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// BELLMOUTH DESIGN TECH TALK WORDS MARK MCVEIGH PHOTOGRAPHY AMCN ARCHIVES THIS SUCKS Getting air into your engine is one of the most important parts of it performing well. It’s also complicated, so here’s the basics S uck, squeeze, bang, blow. Sounds simple enough, doesn’t it? However, these four operations of a normal four-stroke engine must be completed efficiently for it to reach its maximum potential. This issue we’re going to look at the suck or induction phase of running, specifically focusing on bellmouth design. One could be forgiven for ignoring this modest part of the intake system; normally it’s the inlet port that receives the most attention from engineers. However the intake system, excluding the airbox, starts at the bellmouth and it’s this simple part that often gets overlooked in the development process as computational fluid dynamics (CFD), aluminium grinding and flowbench take favour. The design of these wee suckers can have a significant effect on the performance of a race engine, where two-percent more power can make a difference. Intake velocities through a MotoGP engine can be over 150m/s – that’s over 500km/h. Even the smallest bump or imperfection on the bellmouth can have a significant effect on the flow through the intake system. Next time you’re on a plane, look out the window and check out the shape of the jet ducting. Just like the bellmouth, this area is pretty important to airflow through an engine. It’s the profile shape of the bellmouth that is critical and if not optimised it can create a vena contracta – a choking effect which reduces the actual diameter of the flow. The section view through a typical bellmouth (diagram over page) shows how the flow can typically neck down at area AC, effectively restricting the airflow and potentially leaving the engine gasping for air. Engineers use a formula called CD – or discharge coefficient – which is defined as AC/AP. This is simply just the ratio of the measured to the theoretical flow, and is used to gauge the effectiveness of the bellmouth design. It’s pretty obvious that a bellmouth with a radius will flow better that a plain, straight tube. Testing has shown that the flow can improve by over 10-percent just by adding a radius to the end of a bellmouth. Such encouraging gains from a relatively simple change warrant further development in search of the perfect formula. Then there’s the question of what size of radius, taper, intake diameter, exit diameter, length, etc is optimum. You also need to consider the type of radius – simple, elliptical or aerofoil? This matrix of parameters adds up to a bunch of bellmouth options for consideration and can start to get expensive to develop. Which of the thousands of combinations best satisfies all of the performance requirements of the engine? To speed up this process, various computer simulation packages are used – including computational fluid dynamics – to fast-track the development cycle. Most importantly, CFD can simulate and visualise the minute flow details around the bellmouth, which otherwise are not captured using a flow bench, thus capturing horsepower. However, even with computer simulation it’s still a time-consuming process. To speed the process up even further, some MotoGP manufacturers use simulation packages that automate the design process. This involves using a design of experiment (DOE) algorithm that basically thinks for itself, and a system that contains a knowledge base, rules and strategies. DOE systems typically solve engine-performance design problems hundreds of times faster than an engineer. Getting back to CFD, testing in this area has thrown up some interesting results. As the velocity profile below demonstrates, the flow profile extends around the outside radius of the bellmouth. This analysis highlights the importance of wrapping the bellmouth profile radius far enough that the flow is not disturbed at the edges of the radius. All this testing through countless bellmouth designs has thrown up a formula which demonstrates the short, fat bellmouth with an elliptical profile is the most efficient. This shape is also known to reduce fuel-injection spitback when injectors are pointed into the bellmouth. Anyone who has experienced an airbox fire will be glad of this knowledge. What shape is your bellmouth? As with most things in life, motorcycle and engine design is a compromise, and bellmouths are not immune. Packaging for airboxes, cylinder spacing, etc all plays a role in the final specification. Engines are complex beasts and though we’ve tried to simplify the bellmouth design process as much as possible, we still haven’t considered many other aspects involved such as unsteady gas dynamics and pressure waves. We’ll save that for another day. The discharge coeffecient forumla is AC/ AP, which will give a good indication of how effective the bellmouth design will be by measuring the theoretical flow of air. Adding a radius can improve the intake flow by 10-percent – a significant gain for such a small modification. Air passing through the bellmouth can be travelling over 500km/h – that’s pretty fast. At that speed, miniscule imperfections on the profile will upset the flow of air as it passes through. If the air isn’t flowing as efficiently as possible then it can start to have a negative effect on engine performance. AC AP amcn 85/ amcn /84
