What is Volumetric Efficiency?
Volumetric Efficiency is one of the most important factors of an internal combustion engine' performance parameters. It is the ratio of the volume of air/charge drawn into the cylinder/s during the suction stroke to the total displacement of all the cylinder/s at the atmospheric pressure. In simple words, it denotes the engine’s ability to carry out the work efficiently. However, it depends upon the engine speed and load. It also largely depends upon the layout of the inlet and exhaust systems, sizes of the valve ports and valve timing.
A supercharged or turbocharged engine has greater volumetric efficiency than a Naturally-Aspirated (NA) engine which does not use the forced induction method. This is because the turbocharger forces the air into the cylinder at more than the atmospheric pressure. However, the volumetric efficiency of a turbo/supercharged engine depends upon the temperature and pressure in the intake manifold.
The volumetric efficiency of an internal combustion engine indicates the efficiency with which it can move the charge in and out of the cylinders. Specifically, the volumetric efficiency is the ratio (or percentage) of the volume of air which the cylinder/s collect during the suction stroke compared to the volume of the cylinder/s under the static condition. Most naturally aspirated (NA) engines attain the max. efficiency of about 80% at certain engine speed. Generally, most engines attain the highest volumetric efficiency somewhere around their peak torque value i.e. between 2000-3000 RPM.
How to improve the Volumetric Efficiency?
The vehicle manufacturers try to improve the Volumetric Efficiency of an engine in many ways. However, this can be done most efficiently by compressing the incoming charge by using the forced induction method. Hence, the turbocharged/Supercharged engines have more than 100% Volumetric Efficiency. There are many ways by which the engine manufacturers can increase the volumetric efficiency. Alternatively, the manufacturers also use another mechanism called ‘forced cam-phasing’ in the naturally aspirated engines. Most racing cars use this type of mechanism.
A number of high-performance cars use carefully designed and placed air intakes and tuned exhaust systems to improve the Volumetric Efficiency. This arrangement uses pressure waves to push more air in and out of the cylinders. It also brings into action the resonance of the system. Mostly, the Two-stroke engines make use of this model. Some designs also use expansion chambers to reroute the air-fuel mixture back to the cylinder that was emitting it out.
However, the 4-stroke engines deploy a more modern technology called ‘Variable Valve Timing’. It effectively changes the Volumetric Efficiency with the change in speed of the engine. At higher speeds, the engine valves need to open for a longer period of time to move the air/charge in and out of the engine. Thus, the ‘Variable Valve Timing’ helps to achieve this effectively.
Volumetric Efficiency vs Mechanical Efficiency:
Most customers prefer a vehicle with maximum power or better power-to-weight ratio as compared to other vehicles. However, the engine’s mechanical efficiency is also very important. The following paragraph explains what is the mechanical efficiency of an engine and how it affects the power output.
The 'Mechanical Efficiency' is the measured (theoretical) efficiency of the engine compared to its actual (rated) efficiency. However, the engine cannot deliver 100 percent output that it produces. This is because of various factors such as thermal losses to the cooling and exhaust systems, the operation of valves and bearings, and wear and tear of the parts etc. The engine developers cannot achieve an ideal efficiency in the real world. So, instead, some manufacturers specify the mechanical efficiency based on the maximum result from the engine’s performance.
You can achieve 100 percent mechanical efficiency only when the output power is the same as the input energy of the engine. However, the engine loses some of its power while moving the wheels. Some of the power loss takes place through the clutch, transmission, axles, and bearings. Thus, in the end, the output results in lower mechanical efficiency.