An Investigation of
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Cold-starting Problems
Some Notes Relating to the Factors Involved in the Starting of Petrol Engines at Low Temperature S. Interesting Results from Recent Experiments THE starting of an internalcombustion engine at low atmospheric temperatures depends, to a large extent, upon the ability to overcome the friction horse power developed at predetermined temperature and minimum starting revolutions. This frictional value is, in turn, largely dependent upon various physical characteristics of the lubricant employed in the engine, and, in this respect, the viscosity at crankcase temperature is probably of equal importance to the pour-paint value of the oil.
Investigations have been made into this aspect of cold starting, and the results appeal' to indicate that the relation of friction h.p. to start h.p. and engine power at low speeds is the controlling factor.
The lock torque of the starter usually exceeds the Maximum required breakaway torque of the oil, whilst the secondary breakaway torque is less than the first, even at
low temperatures. The power requirements, however, increase rapidly with speed, so that with some oils it is impossible to obtain a speed of even 10 r.p.m. at zero or below zero temperatures, probably on account of the shearing force required for such viscosity as the oils
would possess at low temperature. It should be noted that high-viscosity lubricants actually have a lower breakaway torque than do oils of low viscosity.
The following observations have been obtained from a six-cylindered sleeve-valve engine suitable for bus operation, using lubricants having characteristics in accordance with the table : The friction h.p. of the various oils are plotted in Fig. 1 for a temperature of 15 degrees F., whilst it has been assumed that approximately 50 per cent, of the maximum initial horse power will be available under conditions of cold starting.
It will be quite possible to start readily with oil No. 1 at a starter speed of 67 r.p.m., the lubricant having a temperature of 15 degrees F. (this is illustrated by the friction horse-power curve which lies below the line of estimated initial h.p.), whereas with oil No. 2 the maximum of 17 r.p.m. would probably be insufficient to maintain engine speed
even should sparking occur, as the initial h.p. (maximum) curve indicates that there is not enough power generated to overcome the friction h.p. at this speed and temperature.
Conditions are, of course, greatly improved when a slight amount of dilution takes place, thus lowering the viscosity of the lubricant. Oil No. 1, however, has a viscosity too low for winter service when this fact is borne in mind.
An interesting investigation concerns the power required with oil No. 2 for starting purposes at different temperatures and with differing percentages of dilution. It is, of course, realized that dilution quickly takes place under cold conditions through the use of the choke, particularly on short runs.
This percentage is high for the first 70-100 miles, after which equilibrium is again established, possibly within 15.'0-200 miles. Dilution assists materially in reducing the horse power required to overcome friction at low temperatures.
Examination of Fig. 2 illustrates this point, and it will be seen that lubricant with 15-per-cent. dilution at 15 degrees F., or 7-per-cent. dilution at 0 degrees F. and with no dilution at 15 degrees F., occupy approximately the same relative positions with respect to the estimated initial h.p. curve, whilst they cut the starter h.p. curve at about the same number of revolutions. It is, therefore probable that no starting difficulties would be experienced with oil No. 3, or with oils of similar characteristics after 100 miles, if used in an engine which requires only 30 rpm. to promote firing.
Diluted Oils and Power Requirements.
Fig. 3 illustrates the power requirements with diluted oils at 30 r.p.m. and under differing temperatures, and it will be noted that at temperatures of 0 degrees F., sufficient starter power is generated, but if 60 r.p.m. are required before firing occurs, oil No. 4 may cause starting problems at a temperature of 5 degrees F., although it has operated sufficiently long to have suffered dilution to the extent of 5 per cent.
Not only is it important to obtain sufficient starting torque, but it is essential to provide the moving parts with adequate lubrication imme
diately starting has taken place. Dilution probably occurs, and it is undesirable in that it aggravates the problem by increasing the possibility of the bearings receiving diluted oil. This is caused by the using of the choke in order to obtain the necessary cranking speed.
The flow of lubricant from the pump along the oil pipes is sluggish at low temperatures, and experiments have been carried out in order to ascertain the time taken for the oil to reach the bearings.
Tests commenced with a temperature of 30 degrees F. for all lubricants, and it was found that, with regard to pumping ability, there was little difference between oils Nos. 3 and 4, in spite of the fact that No. 3 has a relatively high pour point, whilst No. 4 possessed higher viscosity at low temperature. At 30 degrees F. both oils reached the bearings in eight seconds, but at 10 degrees F. No. 4 required 30 seconds and No. 3, 18 seconds.
This differene in time was gradually diminished to 0 degrees F., when both oils pumped in 52 seconds. The time occupied by oil No. 5 to reach the bearings increased rapidly with decrease in temperature, due largely to the combination of high pour point and high viscosity at low temperature. The time required at 30 degrees F. was nine seconds, at 10 degrees F., 60 seconds, and at 5 degrees F., 115 seconds.
Consideration of these periods required to establish lubrication indicate that both pour point and viscosity are of equal importance, whilst a desirable mechanical feature regarding the position of the overflow from the pressure relief valve is emphasized
The pressure relief valve should be as near to the pump as possible, for when the lubricant is cold the overflow is large and should be returned to the pump quickly in order to obtain benefit from pump move merit, which tends towards the ren dering of a more fluid oil.
No Marked Heating of Starter Battery.
Incidentally, it is a fallacy, to assume that the starter battery heats appreciably upon the first starter application, as 15-second applications raise the temperature approximately 2I-3 degrees. It is, therefore, desirable to apply the starter at, say, 10-second intervals, should difficulty occur in cold starting, whilst heavy grease should not be utilized to protect the starter-gear splines..
It is not suggested That the engine fuel has no significance in cold starttug, but, whilst there are undoubtedly wide differences with fuels of varying distilling characteristics, it is felt that this aspect is often too greatly emphasized.
In view of the fact that the cranking speed appears to be the main factor in starting from cold, the ultimate solution of this problem will probably be found in the provision of more suitable starting equipment, and detail improvements in the design of the ignition system, induction system and carburetter.