A supercharger can be run in several ways. Superchargers are used to increase the volumetric efficiency of the engine. Supercharging does not result in any combustion problem, rather it improves combustion. It increases intake pressure and reduces temperature ignition delay. However, the supercharger has certain limitations. In this article, I will discuss the limitations of supercharging.
The power output of an engine is limited by knock, thermal, and mechanical loads. usually, one of these limits is reached earlier than the other limits depending upon the type of engine, its design of the structure, cooling arrangements, etc. For spark ignition engines knock limit is usually reached first while for diesel engines the thermal and mechanical load limits are reached first.
Limitations of supercharging in SI engine-
The degree of supercharging in SI engines is chiefly limited by the knock. The increase in pressure and usually in temperature also, because of supercharging reduces ignition delay, and consequently, the engine has a knocking tendency at these pressures.
The knock limit is dependent upon the type of fuel used, mixture ratio, spark advance, and the design features of the engine, of which the valve timing and cooling system are important. Different fuels have different knocking tendencies. Some are more sensitive to increased pressure, others to increase in temperature. For volatile petroleum fuels of high octane number the knocking and pre-ignition tendency is reduced at a very rich and very lean mixture, and the fuels of the same octane value have a different response to supercharging.
The strongest knocking occurs near chemically correct mixture ratios. Very rich and very lean mixtures give non-knocking operation. Usually, supercharged engines are run on rich mixtures to control knock because the lean limits of non-knocking are narrow and require very accurate control of mixture ratio. A slight reduction in lean mixture results in irregular and intermittent engine operation.
The knock limit of the SI engine is also affected by the ignition timings and the thermal load on the engine. At higher intake pressure and temperature, the ignition must be retarded. The temperature of the piston and the combustion chamber is important for the knocking and pre-ignition tendencies of an engine. Usually, for SI engines, the knock limit is reached earlier than thermal load limits, but still latter is quite significant.
The thermal load can be reduced by better cooling or by increasing the valve overlap period so that the clearance volume is scavenged. This results in the reduced temperature of the intake charge because of the passage of air through exhaust and inlet manifolds. A clearance volume scavenged engine thus gives more power at the same thermal load. This effect is more for air-cooled engines.
Limitations of supercharging in CI engine-
The limits of supercharging for compression ignition engines are not due to combustion. The factors that limit the output of the SI engine due to knock, result in a quieter and smoother operation of the CI engine. Any increase in pressure and temperature decreases the ignition delay and the engine runs smoother. The limit of supercharging for the CI engine is reached by thermal loading.
The temperature of the piston and cylinder, if very high, results in scuffing of piston rings and heavy liner wear. The load on bearings is increased due to increased pressure in the cylinder. Durability, reliability, and fuel economy are the main considerations in limiting the degree of supercharging of the CI engine. An increase in maximum pressure in the cylinder decreases the reliability of the engine.
Conclusion–
In this article, I describe the limitations of supercharging. Supercharging is a method of increasing the power output of an internal combustion engine. Please follow us for more details.
