Explaining Volumetric Efficiency

[mikelv]

Before we start, I would like to thank one of our very knowledgeable and helpful members Zippyshoe, who inspired me to write this article after I read his titled:

“Tips on Maximizing Engine Horsepower”


For all DSM’ers out there, this is a must read! I felt VE was an important factor, and wanted to make an addition to his research, allowing users to get a better understanding of this concept. If you can maximize your VE, you can maximize your engine power. Thanks!


Okay, let’s begin. An engine is powered by three basic components that transfer combustion into useable energy: Air, Fuel and Spark. The key proponent to maximizing your engine’s power is to maximize airflow. Mass Air Flow is defined as volume of air entering the engine for a given time period, (usually measured in grams/sec or lbs/min) that is used by the engine for a given time period, (commonly measured in rotations per minute, RPM).

The four primary factors to help maximize airflow are:

1. Intake Manifold Pressure (Boost)
2. Engine Displacement
3. Volumetric Efficiency
4. Engine RPM

This article will focus on Volumetric Efficiency and how to tune and utilize its value and purpose.

Volumetric Efficiency:

Volumetric Efficiency is defined as the charge into and out of the cylinders. More correctly, volumetric efficiency is a ratio (or percentage) of what volume of fuel and air actually enters the cylinder during induction to the actual capacity of the cylinder under static conditions. Therefore, those engines that can create higher induction manifold pressures - above ambient - will have efficiencies greater than 100%.
Volumetric efficiencies can be improved in a number of ways, but most notably the size of the valve openings compared to the volume of the cylinder and streamlining the ports. Engines with higher volumetric efficiency will generally be able to run at higher RPM and produce more overall power due to less parasitic power loss moving air in and out of the engine.
As I mentioned before, one of the most common ways to increase VE, (more air/fuel) is to increase the size of the intake valves and getting the port work done on your head. In the case of us few lucky folks, (turbocharger/supercharger) we can increase VE by raising the air pressure above ambient pressure (vacuum to boost). A more modern technique is variable valve timing, mostly utilized by Honda. What this does is allow the valves to remain open for a longer period of time in reference to engine RPM, allowing your engine to take full at advantage of the specified displacement in every RPM range. At higher RPM, the engine needs the valves to stay open for a greater percentage of the cycle time to move the charge (air/fuel) in and out of the engine.
Volumetric "Efficiency" should in no way be construed to be a measure of engine efficiency, or thermal efficiency of the engine, although it may have an effect on it. For instance, when a standard gasoline engine is at idle or otherwise at less than full throttle, the effective compression ratio of the engine is reduced, resulting in reduced cylinder temperature and pressure during combustion. The thermodynamic laws which apply to all heat engines dictate that the engine will be therefore operating with less than its optimum thermal efficiency.
Stay tuned kiddies for next chapter of explaining how to maximize your engine power, by tuning for your best VE.

Thanks for reading!!