What is volumetric efficiency?

Short Answer:

Volumetric efficiency is the ratio of the actual volume of air or air-fuel mixture drawn into the engine cylinder during suction stroke to the theoretical volume that the cylinder should ideally take in. It shows how effectively the engine can fill its cylinder with fresh charge. Higher volumetric efficiency means better engine breathing and performance.

In simple words, volumetric efficiency tells how much air the engine can actually take in compared to what it could ideally take. It depends on factors like air temperature, pressure, valve timing, and engine speed.

Detailed Explanation :

Volumetric Efficiency

Volumetric efficiency is an important performance parameter of internal combustion (IC) engines. It measures the ability of the engine to inhale the air-fuel mixture (in petrol engines) or air (in diesel engines) during the suction stroke. In ideal conditions, the cylinder should fill completely with air at atmospheric pressure and temperature during the intake process. However, due to various real-life losses such as flow restrictions, valve timing, and heating of air, the actual amount of air entering the cylinder is always less than the theoretical volume.

Mathematically, volumetric efficiency is expressed as:

This ratio is generally expressed as a percentage and gives a direct indication of the breathing ability of the engine.

Meaning of Volumetric Efficiency

The theoretical volume refers to the swept volume or displacement volume of the engine cylinder, which is the total volume displaced by the piston from top dead center (TDC) to bottom dead center (BDC). The actual volume of air or air-fuel mixture that enters the cylinder is usually less than the swept volume because of several losses and restrictions in the intake system.

For example, if an engine cylinder of 1000 cc only draws in 850 cc of air during suction, the volumetric efficiency will be:

This means the engine can only take in 85% of the air it should ideally receive.

Factors Affecting Volumetric Efficiency

  1. Intake Pressure and Temperature:
    Lower intake temperature and higher pressure increase air density, which allows more air to enter the cylinder, improving volumetric efficiency. Conversely, hot intake air reduces efficiency.
  2. Engine Speed:
    At higher speeds, there is less time for air to enter the cylinder, which reduces the amount of charge drawn in, thus lowering efficiency. At very low speeds, poor scavenging also affects efficiency.
  3. Valve Timing:
    Proper opening and closing of intake and exhaust valves help the engine take in the maximum amount of air. Incorrect timing reduces air flow and volumetric efficiency.
  4. Throttle Opening (in Petrol Engines):
    When the throttle valve is partially closed, it restricts airflow and lowers volumetric efficiency. Full throttle operation improves air intake.
  5. Exhaust Gas Back Pressure:
    If exhaust gases do not leave the cylinder completely, the remaining gases reduce the space for fresh charge, decreasing volumetric efficiency.
  6. Air Filter Condition:
    A clogged or dirty air filter restricts airflow into the engine and reduces volumetric efficiency.
  7. Supercharging or Turbocharging:
    Using superchargers or turbochargers increases the intake air pressure, allowing more air to enter the cylinder and thereby improving volumetric efficiency.

Methods to Improve Volumetric Efficiency

  1. Use of Superchargers and Turbochargers:
    These devices compress the incoming air before it enters the engine, increasing its pressure and density. As a result, more air enters the cylinder, which enhances combustion and power output.
  2. Intercooling of Intake Air:
    Cooling the air after compression increases its density, allowing a greater mass of air to enter the cylinder, thus improving efficiency.
  3. Optimized Valve Timing:
    Modern engines use variable valve timing (VVT) to adjust valve opening and closing times according to engine speed, ensuring better airflow and higher volumetric efficiency.
  4. Reducing Air Flow Resistance:
    Smooth intake manifolds and clean air filters minimize air resistance and improve air entry.
  5. Efficient Scavenging in Two-Stroke Engines:
    Proper removal of exhaust gases allows more fresh charge to enter the cylinder, improving volumetric efficiency.
  6. Maintaining Lower Intake Air Temperature:
    Cooler air is denser, so more air mass can enter the cylinder. This can be achieved through intercooling or cold air intake systems.

Typical Values of Volumetric Efficiency

  • For naturally aspirated petrol engines, volumetric efficiency ranges between 70% and 85%.
  • For diesel engines, it is higher, usually around 85% to 90%, because they operate without throttling.
  • For turbocharged or supercharged engines, volumetric efficiency can exceed 100%, since pressurized air enters the cylinders.

This shows that volumetric efficiency directly affects the engine’s power output and performance.

Importance of Volumetric Efficiency

  1. Engine Power Output:
    Higher volumetric efficiency means more air-fuel mixture enters the cylinder, leading to more combustion energy and higher power output.
  2. Fuel Efficiency:
    Improved volumetric efficiency ensures proper air-fuel ratio and better combustion, resulting in less fuel wastage.
  3. Reduced Emissions:
    With better combustion, there is less unburned fuel, which helps in lowering exhaust emissions.
  4. Engine Cooling:
    Proper air intake helps in better combustion and heat management, reducing the chances of knocking and overheating.
  5. Performance Indicator:
    Volumetric efficiency is often used as a measure of the overall performance and breathing ability of the engine.

Example Calculation

If the swept volume of an engine is 2000 cc and the actual volume of air drawn into the cylinder is 1600 cc, then:

This means that only 80% of the theoretical volume of air has entered the engine. Improving the intake system or using turbocharging could raise this percentage.

Conclusion

Volumetric efficiency represents the breathing capacity of an engine and is a key factor in determining its performance and power output. It is the ratio of the actual air intake to the theoretical air intake under ideal conditions. Higher volumetric efficiency means the engine can take in more air, produce more power, and operate more efficiently. By optimizing valve timing, using superchargers, and maintaining proper intake conditions, the volumetric efficiency of an engine can be greatly improved.