EGR PERFORMANCE?

As I understand it, engine per‐ formance (both outputs and eco‐ nomy), is dependent on the amount of clean air (oxygen) available in the combustion chamber to facilitate combustion.

With the recirculation of ex‐ haust gases, is some of the oxy‐ gen not displaced? Will com‐ bustion not be hampered with less oxygen being available?

Furthermore, is it not the ideal situation to keep the tem‐ perature of the air going into the cylinder as low as possible? With the recirculation exhaust gases, virtual flames are blown into the cylinder and heat the air inside in the process.

Thanks for the second, equally in‐ teresting, letter. Your basic under‐ standing of the operation of an ex‐ haust-gas-recirculation (EGR) valve

is correct but you have to remember it almost exclusively operates during part-load conditions and not at full load where maximum power is de‐ manded. The EGR valve is an emis‐ sions-control device that lowers the mono-nitrogen oxide (NOX) emissions of both petrol and diesel engines.

NOX is a harmful gas that forms during the combustion process at el‐ evated temperatures. A way to reduce these toxic emissions is to lower the combustion temperature. One method of achieving this is to send some of the burnt exhaust gases back into the combustion chamber using an EGR valve. These spent exhaust gases are inert and do not contribute to the combustion process but the extra mass (and limited available oxygen) reduces the peak combustion temper‐ ature (much higher than the returned exhaust-gas temperature) and there‐ fore result in lower NOX emissions. Most EGRS are water-cooled today to enhance this effect.

In petrol engines when EGR gases are absent, a cold intake charge boosts efficiency and power at full load as more oxygen is available for combus‐ tion and the tendency of knock which allows more ignition advance is re‐ duced. This is not needed during partload running and, because of the extra exhaust gases, pumping losses across the throttle plate are minimised.

In petrol engines, the percentage of EGR is limited to around 15%; but in diesel engines which run unthrottled with excess oxygen available, it can be as high as 50% of the intake air. my driving pattern. I advised him I travel about 70 km to work and back (Tokai to Parow), mostly in stop-start traffic in eco mode. The technician said the stop-start traffic was the cause of the clutch failing as the car barely got out of second gear on the way to my destination. He recommended I drive in sport mode as the car would hold the revs longer in each gear. This proved to be very uncomfortable and jerky while fuel consumption suffered.

The shudder in second gear has unfortunately now returned at 74 000 km and is worse than ever. I am facing the prospect of again replacing the clutch and am con‐ cerned the warranty may not cover the cost the second time?

When we covered the problems with automatic transmissions in the technical article Problematic auto‐ matic in August 2019, it also focused on the DSG. The article mentioned: “A big mistake that drivers make in vehicles fitted with dual-clutch trans‐ mission is to ride the clutches in stop-start traffic, especially in hilly topography. Remember that it is es‐ sentially a manual clutch with com‐ puter control so creeping up an incline eats away at the clutch in the same way as riding the clutch in a manual car. Employ the handbrake and wait for a slight gap and drive rather than creep to ensure the clutches are fully engaged.”

The above ties up with your find‐ ings and we appreciate it is not al‐ ways possible to change your drive cycle (Cape Town traffic PRE-COVID19 resembled a car park). A torque converter-type automatic transmis‐ sion would be better suited to slog it out in stop-start traffic than the DSG as the fluid coupling is better suited to creeping.

The good news is if the VW war‐ ranty paid for the first replacement, there is no reason why it wouldn’t foot the bill again.