E-G-R stands for Exhaust Gas Recirculation. It’s been around for decades but it’s now becomingly increasingly important in both diesel and spark ignition engines.

Computer-controlled, often externally-cooled, and potentially of much greater use than for just decreasing emissions, EGR is making a major resurgence. So what are the emissions and fuel economy implications of EGR? Some, including reducing full-load exhaust gas temps in turbo engines without the need to run rich air/fuel ratios, are not at all what you’d expect!

So let’s take a new look at EGR.

Exhaust Gas Recirculation

Exhaust Gas Recirculation (EGR) is a process whereby some of the engine exhaust is fed back into the intake.

External EGR is achieved by means of a pipe that connects the exhaust to the inlet manifold, with a control valve interposed in this line to regulate EGR flow.

For exhaust gas to flow in this pipe, the pressure in the exhaust must be higher than the pressure in the intake.

In conventionally throttled spark ignition engines, this pressure differential is present in varying degrees at all loads, and is highest in part-throttle conditions. However, in throttle-less engines like diesels and some direct injection spark ignition engines, this pressure differential is highly dependent on exhaust backpressure. In turbocharged engines, exhaust pressure in front of the turbine is always higher than intake manifold pressure.

Internal EGR occurs when the valve timing is arranged so that there is some back-flow into the combustion chamber from the exhaust, or all exhaust gases are not pushed out of the combustion chamber on the exhaust stroke. Such engines normally have variable valve timing so that internal EGR occurs only when dictated by the ECU; when internal EGR is required, this is achieved by increasing valve overlap.

Internal EGR appears to be a better approach (at least on engines with variable valve timing) as it avoids the need for external pipes and valves, reducing cost and improving packaging. However, external EGR has a significant advantage – the recycled exhaust gas can be cooled before being fed back into the intake. This is termed cooled EGR.

In cooled EGR systems, the amount of EGR flow that actually occurs depends not only on the pressure differential between the exhaust and intake, but also on the pressure drop through the EGR cooler. The actual amount of EGR occurring can be indirectly measured by the intake airflow meter – more on this in a moment.

Traditional EGR Systems

EGR was first widely adopted in the 1970s as a means of reducing oxides of nitrogen (NOx) emissions.

 

In the depicted Nissan system, the amount of EGR depended on two factors – engine load (registered as intake manifold vacuum) and coolant temperature (monitored by a thermal vacuum valve). The higher the manifold vacuum (ie the lower the load), the greater the EGR. EGR was fully activated only at coolant temperatures above 63 degrees C, was partially activated at 40 – 63 degrees C, and was inactive at temperatures below 40 degrees C.

 

This Leyland EGR flow control valve consisted of a diaphragm and a valve. The diaphragm was subjected to manifold vacuum and so valve lift decreased with load.

However, many EGR valves used more complex operation that this - for example, monitoring exhaust backpressure as well as intake manifold pressure, so preventing EGR on the over-run and at idle.

Even in this period, some cars additionally used solenoid control of the vacuum signal to the EGR valve, the solenoid being controlled by a dedicated electrical unit.

Electronically-Controlled EGR Systems

Given that the optimal EGR valve opening varies with load, coolant temperature, pressure differential between exhaust and intake manifold (and other factors), electronically-controlled operation of the EGR valve has obvious advantages. All external EGR systems fitted to current diesel and spark ignition cars use electronically-controlled flow valves.

 

This Bosch spark ignition EGR system uses a vacuum-controlled EGR valve that is electronically controlled by the ECU.

 

This Honda system uses an electronically-controlled EGR valve that incorporates a feedback sensor to determine the actual valve opening that is occurring.

 

Here is the electronically-controlled Honda valve that is used on the Honda Insight.

 

This is a generic diagram of a typical diesel car EGR system. The important point to note is the presence of the airflow meter. By comparing the modelled airflow requirements of the engine (based on rpm, intake manifold pressure, intake air temp and volumetric efficiency) with the actual amount of air being breathed, the amount of EGR that is occurring can be calculated and adjusted as required.

 I wonder would its emissions passed EURO 5...

Of course something has to be done to reduce emissions, but what I don't understand is why they still don't want to use Brown's Gas (a small percentage injected together with the main combustible) for this purpose. The way this reduces emissions is improving the thoroughness of combustion. What could be better?

Quoting from [germex]:

Of course something has to be done to reduce emissions, but what I don't understand is why they still don't want to use Brown's Gas (a small percentage injected together with the main combustible) for this purpose. The way this reduces emissions is improving the thoroughness of combustion. What could be better?

Hey,maybe you could write them a letter about this```hah``

Quoting from [autopartsworld]:


Hey,maybe you could write them a letter about this```hah``

Nah, I won't waste my time doing that. It was more a rhetoric question, as I have an approximate idea about the background.

Quoting from [germex]:

Of course something has to be done to reduce emissions, but what I don't understand is why they still don't want to use Brown's Gas (a small percentage injected together with the main combustible) for this purpose. The way this reduces emissions is improving the thoroughness of combustion. What could be better?

I guess that they think it's too complicated for the ppl with less technical knowledge such as maintain the correct amount of mixture between water and electrolite(Baking soda/KOH) and likewise they have to remapping the ICU,add extra sensor ,safety device to prevent explosion of the H gas,etc.

On second thought, I am trying to make a HHO generator for my own car......

Quoting from [ians]:

I guess that they think it's too complicated for the ppl with less technical knowledge such as maintain the correct amount of mixture between water and electrolite(Baking soda/KOH) and likewise they have to remapping the ICU,add extra sensor ,safety device to prevent explosion of the H gas,etc.


On second thought, I am trying to make a HHO generator for my own car......

You can get these generators ready-made from different sources. Making one isn't easy, because it will have to work in a pulsed mode (besides a few other details) to obtain a high percentage of elemental gases. But buying, installing and using one is very simple. That's why I can't accept that the industry doesn't want to use these devices. They are not even patent protected.

Quoting from [germex]:

You can get these generators ready-made from different sources. Making one isn't easy, because it will have to work in a pulsed mode (besides a few other details) to obtain a high percentage of elemental gases. But buying, installing and using one is very simple. That's why I can't accept that the industry doesn't want to use these devices. They are not even patent protected.

You are absolutely right,it isnt easy making work perfectly....at least worth trying and the challenge behind it.

The good news is that not even patent protected like you said.......