FACTORS IN FUEL ECONOM! ,ND CORRECT CARBURATION.
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Too Little Attention Paid to Petrol Was' Extraneous Reasons for Heavy Consum ture R The Carburetter Not Always to Blame. .. Some Considerations Regarding Mix:Tments.
THE user of commercial vehicles whose bank account showed a steady diminution as the result of some unauthorized person drawing cheques upon it, would very quickly turn his attention to the matter and have it (and, we hope, incidentally, the culprit) arrested, and yet there are thousands of vehicles running which, in effect, are behaving in this unauthorized manner by consuming an immoderate quantity of fuel. Often they appear to do so with apparent impunity, although petrol represents cash just as much as does a draft on the bank.
We have known instances where vehicles which should have been doing 8 to 10 m.p.g. have consistently given only 5 or 6 m.p.g., no particular endeavours haring been made to rectify the trouble or troubles which have caused this excessive consumption. The owner would no doubt be very concerned if he found petrol draining away from the open tap of a tank, but be might just as well have this happen as permit the fuel to be utilized uneconomiCally ; in fact, it might do less harm than being used to excess in the engine, .where it would promote carbonization, overheating, the burning of sparking-plug roints and valves and numerous other troubles.
Do Not Always Blame the Carburetter, In considering this matter of petrol economy, it is usually quite incorrect to throw all the blame 11 rt011 that longsuffering instrument, the carburetter. At a rough estimate, it may he considered that not 50 per cent. of the fuel consumption above what may be regarded as correct is due to carburation. It is unfortunate for the users and makers of carburetters that so many other troubles would appear to reflect on these instruments, and the curse of the carburetter trade lies in the constant demand upon it to service something which does not belong to it.
'Whatever the trouble may be, tinkering with the jets is often the first thing done by drivers and maintenance men, and yet until the carburetter is worn out practically the only things that can happen to it are choking of the jet or jets and flooding, and with the improved filters now available the former seldom occurs, whilst the latter makes itself quickly apparent when the need for correction arises, which, again, is not often.
It must, of course, he remembered that the carburetter employed on a .commercial vehicle is working under conditions vastly different from those obtaining in the case of the private var. The average instrument should easily last the life of the private vehicle, but the bus or coach will run a bigger mileage in a single, year than the average total of a car; 150 miles per day for seven days a
week is quite common. Consequently, carburetters, unless sped ally designed, may be worn out and require renovation in from three to six months.
The original fitting may sometimes have a lot-to do with the matter. For instance, a spring load is often put on the throttle trunnions in such a Manlier that
there is a continual pall to one side, causing rapid wear and subsequent air leakage. In a bus the gears are changed something like .300 times a day and the throttle is being waggled all the time, which serves In accentuate such defects.
A surprising amount of petrol can be wasted in warming up the engine when starting from cold and, in our opinion, much can be said in this connection for a start on coal-gas, a method which is adopted in some garages. This not only permits starting almost immediately but is, we understand, considerably cheaper and avoids oil dilution by cold fuel. • Warming up tile oil in the gearbox and back axle constitutes a definite frictional load upon the engine, and this alone demands the consumption of extra fuel. For this reason, apartfrom other conSiderations,. it pays to utilize thin oil in these units. What with the amount used in starting and in warming up the transmission, a vehicle doing 75 miles per day may give, say, 7.5 m.p.g.; if, however, it runs 140 miles per day it will often give quite easily 8:4 m.p.g.,because what may be called the extraneous consumption is spread over the bigger mileage.
The long petrol-pipe lines on many commercial chassis usually require a lot of attention. Often, when examined, they will he found tobe quite damp with fuel. Now it requires quite a large leakage to cause actual dripping, but a slow percolation may easily cause an evaporation which goes on night and day, depending upon the position of the petrol tap so far as night losses are concerned, and means much wastage.
Purely mechanical troubles, such as binding of the brakes, may exercise a most deleterious effect on fuel economy ; faults in the ignition system resulting in misfiring are also direct causes.
Some of the more knowledgeable Commercial-vehicle owners and drivers each night drain the hot water from the radiators into insulated tanks made on the principle of thermos flasks. The primary purpose of this is probably to facilitate starting in the-morning, but it has the secondary advantage that fuel is thereby saved as the engines do not have to be idled for so long. Most carburetter experts consider that the thermostat is an instrument of the utmost importance as it enables the water to be heated rapidly to its working temperature. The controllable radiator shutter is particularly useful and efficient, but it brings in the human factor.
A frequent cause of excessive fuel consumption is the enriching of the mixture enforced by air leakage past worn valves and valve guides. This may be looked upon as an external method of curing a bad fault. The correct way would be •to fit new 'valves and guides, but this means a certain amount of capital expenditure in material and labour charges, and many users prefer the more insidious method of increasing the consumption of petrol, although, in the end, this probably causes a greater outlay.
Engine design may have much to do with the matter, but it is definitely limited by the quality of the spirit likely to be used. Higher compression means higher efficiency, but limits the choice of fuel, and better fuel means a bigger price.
Petrol Consumption a Variable Factor.
Petrol consumption is a constantly variable factor and must be treated as such. Weather conditions exercise considerable effect upon it,. and in comparing the performance of one vehicle with another of the same make, due allowance must be made for any differences in the proiected.area upon which wind pressure takes effect. Road speed alone may make all the difference between the performance of individual vehicles of similar type, whilst the influence of the driver is most appreciable. Tests have shown that a difference of 10 per cent, in consumption is often due to the manner in which advancing and retarding of the ignition are carried out. In this connection it may be stated that with well-trained drivers variable ignition is a decided advantage, but where it is considered that the driver will not take full advantage of this provision it is better to employ fixed ignition.
To burn petrol completely it must be mixed with about 15 times Its weight of air or 9,000 times its volume. The greatest power is attained when the weigh:: of the petrol is one-twelfth to one-fourteenth of that of the .air, whilst the most power for a given amount of fuel is when the weight is one-Sixteenth of that of the air.
At the top of a suction stroke the space above the piston is filled with exhaust gas and this remaias in the cylinder. The quantity of this gas does not vary considerably ; consequently, at idling speeds the percentage of exhaust gas to mixture is increased to a considerable extent, and this is one reason why it is essential to have a slightly richer mixture for idling, otherwise misfiring and erratic running are apt to occur. Another reason for using a richer mixture P4 the attenuation, or wire drawing, which occurs when the throttle is nearly closed. Small air leaks are of vastly greater importance at slow speeds, vhilst leaking exhaust valves admit more than the usual quantity of exhaust gas.
The presence of deposited fuel in the intake manifold is a great evil. At slow speeds it may remain in the manifold and accumulate to an extent which is equivalent to the amount required for many cylinder charges. Directly acceleration occurs, the whole quantity is swept into the cylinders where, if the temperature be high, it is turned into carbon deposit or is blown out of the exhaust as free carbon, whilst if the temperature be low it may work down past the piston and dilute the lubricating oil. This excess is one of the prime causes of a fiat spot or hesitation while accelerating.
There are certain fundamental requirements in correct carburation. The theorist might think that the mixture should be of uniform quantity or strength throughout the whole range of engine speed, but in practice this is not only almost impossible of attain meat, but if it were possible it would not be advisable. The practicable mixture is one which can combine the greatest power obtainable when due attention is given to the matter of economy. Economy is, of course, required all through the throttle range, but at full throttle the maximum power is necessary, for it may be considered that the only reason for opening the throttle to the full extent is to obtain this power, and it is only shut to decrease speed. In some recently built carburetters, special provision is made for this fuel-enriching. For instance, in the Claudel-ITobson a special power jet, which is brought into operation at or about full throttle, is provided. It has the effect of adding just sufficient petrol to that supplied by the main jet to convert the mixture necessary for economy into the slightly richer one required for maximum power. This obviates the employment of a larger jet to gain the maximum, which would certainly waste fuel at lower speeds.
There is another factor to which consideration ought to be given in every case, as it is already in some. When an engine is comparatively cold the mixture must be fairly rich, but when everything is warmed up it is possible to allow a weakening of the mixture without loss of power and, obviously, with an increase of economy. In the new V and }I instruments of the make to which we have just referred, a 10 per cent. weakening is permitted. The important point is that It must he on a percentage basis at all parts of the throttle curve. The ordinary extra-air device between throttle and engine is not altogether satisfactory and may be very unsatisfactory. The trouble is that it provides a weakening as the throttle closes, owing to the extra suction on the air, and thus interferes with slow running. A good way is to control by reducing the suction on the main and power jets.
Comparatively recent researches into carburation have shown that a cold and wet mixture i8 the best. e34 The latent heat of the wet fuel serves to reduce the internal heat, thus improving the volumetric efficiency. By this we do not suggest that external heating must be altogether eliminated. A certain amount is essential for idling, but petrol fog is much better than petrol vapour. If a modern high-compression engine were to be fed with true petrol vapour and air in the proportion to make a correct mixture the knocking which would ensue would be almost sufficient to blow the heads off the cylinders, bitt in ordinary practice the heat so limits the amount of charge which can be made to enter the individual cylinders, whilst, in addition, the throttle and frictional losses cause wire-drawing, that the result is it is impossible to make a complete charge enter unless a supercharger is employed; therefore, the knocking might not occur to such a drastic degree as we have mentioned.
In a forthcoming issue we shall give the concluding portion of this article, which will deal with certain individual makes of carburetter, indicating the improvements which have been effected during recent years.