston crown with an upward apex (Fig. 4). At top-dead-centre the apex of the cone is just beneath the injector, which is of the multi-hole type. The -fuel spray is directed down the sides of the cone and prevented from inapinging on the cylinder wall by a lip raised on the piston, so that only the smallest practicable distance is left between it and the cylinder head. This arrangement is somewhat disadvantageous in that " squish " turbulence cannot be used; the multi-hoed injector is more liable to derangement than the single-holed type, and the control of the droplet size is less accurate. Mr. R. Wisner, in the paper already referred to, described a combustion chamber known as the "lipped vortex" type (Fig. 5), in which " squish " is used, and only a single' exhaust valve is required, as against two in the G.M. engine. Direct injection seems to lend itself particularly well to two-stroke design as no confined areas in which spent gases may collect are provided. Also, the thermal efficiency is not worsened by heat loss as is occasioned by rushing through narrow orifices. Two-stroke engines are favourable in that heat losses. are low compared with four-strokes, owing to the shortened time available for heat to be dissipated to the coolant. Careful attention to the Cooling is, however, required, for the reason that, although less heat per piston stroke is lost, such strokes occur with doubled frequency in each cylinder. Oil Jets are, therefore, sometimes employed to conduct heat from the piston crown.
By careful attention to the port timing, much of the work which would normally he lost in pushing the scavenge air through the cylinder may be regained. The latter Part of the power stroke does not materially affect the output, so that a fairly early opening of the exhaust port permits the .spent gases to rush away and thereby leave a low pressure in their wake
The inlet air is partially inspired by the exhaust stream, On stationary engines, this feature is actually employed to fulfil_ the whole of the scavenge air induction, and, although the, need for operation at a wide range of speeds precludes this for road-vehicle engines, certain benefit does result by careful attention to this feature.
The opposed-piston engine has much that is of interest, as its fuel econOmy is unrivalled by other types of twostroke, of by Many four-strokes. For Vehicle-engine usage the cylinders are arranged vertically. The lower piston is . connected to the crankshaft in the usual manner, whilst the upper one has a cross-head to which are articulated two connecting rods disposed one on each side of the cylinder. These articulate on crankpins disposed almost diametrically opposite to the lower piston crankpin.
High Thermal Efficiency From
Piston-controlledport Engine (.1n the upper piston is a larger one containing automatic valves.. This piston serves to pump the scavenge air through the cylinder. One piston controls the inlet ports and the other the exhaust, so that fib poppet valves are required and unidirectional flov is obtained. This design owes its high thermal efficiency largely to the favourable areavolume-ratio of the combustion space.
If the unit be imagined as two cylinders placed end to end, it will be seen that the opposed-piston engine is favourable, in that the Cylinder-head surfaces are dispensed with, thus eliminating a source of heat loss. There is, however, a disadvantage to this design. " Squish " cannot he employed, as any reduction in the cylinder•diameter at the combustion space to secure this would seriously impair the straight-through flow of the scavenge air.
By positioning the crank for the upper piston somewhat out of phase from that of the lower one, the inlet timing can be varied in relation th the exhaust to make optimum use of the exhaust depression.
An advantage of two-strokes, which is not always apparent, is that, as modern high-duty engines employ spine form of pump for the supply of scavenge air, it is little more difficult to enlarge it to provide a degree of supercharge, whereas, with a four-stroke-design it is necessary to add a supercharger.
The American two-stroke oil engine to which our contributor refers was fully described in " The Commercial Motor" dated January 28,, 1944. Some edits outstanding features are that it has a high-mounted camshaft which operates the exhaust valves, in Push-rods, and the unit-built injectors.
Each of the latter incorporates its o.wn pump and the unit is so disposed in the cylinder head as to take advantage of the engine's cooling system.
Cylinder scavenging and air charging are carried Out by a blower, a point in connection with which is the' nominal speed at which it is driven. It revolves at only 1.94 times that of the crankshaft fronn which it is driven via helical gearing.
Points of great_ interest in the general design 'are that the main engine block is sci, made that the drive nits can be -assembled at either end, and it can be built up with any number of cylinders; from one to Six, . As an .example of the power output, a three-cylindered 0.M. unit, having a total displacement of 3.4$6. lttre gives a maximum of 72 b.h.p. at the nominal si*.4 of 1,500 r.p.m. • IN Maximum torque is 202 lb /ft st frern
.800 to 1,200