Quote:
Originally Posted by The_bluester
Plenty of people do it...
Glen, I am not sure about EGR being a bypass path for boost on a diesel. Firstly, the only time I know of that the engines I am aware of close the throttle body is on shutdown (To make them stop more smoothly instead of the thud that the high compression ratio can cause) and actually during EGR operation to create a larger pressure difference and draw more exhaust in, given that diesels run without throttling. The other advantage is it can help stop a runway engine, google vids for runaway deisel if you want to see scary stuff. I am not at all sure they would provide much blowoff valve action as it is often the case that the exhaust manifold pressure on a turbo engine is almost the same as the inlet manifold pressure.
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Closing the throttle body plate, is typically a device used to prevent diesel runaway, and completely separate to most EGR valve setups, although the throttle body plate can be used to generate suction into the manifold for EGR gases when the valve itself is opened. The throttle body plate in a diesel, is not a throttle per re as in a petrol car, it is a runaway safety device primarily.
What I was referring to was typical intake over pressure situations, where say during hard acceleration you suddenly have to back off the throttle suddenly. This results in the turbo vanes closing to idle position and the high pressure in the intake pushes back against the vanes, seerking a way out by a the only available path. This sudden reversal has been known to damage the variable vane mechanism and leads to reduced life span of the turbo. Throttle body plate plays no role in this action as it is only used in shutdown to prevent runaway or for EGR suction changes, it does not prevent blow back in the intake.
As far as your claim that exhaust and intake manifold pressures are the same, this is incorrect in the situation I described. The intake is already pressurised when back off occurs, at that point pressure flow at the intake valve face reduces significantly, by nature of less duration of being opened (high rpm suddenly reduced to idle), which means pressure cannot equalise, hence the blowback through the turbo, seeking equilibrium.
The ECM is constantly monitoring the throttle pedal position sensor, when it sees a sudden back off, when the rpm has been above its set trigger point, the ECM commands the EGR valve to open momentarily to bleed off the over boost in the intake (thus protecting the turbo vanes). Because the intake pressure is higher than the exhaust pressure at that time, the over boost flows out the EGR valve to the exhaust manifold.
This has been the operational model of Chysler Jeep, and some GM turbo CRDs for a decade. Of course different manufacturers may take different approaches to this process.