Internal Combustion Engine Theory: A First Look
What Is An Engine?
Simply speaking, an engine is a group of related parts that are assembled in a way to convert energy into motion that, in turn, can be harnessed to do work. Gasoline engines are internal combustion devices that use gasoline as the energy source. Let's build one.
If we were to take a sturdy tin can and spray some gasoline into it, then place a lid over the top and light it through some hole in the side, the explosion would blow the lid quite high into the air. The reason is that the gasoline vapor mixed with air and created a very explosive mixture that in turn created tremendously hot gases that needed to be released. In this case the top blew off.
Now that we know we can move something with the explosion, why not create a mechanical device that can do some work? Therefore, if we hook that can lid to a rod that is connected on its other end to a crankshaft — a device that uses eccentric journals to convert reciprocal motion to rotary motion — and then invert the can and rigidly support it, then the explosion we generate will push the lid and its rod downward, rotating the crankshaft.
Without going into the obvious flaws in the above design, let's move on to how gasoline engines are put together.
Instead of a can we need a strong, rigid cylinder that is drilled into an even stronger piece of metal, usually a heavy casting of iron or aluminum, normally called the cylinder block.
Since we are casting the block, why not design into the casting some passages that can be filled with water to help keep a constant temperature when the engine is running? Also, let's design in some other passages to allow oil to be pumped to lubricate the moving parts.
We can machine the block after casting to create a uniform cylinder of a known dimension. Also, we can drill and tap various holes into the block to accommodate the fasteners that will eventually hold the engine parts together.
The Piston The part that we're going to move with the fuel explosion is the piston. Although older cars used iron pistons, all post-WWII pistons are made of cast or forged aluminum, and are sized to be approximately 10 thousanths of an inch smaller than the cylinder dimension to allow for thermal expansion. Pistons need to be tall enough to avoid tipping sideways as they move up and down the cylinder. They also need to be light to lessen the inertial forces they undergo, so all modern pistons are hollow.
The piston as described above would move quite well down the cylinder bore, but a significant amount of explosive force would "leak" past the sides. In order to limit this problem, pistons are grooved around their circumference with several spaced-apart channels. In these channels are placed spring-steel or iron rings that continuously exert pressure against the cylinder wall, sealing out most of the combustion gases. Most pistons have two compression rings and one oil ring.
About midway down the piston is drilled a precision-sized hole through the diameter to hold the wrist pin, or piston pin.
The Rod The piston is connected to the crankshaft by means of the connecting rod. This is a heavy-duty, high quality device that is pushed down by the piston. At its other end is a circular bearing retainer that rides on the crankshaft as it moves in its eccentric circle. The rod is connected to the piston through the piston pin.
Why does the piston need a pin? Well, if you think about it, the piston travels straight up and down but the rod swings back and forth following the crankshaft. Therefore, the rod must be able to move where it is fastened to the piston and the pin allows that pendulum-like motion.
The Crankshaft The heart of the engine is, arguably, the crankshaft. It has to be strong enough to handle the tremendous force of the moving rod along with the other loads it endures. At its front end there is a pulley that drives the engine's accessories and at the rear end is the flywheel that drives the transmission and remaining drive train. The engine block has cast into it the points on which the crankshaft spins, called journals. In a one-cylinder engine the crankshaft has to be supported in at least two journals, since the forces generated by the explosions need to be distributed in all directions equally without moving the crankshaft's center line.
Bearings Obviously, any moving part like the crankshaft, rod, wrist pins, etc. needs to be designed to create as little friction as possible, otherwise the whole thing would scrape, gall, and heat up so much that the parts would weld themselves together. To accomplish this manufacturers use bearing materials and oil. Crankshaft and rod journals contain bearing inserts made of molybdenum and other special materials, between which run thin pressurized films of oil. More about that later...
Now We Have An Engine, Right?
No, we don't. What we have so far is a block, crankshaft, piston and rod assembly that will turn, but we don't have any way of sealing off the cylinder at the top so that the explosion can drive the piston downward. Several other things are missing as well, such as fuel delivery, exhaust expulsion and an ignition source, but we'll get to that in good time. First, we need to create a combustion chamber in order to have a place for the fuel/air mixture to ignite.
To do this in its simplest form, all we really need to do is bolt a flat piece of heavy metal to the top of the cylinder, leaving space between it and the top of the piston. We'll call this the cylinder head. The cylinder head is removable, of course, but we can't take it off and spray some fuel in, then put it back on and ignite the fuel each time we want a power stroke of the piston, can we? We need a port of some kind to let in fuel and another port to let exhaust out.
The Valves Okay, we've machined a couple ports in the cylinder head, one to let in fuel/air and the other to let out exhaust, but if our engine is going to work we need to have a way to seal them up at the right time. This is done with valves, which are strong metal objects that consist of a stem and a wide, tapered head. We need intake and exhaust valves. The angle of the taper on the valve head is duplicated in its "seat" in the cylinder head port, thus providing a sealing surface.
In order for the valves to properly seal the ports in the cylinder head we need to place springs around the stems and attach them with a clip of some type, commonly called a "keeper."
We are getting closer to a working engine, but we need to figure out some way to open and close the valves at the right time. We'll discuss that shortly, but first we need to address the various cycles the engine must go through, commonly called strokes.
Typical valve assembly for a flathead engine.
All we need to do now is to turn the camshaft and time the valve openings to the correct stroke. Since the crankshaft is turning, why not connect it to the camshaft by either a system of gears or a chain? That's just what manufacturers do, and the components are machined and marked so that during assembly the correct relationship between piston stroke and valve opening, valve timing, is maintained.
An important thing to remember, however, is that the camshaft speed is one-half of the crankshaft speed. In a four-stroke engine each valve opens only every other revolution, so the camshaft rotates one turn for every two turns of the crankshaft.
The mass of the flywheel helps smooth out the motion of the crankshaft.
The flywheel's mass is used to absorb engine vibrations and to keep the crankshaft turning over the next three strokes, thus making the engine run smoothly.
The engine we've assembled above will vibrate pretty harshly, since there's one big power stroke and four changes of piston direction every two revolutions. We need something to dampen out the vibrations. Also, we need something heavy attached to the crankshaft to assist inertial forces in keeping the crankshaft turning long enough to go through all four strokes. In addition, we need something attached to the crankshaft on which we can attach the necessary parts to transfer the engine's power, not to mention start it in the first place.
All these problems can be solved by the use of a flywheel, a large heavy disk that is bolted to the back of the crankshaft. On the circumference of the flywheel we can mount a toothed ring gear that can be engaged by the electric starter. The flywheel's face can be machined and tapped to accept a clutch assembly or, in the case of automatic transmissions, the flywheel itself can be of a lighter design (called a flex plate), augmented by the transmission's torque converter.
One More Thing
Since the crankshaft has a flywheel at one end, its mass will tend to produce forces that want to cause the crankshaft to twist somewhat at the other end, causing vibration. To counteract that vibration, manufacturers utilize a specially-designed balance disk that is attached to the front end of the crankshaft, called a harmonic balancer. This disk is typically made up of two separate pieces that are imbedded in rubber or some synthetic compound. The rubber absorbs differential movement of the two pieces. The size and weight of the harmonic balancer is dependent upon a specific engine's design.
Regardless of an engine's size (displacement), number of cylinders, shape of cylinder bank(s), horsepower, etc., it will contain the same basic parts as the engine we've discussed here. The parts may be arranged in different ways and locations in/on the engine, but you will always find the basic parts, just more of them. A four-cylinder engine will have four pistons, eight valves (at least!), eight lifters, and on and on and on...
Gasoline liquid doesn't burn, but gasoline VAPOR burns, and how! We need to do everything possible to create lots of vapor, starting with mixing the gasoline with air in an ideal ratio - around 14 parts air to 1 part gasoline.
Because the piston (and its rings) in an engine forms a pretty good seal, the fuel/air mixture can be compressed. Under compression the fuel droplets break up into even smaller particles and the temperature of the fuel/air mixture rises, making it easier to ignite. So, if we introduce fuel and air into the cylinder when the piston is down at the bottom and then close the intake valve, it will compress the mixture to the maximum extent.
Hey! If the piston can compress the mixture, that means that when it's moving down the cylinder it can create a vacuum, right? That's right, and we can use that vacuum to draw in the fuel/air mixture by opening the intake valve before the piston starts down.
Now we're getting somewhere. Assuming we're cranking the engine with the starter, the first stroke we will encounter is the intake stroke. The flywheel turns the crankshaft, pulling down the rod and the piston. Simultaneously, we've opened the intake valve, letting in the fuel/air mixture drawn in by the vacuum. The piston reaches the bottom of the cylinder and we close the intake valve.
The piston comes up, compressing the mixture, and completing the compression stroke. When it reaches the top we can ignite the mixture. The gasoline/air mixture explodes with a flame-front (the speed at which the explosion happens) of 2500 feet/sec, roughly the same explosive speed as dynamite.
That explosion forces the piston down in the power stroke. Now the engine is running itself. When the piston reaches the bottom of the cylinder, inertia of the crankshaft and flywheel forces continued rotation. If we open the exhaust valve at this point the upward travel of the piston pushes the burned gases out, creating the exhaust stroke.
There you have the standard, four-stroke internal combustion engine. The four strokes — intake, compression, power, exhaust — each account for one-half turn of the crankshaft. It is interesting to note that the four strokes take two complete turns of the crankshaft, during which only one-fourth of the time the engine is creating power.
How Do The Valves Open?
It should be obvious at this point that we haven't devised a way to open and close the valves. Clearly, we want to have the crankshaft do this work rather than trying to manually open and close them. Engine designers long ago solved this problem.
If we machine a round shaft that lies beneath the valve stem and set its ends in bearings, we have the start of something that will do the job for us. By machining a bump on the shaft, called a cam lobe, the bump can be used to push the stem up as the shaft turns. The size of the bump dictates the amount of lift and therefore the amount of time the valve will be opened. This shaft is called the camshaft.
For reasons of economy we don't want to machine two camshafts, one for the intake valves and another for exhaust valves. Instead, we can place one camshaft in a central location and on that shaft we can machine the intake and exhaust lobes in their proper places. Since we don't want to extend the valve stem or bend it to go over to the camshaft, we can machine a round, bar-like unit that will follow the cam lobe and in turn push the valve stem. This device is called a valve lifter. If we add some sort of length-(page 17 drawing) adjusting mechanism between the lifter and stem, what we now have is the valve train, consisting of the camshaft, lifter, adjuster, stem, spring and keeper.