Gas-Engines and Producer-Gas Plants Part 1

Gas-Engines and Producer-Gas Plants.

by R. E. Mathot.

INTRODUCTION

The constantly increasing use of gas-engines in the last decade has led to the invention of a great number of types, the operation and care of which necessitate a special practical knowledge that is not exacted by other motors, such as steam-engines.

Explosion-engines, driven by illuminating-gas, producer-gas, oil, benzin, alcohol and the like, exact much more care in their operation and adjustment than steam-engines. Indeed, steam-engines are regularly subjected to comparatively low pressures. The temperature in the cylinders, moreover, is moderate.

On the other hand, the explosion-motor is irregularly subjected to high and low pressures. The temperature of the gases at the moment of explosion is exceedingly high. It is consequently necessary to resort to artificial means for cooling the cylinder; and the manner in which this cooling is effected has a very great influence on the operation of the motor. If the cooling be effected too rapidly, the quant.i.ty of gas consumed is considerably increased; if the cooling be effected too slowly, the motor parts will quickly deteriorate.

In order to reduce the gas consumption to a minimum, a matter which is particularly important when the motor is driven by street-gas, the explosive mixture is compressed before ignition. Only if all the parts are built with joints absolutely gas-tight is it possible to obtain this compression. The slightest leakage past the valves or around the piston will sensibly increase the consumption.

The mixture should be exploded at the exact moment the piston starts on its working stroke. If ignition occurs too soon or too late, the result will be a marked diminution in the useful effect produced by the expansion of the gas. All ignition devices are composed of delicate parts, which cannot be too well cared for.

It follows from what has thus far been said that the causes of perturbation are more numerous in a gas than in a steam engine; that with a gas-engine, improper care will lead to a much greater increase in consumption than with a steam-engine, and will cause a waste in power which would hardly be appreciable in steam-engines, whether their joints be tight or not.

It is the purpose of this manual to indicate the more elementary precautions to be taken in the care of an engine operating under normal conditions, and to explain how repairs should be made to remedy the injuries caused by accidents. Engines which are of less than 200 horse-power and which are widely used in a small way will be primarily considered. In another work the author will discuss more powerful engines.

Before considering the choice, installation, and operation of a gas-engine, it will be of interest to ascertain the relative cost of different kinds of motive power. Disregarding special reasons which may favor the one or the other method of generating power, the net cost per horse-power hour will be considered in each case in order to show which is the least expensive method of generating power in ordinary circ.u.mstances.

R. E. MATHOT.

MARCH, 1905.

CHAPTER I

MOTIVE POWER--COST OF INSTALLATION

The ease with which a gas-engine can be installed, compared with a steam-engine is self-evident. In places where illuminating gas can be obtained and where less than 10 to 15 horse-power is needed, street-gas is ordinarily employed.[A] The improvements which have very recently been made in the construction of suction gas-generators, however, would seem to augur well for their general introduction in the near future, even for very small powers.

The installation of small street-gas-engines involves simply the making of the necessary connections with gas main and the mounting of the engine on a small base.

An economical steam-engine of equal power would necessitate the installation of a boiler and its setting, the construction of a smoke-stack, and other accessories, while the engine itself would require a firm base. Without exaggeration it may be a.s.serted that the installation of a steam-engine and of its boiler requires five times as much time and trouble as the installation of a gas-engine of equal power, without considering even the requirements imposed by storing the fuel (Fig. 1). Small steam-engines mounted on their own boilers, or portable engines, the consumption of which is generally not economical, are not here taken into account.

[Ill.u.s.tration: FIG. 1.--30 H.P. Gas-engine and suction gas-producer.]

[Ill.u.s.tration: FIG. 1_A_.--30 H.P. Steam-engine, boiler and smoke-stack.]

So far as the question of cost is concerned, we find that a 15 to 20 horse-power steam-engine working at a pressure of 90 pounds and having a speed of 60 revolutions per minute would cost about 16-2/3 per cent.

more than a 15 horse-power gas-engine, with its anti-pulsators and other accessories. The foundation of the steam-engine would likewise cost about 16-2/3 per cent. more than that of the gas-engine. Furthermore the installation of the steam-engine would mean the buying of piping, of a boiler of 100 pounds pressure, and of firebrick, and the erection of a smoke-stack having a height of at least 65 feet. Beyond a little excavating for the engine-base and the necessary piping, a gas-engine imposes no additional burdens. It may be safely accepted that the steam-engine of the power indicated would cost approximately 45 per cent. more than the gas-engine of corresponding power.

The cost of running a 15 to 20 horse-power steam-engine is likewise considerably greater than that of running a gas-engine of the same size.

Considering the fuel-consumption, the cost of the lubricating oil employed, the interest on the capital invested, the cost of maintenance and repair, and the salary of an engineer, it will be found that the operation of the steam-engine is more expensive by about 23 per cent.

This economical advantage of the gas over the steam-engine holds good for higher power as well, and becomes even more marked when producer-gas is used instead of street-gas. Comparing, for example, a 50 horse-power steam-engine having a pressure of 90 pounds and a speed of 60 revolutions per minute, with a 50 horse-power producer-gas engine, and considering in the case of the steam-engine the cost of a boiler of suitable size, foundation, firebrick, smoke-stack, etc., and in the case of the gas-engine the cost of the producer, foundation, and the like, it will be found that the installation of a steam-engine entails an expenditure 15 per cent. greater than in the case of the producer-gas engine. However, the cost of operating and maintaining the steam-engine of 50 horse-power will be 40 per cent. greater than the operation and maintenance of the producer-gas engine.

From the foregoing it follows that from 15 to 20 up to 500 horse-power the engine driven by producer-gas has considerably the advantage over the steam-engine in first cost and maintenance. For the development of horse-powers greater than 500, the employment of compound condensing-engines and engines driven by superheated steam considerably reduces the consumption, and the difference in the cost of running a steam- and gas-engine is not so marked. Still, in the present state of the art, superheated steam installations entail considerable expense for their maintenance and repair, thereby lessening their practical advantages and rendering their use rather burdensome.

FOOTNOTES:

[A] Recent improvements made in suction gas-producers will probably lead to the wide introduction of producer gas engines even for small power.

CHAPTER II

THE SELECTION OF AN ENGINE

Explosion-engines are of many types. Gas-engines, of the four-cycle type, such as are industrially employed, will here be princ.i.p.ally considered.

=The Otto Cycle.=--The term "four-cycle" motor, or Otto engine, has its origin in the manner in which the engine operates. A complete cycle comprises four distinct periods which are diagrammatically reproduced in the accompanying drawings.

=The First Period.=--_Suction:_ The piston is driven forward, creating a vacuum in the cylinder, and simultaneously drawing in a certain quant.i.ty of air and gas (Fig. 2).

[Ill.u.s.tration: FIG. 2.--First cycle: Suction.]

=The Second Period.=--_Compression:_ The piston returns to its initial position. All admission and exhaust valves are closed (Fig. 3). The mixture drawn in during the first period is compressed.

=The Third Period.=--_Explosion and Expansion:_ When the piston has reached the end of its return stroke, the compressed mixture is ignited.

Explosion takes place at the dead center. The expansion of the gas drives the piston forward (Fig. 4).

[Ill.u.s.tration: FIG. 3.--Second cycle: Compression.]

[Ill.u.s.tration: FIG. 4.--Third cycle: Explosion and expansion.]

=The Fourth Period.=--_Exhaust:_ The piston returns a second time. The exhaust-valve is opened, and the products of combustion are discharged (Fig. 5).

[Ill.u.s.tration: FIG. 5.--Fourth cycle: Exhaust.]

These various cycles succeed one another, pa.s.sing through the same phases in the same order.

=Valve Mechanism.=--It is to be noted that in modern motors valves are used which are better adapted to the peculiarities of explosion-engines than were the old slide-valves used when the Otto engine was first introduced. The slide-valve may now be considered as an antiquated distributing device with which it is impossible to obtain a low consumption.

In old-time gas-engines rather low compressions were used. Consequently a very low explosive power of the gaseous mixture, and low temperatures were obtained. The slide-valves were held to their seats by the pressure of external springs, and were generously lubricated. Under these conditions they operated regularly. Nowadays, the necessity of using gas-engines which are really economical has led to the use of high compressions with the result that powerful explosions and high temperatures are obtained. Under these conditions slide-valves would work poorly. They would not be sufficiently tight. To lubricate them would be difficult and ineffective. Furthermore, large engines are widely used in actual practice, and with these motors the frictional resistance of large slide-valves, moving on extensive surfaces would be considerable and would appreciably reduce the amount of useful work performed.

[Ill.u.s.tration: FIG. 6.--Modern valve mechanism.]

By reason of its peculiar operation, the slide-valve is objectionable, the gases being throttled at the time of their admission and discharge.