Variable compression ratio
Variable compression ratio
The compression ratio of an internal combustion engine refers to the ratio of the maximum volume of the combustion chamber (when the piston is at the bottom dead center) to the minimum volume of the combustion chamber (when the piston is at the top dead center). A higher compression ratio can lead to improved fuel efficiency and power output, but it can also increase the risk of engine knock or pre-ignition. Therefore, variable compression ratio (VCR) engines have been developed to allow for optimization of the compression ratio for different driving conditions.
A VCR engine is an engine that can vary its compression ratio while the engine is running. This is accomplished by using a mechanism that can change the distance between the piston and the cylinder head. There are two main types of VCR engines: those that change the piston stroke length, and those that change the position of the cylinder head.
One way to change the piston stroke length is to use a "sliding crankshaft" mechanism. This mechanism allows the crankshaft to slide up and down within the engine block, which changes the stroke length of the piston. Another way to change the piston stroke length is to use a "split crankshaft" mechanism. In this design, the crankshaft is split into two parts that can move independently of each other. By changing the position of the two parts, the piston stroke length can be adjusted.
Changing the position of the cylinder head is another way to vary the compression ratio. This can be done using a "wedge" mechanism, which raises or lowers the height of the cylinder head. Another way to change the position of the cylinder head is to use a "swinging arm" mechanism. In this design, the cylinder head is attached to a swinging arm that can pivot around a fixed point. By adjusting the position of the swinging arm, the position of the cylinder head can be changed.
There are several advantages to using a VCR engine. First, it allows for optimization of the compression ratio for different driving conditions. For example, a higher compression ratio can be used when the engine is operating at high load or high speed, which can improve fuel efficiency and power output. A lower compression ratio can be used when the engine is operating at low load or low speed, which can reduce the risk of engine knock or pre-ignition.
Another advantage of VCR engines is that they can be used with a wider range of fuels. Different fuels have different octane ratings, which is a measure of their resistance to pre-ignition. By adjusting the compression ratio, a VCR engine can be optimized for different octane ratings, which can improve fuel efficiency and reduce emissions.
Finally, VCR engines can also improve the overall efficiency of the engine. By optimizing the compression ratio, the engine can operate more efficiently, which can reduce fuel consumption and emissions.
In conclusion, a variable compression ratio engine is an engine that can vary its compression ratio while the engine is running. This is accomplished by using a mechanism that can change the distance between the piston and the cylinder head. There are several advantages to using a VCR engine, including optimization of the compression ratio for different driving conditions, the ability to use a wider range of fuels, and improved overall efficiency.