Boost Pressure Change - Diesel Truck Forum - TheDieselGarage.com
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post #1 of 12 (permalink) Old 02-05-2008, 09:21 PM Thread Starter
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Boost Pressure Change

Would like to understand how the boost pressure changes. For instance while in cruise control I started going up a slight incline and noticed the RPM's appeared to stay the same but the boost pressure jumped form less than 10psi to app. 20psi. Still the RPM's remained unchanged and I maintained the set speed. It must have something to do with the extra fuel flow from the cruise control...how does this work?
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post #2 of 12 (permalink) Old 02-05-2008, 09:27 PM
 
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more stres on the engine causes the boost to go up. So while your going up a hill theres more of a load on the engine.
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post #3 of 12 (permalink) Old 02-06-2008, 10:45 PM Thread Starter
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Thanks for your response. What is the stress? Does the boost pressure indicate the compression of air entering the manifold? If the RPM's don't change how does the turbo increase the air pressure since it is the exhaust gases that spins the fan? How much is the inlet air temperature increased in proportion to the boost pressure increase? I know the purpose of the intercooler is to reduce the air temp resulting fromt the boost pressure. Just would like to understand the mechanics involved.
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post #4 of 12 (permalink) Old 02-06-2008, 11:11 PM
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Load/exhaust velocity is what dictates how much boost. Not RPM. You're thinking of a supercharger.

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post #5 of 12 (permalink) Old 02-06-2008, 11:53 PM
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couldn't have said it better myself

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post #6 of 12 (permalink) Old 02-07-2008, 01:23 PM Thread Starter
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Loading the engine increases boost pressure. OK but what I am asking is what mechanically results in the gauge showing a change? Does a doubling of boost pressure also double the air temperature? The manual says not to run at a high boost pressure for extended time periods. Is this because of the temp. becomming so high that the engine is damaged?
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post #7 of 12 (permalink) Old 02-07-2008, 05:05 PM
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As the truck starts up the grade the PCM adds fuel to make more power. The RPM stays the same because the torque converter remains locked. The additional fuel makes more heat in the exhaust, and this heat drives the turbocharger faster. The faster turbocharger compresses more air, resulting in more boost. Boost is just pressure in the intake manifold.

Doubling boost does not necessarily mean the air temp is doubled. It isn't a linear relationship, and it is further complicated because there is an intercooler, which cools the air between the turbo and the intake manifold.

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post #8 of 12 (permalink) Old 02-07-2008, 07:34 PM
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Quote:
Originally Posted by Mark_K
The additional fuel makes more heat in the exhaust, and this heat drives the turbocharger faster. The faster turbocharger compresses more air, resulting in more boost. Boost is just pressure in the intake manifold.
Don't the variable vanes in the turbo adjust to ramp up boost also? Sortof a more aggressive pitch for more air.
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post #9 of 12 (permalink) Old 02-08-2008, 07:33 AM
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Quote:
Originally Posted by ZeroOne
Don't the variable vanes in the turbo adjust to ramp up boost also? Sortof a more aggressive pitch for more air.
Yep, the second (larger) turbo is variable vane'd and will respond as Mark explained.
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post #10 of 12 (permalink) Old 02-14-2008, 02:07 PM Thread Starter
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Thanks for all the responses. I found a source that explained the process of how the turbocharger adds power while keeping the RPM the same. Know that the ratio of air to diesel fuel must remain the same. The turbo spins as a result of the exhaust pressure and temperature. The pressure is proportional to the demand on the pistons. As the vehicle begins to need power the resistance of the piston increases. More compression is required to push the piston back. This increases the pressure and temp of the exhaust gas. In turn the turbo responds to the pressure and temp increase. The computer senses the spin increase and incremenatally adds fuel to the cylinder thus yielding more explosive power. The boost increases power while RPM stays constant. Of course your MPG goes down. A remarkable way of increasing an engine's power using the same displacement.
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