Lesson 1: ‘Understanding The Four Strokes In a Four-Stroke Engine’ (In a Nutshell)
The gasoline engine of today’s automobile is a four-stroke internal combustion engine. It develops its power by using a spark plug to ignite the fuel mixture. As the fuel mixture burns, the expansion of super-heated gases forces the piston down into the cylinder, rotating a crankshaft which the piston is connected to. The process repeats itself in each cylinder hundreds of times a minute; this rotational power is transferred to the rear wheels of the automobile.
Whenever I refer to an electrical switch as being “open”, that means it is “off” . Electricity can not flow of flowing at all.
When I refer to an electrical switch as “closed”, that means it is “on”, or the electricity can flow freely.
Put Ignition Key In, Turn To Crank
You turn the ignition switch to the start position. The ignition switch closes, bridging a small amount of battery current from your cars ignition switch to the “I” terminal on your starter solenoid. The starter solenoid coil is transformed into an electric magnet. This magnetic starter solenoid pulls a small pinion gear from your starter motor out so that it will mesh with the gears teeth on your engines flywheel which is bolted to the back of your crankshaft. Simultaneously, the same electric magnet pulls a large contact switch closed which allows full battery current to flow directly to the starter motor. The starter motor turns that little pinion gear we talked about, and it turns the flywheel and crankshaft causing the pistons to move up and down in their respective cylinders.
And there you have it. Piece of cake so far right? (Grin)
I’m just playing with you…trying to get you to use your noodle. Trust me…by the time I get done explaining everything to you, you’ll have more trouble trying to understand why you never figured this stuff out on your own. Okay. Good. Let’s get back to me making things simple for you again.
Now that paragraph explains the gist of what goes on under the hood of your car when you turn the ignition key. In a couple lessons down the road, we’ll attack that paragraph again but as a team next time. I’ll walk you through it piece by piece. But for now we need to get back to me explaining the basics, so I won’t have to stop to explain every little new term or new principle. That would be too confusing for you. You have heard the ole saying, “You need to have a strong foundation to build a house upon.”.
This is a prime example of that old adage.
Okay…let’s move forward to the basic terminology your going to be hearing a lot more of once I start explaining “the four-strokes”. That is what you came here to read right.
The pistons range of travel is called the stroke. Its lowest point of travel in the cylinder is called bottom dead center (BDC), as the crank continues to rotate it moves the piston up towards (TDC) top dead center, its highest point of travel. If you unbolted the top part of the engine, with any piston at its highest point of travel (TDC), the top of the piston would appear level with the top of the engine block. The combustion chamber would be an area about the size of a clam shell above each piston in that space inside the top portion of the engine called the “head”. This is the small space where the piston squeezes the entire cylinders volume into. Generally you can see the spark plug and both intake and exhaust valve protruding into the combustion chamber area.
As the crankshaft rotates, it rotates a camshaft as well on account of it being directly connected to the crankshaft by means of a gear or chain arrangement. Although the camshaft only rotates at half the speed of the crankshaft, it is considered to be the heart of the engine in that it opens and closes the valves according to piston /crankshaft positions. Major performance modifications are made with the camshaft.
The first stroke of the four-stroke internal combustion engine begins when the rotating crankshaft pulls a piston down the cylinder towards BDC. While that piston is moving downward towards BDC, the camshaft simultaneously opens the intake valve at the same time. The opened intake valve exposes the intake charge to the cylinders low pressure (vacuum) which causes the air and fuel mixture (14.7:1 respectively) at atmospheric pressure to be easily drawn into the cylinder at tremendous speeds reaching a few hundred feet per minute. This is known as the intake stroke.
This is the second stroke of this four-stroke process.
As the piston hovers around BDC getting ready its compressing climb back towards TDC, the camshaft closes the intake valve.
Recall the intake stroke left us with a cylinder crammed full of a volatile mixture of oxygen and gasoline. Now with both valves closed, the cylinder is now completely sealed. In this stroke the piston compresses the entire cylinder’s 40 cubic inches of finely atomized fuel charge into a 5 cubic inch combustion chamber.
As the piston continues to move up towards TDC, it packs its volume into an eighth of its original size, raising pressures and bringing temperatures up to just below the ignition point. The cylinder pressure reaches its near perfect combustible pressure of around 150 pounds or more per square inch — the ignition coil builds up its high secondary voltage and fires as the piston nears TDC. This is the compression stroke.
Power Stroke (Combustion Stroke)
This is the third stroke of the four-stroke fire breathing monster.
At the moment the piston reaches a predetermined point before TDC the coil unleashes 60,000 volts to the spark plug — the electrical spark arcs across the gap igniting the air/fuel mixture — the flame-front races smoothly out and across the combustion chamber — super-heated gases expand rapidly — temperatures rise — cylinder pressures quadruple to 600 psi — the piston is driven down into the cylinder by the expanding gases — connected to the crankshaft, the driven piston rotates the crank and cam assemblies. The reciprocating motion brings the engine to life.
The power stroke has just occurred.
Though the actual flame-front really does move fairly rapidly through-out the combustion chamber once the spark fires off the air/fuel charge, its just a slow process when compared to other volatile ingredients.The process by which the gasoline and air mixture burn during the combustion stroke is actually a slow-burn process by comparison to let’s say…gun-powder, which burns 150 times faster than the air/fuel mix — hence the term, slow-burn, is a very fitting term for our very sluggish automotive engine combustion process.
Surprisingly, most of today’s highly skilled auto technicians still believe the combustion process involves an explosion despite years of automotive research seeking ways to eradicate uncontrolled “explosions” out of the combustion process entirely.
It is however, a disputed concept but common sense and history would seem to dictate a smooth burn as opposed to an explosive one is better. The process of the automotive internal engine combustion is considered to be a slow-burn process at 3/1000th of a second, as opposed to the much more rapidly burning gunpowder…which burns 150 times faster at 1/50,000th of a second.
The fourth and final stroke of this four-stroke process has been responsible from everything to contributing to the smog that lingers over beautiful San Fransisco Bay to the melting of the giant ice caps at our planets poles. The piston reaches the bottom of its stroke (BDC) after the expanding gases of the power stroke force it there. Upon reaching BDC very quickly the exhaust valve now opens and the piston begins to rise again towards the top of its travel (TDC). This time the piston is pushing the high-pressure gases out into the low-pressure area inside the exhaust manifold. This movement cleanses the cylinder of all of its unburned gases and prepares the cylinder for another intake stroke where a fresh air/fuel charge will begin the four stroke process all over again. This is the exhaust stroke.
A piece of cake right?
Good, let’s review it again… briefly this time.
Four Stroke Review
As the crankshaft rotates it moves a piston up and down in a cylinder.
As the camshaft rotates, it opens and closes intake and exhaust valves.
On the Intake stroke, an intake valve opens.
The downward plunging piston creates a suction and pulls a fresh supply of fuel and air into the cylinder through the open intake valve. This is the Intake stroke.
The intake valve now closes.
The piston moves up the cylinder and compresses the air and fuel into a small chamber. This squeezing process causes all the air and fuel molecules to be compacted closer together. This is the compression stroke.
A spark plug ignites the tightly squeezed air and fuel mixture and causes it to burn and expand rapidly. The piston is forced downward in the cylinder from the hot expanding gases. This is the power stroke.
The piston moves upwards again now.
The exhaust valve opens.The rising piston forces all the spent gases out of the cylinder, out through the exhaust valve and out into the exhaust pipe of the automobile. This is the exhaust stroke.
And that is all there is to it.