When we have an automotive concept that involves complex science, we know just who to turn to for a proper explanation. That would be Jason Fenske from Engineering Explained. Today’s concept is all about pressure. More specifically, it’s about the idea of back pressure and whether or not this it’s a good thing inside of an internal combustion engine. Like a number of automotive engineering concepts, back pressure is often misunderstood. So what is back pressure and why do we care about it?
The most basic definition of back pressure is an opposing force acting on a gas that attempts to move through a confined space. For our purposes here, we’re specifically examining the way that exhaust gases leave an engine through the exhaust system. Your engine will pulse the exhaust gases through the exhaust manifold and then through the exhaust pipes. These pulses occur as the individual cylinders work through the four stages of the internal combustion process.
As the pulses move along, they generate an exhaust flow. If you have a restrictive exhaust system, it can generate back pressure that works against the positive flow of the exhaust gas that’s trying to exit your vehicle. As Jason explains, a restrictive exhaust flow that builds up back pressure is only hurting the power your vehicle can deliver because it’s not working efficiently.
However, a little back pressure is a good thing. In fact, it helps. The right size pipe is large enough to breathe well but small enough to create a high exhaust flow. Steps in the exhaust system also create negative pressure waves that travel back to the cylinder and help empty the cylinder of those gases.
All of this depends on things like tubing length, exhaust pipe diameters, the timing of the pressure waves, and other factors like inertial scavenging.
Jason takes to his whiteboard to break it all down. As he often does, his graphical presentation does an excellent and relatively concise job of illustrating what causes back pressure, what back pressure really is, and why back pressure has negative and positive effects on an exhaust system.
This is a short-form video master class on the explanation of exactly what we’re dealing with when we’re talking about back pressure. Watch the video and you’ll become armed with more automotive knowledge to share with your friends.