Thread: 4BD1T Turbo Sizing and Performance Prediction.

Originally Posted by Bush65

Thanks for that. I've been writing this in a word processor and using subscripts and superscripts, which is easier to read than a '^' Sometimes it has worked fine, but sometimes it looks right when I paste into the reply box, but it changes after the reply is submitted.

I should have checked, but I have been pushed for enough time lately.

John,

I'm very grateful for the level of effort you put into these posts... as are all of us playing with turbos, I'm sure. I'm applying the math to my planned VNT setup as you post updates, so if you're happy to keep posting them, I'm more than happy to query things I don't understand or clarify any typos I spot (and am reasonably confident of the answer).

Although not part of a turbo system, here is some information relating to the fuel injection pump and how it relates to the fuel flow rate.

The following 2 pics were copied from the Isuzu workshop manual for the fuel injection pump and relate to the 'RLD' governor on our pumps.
Pic 1


Pic 2


Note that the control rack position has to be moved in the direction of less fuel as engine rpm increases, corresponding to a reduction in VE (volumetric efficiency), and thus a reduction in air, and an increase in pump efficiency.

Also remember that our pumps are calibrated for no turbo in the case of the 4BD1 or the stock turbo for 4BD1T's.

When we add a turbo to a 4BD1, or fit a better performance turbo to a 4BD1T, the turbo increase the air available through increasing the boost pressure, and thereby the air density. So we are then able to increase the fuel at the rpm where air density/boost is increased significantly. The turbo has an effect similar to increasing the VE.

VE is the amount of fresh air drawn into the cylinder / the actual capacity of the cylinder. Here I include the capacity of the combustion chamber with the cylinder. The VE is less than 1.0 due to the induction system, but also because some exhaust gas remain in the cylinder when the induction stroke starts.

So if the ratio of pressure in the exhaust manifold (EMP) to pressure in the inlet manifold (IMP) increases, the VE is reduced. What this means to our turbo calculations is that the assumed VE we use for different rpm points, can be affected by the ratio EMP/IMP. With a tight turbine, the IMP (boost) will build quicker at low rpm, but the EMP will be higher at high rpm and the reduction in VE, with increase in pumping loss will reduce power compared to a larger turbine. Ideally the ratio of EMP/IMP will be little greater than 1.0

An increase in EMP/IMP much over 1.0 also reduces the SFC. So the turbine can make our assumed VE and SFC too optimistic if it is too small. Note also that the back pressure in the exhaust system after the turbine will directly increase the EMP.

The following 2 pics are copies of the calibration curves and table for a 1986 to 1988 Isuzu 4BD1T.

Pic 3


Adjustment point 'A' on the full load curve is the setting of the full load set screw on the side of the governor, the one we wind out so the control rack can travel further in the direction of fuel increase. This effectively shifts the entire full load curve up (the vertical axis is rack position).

BTW, point 'A' is at 900 pump rpm, or 1800 engine rpm (corresponding to max torque).

Pic 4


For this stock 4BD1T the calibrated fuel rate at point 'A' (max torque) is 10.2 to 72.2 cc per 1000 pump strokes.

Point 'D' is at 3000 engine rpm (max power and is 78 cc / 1000 strokes. The later (post 1988) 4BD1T makes about 14% more power at 3000 with a 10% reduction in fuel consumption. Point 'D' is 70 cc / 1000 strokes for the later 4BD1T.

For the stock 1986 to 1988 4BD1, the fuel rate at 1900 engine rpm (max torque) is 67.8 to 69.8 cc / 1000 stroke. At 2600 engine rpm the fuel rate is 70.9 to 74.1 cc/1000 stokes and at 3200 rpm (max power) it is 67.0 to 70.2 cc/1000 st.

With our stock IP we can easily adjust the full load screw to obtain a fuel rate of 140 cc/1000 st. The maximum we can get without changing to larger diameter plungers or other modifications is 180 cc/1000 st.

Unless we fit larger plungers and barrels, the increase in fuel rate is achieved by delaying the finish of injection. So the extra fuel is injected later. What this means is the SFC is lowered.

In an earlier post the values we used for fuel flow was in grams per minute. We then multiplied this by the the A/F ratio to find the air mass flow.

To convert fuel flow in g/min to cc/1000 strokes, we need the specific gravity of diesel fuel, approximately SG = 0.832. Then:

cc/1000 st = (g/min x 1000) / (SG x rpm x 2)

i.e. cc/1000 st = g/min x 600.96 / rpm

For our 3 examples the fuel rate in cc/1000 st is:
(ex a) 90 kW at 3000 rpm

fuel rate = 360.75 g/min x 600.96 / 3000 rpm = 72.26 cc/1000 st

(ex b) 135 kW at 3000 rpm

fuel rate = 541.12 g/min x 600.96 / 3000 rpm = 108.4 cc/1000 st

(ex c) 180 kW at 3000 rpm

fuel rate = 721.50 g/min x 600.96 / 3000 rpm = 144.53 cc/1000 st

To back that up, here's a US EPA tag plate from a 1989 4BD1T. It has 70 mm^3/st (cc/1000 shots) of fuel for 121 SAE hp (90kw). The 126hp rating must be the optimistic one.



However. The engine should also get more efficient with intercooling. So we may get slightly more increase in power than we get increase in fuel.

Originally Posted by Dougal

To back that up, here's a US EPA tag plate from a 1989 4BD1T. It has 70 mm^3/st (cc/1000 shots) of fuel for 121 SAE hp (90kw). The 126hp rating must be the optimistic one.



However. The engine should also get more efficient with intercooling. So we may get slightly more increase in power than we get increase in fuel.

Ha, that tag looks familiar! I certainly hope for a bit more power with the soon to be installer IC.

In my last couple of posts, I started to set a scene before starting on the turbine side of the turbo charger.

If there are any expansion needed or questions concerning the compressor side from what has been posted to date, this would be a good time.

For now I have some pages to share from the best book I'm aware of for turbochargers.

In this post about matching a turbo to a vehicle engine, which would have been more appropriate at the beginning. Apologies in advance for the quality.

Watson & Janota?

Some pages from the section on constant pressure turbocharging. Constant pressure refers to the pressure of the exhaust gasses in the manifold and entering the turbine. The other method is pulse turbocharging.

The 4BD1 turbo is not strictly constant pressure. For constant pressure the exhaust manifold needs a larger volume.

Pulse turbocharging requires the manifold branches to be separated so that the pulses do not interfere. It also needs sufficient number of cylinders to provide pulses that can drive the turbine smoothly, i.e. close together. Ideally the manifold will divide into multiple of three, e.g. well suited to a 6 cylinder engine. With our four cylinder the exhaust valves of two cylinders can be open at the same time, so the high pressure pulse created when one valve opens, is severely reduced by the low pressure from the other cylinder that has an open valve. This affect can mostly be overcome by a properly designed pulse converter.

Pulse and pulse converter turbocharging provides a great advantage over constant pressure turbocharging at low and mid engine speeds, where it can give a much needed torque increase.

Originally Posted by Dougal

Watson & Janota?

Yes.