A Simple Outline of the Principles and Working of Motor Vehicles

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Ahrens-Fox Circulating Pressure System

To improve upon the splash method of oiling, the oil is circulated under pressure directly to the bearings and the excess overflows into the compartments and supplements the lubrication by splash as already described. The Ahrens-Fox lubricating system is an example of this type. The complete motors of the fourand six-cylinder models are shown in Figs. 23 and 24. These motors are of the multiple-valve type, twin exhaust valves and single inlet valves being used, the four-cylinder accordingly having four inlet valves and exhaust valves while the six-cylinder has six inlet and twelve exhause valves.

The manner in which the crankcase oil-pan is divided into compartments corresponding to each cylinder is illustrated by Fig. 25. This also shows the oil pump, which is of the gear type and is positively driven through gearing from the camshaft. The tubes shown in this illustration are the oil stand pipes leading from the oil force chamber and communicating with the main bearings when the oil pan is attached to the crankcase. Fig. 26 is a view of the crankcase after the removal of the oil pan, while Fig. 27 is a bottom view of the oil pan itself, showing the oil pump removed, the pump itself being shown by Fig. 28. The lubricating system is the same in principle and operation in both types of motors and the same system is also employed for lubricating the fire pump.

Fig. 28. Oil Pump, Ahrens-Fox Circulating Pressure SystemFig. 23. Ahrens-Fox Four-Cylinder Motor, Exhaust Side

The supply of oil. which is stored in the crankcase, is kept in constant circulation by means of the gear pump illustrated, a gauge on the dash indicating at all times the pressure under which the oil is being circulated. The capacity of the oil pump is greatly in excess of the lubricating requirements of the motor so that a stream of oil is constantly kept flowing over the bearings as long as the motor is running. A wire gauze screen at the intake of the pump serves to filter the oil each time it is circulated, so that it is kept free of foreign matter and thus retains its lubricating qualities longer. The discharge connection between the pump and the force chamber is made automatically when the pump is attached and the pump is so mounted that it can readily be taken out for cleaning. Slip joints fitted with felt packing are provided at the point where the vertical stand pipes enter the main engine bearings, this packing sealing the joints against leakage when the oil pan is placed in position.

Fig. 24. Exhaust Side, Ahrens-Fox Six-Cylinder Motor. Light Metal is Aluminum Alloy Oil Pan

The lubrication provided directly by the circulation of the oil to the main bearing is supplemented by the splash caused by hook-shaped tubular dippers attached to the lower ends of the connecting rods. These dippers pick up the oil from the covered troughs of the oil pan which are traversed by the connecting rods at each revolution. The entire system is so designed that every part is accessible for inspection and cleaning without disturbing any of the working parts of the motor.

American-LaFrance System

This is also of the combined circulating pressure and splash type of lubrication which has come to be standard practice on modern gasoline engines, supplemented by certain modifications to adapt it better to fire service requirements so that the motor will function properly under a wide range of conditions which vary from comparatively long periods of idling in the fire station to warm up, to continuous heavy duty in pumping. The essentials of the system are the pump, the main distributing pipe with its leads to the bearings and the splash troughs and the oil reservoir which is located in the oil pan. The oil pump is placed at the lowest point in this pan so that the oil returns to it by gravity, first passing through a wire gauze screen to filter it. From the pump the oil is forced into the main distributing pipe, known as a “bleeder tube”, or oil pipe running parallel with the. crankshaft the length of the motor. Leads, or “bleeders” branch from this main feed pipe to the crankshaft bearings and holes are also drilled in these lead pipes so that the oil is delivered to both the bearings and the splash troughs at the same time. With the exception of the main crankshaft bearings to which the oil is fed directly, splash is depended upon to lubricate the remaining parts of the motor, the connecting rods being provided with scoops or dippers for this purpose. An ingenious device serves to regulate the amount of oil delivered to these splash troughs in accordance with the demands of the motor.

Fig. 25. Crankcase Oil Pan of Ahrens-Fox Motor, Showing Pump and Standpipes to Bearings

As the pump is directly driven by spiral gearing from the camshaft, its speed and the amount of oil it delivers increase in the same proportion as that of the motor. This regulation of the oil feed is carried out by notches cut at predetermined points on the front end of the splash troughs. The size of these notches is such that as the speed of the motor increases, the amount of oil discharged into the troughs from the main oil header increases faster than these openings can permit it to escape the result that the oil level in the troughs rises and rod dipamount of oil splashed about by the connecting pers increases. Placing these notches in the forward face of the troughs only also serves to regulate the lubrication in accordance with the grade on which the machine is traveling. When tipped forward as in descending a hill, the oil level is lowered and an excess supply is prevented from reaching the cylinders, whereas in mounting a hill which greatly increases the load on the motor, the oil supply is increased. This varies the oil supply automatically on hills, cutting down the amount when the motor is idling in descending a hill, thus preventing the fouling of the spark plugs and increasing the splash when it is most needed in climbing grades.

Fig. 26. View of Ahrens-Fox Four-Cylinder Moyor with oil Pan Removed

Pistons Recessed and Drilled

To insure proper lubrication of a motor working under heavy load as in pumping or running at high speed to a fire, an amount of oil greatly in excess of the actual requirements must be distributed to the important working parts of the motor, but it is equally important that none of this excess be allowed to find its way into the combustion chambers of the motor where it would be burned to carbon, fouling the spark plugs and valves. This is prevented in the Anterican-LaFrance motor by turning a groove or recess in the skirt of the piston below the lower ring and drilling a series of holes through this groove to the inside of the piston. The lower ring on the piston is termed the oil ring and one of its functions is to scrape the excess oil off the cylinder walls so as to prevent it being carried into the combustion chamber. This excess oil enters the groove, or recess, and passes through the holes to the inside of the piston. By this method, it is possible to splash an amount of oil greatly in excess of the requirements of the motor without having too great a proportion reach parts where it is not wanted. An unfailing indication that too much oil is reaching the combustion chambers of a motor is the condition of the exhaust. When this is accompanied by large [uantities of blue smoke, it shows that oil is being burned as well as gasoline and if not corrected the motor will quickly foul and require taking down to remove the accumulation of carbon. Black smoke at the exhaust is a sign that the fuel mixture is too rich in that there is more gasoline in it than can be burned properly by the motor.

Fig. 27. Bottom View Oil Pan, Ahrens-Fox Motor, with Oil Pump Removed

The main supply of oil is carried in the oil pan and a gauge glass shows the level, indicating the need for replenishment when its level falls while the pressure in the system while the motor is running is shown by a gauge mounted on the sash. As a large quantity of oil is carried in the reservoir, the motor can be run for a considerable length of time without adding to the supply. In addition to the force feed direct to the main bearings, oil pockets are provided so that in case of any failure of the circulating pressure system, the oil level can be raised by the addition of fresh oil to a point where the entire motor is lubricated by the splash alone.

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Automobile Engineering

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(To be continued)




A Simple Outline of the Principles and Working of Motor Vehicles

(Continued from page 1037, Vol. LXV.)

WHEREVER metal surfaces slide or move upon one another, it is essential that they be kept separated by a film of lubricant. The latter serves the double purpose of minimizing the resulting friction and keeping the temperature of the moving parts below the point where expansion due to the heat generated would otherwise cause them to bind. The greater the temperature and pressure to which such moving parts are subjected in operation the greater the need for ample lubrication, so that in an automobile engine lubrication easily ranks as one of the most important essentials. By pressure in this connection, is not meant the pressure in the cylinder but the force acting on the bearing and tending to press the shaft against the latter. The main bearings, which support the crankshaft, afford a good example of this. In modern automobile engines, the m. e. p. (mean effective pressure) generated in the cylinder on the power stroke ranges from 100 to 125 lbs. per sq. in. This pressure is transmitted to the crankshaft through two bearings, i. e., the piston pin or gudgeon,which bears directly in the walls of the piston, or in bushings set in the latter, and the connecting rod bearing which is attached to the crankpin and which through the latter serves to transform linear into rotary motion. The crankshaft bearing also receives this pressure so there is accordingly always a tendency to squeeze out the film of lubricant at each of these points and the film must be constantly renewed by a fresh supply of oil fed to these bearings.

Of equal importance is the film of lubricant which must be maintained at all times between the pistons and cylinder walls. This is subjected to the high temperature of combustion as well as to a high pressure caused by the thrust of the connecting rod which bears the piston against the cylinder wall whenever the rod is in any but a vertical position. As mentioned in a previous chapter, the temperature in the combustion chambers of the cylinders reaches such a high degree that the oil on the parts of the cylinder walls exposed by the downward stroke of the pistons is burned so that most of it passes out the exhaust as a vapor, leaving behind a slight residue of carbon. When this carbon accumulates to any extent on the piston head it causes trouble by bringing about preignition, i. e., the particles of carbon become incandescent and fire the fresh charge of incoming gas before the spark passes at the plug, resulting in a heavy pounding and imposing dangerous stresses on the reciprocating parts of the motor. It is accordingly imperative that the motor be thoroughly lubricated but that an excessive supply of oil be guarded against as if too much of it is carried up by the pistons it results in heavy deposits of carbon.

Splash System

To provide ample lubrication for all internal parts of the motor, the first expedient adopted was to fill the crankcase oil pan to a certain level. This pan is usually divided into compartments corresponding to each cylinder so that the big end of the connecting rod dipped into a pool of oil at each revolution, throwing the oil about violently. Oil grooves turned in the skirt of the piston or oil rings fitted at that point served to carry the necessary supply up on the cylinder walls and to scrape off the excess. This is known as a simple splash system. One of its chief shortcomings was that when freshly filled there was apt to be too much oil and if the motor war run Steadily for a number of hours without replenishing it, the supply ran low. 1’he oil also deteriorates in quality with use so that unless frequently drained the major part of the supply in the crankcase consisted of old oil and an accumulation of dirt—chiefly carbon.

Oil becomes used up in the same manner as any other material subjected to constant pressure and heat, so that after a time, it loses its lubricating qualities. This may readily be demonstrated by a simple test. Dip the fingers in some old oil that has been in use for some time in a motor and rub the fingers together under pressure. It will be found to have a thin and watery feeling and docs not prevent the finger ends from being pressed together, whereas a sample of fresh oil treated in the same manner will be found to form a film on which the fingers slide, keeping them apart regardless of the pressure exerted. It is the viscosity, or body of an oil that gives it its lubricating qualities and the combination of heat and pressure destroys these. Oil remaining in a crankcase also becomes thinned out to a certain extent by the gasoline that condenses in the combustion chambers and finds its way down past the pistons when the motor is run cold. This is often aggravated in cold weather by priming the engine with liquid gasoline to start.

(NOTE—Owing to the press of other matter, the usual instalment of two pages of this department has been unavoidably shortened to one page. With the next issue it is hoped to be able to give our readers the full benefit of Automobile Engineering.—EDITOR.)