Lesser Known Tech: The In and Out of In and Out - EGR Explained

[Drew Dowdell]

Lesser Known Tech:

The In and Out of In and Out - EGR Explained

Today we begin a new series of article on lesser known technology. Many of us gear heads hear of different technologies in vehicles and have a general understanding of why they exist, but sometimes there are things that don't generate much thought because...well.. they seem to always have been there. These articles are to help you understand the background of the technology.... the "how?" and the "why?" that are more than just one or two word answers. In this, our first article, we cover EGR.

Most people even vaguely interested in automobiles know what EGR means, but just in case you're not one of those people, EGR stands for Exhaust Gas Recirculation. EGR is the process of returning some of the hot exhaust gas leaving the cylinder chamber and returning it back into the intake stream.

Simple right? Yes, but why would you want to do that?

IN: EGR was introduced in the 1970s as an emissions control measure. The initial systems were simply on/off vacuum switches depending on the load of the engine. Early systems were lambasted (properly I might add) for robbing engines of performance and causing reliability issues. The maze of rubber hoses found in late 70s through late 80s cars is mostly attributable to the addition of pollution controls.

When a strong intake manifold vacuum was pulled (the less the throttle was opened i.e. you took your foot off the gas) the EGR valve opens to let exhaust gases into the intake manifold. As more throttle is applied, vacuum drops and the EGR valve closes.

That explains the basics behind "how?", but still doesn't explain the "why?".

So why would you want to send exhaust back into the intake? I asked resident powertrain expert Dwightlooi to help explain.

EGR enables much less fuel to be burned during low load situations such as when idling or cruising at a constant speed. When power is not needed, the engine typically only needs to burn a only very small amount of fuel. The problem is that very little fuel with a lot of fresh air means a very lean mixture -- simply: a lot of air, not much fuel. A state known as "running lean". Lean burning increases combustion temperatures, and more relevantly, increases the production of Nitrous Oxide, Nitrogen Monoxide, Nitrogen Dioxide,Nitrate Free Radicals, among others, which are considered smog agents. High cylinder temperatures increase the possibility of premature detonation (pinging/knocking).

By feeding exhaust back into the intake, you are reducing the oxygen content of the intake charge. Therefore, even though the metered fuel amount doesn't change and the volume of gases drawn into the cylinders doesn't change, the fuel-to-oxygen ratio is now not as lean. This allows the engine to use less fuel without getting too hot and/or make too much pollution.

As EGR systems became more sophisticated, they began to actually contribute to the fuel economy performance of the engine rather than detract as early systems did. By introducing inert exhaust gas into the intake manifold at low throttle, it allow for an increase in manifold pressure at a given power output. Increased manifold pressure equals reduced pumping losses (the effort the engine puts into sucking air in) equals greater efficiency.

So what happens when an EGR valve fails? It depends if it's stuck open or stuck closed. An EGR valve stuck in the open position will rob an engine of power during acceleration. In worst case scenarios, it will cause stumbling and hesitation under part throttle. An EGR valve stuck in the closed position will cause a rough idle or stalling at idle. Under acceleration or at cruising speeds it can cause pinging an knocking.

OUT: But even after 30 years of technological advancement to reduce exhaust pollution and increase fuel economy, the EGR valve itself is slowly going the way of the Dodo. The reason? Variable Valve Timing. Dwightlooi continues:

Engines with VVT can dial in huge amounts of valve overlap in the same conditions as you would open an EGR valve. By having both the intake and exhaust valve open at same time you allow exhaust gases to backflow into the cylinder through the exhaust valves even as the intake air is being drawn in. The effect is the same an EGR valve would have accomplished, except you are doing it directly through the exhaust valves instead of feeding gases from the manifold to the intake. It is more precise and no additional hardware over the existing VVT system is needed.

By removing the older EGR valves and designing similar functionality into the VVT system, automobile manufacturers are removing a point of failure in engines while continuing to provide the pollution control and fuel economy benefits the older system gave.

I hope you enjoy this series and if you have any suggestions for topics in future articles, please PM them to me.

[Camino LS6]

Bravo!

This is a nice addition to C&G.