The Four-Stroke Cycle
The intake stroke starts with the piston at the very top of the cylinder, which is called top dead center (TDC). The rotating crankshaft pulls the piston down in the cylinder bore which creates a vacuum. During the intake stroke, the intake valve is opened by the valve train to allow atmospheric pressure to fill the cylinder with an air-fuel mixture.
The ability to fill the cylinder with an air/fuel mixture is measured as volumetric efficiency (VE). During lower revolutions per minute (RPM), VE can approach 100% but as RPM increases the VE will drop. At higher RPM the VE could fall to as low as 80% or more. This is why the horsepower of a naturally aspirated engine will drop at higher RPM.
The intake stroke requires 1800 of crankshaft rotation and ends with the piston at bottom dead center (BDC). It will take two revolutions of the crankshaft, 7200 of rotation, to complete the four-stroke cycle.
The compression stroke begins with the piston at BDC, the cylinder full of an air/fuel mixture and both valves closed. The rotating crankshaft pushes the piston upward towards TDC compressing the air/fuel mixture into the combustion chamber, which is part of the cylinder head.
The volume above the piston at BDC compared to the volume above the piston at TDC is defined as the compression ratio. High-performance engines typically have higher compression ratios; however, higher compression ratios create more heat in the combustion chamber. This extra heat could ignite the air/fuel mixture before the ignition spark, creating spark knock and even engine damage. This is why most engines with a high compression ratio require a higher octane fuel to prevent pre-ignition.
The compression stroke requires 1800 of crankshaft rotation and ends with the piston at TDC. It will take two revolutions of the crankshaft, 7200 of rotation, to complete the four-stroke cycle.
The power stroke begins with the piston at TDC, both valves closed, and the combustion chamber full of an air/fuel mixture. The air/fuel mixture is ignited by an electrical spark from the spark plug and the rapid burn of the mixture creates a high pressure that pushes the piston down towards BDC.
The power stroke requires 1800 of crankshaft rotation and ends with the piston at BDC. It will take two revolutions of the crankshaft, 720 degrees of rotation, to complete the four-stroke cycle.
Power is only produced during the power stroke. The other three strokes are ultimately robbing power. To help smooth out the power impulses, all automotive engines use multiple cylinders.
All the cylinders that share a common crankshaft will fire in two revolutions (7200); therefore, engines with more cylinders produce smoother power due to more power impulses.
This chart shows the degrees of crankshaft rotation between power strokes for each cylinder configuration used in the automotive industry.
|4 Cylinders||Power Stroke Every 1800|
|5 Cylinders||Power Stroke Every 1440|
|6 Cylinders||Power Stroke Every 1200|
|8 Cylinders||Power Stroke Every 900|
|10 Cylinders||Power Stroke Every 720|
|12 Cylinders||Power Stroke Every 600|
The exhaust stroke begins with the piston at BDC and the exhaust valve open. The rotating crankshaft pushes the piston upward towards TDC and the exhaust gases are pushed out the open exhaust valve and into the exhaust system.
The exhaust stroke requires 180 degrees of crankshaft rotation and ends with the piston at TDC. It will take two revolutions of the crankshaft, 720 degrees of rotation, to complete the four-stroke cycle.
The 4 Stroke Cycle Animated
This animation shows the 4-stroke cycle as covered earlier in this lesson. The strokes are numbers as follows:
- Intake Stroke
- Compression Stroke
- Power Stroke
- Exhaust Stroke
Wikipedia – Four Stroke Cycle
Watch the Lesson Video (at top of lesson) to activate the “Take Quiz” button.