Engine performs work
Although this is for cars from the 50's, the basics haven't been changed much and you could use this info for learning how a car engine works. The manner in which the engine performs its work is illustrated below. The crank of the bicycle and the crank in the engine work in similar fashion. When the rider pushes down on the pedal of the bicycle, A, the force exerted on the crank causes the sprocket to be turned. The turning or rotary force thus developed is called "torque." Torque then is that which produces or tends to produce rotation.
What is horse power?
One horse power is the ability to lift 150 pounds a distance of 220 feet in one minute. The total amount of work performed is 33,000 foot-pounds a minute. The same amount of work would be performed if the horse were to lift one pound 33,000 feet a minute, or if his work resulted in lifting 33,000 pounds one foot in one minute.
Due to the fact that in the case of early mechanical devices use was made of the horse as a source of power, the practice of speaking in terms of horse power, with reference to engines, has long had acceptance in the engineering fraternity. In the days when horses were used for power, if one horse could not do the work required, additional horses were added to make available the amounts of power required. Likewise, in the case of early engines, if a single cylinder would not do the work, other cylinders were added.
In automobile engines, when it is necessary to increase the amount of power, other pistons are added. Of course, the bore and stroke of the engine have much to do with the power developed.
Horse power developed by the engine, is used to turn the flywheel and transmission gears, and finally is delivered through the rear axle to the rear wheels, the friction of which, in contact with the road, causes the car to be driven along the highway. It may be said that the horse power developed in the engine is used to do work in propelling the car.
The manner in which the parts of automobile engines are designed and work together to develop power is explained in the following pages of this chapter.
Gasoline engines are made in sizes varying from the fractional horse-power engines used about homes for the operation of small machines, such as the washing machine, cream separator, and the light power requirements, up to engines developing as much as 1,000 or 2,200 horse power for aircraft and marine uses. Most automobile engines fall between the lower power ranges, seldom developing over 200 horse power.
The ability of an engine to do work is dependent upon the power or the horse power developed. This in turn is dependent upon the capacity of the engine. A single-cylinder engine, has a certain capacity. Four similar engines connected together, has four times the capacity for work. As a matter of fact, it would likely develop more than four times the power for the simple reason that the larger number of cylinders makes for a more continuous flow of power, or, let us say, a more even torque.
The size of the engine is ordinarily spoken of as having to do with the bore of the cylinder and the stroke of the piston. The larger these two items, the greater the displacement of the engine and naturally the more fuel which will be drawn in and compressed to be burned, and, of course, the greater the power which will be developed.
Just as it is readily understood that a team of horses will do more work than one horse, so the student of automotive mechanics understands that two cylinders of the same size will do more work than one, and further understands that a larger cylinder or a cylinder with a greater capacity will do more work than a single cylinder of smaller capacity. For this reason we have all engines rated by bore and stroke and the number of cylinders, and this in turn gives the total displacement of the engine and determines in no small Way the capacity of the engine to do real work.
There are two methods of rating horse power of engines, one of these being an arbitrary method of getting the S.A.E. rating which has to do with the licensing requirements in all states and in fact in most countries. The other is the actual power developed which is measured as brake horse power. These features will be discussed at a later point.
If the student will remember that the same kind of action as that illustrated at A and B, is occurring at all times in the engine, when it is being operated under its own power, he can appreciate just how the power passing from the engine into the transmission line may be used to turn the propeller shaft. This in turn will drive the axle shafts which are geared 5 to 1 with reference to the propeller shaft and turn the rear wheels to drive or propel the car forward in the case of forward speed, or in a reverse direction in the case of reverse speed.
The student should now have an elementary understanding of just what is meant by power, torque, and work in relation to the automobile and thus be in a position to understand the need of knowing the theory on which the construction of the gasoline automobile is made.
Engine and ignition time
Only those parts which are essential to engine or ignition timing are shown. Engine timing is done from cylinder Number 1, considering the first throw of the crankshaft and the first rod and piston. The two cams, the two valve lifters, and the two valves belonging to this cylinder are considered for this work. The camshaft gear and the crankshaft gear likewise are essential. Other parts of the engine are nonessential and, for the sake of the study and actual engine-timing process, may be entirely ignored.
In addition to the parts named above, other parts are necessary for ignition timing. These are the timer-distributor, the ignition drive shaft and gears, all spark plugs, spark-retard device, ammeter, ignition switch and the primary wiring, the distributor (part of the ignition head), and the high-tension wiring. These units have all been gathered in compact, yet visible form, in the instruction stand illustrated. The stand does not illustrate engine-building practice, but rather illustrates exact timing of parts, functions of parts, and interrelation of those parts essential to engine and ignition timing.
Diesel engine principle
Fuel is not drawn in with the air but is injected after the air charge has been compressed. Compression ignition is used to fire the mixture. Otherwise the cycle of operation is the same as for the gasoline engine.
Casting cylinder blocks
Since the earliest successes of the motor car, the tendency in cylinder design has been along the line of arranging them all in one block. The earliest designs called for single cylinder castings, mounted individually on a crankcase casting. Next came the cylinders cast in pairs for the four-cylinder engine, and with the development of the six-cylinder engine the cylinders were cast in two blocks, three to a block. Shortly, the manufacturers were casting the four cylinders in one block for the four-cylinder engine, but the practice of casting the six cylinders together was a bit slower in being made practical.
Eight cylinders in V form or in line in a single block are designed and produced without eliciting any comment in modern practice. However, the matter of turning out perfect castings, as complicated as are the castings for a modern motor-car engine, is no slight feat. Not only are the water jackets and other passageways cast about the cylinders for their cooling, but the intake ports and exhaust ports are cored out in the casting. All manner of bosses, brackets, and other parts are cast as part of the single-block casting.
A block so cast is without a multitude of joints and connections, which are needed when cylinders are cast separately and assembled on a crankcase. Perhaps the greatest improvement, however, is one of engine operation. Casting the cylinders in one block helps to maintain an even operating temperature throughout the entire block and assures approximately the same operating temperature to all cylinders.
The number of cylinders in an engine has less to do with the power of the engine than with its smoothness. For instance, it is easily possible to build a 100-horse-power engine with four, six, or eight cylinders. Naturally the pistons in the four would have approximately twice the head area of those in the eight. Each power impulse gained from burning fuel charges would need to be approximately twice as heavy for like engine speeds of an eight. Heavier impulses result in greater strains and consequently more vibration.
While it is true that inherent balance and other engineering data enter into this picture, it is generally conceded that the forces of the power impulses are the largest factor in smooth or rough engines.
In practically all instances, the main bearings of motor-car engines are cast or fitted into the webs and ends of the crankcase. These bearings are usually of the babbitt-lined type, in most cases. Ball bearings have been used with success, but this construction is seldom used for passenger-car engines. Lead-bronze, steel-backed bearings are claimed to show a longer life under conditions of hard service. Airplane and marine, as well as truck engines, are using them with complete success. They are also used with success in passenger-car engines. The babbitt and bronze bearings are what is termed the split-bushing or plain-bearing type.
The upper halves of the bearings appear in the crankcase ends and webs, while the lower halves are bolted onto the upper halves by means of studs set into the crankcase metal. The larger bearing caps are provided with four stud holes. The backs of the bearings may be aluminium, cast iron, cast steel, malleable iron, or drop forgings. The babbitt metal, with which they are lined, may be sweated or spun into the cap, or the cap may be machined to accurate limits and the babbitt sweated onto a brass, bronze, or steel back.
Shims are carried between the two halves of main bearings in many cases of splash-type lubrication but are not commonly used for bearings in forced-lubrication engines. They are thin sheet metal, stamped to the general form of the face of the cap where it joins onto the upper half.
With the success of the motor car and its adoption by the public came an insistent demand to have the heads made so that carbon could more readily be removed. This led to the rather universal practice of making cylinder heads in separate castings so that they might be removed.
The design of manifolds is of interest since the efficiency of the engine is largely dependent on them. For updraft carburetors the intake manifold is fitted with a flange at the centre bottom, and with flanges on the ends where it is attached to the cylinder block.
The exhaust manifold, has been designed to bolt onto the cylinder block, in a manner which connects the upper part with each exhaust-valve port of the eight-cylinder engine. The heater at the centre permits exhaust gases to surround the intake manifold and warm the incoming fuel charge. These manifolds carry three exhaust-port flanges which are connected to the cylinder block. Heat flows from the exhaust passages into the metal of the manifold. Fuel charges, passing through the intake passageways, pick up this heat.
Oil pans and oil sumps
The lower half of the crankcase is made from pressed sheet metal in practically all cases. This makes it light and, at the same time, strong. It will stand the occasional jar or blow better than if it were of cast metal. Being in the lowest position of the parts making up the power plant, it not infrequently is struck by flying stones or receives a blow from some other cause, especially in the out-of-way places and in rough going.
The design and construction of flywheels, on first glance, would seem to be of slight interest. As a matter of fact, much of the stamina of the engine, as well as its flexibility and pickup, is dependent on the flywheel. In the first place, it is essential to the smooth operation of the engine. It stores up the energy received from the explosions within the cylinder, and gives it off at those points where the engine develops no power, otherwise the engine would not run. The first explosion would drive it to bottom dead centre, and without the flywheel to carry it on, it would stop there. High-speed racing engines use very light flywheels.
This duty of keeping the engine turning at one time was of greater importance than in later design. Much more weight is generally carried in the crankshaft than formerly, and where counterbalances are used, they serve to store and give off the required energy. Some manufacturers use two flywheels, one on each end of the crankshaft. This has the effect of placing the entire job in more even balance — similar to the result obtained by the use of counterbalances.
The flywheel serves the purpose of acting as a mounting for the starter ring gear. Sometimes the teeth are cut directly in the flywheel metal, and again the wheel is machined to receive a toothed ring. In the latter case, the ring only is replaced in case of damaged teeth.
The tendency of a rubber band, which has been wound up by twisting, to unwind as soon as released, as in the case of a toy airplane, is well known. If a yardstick is grasped with one hand on each end it is possible to twist or wind it. As soon as pressure is released it snaps back. So it is with a crankshaft. The force tending to wind it is delivered from the piston through the connecting rod on each explosion. Unless a damper is provided it snaps back to position so rapidly as to set up engine vibrations.
Dampers may be placed on the forward end of the shaft outside the crankcase or on one of the forward throws of the shaft within the engine case. They operate on the principle of gradual release of the power stored on the wind-up, allowing the shaft to unwind slowly. Try this with the yardstick, first winding it sharply and then releasing suddenly, and then, on the second trial, hold both ends and release gradually. The higher the engine speed the greater the tension to hold against severe vibration, and the lower the engine speed the less friction required and induced.