Thread: TD5 at altitude (>4000mASL)

I am guessing what I quoted from your post is a typo or grammatical error?
Or are you really saying a turbo charged motor isn't affected by altitude?

Cheers

That is correct........as far as it goes, and the reply gets long winded.

0 boost at MSL is Std Atmo, ie no vacuum, so what the piston feels before ignition is a full charge at "Std" pressure/density. Whether talking in" Hg or psi anything above that is boost. As the engine goes up the turbo will maintain the boost within it's controlled range until it reaches the density/altitude where it just cannot flow anymore air to maintain pressure, though if the waste gate is not properly controlled they will overspeed. The compressor output and the turbine input must balance.

Personally, when I had Td5 and before that the Tdi going over Packhorse pass at around, is it, 6000 to 7000ft my discos didn't even know they were that high; but compared to the situation in this thread he is talking in meters. They were much superior to my NA Rangie V8 which would be panting.

One of the things the Yanks found out with aircraft turbos initially was that unlike mechanical superchargers the turbos would continue to spin up until the waste gate opened. However, in that learning process they spat a few out the back and consequently improved the waste gate control, the compressors and turbines.

Simply, the turbine extracts "Heat", as long as that Heat is available, the compressor will spin, as the density altitude decreases there is less load on compressor tending it increase RPM until the load matches the turbine torque, ie maintain pressure.

An auto diesel turbo is designed to increase power and torque by increasing the air supply,(and injected fuel) will tend to maintain MAP until the compressor simply cannot flow anymore air to maintain the pressure. An auto turbo diesel doesn't know what altitude its at! The turbo will tend to increase pressure till the waste gate opens, or it cannot flow enough air. Luckily in "hot rails" the ECU senses the MAP and injects the correct amount of fuel. Obviously, once the turbo flow limits are reached the amount of fuel injected is reduced, and of course power and torque.

This is where one can start to move into a "risk" zone. If the waste gate doesn't open, and compressor output pressure can't open it, the compressor will be trying to balance the turbine torque, ie increase RPM, unless controlled by another agent. On an aircraft engine the waste gate will one way or another also be barostatically controlled.

For us normal punters high altitude can be simulated by having a blocked/restricted intake tract; ie very high differential pressure across the
filter/s and "overspeed" becomes a risk.

A turbo charger's rotating assembly will always increase or decrease RPM to get the compressor load to match the turbine torque (Heat input). That's why a turbo is one of those things which provides something for nothing

On my Bimmer I've made running out of puff a virtue (for me). I ended up using a small turbo to give me low RPM grunt. It runs up quickly to 12psi then as the engine rpm increases the turbo slowly comes back to 7psi. The turbo simply cannot flow enough air. For the short durations that it happens it is not a problem.

Apologies for the long winded reply, and apologies if it still doesn't come across. I've reviewed and corrected it a few times. This medium is a bit limiting.

The long and short of it is: A turbo charger's rotating assembly will always increase or decrease RPM to get the compressor load to match the turbine torque (Heat input)