One of the wide range of services offered by EASL is Computational Fluid Dynamics (CFD). With CFD, the interaction of two fluids, the flow of air over an object, or the heat transfer through a material can be analysed for a number of purposes. The uses for CFD are almost as varied as the services EASL offer.
Here, we are going to look at how CFD analyses can be utilised to improve the flow of exhaust gases through an exhaust manifold for a piston engine and is based upon a basic understanding of the four-stroke cycle and valve timing.
It’s a common knowledge that an exhaust system is there to take the noxious gases away from the occupants of a vehicle, but what is not realised by most is that exhaust systems can restrict the amount of power an engine can output if designed poorly.
The part of the exhaust system where the design is most crucial is called the exhaust manifold/header.
To understand how CFD can be used to extract more power out of an engine, the theory of exhaust manifold design needs to be understood.
At the end of the power stroke, the pressure in the cylinder is higher than the pressure in the exhaust port and exhaust pipes. When the exhaust valve opens, the pressure difference causes the exhaust gases to flow into the exhaust.
With this pressure difference, as the gases flow into the exhaust port there is a pressure spike within the port.
This spike creates a pressure wave that flows down the exhaust. It isn’t until the later stages of the exhaust stroke that focus switches to the pressure wave. When the pressure wave does come into focus though, is when the length and diameter of the pipes comes into play as this defines the rpm at which this pressure wave is most useful. The length and diameter of pipe also affects the gas flow, but more on that later.
When the pressure wave comes across a change in cross-sectional area, a reflected pressure wave is generated.
This reflected pressure wave is most often negative. The trick with exhaust headers is to tune the length and diameter of the pipes such that this negative pressure wave arrives back at the exhaust valve as the intake valve opens at the desired rpm.
During valve overlap, this negative pressure wave will help to suck the fresh intake charge into the cylinder, this is known as exhaust scavenging.
Although, if the exhaust scavenging effect is too strong, fresh intake charge will be pulled into the exhaust; reducing power. This is known as over scavenging. To help analyse this pressure wave, acoustic analyses are ran instead of CFD analyses.
Back to the initial and middle parts of the exhaust cycle. Initially when the exhaust valve opens, due to the high pressure differential the gas velocity is high and tails off through the exhaust cycle as the pressure inside the cylinder reduces to near atmospheric pressure.
It is through this stage of the exhaust cycle that the gas flow inside the header needs to be optimised as this is when turbulent flow is most likely to occur.
CFD is used here to analyse the gas flow making sure that there is no build up in pressure in concentrated areas, and to make sure the gas velocity and temperature is as constant as possible. With the gas flow, the focus is on flow separation around the bends.
Flow separation is where the gas peels away from the boundary layer and causes the gas flow in this area to turn into a vortex, which will inhibit the gas flow.
In exhaust headers, a cause of this is too tighter bend radii where the solution is to enlarge the radius of the problematic bend. However, that is not always possible due to packaging constraints.
The flow is also affected by the diameter and length of the pipes. If the pipes are too large then the gas will travel too slowly and won’t be able to evacuate the cylinder in time. If the diameter is too small, then the gas will choke at high rpms.
Since the minimum diameter is governed by the diameter of the exhaust port, and it is good practice to have the primary pipe diameter slightly larger than the exhaust port, the starting point for the diameter is immediately obvious.
From here, the shorter the pipes, the better it is for getting the exhaust gas out of the system. But that affects the rpm at which scavenging is effective. As a general rule of thumb, the higher the chosen rpm, the shorter the header needs to be.
Exhaust header design/tuning is all about finding the balance between pipe diameter and length to make sure the exhaust gas does not choke and to make use of the negative pressure wave at the correct rpm. Smoothing the airflow through the exhaust to keep the amount of turbulent flow down to a minimum is also a priority.
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