Greater Power Greater Economy
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By
R. W. Bent, M.I.Mech.E. UNLIKE the private car engine, the commercial vehicle power unit has never been handicapped by a taxation system based on rated horse-power. Now that cars first registered in 1947 pay a flatrate tax of £10, designers are given more scope in the production of larger and more powerful engines
In the commercial vehicle world the main restrictions on engine size have been weight, and to a lesser extent fuel consumption. On larger vehicles employing oil engines, the increase in size and power developed have steadily progressed, particularly tn the past few years. Fig. 1 shows power curves marking definite stages in the development of the oil engine.
Thus, when first introduced in the early thirties, a maximum of about 85 b.h.p. was considered adequate. Some 10 years later 100 b.h.p.• was commonly developed, and to-day about 125 b.h.p. is generally adopted for heavy goods vehicle and doubledeck bus engines Rapid Acceleration The increase is largely to meet the requirements of rapid acceleration and to obviate the need for frequent gear changing It is due to the fact that the larger engine can pull a higher gear, that there is little or no increase in the overall fuel consumption. In addition, the larger engine, working usually well within its rated power, has increased the periods between overhauls and thus reduced maintenance costs.
The question asked by many operators is whether there is scope for even more powerful engines, and what is the best means for obtaining such additional power.
The b.h.p., which is the work done per minute at the flywheel, divided by 33,000, depends on two factors: (a) the mean effective pressure (m.e.p.), and (b) the volume swept out per minute.
The m.e p. in the cylinder is frequently confused with the maximum. pressure, and it is often thought that because the oil engine has a maximum pressure about twice that of the petrol unit, its mean pressure must also be higher. The reverse is actually. the case, and the m.e.p. of most oil engines is about 100-110 lb. per sq. in., whereas many petrol engines reach 120-130 lb. per sq. in.
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The maximum pressure, which depends on the compression ratio, is only one of the factors governing the tn.e.p. A higher compression ratio", will result in a greater maximum pressure, and therefore increased m.e.p., but there are limits to the compression ratio, even in oil engines.
Unlike petrol engines, there are no detonation problems, but high-compression ratios result in increased piston and bearing loads, and consequently, to deal with these forces, greater strength and weight of the whole structure are needed. It is for these reasons that most makers have adopted a compression ratio of about 16 to I.
One important aspect, in which modern engines are superior, is in respect of combustion efficiency. This refers to the completeness with which the fuel and air are burnt, so generating the maximum quantity of heat per cycle By careful attention to the shape of the piston crown and cylinder head, turbulence is promoted so as to ensure complete mixing of the fuel with the air.
Volumetric efficiency, expressed as the ratio of the air drawn in per stroke to the swept volume when the air is at normal temperature and pressure, is also an important factor in respect of the power developed. This efficiency decreases at high speeds owing to the restrictions caused in the valve passages, etc and apart from attention to such details, the designer can do little to improve the charging of the cylinders Supercharger Difficulne!, A marked improvement can be achieved by supercharging, and where greater power is required for a given size of engine, this is the most practical way to secure it. Supercharging, however, has not made much headway. because, apart from the extra cost involved, it absorbs some power to drive the blower, unless it be driven by the energy of the exhaust gases: it also increases fuel consumption.
An outstanding example of the increased power resulting from supercharging is to be found in the Crossley 8.6-litre six-cylindered oil engine, the output of which, without supercharging, is 100 b.h.p. at 1,700 r.p.m, Engine torque is 358 lb.-ft. at 1,000 r.p.m. Supercharging raises the output to 150 b.h.p. at 1,750 r.p.m., and engine torque to 484 lb:-ft. at 1,000 r.p.m. The exact net gain is a matter for speculation, but there is no question in this case that the overall results fully justify the modifications involved.
The volume swept out per minute depends on the piston area and the mean piston speed. The piston area is proportional to the diameter, squared; for example, a cylinder 6 ins, in diameter will have four
times the area of one having a 3-in. bore (Fig. 2): There are, however, practical limitations to the size of the bore, and these are mainly concerned with weight and cooling
Thus, in the example cited, although the piston area was increased four times, the weight would he nearer eight times that of the smaller engine. Thisineans that the gain in power is at the expense of a much lower power-to-weight ratio.
Cooling difficulties have already occurred in some oil engines, owing to the high temperatures attained by the piston crown. Fig. 3 shows typical temperatures for pistons of 4-in. and 5-in, bore cylinders. The increase in crown temperature means that, apart from the normal troubles associated with incipient overheating, the strength of the piston decreases, with the consequent likelihood of early failure. So far, piston design has managed to keep step with the larger and more powerful engines, but any further enlargement of the bore will be difficult to cope with unless such an expedient as cooling by oil jets on the underside of the piston crown be used.
Maximum Revolutions
The mean piston speed can be raised by increasing the engine revolutions or the piston stroke. Neither method is a satisfactory proposition, as maximum revolutions are limited on the oil engine by the smoke point, usually about 1,800 r.p.m. Lengthening the stroke means higher inertia stresses as well as increasing the overall height of the engine.
A. method of raising the power output which designers may decide to pursue, is to use eight-cylindered engines. So far, this type has been used to only a limited extent on oil engines,, and is mainly confined to only one make of petrol engine—the Ford. As already stated, the lirnitatioa. in bore-size appears to be rapidly approaching on six-cylindered engines, so that an increase in the
number of cylinders would seem to be the only alternative.
Considering the points in favour of eight cylinders, the increase in power, although entailing a heavier unit, actually improves the power-to weight ratio. This is because the smaller bore allows pistons of less weight to be used and the reduced stresses means lighter reciprocating and rotating parts throughout. The smaller cylinders improve the cooling, particularly of the piston, and avoid failures due to heat stresses.
A further advantage is the more even torque of the eight-cylindered engine. Fig. 4 shows the torque variations for sixand eightcylindered engines. It will be seen that the range for the eightcylindered unit is considerably less than for the six-cylindered unit. In practice this gives a smoother start from rest and better acceleration. Turning to the possible disadvantages, the 'fitcreased length of eight cylinders against six would seem to entail some sacrifice of loading space on both goods and passenger vehicles. Of course, the addition of two extra cylinders does not mean a one-third increase in length, but only about 15 per cent, if car engines can be taken as a criterion. This assumes that the in-line arrangement is retained, but other layouts such as arranging the cylinders in V formation, or at 180 degrees, are feasible. The V system does not lend itself to the forward-control vehicle, but the opposed-cylinder type could be accommodated,
Main Obstacle
The increased use of the underfloor engine would present no difficulties with the in-line design, as longitudinal space is not limited. In fact, this seems to remove the main obstacle to the use of eight cylinders.
The question of fuel consumption is a point that will weigh with operators.. On eight-cylindered petrol engines, mixture distribution has been a difficult problem, but the oil engine is immune from carburation. troubles. . Although a larger engine will naturally consume more fuel, theoverall consumption is not likely to be appreciably greater, owing to the ability of the unit to pull a higher gear. Under some conditions, it could well be less.
The question of maintenance will not worry operators who have had experience of fourand sixcylindered units. They realize that more parts do not necessarily mean more troubles, in fact With' the components less highly Stressed, the maintenance periods can be extended.
On balance, therefore, there would appear to be a strong argument for investigating the poSsibilities of eightcylindered in-line engines for securing the higher performance desirable in large capacity lorries and buses.