Common metric units of exhaust back pressure include kilopascal kPa —which we use in this paper—and millibar mbar , the latter being equal to hectopascal hPa. Common customary units include inch of water column in H 2 0 and inch of mercury column in Hg.
The following relationship exists between these units:. While back pressure considerations have always been faced by designers of exhaust systems, increased interest in exhaust pressure has been caused by fitting diesel engines with diesel particulate filters DPF , and the introduction of complex aftertreatment systems in general.
Installation of DPFs often raises concerns about increased exhaust back pressure. In normal circumstances, the pressure drop levels caused by an exhaust muffler and by a properly designed DPF can be actually similar.
The change in back pressure is less than 1 kPa with a clean filter. However, most of the exhaust gas pressure drop over a DPF tends to be caused by the accumulated soot, rather than the filter substrate. Problems arise if regeneration of the DPF does not occur on a regular basis, causing its pressure drop to increase to unacceptable levels. This can lead to an increase in fuel consumption, PM and CO emissions and exhaust temperature.
The increased exhaust temperature can result in overheating of exhaust valves and the turbine. An increase in NOx emissions is also possible due to the increase of engine load.
Other effects on diesel combustion are possible, but depend on the type of engine. Increased back pressure may affect the performance of the turbocharger, causing changes in the air-to-fuel ratio—usually enrichment—which may be a source of emissions and engine performance problems. The magnitude of the effect depends on the type of the charge air systems. 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. Sign up to get the latest performance and luxury automotive news, delivered to your inbox daily! I agree to receive emails from Motor Authority. I understand that I can unsubscribe at any time. Privacy Policy. Contribute: Send us a tip Contact the editor.
People who read this, also read: What's the difference between turbo lag and boost threshold? Can a catback exhaust actually make your car faster? Here's how Mobil 1 ensures it makes a 20,mile claim 'Engineering Explained' tackles Koenigsegg's camless engine. Share This Article:. So it becomes clear that too much back pressure is bad.
And too little back pressure is bad. You need just the right amount. But what if your engine is putting out more than the numbers shown in the table?
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