CHAPTER XIILIQUID FUELS

Fig. 67.—Petrol Engine Brake.

Example:—An engine being tested by a brake applied to the flywheel as shown in the sketch (Fig.67) exerts a pull of 50 lbs. at a speed of 2,000 revolutions per minute. If the length of brake arm is 30 inches, calculate the brake horse-power developed.

Work done per minute = 50 × 6·28 × 30/12 × 2000 ft. lbs.B.H.P. = (50 × 6·28 × 30/12 × 2000)/33,000 = 47·5

Work done per minute = 50 × 6·28 × 30/12 × 2000 ft. lbs.

B.H.P. = (50 × 6·28 × 30/12 × 2000)/33,000 = 47·5

Rated Horse-Power.—For taxation purposes the Treasury makes use of a formula for the rating of petrol engines according to their probable horse-power. This formula is based on a certain speed of the piston whichwas regarded as a limiting value some years ago (when the formula was first proposed) and on the attainment of a certain effective pressure in the cylinder.

Horse-power from the Treasury formula = 0·4 d2n.Where d = diameter of cylinder in inches,n = number of cylinders.

Horse-power from the Treasury formula = 0·4 d2n.

Where d = diameter of cylinder in inches,n = number of cylinders.

With modern engines much greater horse-power is obtained, and a near approximation to the true output is obtained by using what is now known as the Joint Committee’s formula.

Brake Horse-Power = 0·46 n (d + s) (d - 1·18)Where d = diameter of cylinder in inches.s = length of piston’s stroke in inches.

Brake Horse-Power = 0·46 n (d + s) (d - 1·18)

Where d = diameter of cylinder in inches.s = length of piston’s stroke in inches.

This formula is only to be used in an attempt to predict theprobablemaximum horse-power which any engine will give. It must not be confused with the ordinary brake horse-power formula.

Example:—Find the probable maximum horse-power of an engine having four cylinders each 3 in. bore and a piston stroke of 4 in. What would be its horse-power for taxation purposes?

By Joint Committee’s formula—B.H.P. = 0·46 × 4 (3 + 4)(3 - 1·18) = 1·84 × 7 × 1·82=23·35By Treasury formula—B.H.P = 0·4 × 32× 4 = 0·4 × 9 × 4 =14·4

By Joint Committee’s formula—

B.H.P. = 0·46 × 4 (3 + 4)(3 - 1·18) = 1·84 × 7 × 1·82=23·35

By Treasury formula—

B.H.P = 0·4 × 32× 4 = 0·4 × 9 × 4 =14·4

Indicated Horse-Power.—The horse-power which an indicator would show as being developed inside the cylinder of a petrol engine, above the piston, would be called theindicatedhorse-power, and should always work out a greater number than the brake horse-power or power available at the engine flywheel, because some of the power liberated from the combustion of the petrol within the cylinder is lost in friction of the piston and bearings.

The Indicated Horse-Power or I.H.P. = (Pe× A × L x Ne)/33,000.Where Pe= mean effective pressure from the diagram, in lbs. per sq. inch.A = area of piston in square inches = 0·7854(diameter of cylinder)2L = length of stroke of piston, in feet.Ne= number of power impulses per minute delivered to the crankshaft.

The Indicated Horse-Power or I.H.P. = (Pe× A × L x Ne)/33,000.

Where Pe= mean effective pressure from the diagram, in lbs. per sq. inch.A = area of piston in square inches = 0·7854(diameter of cylinder)2L = length of stroke of piston, in feet.Ne= number of power impulses per minute delivered to the crankshaft.

Since a four-stroke engine gives one power impulse to the crankshaft in every two revolutions, it follows that Neis equal tohalfthe number of revolutions per minute for a single-cylinder engine of that type, andtwicethe number of revolutions for a four-cylinder engine. A four-cylinder two-stroke engine might be arranged to give eithertwo or fourimpulses per revolution of the crankshaft—depending upon the arrangement of the cranks.

Example:—A four-cylinder four-stroke engine runs at a speed of 2,000 revolutions per minute and the mean-effective pressure in the cylinders is 75 lbs. per square inch. Calculate the indicated horse-power if the cylinders are 4 in. × 4 in.

I.H.P= (Pe× A × L × Ne)/33,000= (75 × 0·7854 × 42× 4/12 × 4000}/33,000= {75 × 12·56 × 4000}/99,000 =38

The Indicator Diagram.—At the commencement of this chapter we explained that the work done by a force was measured by multiplying the number representing the magnitude of the force (in pounds) by the distance through which it had acted (measured in feet). This product gave us the quantity of work done in foot-pound units. Thus “quantity of work done” is really the product of two numbers, just as the area of a rectangular floor space is measured by length times breadth. In symbols we write W = F × S where F is the magnitude of the force or resistance in pounds and S the distance through which it has acted, in feet. It is interesting to contemplate this symbolical expression W = F × S together with the expression Area = Length × Breadth, because it gives us a new idea for measuring work. Imagine a diagram of the kind shown in Fig.68, in which the curved line AB has been obtained byplottingvalues of F and S for any imaginary case. The diagram is supposed to represent pictorially how the particular force under consideration has varied in magnitude as it has traversed a space represented, to some scale, by the length DC. It is clearly seen that the force has beendecreasingin anirregularmanner from some large value represented by the height DA to a small value represented by the height CB. We now proceed to show that the shaded area ABCD measures the total amount of work done by this force.

Fig. 68.—Force-space orWorkDiagram.

Considering for a moment just the small stripefdcof the diagram we see that it is easy to find a rectangleabcdequal in area to it. Now theheightof this rectangle will be theaverage valueof the force while it traversed thespacecd, and hence the area of the rectangleabcdgives thework doneby the force in passing fromctod. Similarly by dividing up the whole diagram we would obtain a number of little rectangles each equal in area to the magnitude of the work done from point to point. Thus the whole area ABCD gives the whole work done. To measure the work done in an engine cylinder we must use some form ofindicator. An indicator is an instrument which traces out a diagram on whichabscissæ(or horizontal distances) represent displacements of the piston andordinates(or vertical distances) represent the pressures acting on the piston.

Fig. 69.—Petrol Engine Indicator Diagram. Four-stroke Cycle.

Ordinary steam engine indicators with pencil motion and paper drum are not suitable for use with fast running petrol engines. The moving parts of these indicators are too heavy and their springs too sluggish in action to keep correct time with these high speed engines. Again, there is too much friction between the pencil and the paper drum, as well as in the lever joints. Therefore special indicators have to be used, in which the diagram is traced out by a beam of light reflected from a mirror on to a ground glass screen or photographic plate. One corner of the mirror is tilted in time with the movement of the engine piston by means of a special reducing mechanism, and another corner of the mirror is tilted in a direction at right angles to the first by means of a very short thin rod kept in contact with a metal diaphragm subjected to the pressure of the gases in the engine cylinder. A beam of light is thrown on to the mirror from a lamp, and after reflection traces out the diagram on the screen or plate. Such an instrument would generally be described as amanograph. An indicator diagram from a four-stroke engine is shown in Fig.69. The line ABC represents the suction stroke of the piston during which the pressure of the gases in the cylinder falls a little below that of the atmosphere. Atmospheric pressure is shown by the height of the line LLabove the base, or line of zero pressure (perfect vacuum). The inlet valve can be opened at B and closed at D after the crank has turned the bottom dead-centre and begun the compression stroke. The line CDE represents the compression stroke of the engine, during which the gases are compressed and their pressure rises. The height of the point E above the line LL gives thecompression pressureto the scale of the diagram. Ignition occurs at E, and results in an instantaneous rise of pressure to F due to the explosion, which is, however, quickly followed by expansion to G. The exhaust valve opens at G, the gases arereleased, and the pressure falls still further to point H. The line HA represents the exhaust stroke of the piston, and the exhaust valve would be closed after the crank had passed its upper dead-centre and commenced the suction stroke. The distance marked (x) on the diagram measures theclearancevolume (or volume of the space above the piston containing the valves and referred to as the combustion chamber) to the same scale that the lengthof the diagram measures the volumetric displacement of the piston. The volume traced out by the piston during any working stroke is measured by multiplying the area of the piston in square (centimetres/inches) by the length of the stroke in (centimetres/inches) the product giving us the capacity of the cylinder in cubic (centimetres/inches). The area of the diagram HEFG gives the work done during one cycle of operations, and the area of the small diagram ABCD gives the work lost in taking in and expelling thecharge. The small area should be subtracted from the large one to get the useful work done per cycle of operations. The area of the diagram HEFG may readily be obtained by finding its vertical height at a number of equidistant points, and from these measurements ascertaining the average ormeanheight of the diagram. The average height of the diagram (in inches) multiplied by its length (also in inches) gives the area in square inches.

Fig. 70.—Indicator Diagram from a Two-stroke Engine.

The average or mean height of the diagram also gives what we term themean effective pressureacting on the piston, and constitutes the Peof the indicated horse-power formula above. The area ABCD is always small and generally neglected with four-stroke engines. There aretwo separate diagramsfor a two-stroke engine. The diagram for the working cylinder is A1B1C1D1in Fig.70, and that for the crankchamber is E1F1G1H1. The effective work done per cycle is measured by the difference in the area of these two diagrams. The piston uncovers the exhaust port at B1and closes it again at C1; it uncovers the inlet port at F1and covers it again at G1. From F1to G1the charge is being delivered from the crankchamber to the working cylinder. The area of the loop E1F1G1H1is larger than the corresponding portion of the four-stroke diagram and should not be neglected.

Important factors in the choice of a liquid fuel for use in portable internal combustion engines are: (1) low cost; (2) ease and safety of transportation or storage; (3) high volatility, i.e., readily convertible into vapour; (4) non-corrosive action on metals; (5) high heat efficiency; (6) ability to give satisfactory results in existing types of internal combustion engine.

Petrolis a liquid fuel composed of carbon (C) and hydrogen (H) in chemical combination. The principal method of producing petrol is by distillation of crude petroleum. The best mixture to use in a petrol engine is one composed of 2 cubic feet of petrol vapour to every 98 cubic feet of air. Petrol does not require any heat to vaporize it under ordinary atmospheric conditions.Pre-ignitionof the charge is liable to occur if the compression pressure exceeds 100 lbs. per square inch. It does not corrode or deteriorate metal parts, but leaves a black carbon deposit if not properly burned. Its volatility is high and its specific gravity is low, being about 0·71. An average figure for the calorific value of petrol would be 20,000 B. Th. U. per lb. Petrol is very expensive and also needs care in handling. Private motorists are not allowed tostorepetrol or benzol.

Benzolis a liquid fuel containing more carbon (C) and less hydrogen (H) than petrol. The principal method of obtaining benzol is by distillation of coal tar. The strength of the mixture should be such that a little more air is supplied in proportion to the quantity of fuel used than isrequired for petrol. Generally, it may be said that when an engine has been running on petrol and is changed over to benzol the size of the carburettor jet orifice should be slightly reduced and the weight of the float increased—no other changes need be made anywhere. Benzol is very volatile and also highly dangerous to handle, on account of its low flash-point. It often contains impurities which attack the metal parts of the engine and gum up the valves. It is more liable to deposit carbon than petrol. Benzol attacks rubber, and paint on coachwork. It is as expensive as petrol at the present time. The specific gravity of benzol may be taken as 0·88 and its calorific value as 19,000 B. Th. U. per lb. It may be compressed above 100 lbs. per square inch without pre-igniting.

Alcoholis a liquid fuel composed of carbon (C), hydrogen (H), and oxygen (O). The principal method of obtaining alcohol is from the fermentation of vegetable matter, such as potatoes, beetroot, etc. About 6 cubic feet of vaporized alcohol to every 94 cubic feet of air should be used. The volatility of alcohol is very poor compared with petrol or benzol, and it generally contains some water in suspension. It will stand double the compression pressure of petrol without pre-igniting. Alcohol is not so liable to deposit carbon as petrol or benzol, but is very liable to cause rust. It is not obtainable as a fuel in Great Britain at present, owing to the high duty on it. Engines for use with alcohol ought really to be specially constructed for the purpose. Its calorific value is only 12,000 B. Th. U. per lb., and its specific gravity is 0·82. Alcohol requires to be heated before it will vaporize, this heat generally being obtained from the exhaust gases after the engine has been first started up. Alcohol is fairly safe to handle or store.

Paraffinis obtained during the distillation of petrol from crude petroleum, and consists of carbon (C) and hydrogen (H) inalmostthe same proportions as petrol. Itsvolatility is low, and it requires heat to vaporize it. The heat required for vaporization is usually obtained from the exhaust gases after the engine has been got running. In starting up a lamp must be used for heating the vaporizer of the carburettor. Paraffin will stand a little higher compression than petrol before pre-igniting. The specific gravity of paraffin may be taken as 0·80 and its calorific value as 18,000 B. Th. U. per lb. It is much cheaper than either petrol or benzol, being only about one-third of the cost. The chief objections to its use are its smell and the greasy character of the stain left by it on coachwork or clothes; also the difficulty of having to heat the vaporizing chamber of the carburettor. It is much safer to handle and store than either petrol or benzol, and requires about the same proportion of air to form an explosive mixture as that given for petrol. The range of variation of strength in the mixture which is permissible with paraffin is much less than with either petrol, benzol, or alcohol. Alcohol has the greatest range of variation in mixture strength. Paraffin is also very liable to deposit carbon, owing to the small range of variation permissible in the strength of the mixture.

Thermal Efficiency.—In the foregoing notes we have used certain terms which have not previously been explained, and therefore it is necessary to give one or two definitions.

TheSpecific Gravityof a fuel is the ratio of the weight of one gallon of the fuel to the weight of one gallon of water. As a gallon of water weighs 10 lbs., it will be evident from the above notes that a gallon of petrol only weighs 7·1 lbs., whereas a gallon of benzol will weigh 8·8 lbs. (approx.), hence it is not surprising to learn that moremileage per gallonis obtained with benzol than with petrol, even though the calorific value of benzol, per lb., is less than that of petrol. Sometimes the specific gravity is referred to as thedensityof the fuel, but this is only correct when grammesand centimetres are being used. The density of any fuel is the weight of 1 cubic foot expressed in pounds or, in general terms, the mass of unit volume of the fuel. Thedensityof petrol in English units would be about 44 lbs. per cubic foot.

OneBritish Thermal Unitis the quantity of heat required to raise the temperature of 1 lb. of water by 1 degree (Fahrenheit scale) when the temperature of the water is about 60°F.

TheCalorific Valueof any fuel (reckoned on the British system of units) is the amount of heat (expressed in British Thermal Units) which will be given out by 1 lb. of the fuel when it is completely burned. The liquid fuels we have to deal with are hydrocarbon compounds, and when completely burned the whole of the carbon is burned to carbon dioxide (CO2) and the hydrogen to steam (H2O), leaving no residue. By means of acalorimeterwe can experimentally determine the calorific value of any fuel.

It has long been known that work can be turned into heat, and the petrol engine is a good example of the reverse process which consists in turning heat into work. In a steam engine and boiler plant the heat of the fuel is liberated under the boiler, and then a portion of it gets transferred to the water in the boiler and forms steam, which is then taken to the engine and does work in the cylinder, the whole being a wasteful process. The petrol engine is aninternal combustionengine, or one in which the fuel is burnt inside the engine cylinder itself and converted directly into work. From every British Thermal Unit of heat liberated by the combustion of the fuel in the cylinder we should be able to get 778 foot-pounds of work if thethermal(or heat)efficiencyof the engine was 100 per cent. The thermal efficiency (η) of any engine may be defined as the ratio which the heat equivalent of the work done per minute by the engine bears to the heat which wouldbe liberated by the complete combustion of the quantity of fuel admitted to the cylinder per minute. Thus—

η = ((Horse-power of the Engine × 33,000)/778)/((Number of pounds of fuel consumed per minute) × (Calorific Value of the fuel))

Example:—An engine developing 30 horse-power uses 0·50 lb. of benzol per minute. What is its thermal efficiency? The calorific value of benzol may be taken as 19,000 B. Th. U. per lb.

η = (30 × 33,000/778)/(0·50 × 19,000) = 0·134, or 13·4 per cent.

Many of the troubles that are likely to arise have already been referred to in previous chapters, but the following additional notes may be found useful.

1. Engine refuses to start.

Care must be taken to observe exactly what happens, and one cannot do better than ask oneself mentally some of the following questions.

(a)Is the ignition “on”?

If a magneto is fitted the earth connexion should be open, but if a coil and accumulator are fitted the earth connexion should be closed.

(b)Is the petrol reaching the carburettor jet?

Before removing the jet for the purpose of examining and cleaning it, it would be advisable to ascertain whether the petrol was reaching the float chamber. Provided there is a reasonable amount of petrol in the tank and the tap is turned on, there must be a stoppage either in the petrol filter, the petrol pipe, or the bottom portion of the float chamber. Examine the filter and float chamber before disconnecting any pipes.

(c)Is there a good compression in all the cylinders?

If there does not appear to be any compression in any of the cylinders, it is probable that the carburettor throttle is closed and no air or gas can enter the cylinders. If there is a good compression in some cylinders and a poor one or none at all in others then—

(1) One or more of the valves may be held off its seat by dirt, by distortion, or by some derangement of the valve gear. Examine the valve gear externally, turning the engine slowly to watch its action. Afterwards remove valve caps and inspect valves if necessary.(2) One or more of the sparking plugs or valve caps may be short of its washer. In this case the blow will be heard as the engine is turned round by hand.(3) A piston may be cracked or broken or a cylinder cracked.(4) A cylinder may have got badly worn and the rings on the piston jammed so that they no longer keep it gas-tight.

(2) One or more of the sparking plugs or valve caps may be short of its washer. In this case the blow will be heard as the engine is turned round by hand.

(3) A piston may be cracked or broken or a cylinder cracked.

(4) A cylinder may have got badly worn and the rings on the piston jammed so that they no longer keep it gas-tight.

(d)Is the engine very stiff to turn over?

Stiffness is due as a rule to lack of oil on the cylinder walls, caused by absence of oil in crankchamber or the film of oil on the cylinder walls having been washed off whenprimingthe engine with petrol in attempting to start it. If a connecting rod is bent, or the crankshaft distorted or a piston ring broken, stiffness will also be noted. Very often by removing the valve caps and pouring a teaspoonful of oil or paraffin into each cylinder the engine may be freed by vigorously turning the starting handle by hand until the cylinders and pistons are well lubricated.

(e)Is there any sign of an attempt to fire the chargesuch as an occasional puff of smoke from the exhaust or inlet, or an occasional jerk round of the engine as you turn the starting handle, or an occasional “bang” in the exhaust box?

If the ignition is “on” and the carburettor jet clear, the compression good and the engine quite free, yet there is no sign of a “fire” from any of the cylinders, it is possible that air is leaking into the induction pipe through a faulty joint or any one of the following ignition troubles may have occurred:—

(f)Defective sparking plug or plugs.This may arise from water or oil or dirt between the plug points; or from faulty insulation in the body of the plug. To test whether the plugs are at fault an easy method is to take a screwdriver with a wooden handle and place the metal blade on the terminal of the plug, letting the point come about one thirty-second of an inch from the metal of the cylinder or any of the pipes; when the engine is turned by hand the spark will be seen to pass across this improvised gap if the magneto and leads are in order.

(g)Defective electrical connexions.

The high tension cables may be broken, or disconnected, or short-circuited. Theearthwire may be short-circuited (i.e., in electrical contact with some other wire or metal fitting). There may be a short-circuit in the ignition switch.

(h)Defective magneto or coil.

The low tension contact breaker lever may be jammed so that the make and break is inoperative, or one of the carbon brushes may have got broken. Occasionally one finds the magnets of the machine have lost their power; or there is some electrical defect in the armature or condenser. The battery may have become exhausted. The trembler blade may be stuck up.Water may have found its way on to the high tension electrode or into the safety spark gap.

2. Engine starts up fairly well, runs a little, and then stops.

Take care to notice the manner in which the engine runs and stops. Note whether it runs regularly or irregularly and for how long a time.

If the engine runsregularlywith all cylinders firing, then probably the exhaust is choked or the petrol supply fails. Failure of the petrol supply may be due to the use of too small a jet in the carburettor, too low a level in the float chamber, or to partial stoppage in the pipe line. Another cause of this trouble of intermittent running would sometimes be loss of battery power when using coil ignition, i.e., batteries want recharging.

If the engine runsirregularlythe trouble is probably due to too much oil in the cylinders causing the plugs tomisfire, the presence of water or dirt in the petrol, a defective valve, a broken carbon brush, or poor electrical contact somewhere in the magneto, the low tension contact breaker (coil), or high tension distributor (coil).

To ascertain whether the engine is firing regularly on all cylinders, or to detect which cylinder ismisfiring, the best procedure is to open the compression taps in turn while the engine is running and in each case speed up the engine while you have the tap open. Cylinders which are firingwellgive a sharpcrackingnoise, those which are not firing merely give ahissingnoise. If no compression taps are provided, each plug must be short-circuited to the frame in turn by the screwdriver method given above. The short-circuiting process causes a reduction in engine speed except on that plug which is already not firing. The method is not so good as the compression tap process, because the plugs often get oiled up during the short-circuiting process and the difficulty is accentuated.

3. Timing the Ignition.

My colleague, Mr. Oliver Mitchell, has pointed out to me that it is often impossible to tell directly when the piston is exactly at the top of its stroke, and he recommends a study of the accompanying Valve Setting Diagram (Figure 71). From this it will be seen that it is sufficiently near to bring the engine first of all to such a position that the exhaust valve hasjust closed; then make a chalk mark on the flywheel and give the engine one complete turn round; the piston will then be in the firing position if the flywheel is turned a shade backwards. Another method would be toretardthe ignition fully and time it so that the sparkoccurred one complete revolution after theinletvalve hadjust commenced to open. When either valve is closed its tappet can be felt to befree, the amount of freedom depending upon the clearance between the tappet head and valve stem.

Fig. 71.—Diagram of Valve Setting.

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