Volumetric efficiency is the measure of success with which air supply is inducted into an engine. This is a very important parameter because it indicates the breathing capacity of the engine. Specifically, volumetric efficiency is defined as the volume flow rate of air into the intake system divided by the rate at which the volume is displaced by the system.
Volumetric efficiency is one of the most important parameters which determine the performance level of a four-stroke engine. Because four-stroke engines have a particular suction stroke, volumetric efficiency is an indicator of the engine's breathing ability (the ability to which the engine utilizes the air and puts out power), according to "Internal Combustion Engines" by V. Ganesan. More powerful and more effective engines take in the most air.
The definition of volumetric efficiency can be phrased in many different ways but the general idea is the same, according to V. Ganesan. It is the ratio of the actual mass of air taken into the engine during a given period of time to the theoretical mass of air which should have been taken in during that same period of time, based upon the total piston displacement of the engine, and the pressure and temperature of the surrounding atmosphere.
Volumetric efficiency is primarily affected by four variables. These variables include the density of the fresh charge, the exhaust gas in the clearance volume, the design of the intake and exhaust manifolds, and the timing of intake and exhaust valves.
Density of Fresh Charge
As air supply, or fresh charge, arrives in the hot cylinder, heat is transferred to the cylinder from the chamber walls and the hot residual exhaust gases. This raises the temperature of the cylinder, creates a decrease in the mass of fresh charge admitted, and reduces the engine's volumetric efficiency. Volumetric efficiency is increases with increased density (or lower temperature and higher pressure of fresh charge).
Exhaust Gas in Clearance Volume
When the piston moves from certain exhaust products on the intake stroke, the products expand and occupy a portion of the piston displacement greater than the clearance volume. This reduces the available space to the incoming charge, according to V. Ganesan, thus negatively affecting the volumetric efficiency.
Design of Intake and Exhaust Manifolds
The intake manifold needs to be designed so that it brings in the maximum possible fresh charge while the exhaust manifold needs to be designed so that it allows for exhaust products to properly escape, according to V. Ganesan. If the manifolds are not designed properly, volumetric efficiency is negatively affected.
Timing of Intake/ Exhaust Valves
Valve timing is the regulation of the points in the cycle at which the valves are set to close and open, according to V. Ganesan. Time should also be allotted for the actual opening and closing of the valve. Valve timing affects the engine's air capacity and the mass of air that can be taken into a cylinder (thus affecting the engine's volumetric efficiency).
- Photo Credit "Jaguar V12 engine" is Copyrighted by Flickr user: dave_7 under the Creative Commons Attribution license.
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