M. P. G.
Page 51
Page 52
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THAT petrol consumption, as an item of fleet maintenance, is of paramount importance, is too well known to warrant comment. What is not so well known, however, is the number of small details which contributes to the difference between a good fleet average and a bad one; in this article the matter will he discussed from two points of view.
The writer is Tortunate in that he has had the opportunity f o r
dealing with petrol consumption not only in fleet maintenance, but also as a carburetter engineer, and is, therefore, fully aware of the difficulties encountered in both directions.
One of the most outstanding features appears to be that, in far too many cases, petrol consumption, good or bad, Is credited as being entirely the affair of the particular carburetter concerned, and, where the consumption is above the average, much time is often spent in attempting to " tune " the instrument by the use of smaller jets, etc. In some eases, even, the jets are hammered up and other methods of a like nature are resorted to, whilst mechanical details which are possibly the prime cause of the trouble are ignored.
M.P.G. Return a Good Barometer.
Most fleet engineers will agree that figures for mileage per gallon of fuel, whether taken over periods of a day, a week or a month, must he read in conjunction with the mileage covered In the period, the class of work and mechanical condition.
--. Whenever there appears to be a sudden fall in mileage per gallon in connection with a vehicle, the performance of which has hitherto been good, it is a sound indication that all is not well. Indeed, these figures form a good barometer of the fleet efficiency generally, and, if closely watched, will lead to many small defects being discovered before damage is done.
Apart from the almost inevitable temporary rise in fuel consumption which usually follows engine and general overhaul, a good fleet average can be obtained only by close attention to detail in maintenance. Efficient docking at set intervals reduces running repairs to a minimum and does much to eliminate those troubles which react adversely
upon petrol consumption. Stiffness of the brakes, engine and transmission, and clutch-slip and tyre pressures are points which need most watching, so far as the chassis is concerned, and where road tests are carried out before or after overhaul, tests for deceleration should be made to ascertain whether the vehicle is perfectly ' free" as regards rolling resistance, This should be done on a hard, level road, the test consisting of checking the time taken to coast In neutral from a given speed—say, 30 m.p.h. to 5 m.p.h. An average figure for the type of vehicle concerned can be obtained, and it should remain the test figure for comparison.
Leaks in the petrol tank and feed line usually make themselves evident at once when the machine is stationary, the same applying to flooding of the carburetter. Air cleaners should be examined regularly and carefully cleaned. Where they are being fitted for the first time, care must be taken to ensure that they are of sufficient capacity to pass the desired quantity of air without putting any extra depression on the jet, otherwise consumption will at once be adversely affected.
Possible Reasons for Abnormal Consumption.
An overhauled engine which fails to give a normal consumption figure after running " free " should be checked for wrongly timed ignition or valves. It is well known that troubles of this nature should be evident to the driver by the performance of the vehicle, and should be reported as such, but it is surprising how long such a condidition can obtain before being noticed by some drivers. Where possible, it is aesirable that the carburetter repairs and adjustments (with the exception of such details as adjusting the idling, etc.) should be in the hands of one or two men, according to the size of the fleet, who should be responsible for the condition and adjustment of all carburetters. The practice of allowing any mechanic to " ttine " a carburetter canhot be too strongly deprecated.
One or more Spare " master " instruments should be kept on hand (preferably in the foreman's office) and should carry the required specification. When the efficiency of the carburetter in service is in doubt, a "master" carburetter can be fitted and the matter settled.
To keep spare carburetters simplifies maintenance in other directions, because it is obviously more desirable that a mechanic, other than the carburetter specialist, should change an instrument, rather than attempt to "tune" it. Where the fleet is large and is composed of one type of vehicle, it is advisable to follow the practice of the London General Omnibus Co., Ltd., of maintaining one setting for all the units, and attempts to cure engine faults by means of different settings should not be tolerated.
Carburetter Maintenance.
Turning now to the purely carburetter side of maintenance, it is advisable to have'a test stand fitted up, where float levels, suspected leaks, etc., can be checked. This will consist, in the essentials, of a small bench carrying an angle-iron frame to which carburetters can be clamped. A drain pan must be fitted under the frame to catch petrol dripping from the instruments under test, and a pipe should be attached to the lowest portion of the pan; through which the petrol can pass into a can beneath the bench.
The head of petrol for testing float levels is best arranged by a gravity tank which can be adjusted to different heights, the maximum height necessary being 6 ft. above the needle valve of the carburetter under test, measured from the petrol level in the tank. This head of petrol is equal to the pressure of 1.92 lb. per sq. in. (say 2 lb.), delivered by a mechanical fuel pump of the well-known A.C. pattern. This pressure varies slightly with the type of engine, but 2 lb. is an average figure.
Where gravity fuel feed is employed on the vehicles, the test tank must be set to a height similar to that on the vehicle concerned. Thus, if the carburetter under test belongs to a vehicle equipped with an A.C. fuel pump, the test tank must be adjusted to the 6-ft. position, whilst, if the machine has gravity feed, the average height must be measured on the vehicle from the needle valve to the average height of fuel in the tank or A-utovac, and the corresponding distance must be adjusted on the test stand.
Instead of the adjustable gravity tank, a pressure test tank may he used. This consists of a small airtight petrol tank bearing a handoperated pressure pump and gauge. The tank is usually attached firmly to the bench and holds about two gallons. It is, of course, necessary to insert a tap in the petrol pipe between the tank and the carburetter union, so that adjustments can be made to the float mechanism, etc., without having to release the pressure.
The carburetter to be tested is rigged up, the petrol pipe attached and the tap turned to the "off " position, the pressure then being raised by means of the hand pump to the required figure.
Obtaining the Head of Fuel.
Where it is desired to obtain the head of fuel from the known pressure, the following formula can he used :— h =p x 144/d.
Where h=head of fuel in feet. p–pressure gauge reading in lb. per sq. in. d=density of fuel con cerned.
Therefore p= h x d/144.
The density of the fuel is obtained by multiplying the density of water by the specific gravity of the fuel; thus ,for average .740 it=62.35 x.740=46 lb. per cubic ft.
For benzole mixture of approximately .770, the density =62.35 x.770 =48 lb. per cubic ft.
The use of a Brown and Barlow flowmeter is recommended for checking the effective sizes of jets. Alternatively, a constant-head gravity tank can be fitted up, and the jets flowed under a 50-cm. head (measured from the jet orifice to the constant level in the tank).
Using a Flowmeter.
The time must be checked with a stop watch, and the amount of petrol flowing in a minute carefully measured in a beaker calibrated in ccs. This is, however, a lengthy method, and for convenience and c2 rapidity the flowmeter cannot be surpassed.
It must be stated most emphatically that in no circamstances should the jet orifice be tampered with. This practice is giving so much trouble that one well-known concern is manufacturing hardenedsteel jets in an endeavour to ,combat the evil.
Except in the case of the thin plate jet orifice, which is made by a special process, jets are almost invariably drilled and finished to size with a fine broach, which gives a parallel orifice with a perfect finish. Therefore, the opening of jets with a tapered reamer necessarily results in a tapered orifice, which is extremely undesirable.
A Practice to be Condemned.
Furthermore, the practice of hammering up the tip of a jet with a round punch, in order to close the orifice, and then filing the top smooth, is particularly reprehensible, as the flow characteristic of the jet is completely destroyed. With a jet having a high lengthdiameter ratio, it results in bulging in the tip of the jet, so that the effective orifice is converted into one having a length-diameter ratio of unity or less, and a flow like a corkscrew, whilst, with a thin plate type of jet, the orifice will bulge inwards, and the jet will be useless.
In order to clear up any misunderstanding regarding the flow of jets, it is necessary to lay stress on the fact that because a jet will pass the required amount, as indicated on a flowmeter, after having-been abused in this manner, it is by no means proved that the jet will function correctly under carburetter operation, "While on the flowmeter the jet is delivering petrol under a constant head of 50 cm, or 19.7 ins., of petrol, at constant temperature and viscosity. Under normal conditions of carburetter operation, the suction head will vary between a maximum of about 70 ins, of petrol and a minimum of, perhaps, 11 in.; if the carburetter be situated near the exhaust pipe, the temperature and viscosity will also vary considerably. It is, therefore, apparent that there are factors governing the flow, other than those obtainable on the flowmeter.
• The flow characteristic of a jet depends upon the operating head, the temperature and viscosity of the fuel concerned, and the-shape of the orifice and its approaches. As the last-named is the only constant factor, the carburetter designer is compelled to evolve a jet that will cover, as closely as possible, the vagaries of the other three, and, at the same time, give proper compensation under the conditions afforded by the carburetter.
When alteration of the orifice structure takes place, the whole compensation-balance of the carburetter is overthrown, which results in loss of power, overheating, burnt valves and often high consumption, and such treatment, in the long run, is far more costly than keeping the orifice strictly untouched and changing the jets when the occasion arises.
In this connection it is necessary to explode the fallacy that proportionately small jets and a resulting weak mixture invariably represent more miles per gallon. This is by no means the case and better consumption can often be obtained by fitting larger jets, due to the fact that, with a weak mixture, the engine is not developing its full power, and, consequently, lower gears have to be used more frequently.
Setting the Idling Adjustment.
The idling adjustment should, in every case, be set as "lean" as possible, consistent with steady idling and freedom from flat spots at small throttle openings. The influence of the idle and change-over system is noticeable far up the partthrottle curve and has a marked effect upon consumption.
As consumption is, to a great extent, dependent on driving, drivers should be urged to (a) avoid undue movement of the accelerator when driving, and not to use full throttle when the required speed can be maintained on half throttle. The following may well be made a slogan and placarded around the running shop :—"Drive with the accelerator, not on it,"