Volumetric Efficiency

[JDMFantasy2k]

Glad to see the new tech articles. I love learning new stuff, especially complex stuff like that. I figured i'd give my 2 cents and post my essay on volumetric efficiency i wrote for my creative writing class. Althoguh most of you know these basics, i figure it could be good for noobs.

Volumetric Efficiency of Gasoline Engines
Will Starzec

Many people who drive use their cars without realizing how they work. The engine, the most complex part of the automobile, has left many clueless as to how it works. Volumetric efficiency directly affects an engine’s performance as much as or more than mechanical parts. Fortunately, there are devices and technologies that can help improve volumetric efficiency.

First, let’s see how a basic gasoline engine works. A gasoline engine uses moving pistons inside a cylinder to make power. These pistons are attached to rods which transfers the power to the crankshaft and out of the engine. The force created inside an engine comes from a controlled explosion. This explosion is caused by a mixture of gasoline and air and is ignited by a spark. This explosion forces the piston down and rotates the crankshaft. A mixture of gasoline and air is introduced into the engine by a series of pipes and valves. Consequently, the spent gasses from the combustion process are expelled through a similar series of valves and pipes.

Understanding an engine’s basic principles allows us to determine what volumetric efficiency actually is. Volumetric efficiency, or VE, is a measure of how efficient an engine is at moving gasses through itself. Since every engine has a specific displacement (the volume of one cylinder in the engine multiplied by the number of cylinders), an engine with a VE of 100% would be moving an amount of gasses equal to its displacement. However, it is nearly impossible for a stock engine in a production car to have a VE of 100% because certain limitations. Mechanical designs and the laws of physics limit VE to about eighty-five percent on most engines. As a rule of thumb, the more efficient you make the engine, the better it will perform. An engine’s powerband is directly related to an engine’s VE. According to Chan’s views, If your motor moves air at peak efficiency between 3000-5000 rpm, that’s known as your peak usable powerband. That actually has a direct relation to volumetric efficiency. Most motors will not see 85% VE throughout the entire rpm range (Why There’s No Replacement).

Although many car enthusiasts don’t know it, many of the performance parts they buy for their engines are targeted towards a common goal: improving VE. Modifications vary from mild to wild, in terms of performance benefits and price. The first modifications many enthusiasts buy for their vehicles are called basic performance upgrades, or BPU. A cold-air intake, performance exhaust, and a header are three BPU upgrades that will allow the engine to breathe easier. These three modifications are priced reasonably and deliver a small to medium performance gain.

The next step to increasing VE is to change and rework the internal engine components themselves. Changing the camshaft(s) (a rod-like device that controls the lift and duration of the valves in the engine) allows the engine to induce and expel gasses faster, and equates to better efficiency. Other processes like porting and polishing are used to increase efficiency through the cylinder head. However, more performance costs more money, as these modifications are expensive. Another internal modification that increases VE considerably is stroking, or boring the engine. Both of these processes increase the volume of the cylinders in the engine, allowing the engine to move more gasses per cycle.

People that are looking for large power gains usually buy a forced induction system. Forced induction systems include turbochargers, superchargers, and nitrous systems, and are the most expensive and most effective modifications. They are able to increase VE unlike any other modification. All of the devices are external and actually pressurize the air traveling into the engine. According to Chan’s calculations, “with 9 psi of boost, we see almost a 60% in airflow, which translates to roughly a 60% increase in power”( Why There’s No Replacement).

One of the more simple forced induction systems is Nitrous oxide. Nitrous is actually liquidized nitrous oxide (N2O) gas in a bottle. When the gas is injected into the engine, it cools the intake air and allows for more air to be crammed into the cylinder. Because N2O is an oxidizer, it makes combustion more powerful.

Perhaps one of the most economical devices for increasing VE is the turbocharger. A turbo is a basic air compressor that is powered by exhaust gasses. The intake pressurization caused by the turbo enables the engine to induce more air than it would ever be able to in its naturally aspirated trim. The pressurization of the engine intake is commonly called “boost”. A turbocharger will create boost under a certain amount of throttle, and it’s possible to drive some cars without even reaching boost. Turbos are a great invention because they allow you to make power on boost and save fuel off boost.

Similar to a turbocharger, a supercharger is a device that also pressurizes the intake air. However, a supercharger is driven directly by the engine and doesn’t create backpressure in the exhaust, as a turbo does. The trade off is that a supercharger requires power to run. According to Kojima, a supercharger’s positive pressure across the engine has inherent advantages:
Unlike a turbocharger, a supercharger has no turbine to be an impediment on exhaust gas flow. Because of this, the supercharger maintains a
positive pressure across the engine with the intake pressure higher than the average exhaust pressure. With this positive pressure differential across the engine, the volumetric efficiency is almost always increased. A VE of up to 110 percent is possible with a supercharger (259).

Realizing that VE is critical to engine performance, some car manufacturers developed technologies to increase VE. GDI, or Gasoline Direct Injection, is one of these technologies. GDI engines operate like regular gasoline engines, but have some characteristics of diesel engines. Like a diesel engine, GDI engines inject fuel directly into the cylinders, making the engine more efficient and powerful. A GDI engine can have “up to 40 percent more fuel economy than a conventionally injected engine, 10 to 15 percent higher power and torque, and 20 percent lower carbon dioxide emissions” (Kavanagh). Kavanagh continues by saying “In addition the injector, Mitsubishi moved the intake port, making it nearly vertical. This further enhances volumetric efficiency and creates a tumbling motion in the cylinder.”

Another amazing technology that has been developed to improve VE is variable valve timing, or VVT. VVT is a system that changes the lift and duration of the valves in the engine. Many manufacturers have their own variations of VVT, the most popular being Honda’s VTEC (Variable valve Timing and lift Electronic Control) system. High performance “VTEC” equipped engines typically have a VE of about ninety- percent. This
system allows the engine to have great fuel economy and a great deal of power. Chan describes Honda’s VTEC system in depth by saying:
The way it works is to allow the motor to have more than one cam profile.
As the motor needs less air at lower rpm, VTEC uses a less aggressive
cam profile in that range, and when the motor reaches a certain rpm,
the big cam lobe engages and opens up the valvetrain to allow more air to
pass through for high rpm operation. This can improve VE% over a
broader rpm range (Why There’s No Replacement).

In conclusion, volumetric efficiency is a key factor in an engine’s power output, fuel economy, and emissions. Many people modify their cars with parts that improve VE, thus increasing power. In recent years, manufacturers have developed technology to make engines more volumetrically efficient. These advancements allow engines to be more powerful, more fuel efficient, and more environmentally friendly. Lastly, cars with efficient engines are very fun to drive.

Works Cited


Chan, Kevin. Why There’s No Replacement for Displacement. 28 Aug. 2003.

Speedupgrade.com. 18 Dec. 2004

<http://www.speedupgrade.com/articles...92608/5164.htm>


Kavanagh, Jason. “Gasoline Direct Injection.” Sport Compact Car Nov. 2002:

237-242.


Kojima, Mike. “Suck, Squish, Bang, Blow.” Sport Compact Car Aug. 2001: 258-

266.