Excerpts from MORDEN MECHANICAL ENGINEERING

By Staff
1 / 3
Fig. 2Clerk Two-stroke Cycle Engine
2 / 3
Fig. 1.Four-stroke Single-acting Engine
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Fig. 3 The Day Gas-engine

The following begins a series of articles from the 1923 edition
of Modern Mechanical Engineering, on the subject of gas engines.
The original articles were sent to us by Jan van der Gugten, 2633
Ware Street, Abbotsford, B.C., Canada V2S 3E2, who thought our
readers would find them of interest.

CHAPTER 1: Introduction

Historical Summary.Space permits of a very
brief reference only to the early history of the
internal-combustion engine; for a full account the reader may
consult Vol. I of Sir D. Clerk’s The Gas, Petrol, and Oil
Engine. Probably the earliest instance of the use in Britain of an
explosion to obtain mechanical effect is that of the primitive
cannon used by Edward III about A.D. 1327. Three hundred and fifty
years later C. Huygens, followed by Papin and d’ Hauteville,
endeavored to obtain continuous motion from successive explosions
of gunpowder in a cylinder fitted with a piston; and much later
still, viz. in 1820, Farish of Cambridge constructed a small engine
intended to be driven by gunpowder; this explosive is, however,
quite unsuited for use in internal-combustion engines for both
thermal and practical reasons.

In 1820 Cecil, also of Cambridge, made what was probably the
first actually working gas-engine, using as his fuel an explosive
mixture of hydrogen and air; thereafter appeared in succession
Brown’s ‘gas-vacuum’ engine, Wright’s engine,
Barnett’s engines, and the very singular ‘free-piston’
engines of Barsanti and Matteucci and of Otto and Langen; and in
1860 the once popular, though very uneconomical, Lenoir engine,
which may be fairly described as a double-acting steam-engine using
a mixture of coal-gas and air in lieu of steam; Hugon (1865)
effected improvements in the details of Lenoir, but its still high
fuel consumption caused its abandonment in favour of the much more
economical, though mechanically objectionable, Otto and Langen
free-piston type. It should be mentioned also here that in 1873
there appeared in America the theoretically efficient ‘constant
pressure’ Brayton gas-engine, whose success was prevented by
practical difficulties; the modern highly economical Diesel
oil-engine illustrates the principle embodied in the Brayton of
burning fuel at (approximately) constant pressure during its
admission to the combustion chamber.

The final step in the evolution of the modern four-stroke cycle
internal-combustion engine was made in 1862 when de Rochas first
laid down explicitly the procedure to be adopted to obtain maximum
economy in the working of gas-engines, comprising suction,
compression, explosion and expansion, and exhaustall performed
within the working cylinder.

A four-stroke single-acting engine of the simplest type is
illustrated diagrammatically in Fig. 1. It includes an open ended
cylinder fitted with a closely fitting piston which drives a crank
by means of the usual type of connecting-rod; in the upper end, or
‘combustion chamber’, of the cylinder a pocket is formed
containing the inlet valve A, and exhaust valve B, the latter being
forcibly raised at suitable intervals by a cam C and roller-ended
tappet-rod D. Suppose the engine to be turning, and the suction
stroke about to commence. The piston in its descent creates a
partial vacuum above it, the inlet valve A accordingly opens and
fresh mixture passes into the cylinder; this is the suction stroke.
When the piston has reached the bottom of its stroke and commenced
to return the inlet automatically closes, and the upper charge is
compressed into the upper part of the cylinder and valve pocket;
this is the compression stroke. The mixture is exploded by the
ignition plug at, or near, the instant of greatest compression, and
the piston is next driven downwards, thus performing the expansion
or working stroke. Near the bottom of this stroke the cam C lifts
the exhaust valve B, and the burnt gases escape into the
atmosphere; B remains open throughout the whole of the succeeding
or exhaust stroke; this cycle is repeated indefinitely so long as
the engine continues running. The cam-shaft is driven by gearing at
half the crank-shaft speed, and is accordingly frequently referred
to as the ‘half-time’ shaft. In all modern engines the
inlet valve is also cam-operated, an increased volumetric
efficiency and increased speed being thus obtained.

The Otto Silent Gas-Engine.It was not, however,
until 1876 that Dr. Otto produced his world-famous ‘silent’
gas-engine, working exactly upon the four-stroke cycle as laid down
by de Rochas fourteen years earlier. Otto realized the de Rochas
cycle in a practical and most successful manner and employed flame
ignition. The gas consumption of his engines was at once found to
be much lower than had ever previously been attained, averaging
only from about 24 to 30 c. ft. of coal-gas per brake horse-power
hour. The introduction of the ‘Otto silent gas-engine’
marked the beginning of the era of the internal-combustion engine
as a prime mover of world-wide importance.

The Otto silent gas-engine was taken up in England by Messrs.
Crossley Brothers, Ltd., of Manchester, and progress was rapid. In
1878 a gas-engine of 3 h.p. was regarded as large; in 1881 a 20
h.p. engine was considered as remarkable; in 1898 the largest
gas-engine built was of 220 i.h.p. The high cost of coal-gas as a
fuel probably restrained the growth of engines, but the possibility
of utilizing the hitherto wasted blast furnace gases for power
purposes first demonstrated by Thwaite in 1895, and the evolution
of the gas producer by J.E. Dowson and others between 1878 and
1903, provided engineers with suitable gases for power purposes
which could be cheaply and easily produced in very great
quantities. These discoveries provided the necessary stimulus to
the construction of very large engines.

Thus in 1899 the Socit Cockerill of Seraing (Belgium) had in
operation a single-cylindered single-acting four-stroke engine of
51.2 inch bore and 55.13 inch stroke running on blast-furnace gas
at 90 r.p.m., and developing 600 b. h.p.

At the end of 1910 the Nuremberg Company (M.A.N) had in
operation double-acting gas-engines aggregating roundly 450,000
b.h.p., and including two-cylindered tandem units developing 2500
b.h.p.. While in 1922 the Premier Gas Engine Company Ltd. of
Sandiacre (Notts.), had designed an eight-cylindered two-crank
double-acting four-stroke engine of 6000 h.p. (pp. 50,51).

The Two-stroke Cycle.In the Otto, or Beau de
Rochas, or ‘four-stroke’ cycle the utilization of the same
cylinder alternately for pumping and power output, though
exceedingly convenient as a simplification in design, introduces
the disadvantage that a working impulse is obtained once only in
every four strokes of the piston; the crank-shaft speed is
consequently far from uniform unless one, or frequently two, very
heavy flywheels are fitted which by their momentum maintain the
rotation only slightly diminished during the exhaust, suction, and
compression periods.

Attention was accordingly quickly directed to the problem of
increasing the frequency of the impulses, and as early as 1878, Sir
D. Clerk produced his first two-stroke cycle engine, quickly
improved upon in a second design of which an example was exhibited
at the Paris Electrical Exhibition of 1881. This engine is
illustrated diagrammatically in Fig. 2. A is the power cylinder
containing at its outer end exhaust ports E, , overrun by the
piston C when near the end of its out-stroke. The pump-cylinder B
is, fitted with a piston D driven by a crank about 90° in advance
of the power crank; on its out-stroke D draws into B a charge of
mixed gas and air through the sliding valve H and pipe, W. When D
commences its in-stroke the charge in B becomes slightly
compressed; as soon as its pressure exceeds that in the power
cylinder A it is delivered into the combustion chamber G through an
automatic inlet valve. The power piston next returns, first cutting
off the ports E, E, and next causing the automatic inlet valve to
close; the entrapped fresh charge is then compressed into the
chamber G, fired, and the working out-stroke follows.

Thus every out-stroke of C is a working stroke, and the engine
accordingly gives one impulse every revolution.

The working impulses being thus twice as frequent as in a
four-stroke engine, an ideal two-stroke cylinder should develop
twice as much power as a four-stroke of the same bore, stroke, and
speed. Practically, however, the two-stroke cycle suffers from
drawbacks which prevent the realization of this ideal; thus in the
four-stroke cycle the inlet valve is opened slightly before the end
of the piston stroke, and remains open throughout the whole suction
stroke and usually for a short period thereafter; the charging of
the cylinder thus continues during about 220 of crank-shaft
revolution. On the other hand, in the two-stroke engine charging
must be effected during the short interval elapsing between the
uncovering and re-covering of the ports E, E, which ordinarily
occurs in about 80 of crankshaft revolution; hence the duration of
charging in the four-stroke is about three times as great as in the
two-stroke engine. Though this drawback is somewhat reduced by
providing large inlet area, the two-stroke engine is nevertheless
in general incapable of such effective charging as the four-stroke,
and more power is also absorbed in the charging operation.*

Again, in the four-stroke engine, the exhaust valve is open
during about 240° of crank-shaft revolution, and the burnt gases
are moreover positively expelled by the piston during the whole
exhaust stroke, the combustion chamber alone remaining filled with
burnt gas at, or often slightly below, atmospheric pressure. In the
two-stroke the exhaust has to be effected while the crank-shaft
turns through about 80°, and the whole cylinder remains filled with
burnt gases which, while being assisted in their exit by the
incoming fresh charge, heat it, thus reducing its density and
diminishing the ‘volumetric efficiency’ of the engine.
Further, it frequently happens in small two-stroke engines that
there is some loss of fresh charge by direct passage through the
exhaust ports; in large engines this is avoided by means described
later. Clerk improved the scavenging bf his engine and avoided loss
of fresh mixture through the exhaust ports by forming his
combustion chamber G (Fig. 2) in long, conical shape, and admitting
the charge under slight pressure at the apex. As early as 1884 his
engines were built by Messrs. L. Sterne & Company Ltd., in
sizes ranging from 2 to 12 HP, running at 200 to 130 RPM, and
showing for that period considerable economy in fuel consumption,
ranging from 29 c. ft. of coal-gas per indicated horse-power hour
in the smallest, to about 20 c. ft. in the largest sizes.

The two-stroke engine, with suitable modifications, has long
been firmly established, both in the very largest types of
gas-engine represented by the Koerting and Oechelhauser and Diesel
designs, and also, in a simplified form next to be described, in
immense numbers as the power unit of many motor-boats and of the
very popular two-stroke motor-bicycle.

The Day Gas-engine.The very important
simplification just referred to was made by Day in 1891, and
consisted in using the crank chamber as a charging pump, thus
dispensing altogether with the separate pump B of Fig. 2.

The Day gas-engine is illustrated diagrammatically in Fig. 3 in
two forms, that on the left showing the ‘two port’ type
used e.g. in the well-known ‘Bolinders’ marine engine,
while the right-hand view illustrates the three-port type as
universally employed in motorcycle applications.

Referring firstly to the two-port type, the crank-chamber is
made quite airtight, and the ascent of the piston accordingly
creates in it a partial vacuum causing mixed gas and air to enter
through the non-return valve shown; on the descent of the piston
this charge is compressed to four or five pounds per square inch
pressure. When near the bottom of its stroke the piston first
overruns the exhaust port shown, permitting the discharge of the
burnt gases, and immediately afterwards uncovers the inlet port,
whereupon the compressed fresh charge from the crank-chamber at
once enters the cylinder, and assists in the removal of the exhaust
gases. The ‘hump’ shown on the piston deflects the entering
stream of fresh mixture upwards, and thus minimizes loss by
short-circuiting through the exhaust port. The piston next ascends,
cuts off the ports, and compresses the entrapped charge into the
top part of the cylinder, where it is fired at or about the instant
of greatest compression, and the working stroke follows.

The two-port type requires a valve, but the three-port engine is
entirely valveless. The ascent of the piston causes a partial
vacuum in the crank-chamber as before; when near the top of its
stroke, its lower edge overruns the port shown, and a charge of gas
and air immediately rushes into the crankchamber. The piston next
descending-cuts off this charging port, and the mixture is
compressed in the crank-chamber, the subsequent action being
exactly as in the two-port type. Thus every down-stroke is a
working stroke, and in the three-port form the engine is of the
simplest possible character, the only moving parts being the
piston, connecting-rod, and crank-shaft. It will be observed that
these engines will run equally well in either direction; this is a
valuable property in marine applications, where ready reversibility
is essential, and they are accordingly largely used for the
propulsion of motor-launches, particularly in America.

For a fuller account of the history of the evolution of the
modern internal-combustion engine, and of the labours of many other
distinguished engineers, as Ackroyd, Atkinson, Daimler, Diesel,
Robson, & c., reference must be made to the large special
treatises, as, e.g., those of Clerk* and of Bryan Donkin. The
theoretical aspect of these engines is also dealt with in a
separate article of this work entitled ‘Applied Heat’, Vol.
IV, p 123. We proceed therefore to briefly refer to the gaseous
mixtures now ordinarily employed as fuel by gas-engines.

CHAPTER II: Fuels

Gaseous FuelsThe principal gaseous fuels used
in Great Britain are:

(1) town’s gas, often termed ‘coal-gas’;
(2) producer gas; (3) coke-oven gas; (4) blast-furnace gas. In
America (5) natural gas is also used.

1. Town’s gas obtained by the distillation of coal in closed
retorts, and subsequently enriched by the addition of other gases,
and particularly of ‘water-gas’ (consisting mainly of
carbon monoxide and hydrogen), is still the principal gaseous fuel
of the very numerous small-powered stationary gas-engines so widely
used for miscellaneous industrial purposes; it is an excellent fuel
for gas-engines of any size, but its relatively high cost restricts
its use in general to the smaller-powered units. Until recently
companies were compelled to supply gas of a standard illuminating
power, but the introduction of the incandescent mantle, and the
largely increased use of gas for heating and power purposes,
rendered its heating value of the first importance. Accordingly in
1920 an Act was passed empowering companies to sell gas at a price
proportioned to its heat value, 100,000 British units of heat*,
termed one ‘therm,’ being taken as the unit of supply.
Thus, in 1922, the heating values of the gas supplied by the
principal London companies were: Of the South Metropolitan Gas
Company: 560 B.Th.U. per cubic foot. Of the Gas, Light & Coke
Company: 500 B.Th.U. per cubic foot. Of the Commercial Gas Company:
475 B.Th.U. per cubic foot.

To ascertain the heat supplied in therms to a consumer one has
therefore to multiply the number of cubic feet of gas used by the
heating value per cubic foot, and divide by 100,000. Town’s gas
is a mixture, in somewhat variable proportions, of the combustible
gases hydrogen, carbon monoxide, methane (CH4), and other
hydrocarbons, with small quantities of the incombustible gases
carbon dioxide, oxygen, arid nitrogen.

2. Producer gas is a mixture of gases formed by the combustion
of almost any carboniferous fuel with a limited supply of air, and
consists principally of carbon monoxide, hydrogen, and carbon
dioxide, with a large quantity of atmospheric nitrogen.

In the most usual form of ‘suction producer’ anthracite
or coke contained in a cylindrical fire-brick lined
‘producer’ is maintained incandescent by air drawn through
it by the engine suction. Steam is also injected into the fuel, and
the issuing mixture of hot and smoky gases after being cooled and
cleaned is mixed with a suitable quantity of fresh air and supplied
to the engine. For further details of this important subject the
reader should refer to the article on ‘Gas Producers’ in
this volume. Producer gas is now used all over the world, a great
variety of refuse material being utilized as fuel as, e.g., peat,
sawdust, straw, coconut shells, cotton and sunflower seeds, tea
prunings, rice husks, etc. The heat value per cubic foot varies
with the fuel from which the gas is made; using anthracite or coke
in a suction producer an average value is about 130 B.Th.U., the
gas usually requiring for complete combustion rather under 1 cu.
ft. of air per cubic foot.

Its heat is thus very much less than that of town’s gas;
nevertheless, as explained here under, it forms a most satisfactory
fuel for gas-engines; using gas coke a suction producer may be
expected to give roundly about 180,000 c. ft. of gas per ton of
coke burned.

3. Coke-oven GasIn the production of coke for use in
blast-furnaces, foundries, and general metallurgical processes,
large quantities of gas (about 10,000 c. ft. per ton of coke made)
of high heat value are evolved. The gas varies considerably in
composition, and consists of a mixture of the same gases as
producer gas, but is characterized by its high hydrogen and methane
content, the hydrogen varying from about 30 percent to 60 percent
and the methane from about 20 percent to 25 percent of the whole
volume. Such gas requires for complete combustion from 3 to 4 c.
ft. of air per cubic foot, and has a heat value ranging from about
350 to 450 B.Th.U. per cubic foot. Though a valuable fuel for
gas-engines, its variable constitution and high hydrogen content
introduce some practical difficulties in its use, but these are met
by providing special means of rapidly adjusting the admission of
gas and air, while the tendency to pre-ignition due to the high
proportion of hydrogen may be overcome by Clerk’s method of
admitting a small quantity of cooled exhaust gases with each fresh
charge*.

4. Blast-furnace GasIn 1895 the late Mr. B. H. Thwaite showed
that the hitherto waste gases produced in large quantities by
blast-furnaces could be used in gas-engines, and his discovery
greatly stimulated the development of the very large gas-engines
now becoming common. The gas is of low heat value, averaging only
about 100 B.Th.U. per cubic foot, and requiring for complete
combustion about 0.75 c. ft. of air per cubic foot. Roundly, some
180,000 c. ft. of gas are ordinarily produced per ton of fuel
burnt, the average composition in Great Britain being (by volume)
carbon monoxide 25 percent, hydrogen 2 percent, nitrogen 66
percent, carbon dioxide 6 percent, traces of methane, etc., 1
percent. The gas must, of course, be cooled and cleaned before
use.

5. Natural GasVery frequently associated with the oil deposits,
vast reservoirs of natural gas exist over a wide area in the United
States and elsewhere, as, e.g., the neighborhood of the Caspian
Sea; a small supply also exists in Great Britain at Heathfield, in
Sussex, and it has been thought that considerable quantities are
probably obtainable in this locality. In the United States the
amount obtained annually is immense; e.g., in 1903 over 240,000
millions of cubic feet were used for heating, lighting, and power
purposes. Natural gas often contains from 90 to 95 percent methane
(CH4), with small quantities of ethane and olefines, and traces of
water and oil; it is an ideal gas for use in internal-combustion
engines. Considered as methane, its heat value is very high, viz.
1070 B.Th.U. per cubic foot, and requires per cubic foot for
complete combustion 9.5 c. ft. of air; actually its heat value is
usually in the neighborhood of 950 B.Th.U. per cubic foot.

TABLE I

Average Heat Values of the usual Gaseous Mixtures

Fuel.

Average Values.

B.Th.U. of Heat per Cubic Foot at 60° F. and 760 mm.

Cubic Feet of Air per Cubic Foot of Gas for just Complete
Combustion.

B.Th.U. of Heat per Cubic Foot of Mixture with Air.

Average petrol (as vapour)

4650

49

95

Natural gas

950

9.5

90

Town’s gas

500

5.5

77

Coke-oven gas

400

4.0

80

Suction-producer gas from anthracite or coke }

130

1.0

65

Blast-furnace gas

100

0.75

57

In Table 1 some average figures relating to the above-mentioned
gaseous fuels are collected together for general reference; figures
for average petrol are included for comparison. It will be observed
that, notwithstanding the large variation in the heat value per
cubic foot of gas, the heat value per cubic foot of mixture with
air varies between the much narrower limits of 57 to 95, due to the
greatly differing volumes of air necessary to cause complete
combustion in the several cases. It is mainly for this reason that
the poorer gases prove so useful as engine fuels.

Further, excepting in the very special, relatively small
petrol-engines of motors and aircraft, experience has shown that in
order to avoid trouble from overheating of pistons and cylinders it
is necessary to limit the amount of heat supplied to the engine. A
convenient method of estimation is to take the heat supplied per
cubic foot of working stroke swept by the piston, and for
gas-engine cylinders British practice in general allows only about
50 B.Th.U. per swept cubic foot with cylinders up to 20 in. in
diameter, falling to from 35 to 40 B.Th.U. per swept cubic foot
with 30-inch cylinders. This restriction limits the practically
attainable (indicated) mean effective pressure in such engines
during continuous running, to from about 65 to 55 lb. per square
inch. The reduction in heat supply is obtained by increasing the
admixture of air; thus, with town’s gas, usual proportions are
9 to 10 volumes of air to 1 of gas; and similarly in other cases.
See equations 8 and 9 and Table III of page 31.

This concludes the first two chapters of the Gas Engine section
of the 1923 edition of Modern Mechanical Engineering. Additional
chapters will follow in future issues of GEM.

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