What is Engine
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"EFFICIENCY"?
By R.W. Bent, M.I.Mech.E.
The Word Efficiency, as Applied to Power Units, Can be Misleading Unless Correctly Defined in Terms of Overall Merit WHEN announcing a new vehicle, manufacturers frequently state, "Our new chassis is fitted with a highly efficient six-cylindered engine." The operator reading the words "highly efficient" may visualize a more powerful unit than its predecessor, one halting a longer life between overhauls or lower fuel consumption.
It may be that the manufacturer's sales department desires to convey all these points, but to the engineer the term efficiency has a definite meaning or rather several, as there are many forms of efficiency as applied to an internal combustion engine
Output in Heat Units What may be termed the bask efficiency, is the thermal efficiency, because we are dealing with heat e.ngines.in which the ratio of output to heat input is of fundamental importance. The power supplied to the engine is the heat in the fuel, the heat or calorific value of all hydrocarbon fuels being approximately 10,000 per lb_ of fuel. The work done by the engine is a measure of its power output, but it is necessary to express the work in heat units, 1 C.H.U. being equivalent to 1,400 ft.-lb. of work.
Fig. 1 shows the thermal efficiency of a six cylindered oil engine developing about 125 b.h.p. It is termed brake thermal efficiency to distinguish it from the indicated thermal efficiency. The former is based on the brake horse power. and the latter on the indicated horse c6 power, the difference being due to frictional losses.
It will be apparent that Ay claim of 100 per cent. efficiency is absurd, but the query arises as to how the remainder of the heat is dissipated. Fig. 2 shows the distribution of the heat supplied to a typical oil engine, and it will be seen that nearly twothirds of the heat input passes out as exhaust gases and cooling losses. These losses are inevitable, although the proportion may be increased slightly if, for example, the engine be operating at too low a temperature.
Any gain in thermal efficiency can be translated into a lower fuel consumption, or more power with the same consumption. The most direct way of achieving this is by raising the compression ratio, but in practice this has very definite limits. With petrol engines detonation accompanies too high a ratio, although the use of high-octane fuels enables the detonating point to be raised. As an example, if the octane value of a fuel be increased from 70 to 90 the compression could be raised by approximately two ratios, say from 6 to 1 to 8 to I.
With oil engines the problem of detonation, as such, does not exist, but there are practical limits to the compression ratio. High ratios mean reduced clearance volumes and the increased pressure results in greater stresses on pistons, connecting rods. crankshafts and other engine components. Furthermore, the gain in efficiency becomes less at the higher ratios. Thus, by increasing the ratio from 5 to 7 there is a gain . in efficiency of about 5 per cent. but from 13 to 15 the increase is less than 3 per cent.
Instead of quoting the brake thermal efficiency, the manufacturer generally expresses the consumption in pints or lb. per b.h.p. hr. This is another way of stating the efficiency, which can be found if the calorific value of the fuel be known. If the consumption is given in pints per hr. the specific gravity of the fuel must be known to convert it to lb./hr.
For example. suppose an engine consumed 0.4 lb. of fuel per b.h.p. hr., the calorific value of the fuel being 10,500 C.H.U. per lb., then the heat in the fuel — 0.4x 10,500 = 4,200 C.H.U.
Petrol and OH Engines
It will be noted that the consumption increases slightly at both low and high speeds. This is in contrast to the petrol engine where, particularly at low speeds, the consumption is much higher in terms of pints per b.h.p. hr.
The second efficiency affecting the engine performance is known as mechanical efficiency. This is defined as the ratio of the useful work done to the work developed in the cylinder. A working formula is the brake horse power (b.h.p.) divided by the indicated horse power (i.h.p.). In practice the i.h.p. is not easily ascertained, because it usually involves finding the average pressure in the cylinder.
The mechanical efficiency is a measurement of the frictional losses in the engine and the losses involved in charging and exhausting the cylinder. These losses can be found on some types of electric brake or dynamometer by measuring the power required to turn the engine at
any given speed. Thus, suppose an engine develops 62 b.h.p. at a certain speed, and the power to rotate, or motor it, as it is called, at the same speed is 11 b.h.p. The i.h.p. then equals 62 + 11 73, and the mechanical efficiency is 62 ÷ 73 = 0.85 or 85 per cent.
Another method of finding the losses consists of cutting each plug or injector in turn, and measuring the power developed. On a fourcylindered engine if X=b.h.p. with four cylinders firing; Y=b.h.p. with three cylinders firing; Z=i.h.p. of one cylinder; F=frictional h.p. of one cylinder.
Then X = 4(Z — F) = 4Z — 4F; X—Y=Z. An approximate test can be made by shorting one cylinder and multiplying the frictional horse power by the number of cylinders.
Mechanical Efficiency
The mechanical efficiency depends on the general design, lubrication, load and speed. As an example, an engine fitted with a ballor rollerhearing crankshaft or connecting rods, would have a higher mechanical efficiency than one using the normal plain bearings, but the extra cost and complication would probably not be justified. Most modern engines have a mechanical efficiency of about 90 per cent. which is reduced slightly at higher speeds.
A feature which has a considerable effect upon the performance, especially at high speeds, is the volumetric efficiency. This is a measurement of cylinder charging, and is defined as the ratio of the volume of the air drawn in at normal temperature and pressure to the swept volume.
Filling the Cylinders
Thus, if the volume of air drawn in on the suction stroke were equal to 80 per cent, of the swept volume (Fig. 3), the volumetric efficiency would be 80 per cent. Because the power of an engine at any given speed depends on the amount of air used per .cycle, the volumetric efficiency has a direct bearing on the performance.
As the engine speed increases, the time for charging the cylinder is. reduced, and the effect of restrictions imposed by the valve ports becomes more pronounced. This is the chief reason why the power-curve peaks and even tends to fall slightly, although the engine r.p.m. is still increasing. With oil engines, this peak is not usually reached as the speed is governed well below the maximum attainable. This ensures that the life of the engine is prolonged between overhauls, and at the governed-speed the engine is consuming the maximum amount of fuel it is able to burn efficiently.
A further factor affecting the volumetric effitiency is the temperature of the air or mixture entering the cylinder. If the temperature be raised, the volume increases and a reduced quantity is induced. This result is likely to be more pronounced on petrol engines as heating of the mixture is necessary to ensure adequate vapourization.
The obvious way. of increasing this efficiency is to raise the pressure of the entering air, as for example. by supercharging. This system has not, so far, found favour on commercial vehicle engines on account of the additional cost and increased loading of the engine parts. It would, however, enable a smaller engine to be used, the extra power being obtained from the improved volumetric efficiency as more fuel would be needed to burn the greater quantity of air.
These three efficiencies, thermal, mechanical and volumetric, are the principal ones affecting engine performance. The first is the most important, but it is necessary to mention one point when considering the fuel consumption in pints per b.h.p. hr. Although this does permit a direct comparison, with engines of varying capacity, it should be remembered that these figures are obtained under full-load, a condition which seldom occurs in service.
Value of. Road Tests It is in this connection that figures of actual consumption under normal running conditions, such as those given in the tests conducted by this journal are of particular value. Road tests also reveal other important performance items which have no standard efficiencies. Such factors as ease of handling, suspension and reliability can be ascertained only under working conditions.
Braking performance is often stated as a percentage, 100 per cent. being a de-celeration equal to that of gravity-32 ft. per sec. per sec. It is rather surprising that a similar method is not used to measure the acceleration as an alternative, or additional to the normal method of stating the time required to reach a given speed.