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EFFICIENCY OF INTERNAL-COMBUSTION ENGINES.

Preliminary Report of the Committee appointed on the 6th of November, 1903, to consider and report to the Council on the Standards of Efficiency of Internal-Combustion Engines.

Preliminary Report of the Committee.

(Adopted by the Council 21 March, 1905.)

Standards of efficiency of heat-engines are required to indicate the extent to which the heat supplied is utilized in producing mechanical energy, in any given engine, and for the comparison in this respect of engines of different types and construction.

A committee appointed by the Institution has already reported1 on such standards for steam-engines, and it is desirable to state briefly the results they arrived at before considering in what respects the case of internal-combustion engines requires a different treatment.

The Steam-Engine Committee pointed out that, ordinarily, for practical purposes the efficiency might be measured by the number of pounds of feed-water required per I.HP.-hour, or more exactly by the number of thermal units supplied to the engine per I.HP.minute. The hour and minute units are selected for convenience.

1 Minutes of Proceedings Inst. C.E., vol. cxxxiv. 1898, p. 278.

Nevertheless, for scientific purposes, and to a great extent for practical purposes also, it was more definite to state the efficiency as a ratio. Several such ratios have been used, corresponding to different ideas as to the object for which a statement of efficiency is required. The most fundamental of these ratios is that of the work done to the heat supplied, which is termed the absolute efficiency.

Absolute thermal efficiency

=

Heat converted into work
Heat supplied

In comparing the relative value of heat-engines of different types, -for example, in comparing steam-engines and gas-engines-the absolute thermal efficiency is necessarily used.

In any actual thermal engine a large part of the heat supplied is unavoidably wasted; that is, it could not be utilized by any conceivable engine in the conditions under which the actual engine necessarily works. Hence, in the ratio expressing the absolute efficiency, the unutilized or wasted heat is made up of two portions, (a) a portion depending on the available temperature-range which would necessarily be wasted even in an ideally perfect engine; and (b) a portion wasted in consequence of defects of operation in the actual engine considered. A knowledge of the amount of this latter portion is very important to engineers. It is, therefore, convenient to have a statement which expresses the ratio of the heat utilized to that which would be utilized in an ideally perfect engine working with the same temperature-range. The ideally perfect engine, for any given temperature-range, is one working with the Carnot cycle and losing no heat by conduction or radiation. By comparing the heat utilized by an actual engine with. that which would be utilized by a Carnot engine having the same temperature-range, a measure is found of the waste due to more or less avoidable defects of the engine itself, or to the nature of the particular working fluid employed, and independent of the necessary waste. The thermal efficiency of an ideal Carnot engine, T1 — T2 as is well known, is given by the very simple expression depending on the absolute temperature limits only. Hence using the Carnot engine as a standard

Relative efficiency =

Heat utilized

Heat utilized in Carnot engine'

1

T1

There is here introduced the idea of a "standard engine of comparison," and the question arises whether in any given case the Carnot engine is the best standard of comparison. There is one

adiabatic operation in the Carnot cycle which cannot be performed in any actual steam-engine, although ideally an equivalent result would be obtained by using a regenerator. Actually in the fourth operation of an actual engine heat is received at rising temperature. Rankine and Clausius described a cycle like that of the Carnot engine, except that in the fourth operation heat is received at temperatures varying from the lowest to the highest of the temperature-range, and showed how the efficiency of an ideally perfect engine working with initially saturated steam and using this cycle could be calculated. If a Rankine engine, working with the same temperature-range as the actual engine, is taken as the standard of comparison, then,

Relative efficiency1 =

Heat utilized

Heat utilized in Rankine engine

It may be pointed out that the absolute efficiency and the two. relative efficiencies described are all quantities of the same kind. In the case of the absolute efficiency, the standard engine of comparison is a Carnot engine working to the absolute zero of temperature as a lower limit. This is a condition not realizable in actual engines, but necessary if all the heat is to be converted into work.

For steam-engines working under ordinary conditions, the difference of the relative efficiency, whether the Carnot or the Rankine engine is taken as the standard of comparison, is inconsiderable, and for practical purposes the latter is generally the more instructive. A modification of the ordinary Rankine cycle may be made for the case of an engine using superheated steam.

The actions in gas or internal-combustion engines are different from those in a steam-engine, so that the Committee finds that different standards of comparison are required. In the steamengine, the bulk of the heat supplied to the engine is received at a constant higher temperature and rejected at a constant lower temperature, and both these limits of temperature can be observed directly with little difficulty or inferred accurately from observations of the pressures. For saturated or wet steam the relation of pressure and temperature is exactly known. It is easy, therefore, to determine the temperature-range in a steam-engine and to choose as a standard of comparison an ideal engine having the same temperature-range. But it is otherwise with the internal

1 The Committee on the Thermal Efficiency of Steam-Engines have used the term "efficiency ratio" for the ratio of heat utilized to that utilized in the standard engine, namely, in the case of steam-engines, the Rankine cycle engine

combustion engine, of which the gas-engine may be taken as the type.

In internal-combustion engines, the additions of heat are made at rising temperature, and an indefinitely small amount is added at maximum temperature. Also, to put the point broadly, neither the lower nor the higher temperature of the cycle can be directly observed or calculated, except on certain assumptions and to a certain degree of approximation. Hence, it does not seem desirable to take as the standard of comparison for such engines an ideal engine having the same temperature-range. It is possible, as will be seen later, to take as the utilized temperature-range the temperature-range during adiabatic compression of the cylinder contents. Then an ideal cycle can be found not widely different from the cycle of the actual engine.

PHYSICAL CONSTANTS FOR GASES.

In the steam-engine, the working fluid has a constant composition, and its physical properties depend only on the pressure and temperature according to laws definitely ascertained. In internal-combustion engines the working fluid consists of a mixture of air and other gases, of varying composition, the physical properties of which at the high temperatures and pressures involved are not well determined. It is desirable to examine how far the physical constants required for the reduction of gas-engine experiments are known, and to what extent they probably vary in the conditions present in gas-engine working.

Table I. gives the densities of those gases most important in considering questions of gas-engine efficiency.

Calorific Value.-Table II. gives the calorific value of 1 lb. of the most common combustible substances, and illustrates the difference between the higher calorific value, in which all the products of combustion are cooled down to 32°, and the lower calorific value, when the steam produced by combustion escapes as steam, carrying away its total heat of formation.2

1 The temperature-range in a gas-engine, with a compression-ratio of 5 to 1, may be from 200° to 2,800° F. The thermal efficiency of a Carnot cycle for this range would be 0.80, or twice the efficiency of the constant-volume cycle for a compression-ratio of 5 to 1.

2 The lower value is calculated from the higher by deducting the total heat of the steam produced, reckoned from 32° at 212° F.-that is, 1,146 B.Th.U. per pound of steam produced.