Internal Combustion Engine Theory: A First Look - Page 2

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.

Summary

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...

Four-Stroke Cycle

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.