The text following describes the four-stroke-cycle during ideal conditions. It is hypothetical. It implies that the working medium is just air and therefore no chemical reaction occurs during the cycle. It implies that the cycle is adiabatic, that no heat is either gained or lost during the cycle. It also implies that the working medium has no mass and therefore no inertia.
One stroke is defined as the piston moving from the point where it is highest in the cylinder to the point at which it is the lowest in the cylinder, or vice-versa.
I would like to straighten out some terminology and give a little history. There is no such thing as a 4-cycle engine. It is properly termed a 4-stroke-cycle engine, or just 4-stroke for short. The piston travels through 4 strokes in order to complete 1 cycle. A cycle is defined as the succession of operations in the engine cylinder, which constantly repeats itself. This is also known as the 'Otto' cycle as it was originally patented by Nicholas A. Otto in 1876. Its first practical demonstration was made in 1878. Credit should be given, however, to Beau De Rochas, a French railroad engineer. In 1862 De Rochas published a pamphlet describing the cycle, many years before Otto's patent.
1. Intake Stroke
The first stroke is called the 'Intake' stroke. The stroke begins when the piston is at TDC (Top Dead Center, the highest the piston can go). At this time the intake valve(s) are open and the piston has just begun moving downward. The pressure differential caused by the downward moving piston forces the air/fuel mixture through the lower pressure intake tract and into the combustion chamber and cylinder. This pressure differential is caused by the 200 miles of atmosphere above us, which weighs 14.7 pounds per square inch (at sea level).
2. Compression Stroke
The second stroke is called the 'Compression' stroke. After the piston has reached BDC (Bottom Dead Center, the lowest the piston can go) it begins moving upward. At this point both valves are closed. The rising piston reduces the total volume available in the cylinder/combustion chamber. This leads to a rise in pressure that is proportional to the amount the air is compressed.
3. The Power (Combustion) Stroke
The third stroke is called the 'Power' stroke. After the piston has again reached TDC the power stroke begins. Both valves are closed and ignition begins. This forces the piston down and is where the power is produced.
4. The Exhaust Stroke
The fourth stroke is called the 'Exhaust' stroke. It begins when the piston has again reached BDC. The exhaust valve(s) is open during this stroke. The piston begins moving up and forces the exhaust gases out of the cylinder. When the piston reaches TDC the cycle starts over, at the intake stroke.
A way to remember . . .
A rather odd sounding yet easy way to remember the four strokes is by the term 'Suck-Squeeze-Bang-Blow'. These represent the four strokes, 'Intake-Compression-Power-Exhaust'.
The Six-Stroke Cycle - What is really happening?
In reality, if the valves opened at exactly at either TDC or BDC the engine would run poorly and would not be able to rev very high. Since heat will be created and the working medium is an air/fuel mixture, a mechanism is required to accommodate these. The actual 'strokes' do not start at TDC or BDC. They are defined by the points at which the valves open and close.
The extra two 'strokes' are not strokes at all. They are actually events separate from the four strokes, however they are very important events during the cycle. The first event is known as 'overlap' and happens at TDC when both the intake and exhaust valve(s) are open. This occurs at the end of the exhaust stroke/beginning of the intake stroke. The second event is called 'blowdown' and spans the time from when the exhaust valve(s) opens until the piston has reached BDC on the power stroke.
1. Intake Stroke
The intake stroke begins when the intake valve(s) opens. This may be a number of BTDC (before top dead center). As an example we will say it starts 30 degrees BTDC (before top dead center). When the intake valve(s) starts opening the exhaust valve(s) are also opening, but it is closing quickly. The intake stroke continues as the piston reaches TDC and then until it is in the bottom of the cylinder at BDC and on until the intake valve(s) closes sometime ABDC (after bottom dead center). As an example we will say it ends 60 degrees ABDC. The reason it closes ABDC is because due to the inertia of the incoming air/fuel charge it is possible to use this inertia to fill the cylinder to more than 100% (by weight), a sort of inertial supercharging.
2. Compression Stroke
The compression stroke begins as soon as the intake valve(s) closes, 60 degrees ABDC in our example. The piston is rising in the cylinder. When it reaches TDC the compression stroke stops. The reason it stops at TDC is once the piston has crested and begins moving downward the medium is no longer being compressed because the piston is now moving downward and is increasing the volume of the combustion chamber/cylinder. As you can imagine, the engine cannot compress the charge until both valves are closed, meaning there is nowhere for the gases to leak out. The compression ratio that is measured from the point where the intake valve(s) closes is called the effective compression ratio and is explained later.
3. Power Stroke
The power stroke begins at TDC. Sometime before TDC the spark plug has ignited the air/fuel mixture in the cylinder. We will say this has occurred 20 degrees BTDC. The cylinder pressure and temperature is rising rapidly due to the burning of the air/fuel mixture and, once the piston is moving downward, begins doing work on the piston. This forces it downward and the connection rod and crankshaft convert it from reciprocating motion to rotary motion. The power stroke ends when the exhaust valve(s) opens.
4. Exhaust Stroke
The exhaust stroke begins the moment the exhaust valve(s) opens. As an example we will say this happens 60 degrees BBDC (before bottom dead center). The stroke continues as the piston reaches BDC, while it moves from BDC back up to TDC, and then on until the exhaust valve(s) closes. We will say it closes 30 degrees ATDC (after top dead center).
5. The '5th Stroke'
You may have noticed that there is a period of time when both the intake and exhaust valves are open. This is sometimes referred to as the '5th Stroke'. It is also known as 'Overlap' or 'Valve Overlap'. Overlap occurs from the time the intake valve(s) opens until the exhaust valve(s) closes. As it begins the exhaust valve(s) is open a little and the intake valve(s) is just starting to open. As the intake opens more, the exhaust closes more. Overlap ends when the exhaust valve(s) closes completely. During overlap the piston is at TDC and is not moving very much relative to the number of degrees the crankshaft is turning. This phenomenon when piston motion has nearly stopped is known as 'Piston Dwell' or just 'Dwell'. Having the intake valve(s) open as well as the exhaust, combined with dwell creates a time when the air/fuel mixture is entering through the intake valve(s) and the exhaust gases are moving out the exhaust valve(s). The inertia of the air/fuel charge helps it push the burnt gases that remain in the cylinder, known as 'clearance' gases, out the exhaust. This is important because any burnt gases that are allowed to stay in the cylinder take up space where air/fuel mixture could have been. In addition, the high temperature of the clearance gases will heat up the incoming air/fuel charge making it less dense. It may also lead to detonation. Using the inertia of the gases leaving the cylinder it is possible to actually help the incoming air/fuel mixture to be drawn into the cylinder. It is also possible to use pressure waves that are traveling up and down the exhaust to help draw in fresh charge - more on that later.
6. The '6th Stroke'
The time allowed for creating power is from TDC to the point that the exhaust valve(s) opens, 60 degrees BBDC in our example. This only gives 120 degrees in which the pressure in the cylinder pushes the piston down and creates power. This may seem counter-productive but in reality it allows the engine to make more power. As the exhaust valve(s) opens any pressure left in the cylinder is blown out the exhaust. This period is known as the 'Blowdown Period' or just 'Blowdown'. The idea here is to get as much pressure as possible out the exhaust before the piston hits BDC and begins moving upward. If the exhaust valve(s) were to open at BDC the piston would be responsible for pushing all the burnt gases out the exhaust valve(s). The pressure in the cylinder would actually be pushing on the piston as it comes up, causing it to resist moving upward. This resistance is termed 'Pumping Losses'. A pumping loss occurs anytime the engine has pressure above ambient acting on the piston while it is moving upward. Therefore it is extremely important to get as much pressure out of the cylinder before the piston begins moving up on the exhaust stroke. It is a fine balance between holding the pressure in the cylinder to force the piston down, creating power, and opening the exhaust valve(s) to get the pressure out of the cylinder to avoid pumping losses. This compromise is one factor to consider when choosing a camshaft.
