I am trying to find understand the basic workings of a modern Diesel engine.

Specifically the 3.6 TDV8 - if that makes any difference.

I have read up, and looked at schematics, but would like some assurance of my understanding from those that know.
So which, if any, of my observations are correct?

1) The throttle (higher/lower rpm) is dictated NOT by a throttle body or carburettor that adjusts the flow of air to the cylinders, but by the accelerator pedal requesting the ECU provide more fuel, via the injectors, to the cylinders.

2) Exhaust gases when leaving the cylinders flow thru a turbo, which in turn provides above atmospheric pressure air to the cylinders.

3) The EGR valves allow some exhaust gas (before it enters the turbo) to be mixed with the incoming filtered fresh air and returned direct to the cylinders for reignition.

4) The condition of EGR valves at rest, engine start, and idle is CLOSED (inhibiting the return flow of exhaust gas to the cylinders). With rising engine rpm the EGR valves will open to allow increasing volumes of exhaust gas to flow back into the cylinders.

Q1 - Do all exhaust gasses, eventually, have to pass through the turbo to exit the tail pipe ?
Q2 - Does all inducted air, both fresh from the air filter, and exhaust from the EGR, flow through the intercooler prior to entering the cylinders ?
Q3 - Is all inducted air from the air filter pressurised by the turbo ? ie is the turbo ‘full time’ throughout the rev range, or does it engage and disengage ? If not, is there another path for inducted air other than thru the turbo housing ?

Any input appreciated

Yes, No, Yes 2012 TDV8 Vogue (L322)

Point 4 is incorrect. Some exhaust gas is returned via the EGR valves direct to the inlet manifold when the engine is idling or under light load as when cruising but never under high load.

Kevhdkent wrote:

1) The throttle (higher/lower rpm) is dictated NOT by a throttle body or carburettor that adjusts the flow of air to the cylinders, but by the accelerator pedal requesting the ECU provide more fuel, via the injectors, to the cylinders.

Modern diesels also have a throttle body with a throttle flap. The main reason is to create a low pressure area to assist with EGR scavenging.

2) Exhaust gases when leaving the cylinders flow thru a turbo, which in turn provides above atmospheric pressure air to the cylinders.

Yes, the manifold flows into the turbo. The amount of exhaust driving the turbine is controlled by the wastegate or in variable geometry turbos, the actuator

3) The EGR valves allow some exhaust gas (before it enters the turbo) to be mixed with the incoming filtered fresh air and returned direct to the cylinders for reignition.

Not really. EGR ingress is downstream of the turbo, not upstream. Also see answer to number one.

4) The condition of EGR valves at rest, engine start, and idle is CLOSED (inhibiting the return flow of exhaust gas to the cylinders). With rising engine rpm the EGR valves will open to allow increasing volumes of exhaust gas to flow back into the cylinders.

The opposite of this. EGR is used at low engine RPM when the throttle butterfly is only partially open thus creating a low pressure area to help draw in the EGR. At higher engine speeds the EGR closes.


Q1 - Do all exhaust gasses, eventually, have to pass through the turbo to exit the tail pipe ?

Not exactly, the wastegate allows gases to effectively bypass the turbo although in most cases the exhaust does still enter the turbine housing.

Q2 - Does all inducted air, both fresh from the air filter, and exhaust from the EGR, flow through the intercooler prior to entering the cylinders ?

NO. EGR is cooled by the EGR cooler. Only filtered, metered air passes the intercooler.
Q3 - Is all inducted air from the air filter pressurised by the turbo ? ie is the turbo ‘full time’ throughout the rev range, or does it engage and disengage ? If not, is there another path for inducted air other than thru the turbo housing ?

This is controlled by engine speed and the wastegate. The turbo is only engaged at rising engine speed and limited in operation by the wastegate

The turbo is always spinning but at lower RPMS it's not generating positive manifold pressure.
One of the problems with EGR's is that when worn or fouled they don't close or don't close fast enough when you get on power so excess exhaust is vented to the intake. A throttle plate on a modern diesel engine should also prevent a runaway engine if the turbo seals let go. Saw that on an old Discovery once...... 2012 TDV8 Vogue (L322)

It appears from possible faults codes that the EGR throttle on the 3.6 TDV8 can be closed under pcm control when unexplained operation occurs in order to prevent runaway but none of the 2.7 TDV6, the 3.0 TDV6 or 4.4 TDV8 will attempt to close the butterfly to prevent runaway, unless someone like myself has installed a mechanism to close the butterfly outside of pcm control. My 4.4 TDV8 closes the EGR butterfly for 15 seconds every time the ignition is switched off. I did a similar modification to my previous TD5 D2.

It was along time ago - likley a D1. 2012 TDV8 Vogue (L322)

How did you do that mod and why, may I ask? Christian.


Current Cars
2011 Range Rover Vogue SE
2020 BMW M4 Competition
2019 BMW X4

My RR Blog: www.facebook.com/L322Project
or
https://www.fullfatrr.com/forum/topic61540.html

Caesium wrote:

Kevhdkent wrote:

1) The throttle (higher/lower rpm) is dictated NOT by a throttle body or carburettor that adjusts the flow of air to the cylinders, but by the accelerator pedal requesting the ECU provide more fuel, via the injectors, to the cylinders.

Modern diesels also have a throttle body with a throttle flap. The main reason is to create a low pressure area to assist with EGR scavenging.

2) Exhaust gases when leaving the cylinders flow thru a turbo, which in turn provides above atmospheric pressure air to the cylinders.

Yes, the manifold flows into the turbo. The amount of exhaust driving the turbine is controlled by the wastegate or in variable geometry turbos, the actuator

3) The EGR valves allow some exhaust gas (before it enters the turbo) to be mixed with the incoming filtered fresh air and returned direct to the cylinders for reignition.

Not really. EGR ingress is downstream of the turbo, not upstream. Also see answer to number one.

4) The condition of EGR valves at rest, engine start, and idle is CLOSED (inhibiting the return flow of exhaust gas to the cylinders). With rising engine rpm the EGR valves will open to allow increasing volumes of exhaust gas to flow back into the cylinders.

The opposite of this. EGR is used at low engine RPM when the throttle butterfly is only partially open thus creating a low pressure area to help draw in the EGR. At higher engine speeds the EGR closes.


Q1 - Do all exhaust gasses, eventually, have to pass through the turbo to exit the tail pipe ?

Not exactly, the wastegate allows gases to effectively bypass the turbo although in most cases the exhaust does still enter the turbine housing.

Q2 - Does all inducted air, both fresh from the air filter, and exhaust from the EGR, flow through the intercooler prior to entering the cylinders ?

NO. EGR is cooled by the EGR cooler. Only filtered, metered air passes the intercooler.
Q3 - Is all inducted air from the air filter pressurised by the turbo ? ie is the turbo ‘full time’ throughout the rev range, or does it engage and disengage ? If not, is there another path for inducted air other than thru the turbo housing ?

This is controlled by engine speed and the wastegate. The turbo is only engaged at rising engine speed and limited in operation by the wastegate

Sounds like the EGR's could be sticking open or you've got a faulty throttle body which isn't unheard of. The stepper motors can fail. Christian.


Current Cars
2011 Range Rover Vogue SE
2020 BMW M4 Competition
2019 BMW X4

My RR Blog: www.facebook.com/L322Project
or
https://www.fullfatrr.com/forum/topic61540.html

GraemeS wrote:

It appears from possible faults codes that the EGR throttle on the 3.6 TDV8 can be closed under pcm control when unexplained operation occurs in order to prevent runaway but none of the 2.7 TDV6, the 3.0 TDV6 or 4.4 TDV8 will attempt to close the butterfly to prevent runaway, unless someone like myself has installed a mechanism to close the butterfly outside of pcm control. My 4.4 TDV8 closes the EGR butterfly for 15 seconds every time the ignition is switched off. I did a similar modification to my previous TD5 D2.

The throttle bodies are both open in the static position with ignition on. When ignition is turned off they both close slowly in unison, then immediately open where they remain.
Is this correct ?

[quote="Kevhdkent"][quote="Caesium"]

Kevhdkent wrote:

The purpose of this exercise is I’m trying to understand the conditions that caused a critical drop in power while motoring at speed, reducing revs to 500rpm but without stalling the motor. Then complete non-starter, UNTIL the EGR to intake manifolds were disconnected ?

Could the EGR’s fail to open at speed ? Could this cause both lack of power at 60mph, and then refusal to start ?
Starting only occurring when clean air is allowed direct into the manifold ?

Caesium wrote:

How did you do that mod and why, may I ask?

You seem to encounter a lot of runaway diesels.