Thread: Supercharge a 4BD1?

Originally Posted by Dougal

Feel free to walk me through it.

We have done this dance before dougal, with turbo charging and LPG fumigation. NB my Isuzu hasn’t melted a piston yet.
Not prepared to get into yet another bagging match with you

Oh well, for those who are interested, here's a VW TDI thread on boost vs backpressure in their turbos.

This one has some very interesting plots with boost (blue) and backpressure (red) through 3rd gear acceleration.
Exhaust & Intake Pressure Measurements - TDIClub Forums

This plot shows it quite nicely, when all the conditions line up nicely you end up with boost overlapping backpressure, but in acceleration that only happens for a little bit.

Originally Posted by Dougal

This plot shows it quite nicely, when all the conditions line up nicely you end up with boost overlapping backpressure, but in acceleration that only happens for a little bit.

Dougal
Although an informative post you have omitted to point out that the exorst gas pressures measured in this example are post turbo. Therefore a weeeee bit misleading to have posted this as an example.
As many already know that it is the pressure difference from pre turbo and post turbo that "spins the wheel" and that moving a gas from a smaller volumetric area to a larger volumetric area will reduce pressure accordingly.
Pre turbo pressure is the measure of the lack of cylinder scavenging, thus the deduction in efficacy that a turbo introduces. Regardless of the turbine efficacy of the turbo its self.
A super charger will not have these difficulties to over come. They do have other difficulties but.

Originally Posted by 85 county

Dougal
Although an informative post you have omitted to point out that the exorst gas pressures measured in this example are post turbo. Therefore a weeeee bit misleading to have posted this as an example.

They are manifold pressures, not post turbo. EMP = Exhaust Manifold Pressure.

You'd need a plugged muffer to have 20psi after the turbo.
The post turbo pressures are shown in that green line at the bottom of the plot. The one reading very little pressure. The red and blue are as previously explained total backpressure and boost.

so how does the turbo produce more boost on the intake than the exhaust side, is it something to do with the exhaust gasses cooling therefore shrinking in the tail pipe and therfore 'sucking' the exhaust gasses out. creating a low pressure system to increase the flow and therefore speed up the impeller

Originally Posted by mopar

so how does the turbo produce more boost on the intake than the exhaust side, is it something to do with the exhaust gasses cooling therefore shrinking in the tail pipe and therfore 'sucking' the exhaust gasses out. creating a low pressure system to increase the flow and therefore speed up the impeller

It's because turbines feed off both heat and pressure. The more heat you have (i.e. hotter exhaust), the less pressure they require.

Originally Posted by Dougal

It's because turbines feed off both heat and pressure. The more heat you have (i.e. hotter exhaust), the less pressure they require.

I think both you and 85 County are slightly skewed in your explanations. My take is that when you add heat to a mass flow, you increase its pressure, and therefore its velocity as it escapes into the turbo. Mass x velocity = momentum, which is the physical quantity that changes through the turbo. This momentum is added to the turbo vanes and then transferred to the inlet stream, less any losses in friction, ie it is conserved. Turbo lag relates to the momentum that is stored in the turbo's spinning shaft before being transferred to the inlet stream.

As the mass flow should be the same through the inlet and outlet sides of the turbo, the best measure of change is the temperature difference from one side of the turbine wheel to the other. Heat is not quite the correct term for temperature. Heat is a mathematically derived term from mass, specific heat and temperature difference. At least that is what I learnt in physics classes. It cannot be measured directly.

Just my attempt to clarify (???) the explanation.

Originally Posted by bee utey

I think both you and 85 County are slightly skewed in your explanations. My take is that when you add heat to a mass flow, you increase its pressure, and therefore its velocity as it escapes into the turbo. Mass x velocity = momentum, which is the physical quantity that changes through the turbo. This momentum is added to the turbo vanes and then transferred to the inlet stream, less any losses in friction, ie it is conserved. Turbo lag relates to the momentum that is stored in the turbo's spinning shaft before being transferred to the inlet stream.

As the mass flow should be the same through the inlet and outlet sides of the turbo, the best measure of change is the temperature difference from one side of the turbine wheel to the other. Heat is not quite the correct term for temperature. Heat is a mathematically derived term from mass, specific heat and temperature difference. At least that is what I learnt in physics classes. It cannot be measured directly.

Just my attempt to clarify (???) the explanation.

Of course mass is conserved, I'm not sure why you've bought that up as no-one has stated otherwise

The turbine in a turbo is dealt with in the brayton cycle. Here's a nice summary:
http://www.engr.sjsu.edu/ahashemi/Brayton%20Cycle.pdf

The energy to be concerned with is enthalpy (symbol is "h", units Joules), it includes the heat, kinetic energy and pressure of the gas flow. Follow the maths through (it's all in the PDF linked) and you'll see that the more heat energy you have, the less pressure you require to extract the same amount of work.

Which is exactly what the measurements taken by myself and others show well.
Accelerating from cold I have twice as much backpressure as boost.
At cruise with ~430C EGT I have around 1.5 times as much backpressure as boost.
Under high load and low rpm situations (600+C EGT) I have a small window of more boost than backpressure.