Thread: Ideal boost at cruising speed

Originally Posted by djam1

Hope this is not too far of subject
I have a TD5 with a VNT and find that it runs max boost 1.3 Bar at cruising speed
Often wondered if that was a good thing?

One atmosphere is also approximately the ambient pressure at sea level and is equal to 14.7 PSI or 1.01325 Bar.

From what I am reading here on this thread, your setting could be too high also.
.

Considering the manifold gets very hot, whats the easiest way to measure TIP?
Just a mechanical gauge (eg 50 PSI) with copper tube?

Steve

Originally Posted by steveG

Considering the manifold gets very hot, whats the easiest way to measure TIP?
Just a mechanical gauge (eg 50 PSI) with copper tube?

Steve

Yeah. Copper/steel tube to get away from the manifold heat, then plastic/rubber line and into a gauge. You need twice the boost for gauge scale in the worst conditions. So 50psi should be good.

Originally Posted by djam1

Hope this is not too far of subject
I have a TD5 with a VNT and find that it runs max boost 1.3 Bar at cruising speed
Often wondered if that was a good thing? the fact that everything is under pressure all the time

Several issues/points:
If you make tuning improvements, which lead to better performance, e.g. improve volumetric efficiency, then for the same power/torque, the boost pressure (at full load) should reduce. Those who highly modify diesel engines for competition use delight in seeing lower boost pressure while tuning, so long as performance is same or better, i.e. lower boost pressure is a goal.

Generally the drive pressure (TIP) has to increase to achieve an increase in boost pressure. During the exhaust stroke the piston is apposed by the pressure in the exhaust, thus power/torque is wasted pumping the exhaust out of the cylinder. This loss can be considerable and has to be made up for by burning more fuel on the power stroke of another cylinder. On the other hand boost pressure pushes down on the piston during the intake stroke - this is why we are would like boost pressure to be higher than drive pressure - the ratio, drive/boost is indicative of the overall efficiency of the turbo system, ideally it is less than 1 or just over 1 at max load.

Drive pressure (and EGT) must increase if back pressure (restrictive exhaust system) is higher. It will also increase if the turbo is driven out of its best efficiency range. IMHO these (particularly the exhaust system) are why Dougal's drive/boost ratio is what he reported in an earlier post, and not lower.

A restrictive turbine housing will choke the exhaust flow and increase drive pressure. Some VGT turbos come from small diesel engines and have a smallish (for a high output 3.9 litre diesel) turbine housing and this will be felt if the nozzle vanes are not opening at higher drive pressure. We haven't been told, in this thread, what size VNT turbo you or the OP have.

High drive pressure results in a greater mass of residual exhaust gas in the cylinder when the exhaust valve closes during the intake stroke - i.e. less oxygen available to burn extra fuel for increased performance and efficiency. This is more important for those with the earlier (pre 89) 4BD1's because the valve timing for them is better suited for a NA rather than a turbo engine like your TD5. After late 88 the 4BD1 valve timing is better suited for a turbo diesel.

At cruise, the power is down, and the boost should reduce, and the drive pressure also reduce and in particular it should be less than boost. For economy at cruise you would aim for those figures to be low (lower = better efficiency).

Hope that helps and doesn't alarm.

Originally Posted by Bush65

At cruise, the power is down, and the boost should reduce, and the drive pressure also reduce and in particular it should be less than boost. For economy at cruise you would aim for those figures to be low (lower = better efficiency).

Hope that helps and doesn't alarm.

My drive pressure results (gauge line is currently broken, so not recent) show the drive pressure is opposite to drive temperature. The lower your exhaust temp, the higher the drive pressure must be in relation to boost.

Dr Afred Buchi even developed a law for this.

Originally Posted by Dr Alfred Buchi

choked flow excepted, the turbine pressure ratio in relation to the compressor pressure ratio is a function of the total turbocharger efficiency and the turbine inlet temperature.

Higher exhaust temperature gives you a better boost/drive pressure ratio. I was hitting 2:1 in the worst conditions (accelerate hard from rest with a cold manifold, 40psi drive pressure for 20psi boost).
I was hitting 1.5:1 at cruise with 420C EGT (12-13psi drivepressure for 8-9psi boost).
At 600C EGT I was getting 1:1. 20psi drivepressure for 20psi boost.
Above 600C if I could kept the rpm below 2000 (to minimise the effect of my restrictive exhaust) I could get down to 18psi drive pressure for 20psi boost.

Applying this to cruise conditions mean you will never get a good boost/backpressure ratio because the EGT is too low. The turbo has to extract more energy as mean static pressure and less as temperature.
So our best efforts at reducing the parasitic loss becomes reducing boost (open your VNT vanes more) to reduce boost and drive pressure.

But zero of each isn't the answer either. There is a sweet spot and I'm not sure exactly where it is. But it's somewhere between 5-10psi on my engines. Fitting a larger turbine housing to my Isuzu decreased my drive pressure and cruising boost, but increased my fuel consumption. My best fuel economy to date has happened with that tiny T25 turbo pulling 12-13psi drive pressure for 8-9psi boost at 100km/h. My current turbo setup is closer to 7psi boost at 100km/h and I don't get many fill-ups hitting 10km/l with it.
I suspect the ideal ratio may come down to a target A/F ratio more than a target boost. It is a complex situation trading off recovered exhaust energy against parasitic loss.

Until recently I haven't had a VNT engine to test this on. I do now have a 2.0tdi Skoda (VW engine) and that pulls around 5psi at 100km/h cruise. It should be possible to log EGT with the software interface I have, but I haven't done that yet. I intend to fit a VNT to my other work car (I have the turbo) and possibly my Isuzu. But I have a lot more work to do before then on all fronts. Work, personal and boring maintenance stuff.

Guys
The VNT I have is the Pete Bell / turbo technics designed unit for the TD5
I must do some further checking I just recall that the boost was up pretty well while cruising.
The EGT are very good overall usually less than 450 degrees I do however live in a very flat area and don't regularly tow

Originally Posted by djam1

Guys
The VNT I have is the Pete Bell / turbo technics designed unit for the TD5
I must do some further checking I just recall that the boost was up pretty well while cruising.
The EGT are very good overall usually less than 450 degrees I do however live in a very flat area and don't regularly tow

GT2256V?

Too low can be a problem for EGT.

Back on the original topic, at 100 km/h with no headwind, no trailer and a reasonable load mine sits on about 5-6 psi.