Posts Tagged ‘Cosmos’

Flow Simulation and the 75 Dollar Question

Friday, June 22nd, 2012

Is it worth the extra 75 dollars for a long tube header versus a short tube?

Let’s start answering this by examining how an exhaust header works, and why you would want one.  Headers are one of the easiest bolt-on accessories you can use to improve an engine’s performance. The goal of headers is to make it easier for the engine to push exhaust gases out of the cylinders.

 

To further understand why the exhaust manifold has an impact on performance let’s review the  combustion cycle of a gasoline engine.

  1. The intake stroke-  Starts with the piston at the top of the cylinder.  As the piston moves downward the intake valve opens allowing the air fuel mixture to enter the cylinder.
  2. The compression stroke-  Moves the piston back up to compress this air fuel mixture, causing the ignition of the air fuel mixture to be more powerful.
  3. The combustion stroke –  When the piston reaches the top of the cylinder, the spark plug emits a spark to ignite the gasoline. The gasoline charge in the cylinder explodes, driving the piston down.
  4. The exhaust stroke- Once the piston hits the bottom of its stroke, the exhaust valve opens and the exhaust leaves the cylinder to go out the header.

During the exhaust stroke, back pressure robs the engine of power. The exhaust valves open at the beginning of the exhaust stroke, and then the piston pushes the exhaust gases out of the cylinder. The more resistance there is to the piston expelling the exhaust gases, the greater the power loss.

Once the exhaust gases exit the cylinder they end up in the exhaust manifold. In a four-cylinder engine, all cylinders utilize the same manifold. From the manifold, the exhaust gases flow into one pipe toward the catalytic converter and the ­muffler. The idea behind an exhaust header is to eliminate the manifold’s back pressure. Instead of a common manifold that all of the cylinders share, each cylinder gets its own exhaust pipe. Old hot-rodder intuition, gut feel, and experimentation lead to each pipe being the same length, and using a two into one set up. Two into one specifies that the pipe leading from two cylinders merge into one.  In the case of a four cylinder, pipes from cylinders 1 and 2 lead to one pipe, and pipes from cylinders 3 and 4 lead to one pipe.  Those two pipes then merge again into the collector. The two into one method “smoothes” the flow through the pipe causing less turbulence when the flow fields merge.  These pipes come together in a larger pipe called the collector. By making them the same length, it guarantees that each cylinder’s exhaust gases arrive in the collector spaced out equally so there is no back pressure generated by the cylinders sharing the collector. Basically Header=Power, and we all want more power.

The 75 dollar question arose from my sister.  She is considering replacing her stock exhaust manifold with an after-market header, and was wondering what was the best “bang for the buck”.  After researching the topic extensively we found that across all the after-market brands the designs seemed to be the same regarding pipe routing, materials, etc.  So the main question came down to should she buy the “short tube” or “long tube” header?

Both the “long tube” and “short tube” headers have equal length pipes from the engine block to the collector.  Both ran a two into one method.  The long tube header however claims that since it is longer by design there would be less back pressure due to a smoother flow.  The differentiator was about 75 dollars, and the fact that the “long tube” header would need the catalytic converter to be moved and remounted by a muffler shop.  The “short tube” header is a direct bolt in.

I couldn’t resist turning to Flow Simulation to solve this question.

We purchased the long and short tube headers, and removed the stock manifold to be able to accurately take measurements from them.  The models are close but not exact without a reverse engineering tool such as a scanner or arm.

After the models were completed the next step became the boundary conditions.  I was able to find a good reference guide located on line from www.donaldsonexhaust.com.  Given the engine Horsepower, cubic inch displacement, and operating RPM I was able to determine Intake airflow, and exhaust gas flow in CFM.

This calculated the exhaust gas CFM to be 520.00 CFM, or 130.0 CFM per port. Please see the hand calculations below.

Yes Engineers Still Do Hand Calcs

Knowing the CFM of the exhaust leaving the cylinder allows us to compare pressure drop from the inlet to outlet across the three manifold models.  The stock exhaust will be the base line for comparison.

 

Model Set Up:

 

Inlet Condition:                130 CFM per inlet port

Outlet Condition:             Environmental Pressure

Surface Goals:                   Each Inlet Goal – Static Pressure / Mass Flow Rate

Outlet Goal – Static Pressure / Mass Flow Rate

Results:


Stock Flow Path

Stock Pressure Gradient

Short Tube Pressure Gradient

Short Tube Flow Trajectories

Long Tube Pressure Gradient

 

Summary:

 

The “short tube” header is hands down the best value.  Both after-market headers showed a drastic decrease in pressure drop over the stock manifold however, the “long tube” header only had an edge over the “short tube” pressure by 0.019 PSI.  As a bonus the “short tube is a direct bolt in, not requiring the existing catalytic converter to be moved.  As Engineers we are always worried about time and money, and are often faced with a decision regarding these two factors.  From my engineering background and proof provided by flow I recommended the “short tube” header.

Robert Warren

Elite Application Engineer CAE Technical Specialist 3DVision Technologies

Dimension Printed Simulation Verified Blow Off Valve Adapter

Wednesday, June 29th, 2011

Adding a new Blitz Blow Off Valve (BOV) to an aftermarket turbo system lead to no clearance between the valve and the hood of the vehicle.  An adapter was needed to drop the BOV from the high pressure pipe outlet to between the twin cooling fans behind the radiator.

 

High Pressure Pipe Assembly

High Pressure Pipe Assembly

 

BOV Adapter

BOV Adapter

The problem statement is as follows:

A custom adapter was developed to accommodate hood clearance.  Before final fabrication out of aluminum a prototype was “printed” using a Dimension Rapid Prototype Printer.  The printed ABS parts are inherently porous and needed to be sealed in order to hold pressure.  The part was dipped quickly  in acetone and then washed thoroughly with soapy water to seal the pores.  The part was then tested to 110 PSI on a test bench before failure.  A second part was then tested on the car.

Because operating pressure is only 10 PSI, a FOS of 10 was provided by the design.

110 PSI Failure

110 PSI Failure

The second consideration is that the BOV is cantilevered off of the high pressure pipe bung .  The BOV weighs approximately 1/8th of a pound. Adding this to the loading still produced a FOS of   5.

Combined Load

Combined Load

Simulation verified the physical test results and showed that the printed part holds up to the design requirements. The printed ABS adapter works so well an aluminum version was never fabricated.  110 passes down the 1/4 mile drag strip, 1000′s of miles, and 4 autocross seasons, and the little plastic adapter keeps on going.

Robert Warren

Elite Application Engineer CAE Technical Specialist 3DVision Technologies

Mesh Control: As Easy as 1-2-3

Wednesday, April 13th, 2011

When is the last time you saw this warning dialog when meshing?  If your primary responsibilities include FEA, chances are it was as recently as yesterday.

2011-0412a Mesh Failed

As we continue to design and analyze increasingly complex models, our need to access mesh controls will increase proportionally.  Wouldn’t it be nice if accessing those tools were made simple?  Amazingly enough, they already are!  At the conclusion of a failed mesh, just click on the Mesh Failure Diagnostic button.

2011-0412b Failure diagnostics

Yes, it is that simple to access the Mesh Failure Diagnostic tools.  You’ll notice that this opens up the Simulation Advisor in the Task Pane.  The Simulation Advisor is a great tool for beginning and experienced Simulation users, alike.

Here are the 1-2-3′s of using the Simulation Advisor for applying mesh controls.
1. Select one of the parts from the list of ‘failed to mesh’ parts.
2. Click the Mesh Control button.
3. Apply the local mesh control desired to the part by either changing the slider bar or typing a value for the mesh control you wish to apply.

2011-0412d Mesh Control 123

Now click the green check mark to OK your mesh controls.  SolidWorks Simulation will mesh the component you just applied mesh control to.  Notice that you can apply mesh control to several components in one step by adding (clicking) more components to the ‘Selected Entities’ dialog box.  Another nice feature is that the Simulation Advisor window will stay open as long as you have parts that failed to mesh, allowing you to continue applying controls until your entire model has meshed.  Try using the Simulation Advisor the next time you encounter a model that is difficult to mesh!

Bill Reuss

Elite Application Engineer CAE Technical Specialist 3DVision Technologies

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