BUSTION ENGINES.) Mechanical efficiency is the ratio of work output of the engine to the work developed within the engine cylin ders. The difference is accounted for by the friction and pumping losses within the engine itself. Of the various friction losses, the piston friction is the largest. Con siderable work is also expended in pumping the gases into and out of the cylinders, particularly at part throttle. Automobile engines vary from 75 to 93% mechanical ef ficiency at full load.
To decrease the friction losses in an engine, bearing materials with low-friction properties have been developed, and also more thorough lubrication of moving parts has decreased friction to as low a figure as seems feasible with present design of engines.
The pressures developed at the bearing surfaces have necessi tated certain minimum limits for bearing sizes, with due regard for durability. While the bearing friction losses for a given size of engine have been decreased but little, the mechanical efficiency in many cases has been increased by increasing the power output of the engine without a corresponding increase of friction. (See section, Metals in the Motor Car.) Volumetric efficiency is the ratio between the volume of air drawn into the engine for each two revolutions (one cycle), and the engine displacement, or volume of the cylinders swept by the pistons.
In the operation of an automobile, more power is required to bring the vehicle up to any predeter mined speed than to keep it moving at that speed (except the maximum speed of the automobile). The difference between the power of the engine at any speed and the power required to drive the automobile at a speed corresponding to this engine speed on level roads, represents the power available for accelera tion and hill climbing. To increase this difference, for starting purposes and on other occasions when greater power is needed, the change gears in the transmission are used. (See section, Transmissions.) Nearly all cars have their rear axle gear ratio so chosen that the engine will develop very nearly its maximum power at approximately 75% of the maximum speed of the car when in direct drive. Then the maximum hill-climbing ability or the maximum accelerating ability will come at the speed of maximum torque, or for most American cars, about 20 to 25 m. per hour.
The power required to drive an automobile on a level road is utilized to overcome two resistances to motion—rolling or fric tion resistance, and air resistance. The rolling resistance is caused by friction in the transmission, universal joints, differential, rear wheel and front wheel bearings, and also of the tires on the road. The air resistance is caused by the pressure of air against the front of the car and the partial vacuum behind it, and by the friction of the air as it passes over the various surfaces. The rolling resistance is more or less constant, regardless of the speed, while the air resistance increases as the square of the speed. In other words, at 6o m.p.h. the air resistance is four times as great as it is at 3o m.p.h. In their search for means of overcoming or minimizing the effect of increasingly high wind resistance at increasingly high speeds, designers have evolved the "streamlin ing" of car bodies, by means of which the frontal area is lessened and the vacuum behind is decreased.
The engine, with its lubrication and cooling, and the carburetion and ignition systems, function as a unit to produce power, and are commonly known as the power plant. In addition, a mechanism is necessary to transmit the power produced by the power plant to the wheels of the vehicle in order that they, rotating, may push the car along the road. This mechanism is the transmission, and it consists of several units: (I) the clutch, (2) the gear-set, gear box or transmission, (3) the propeller shaft and universal joint, (4) final drive, differential, rear axle.
The clutch enables the operator to connect or dis connect at will the engine and the mechanism used for transmitting the motive power to the driving wheels. The clutch is disengaged when one desires to stop the vehicle without stopping the engine, or when it is necessary to change the ratio of the gear in the transmission. Of the many forms of clutches that have been used, the three types—the cone, fig. 14, multiple disc, fig. 15, and single plate, fig. i6, —have been used more than any others. The cone clutch is made up of a short portion of a cone with an in cluded angle of about 12° to 15°, and is usually faced with leather or asbestos fabric. Provision is made so that this cone can be moved axially in and out of contact with a corresponding conical surface in the engine fly-wheel. A spring or springs are employed to keep the clutch engaged with the fly-wheel when it is not held out of engagement by the operator. The multiple disc clutch is made up of a plurality of plates or rings, half of which, called the driving discs, are connected by splines to the fly-wheel, the other half being similarly connected to the main drive shaft of the trans mission. Sufficient spring pressure is provided so that the friction between the driving and driven disc surfaces will transmit the full power of the engine without slipping. The single plate clutch has largely superseded all other forms, though some multiple disc units still appear in Europe. This clutch is made up of a single plate which is keyed to the transmission main drive gear shaft and is usually spring-clamped between a flat surface on the fly-wheel and a pressure plate arranged so that it is also driven by the fly wheel. The required pressure to prevent slipping is provided, either by a plurality of springs at the back of the pressure plate, of can speed. For normal driving conditions on level roads, however, the power required is less than the maximum power supplied by the engine, and for this condition it is desirable to run the engine at a much lower speed in order to give economy of operation. To fulfill these two conditions—maximum power and economy—it is necessary to incorporate between the engine and the road wheels a device which will permit a change of the ratio of engine speed to car speed. This is called the transmission. An ideal transmission would fulfill the above two conditions completely, but at present mechanical difficulties, cost, size, weight and complicated operation, make such an ideal transmis sion impracticable.