Overcoming Diesel-fuel Freezing
Page 43
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WINTER is officially here, and the nights are already drawing in If this winter is anything like as bad as the last one, diesel-vehicle operators will be starting to think about the troubles they experienced at the beginning of this year. This being so, a survey of the fuel-freezing troubles experienced during the severe conditions of last winter and which has been made by C.A.V. Ltd., is particularly appropriate. This has been based on detailed reports from their service engineers working in all parts of the British Isles.
The most common trouble reported was blockage of the fuel feed at the primary filter, between the vehicle fuel Lank and feed pump: Based on improvements to this part of the fuel system, C.A.V. have made recommendations which, if generally adopted, should render diesel vehicles less vulnerable to arctic conditions and hence reduce field-service problems in the future.
The effect on fuel oil of the lowtemperature air encountered was the forrnatieth of wax, this having an appearance often described as "jelly " or white soap flakes ". Waxing occurred in those parts of the fuel system located in an exposed position on the vehicle chassis.
Of the exposed parts, primary filters with gauze and felt inserts were the main point of restriction.
Out of a total of 348 vehicle failures reported, there were 107 in which the engine would not start and 241 in which it stopped after a short distance had been covered. The following is a summary of the faults recorded in the survey:
58 failures with blockage of the main filter.
229 failures with blockage of primary filter between fuel tank and feed pump.
46 failures with "freezing" in p:pe between fuel tank and primary filter.
8 failures with " freezing" in pipe between feed pump and main filter.
50 failures with " free7ing " in fuel tank.
6 failures with blockage of feed pump Of the 58 blocked main filters, about half were on vehicles having this unit located directly in the air stream from the radiator fan. From reported cases of all other forms of blockage, it appears that most makes and types of vehicle were similarly affected irrespective of the fueloil brand used. Three methods of cure were adopted:— ( I) Cleaning or removal of gauze primary filter insert.
(2) Lagging of fuel tank, exposed pipes and primary filter bowl.
(3) Diluting fuel With petrel or paraffin.
In 26 per cent of the cases of failure traces of water were found in the fuel system.
From the foregoing it will be evident that the ideal means of primary protec tion should not only provide adequate safeguard against damage to the feed pump and prevent blockages of the suction-side pipework caused by exceptionally large contaminents, but It should also assist in removing water from the fuel and must not choke at temperatures down to the pour-point of the fuel.
Under normal temperature conditions, gauze primary filters of the types commonly used are quite satisfactory. But the rapid choking which occurs in very cold weather is caused by inadequacy of gauze inserts to deal with large quantities of wax precipitated in the fuel. This problem is heightened by the fact that, to prevent pipe blockages, the filter must be installed close to the fueltank outlet, where it is not normally warmed by the engine.
T he need for a separator which is functionally equivalent to a gauze filter, but which is non-choking, has been known for some time and now the results of the survey have demonstrated the urgency of the requirement.
he solution put forward by C.A.V. is to substitute a small " sedimenter " for the gauze primary filter. The model offered is their new Type SS sedimcnter, which is a modified version of the simple sedimenter used originally in the Filtrap system of water. separation announced some 18 months ago for the prevention of corrosion in fuel-injection equipment. It is suitable for fuel flows of up to 10 gallons an hour.
A sedimentcr is more effective than most primary gauze filters in removing solid particles and is completely nonchoking at all temperatures at which it is possible for the fuel to pass through the pipes.
The SS unit removes all the larger water droplets and hence prevents blockage by ice. It has the further advantage over some current primary filters that the fuel flow is unimpeded even when the water collected is frozen, provided that the sedimenter chamber has not become overfilled with water.
Principal components are a hollow conical-shaped element enclosed in a cylindrical container. Diesel fuel oil, fed into the container through a side connection in the head casting, flows clown the outside of the element and passes at low velocity through a clearance between it and the container wall. It then flows upwards inside the cone to the outlet ports, whilst solid particles and water droplets are deposited in the sedimenter chamber. This is a bowl of heat-resisting glass forming the lower half of the container. The transparent sides enable the water level to be inspected and there is a drain plug in the base. A by-pass air bleed is incorporated in the head.
The sedimenter should be installed with the shortest possible length of feed pipe from the fuel tank and in such a position that there is some form of windshield. The latter must not prevent regular visual inspection of the contents of the transparent bowl.
Pipelines should be of liberal bore diameter and free of sharp elbows: There. should be no bends in which water could collect, especially between the tank and the sedimenter. This and other exposed parts of the pipe-line may be lagged, preferably with asbestos, to allow the use of a blowlamp for heating in emergencies.
There should be no difficulty caused by cold choking of final filters of the C.A.V. paper-element type, installed between the pressure side of the feed pump and the injection pump, if they are located where engine heat is received quickly after starting and the fitting of genuine replacement elements has been carried out at the recommended intervals. During exceptionally severe weather the engine should be idled for a few minutes before the vehicle is driven away. • J.F.M.