Tuesday, June 17, 2014
Engine Mods: It All Starts With Airflow- Part 1
Have you ever noticed the big blower sticking out of the hood of a hopped up hot-rod? Or how about the loud exhaust when the throttle is opened on a high horsepower monster machine? Have you ever wondered why the big intake system or loud exhaust on these powerplants? Its not all about chrome, looking cool and noise- no really, its not.
First, lets get one scientific concept perfectly straight- there is no such thing as suction or vacuum. An area of higher pressure can push, or exert force to balance a difference in pressures, but never can a lower pressure area exert a negative, or pulling force. An area of lower density and thus lower pressure air can be created by increasing the volume of a container and trapping a fixed amount of air. The overall density and pressure drops when we make the volume larger, but the same amount of stuff is inside it.
So, if you create an area of lower pressure, then open a valve, atmospheric pressure outside PUSHES air into that space. This is how the engine works- the piston moves down in the cylinder with the intake valve open and the air follows in behind it, filling the space with air. Now that we have the Physics portion of the lesson out of the way...
An internal combustion piston engine (most car, truck, ship, train and equipment engines are such) is first and foremost an air pump. Most engines will move hundreds of cubic feet of air through the engine for every gallon of fuel that gets burnt. Air flows in through the air filter, gets measured, metered and used as an oxygen source for combustion of fuel, then becomes exhaust gas and exits through the tailpipe and muffler.
The engine must use some of its own output to move air in and out of itself, thus the easier it is to move that air, the better the overall efficiency of the engine. Knowing that the engine is simply an air pump, it would make great sense, would it not, that making it easier to move all the air it can in and out would improve performance in many ways. There are many ways to achieve the goal of assisted airflow, thus improving efficiency. Any race engine builder you happen to talk to will talk of porting and polishing, high-lift and longer duration cams, tuned intake and exhaust, headers and air filters. All of these mods are ways to move more air into and out of the engine easier.
There are many points in the air-flow path into and out of the engine where one can make improvements. By adding or subtracting parts, increasing opening size and reducing resistance to flow, horsepower, torque, and efficiency in the form of power:fuel ratios all increase.
The two most common upgrades for all engines are still the best these days. A bigger air filter connected to a cold-air ducting system combined with a larger or easier flowing exhaust can add significant power without internal engine modifications. Intake and exhaust can add a quick, easy, 10% increase, or more, to an engines output, and thus a similar decrease in fuel use to do the same work. This means that an engine makes more power from the same amount of fuel because less energy is wasted to move its own air.
Now, all these mods are based on an engines ability to move air at a maximum or about 14 PSIA (absolute pressure) from atmospheric pressure. If we can increase that drive pressure, we can increase the flow of air, therefore we can add more fuel to the additional air and create even more power from the same sized engine.
The idea of pressurizing the intake air for engines was conceived almost simultaneously with the design of engines themselves. By adding a compressor which can move air at higher efficiency than the engine itself can, we can add substantial power and even overall efficiency just by increasing the air-charge into each cylinder.
Come back next time for part 2 of this article for the breakdown of types of compressors used for engines, how they work and what it means for engine performance. Til then, MW out... :-)