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Understanding Port Swirl In Diesel Engines
In any internal combustion engine whether it be gas or diesel, port swirl plays an important role in the combustion process. But how much do you really understand about something so important?
By Bob McDonald
What is port swirl? Actually this is a little contradictive. The port doesn’t actually swirl, it is the design of the port that causes air to swirl! Swirl is what is taking place when the intake charge exits the intake valve and enters the combustion chamber. As the air enters the combustion chamber and the piston travels down in the bore, it causes a spiraling effect. The air in the spiraling effect actually travels around the bore such as water goes around in a flushing toilet. This effect is used in helping to fill the cylinder. It is swirl that controls the engine’s output and rpm.
To help explain this further we will use a flowbench to demonstrate the differences between gas and diesel and see the effects of port swirl in the combustion process. In a typical modern day internal combustion engine you have four strokes: intake, compression, power and exhaust. In a naturally aspirated gasoline engine, just before the piston is top dead center, the intake valve begins to open to start filling the cylinder with air and fuel.
As the piston starts down the bore, the intake valve fully opens and the air-fuel mixture gets drawn in as the piston nears bottom dead center. With the design of the port, as air passes through and exits the intake valve, it enters the cylinder in a swirling motion. In all of the heads that I have flow tested, the manufacturers incorporate a level of swirl into the port for the application.
I keep mentioning application because there are different levels of swirl found in gasoline engines depending on whether it is a stock, mild or more wild application. On the flow bench you will often see that porting cylinder heads for more cfm causes the swirl percentage to drop. This happens because you are reshaping the port’s design for more airflow and most of the time the head porter is trying to straighten the port. Air tends to flow better with less bends and restrictions (see illustration).
On a flow bench, swirl is measured with a swirl meter which will, in turn, be measured in rpm. In a typical small block Chevy, swirl ranges anywhere from 700 to 1,400 rpm, in accordance with how much lift is measured at the valve. This generally takes place from approximately .002˝ to .007˝ of valve lift.
Now in a diesel cylinder head, the swirl from the intake port entering the combustion chamber can be as much as three times the rpm of gasoline cylinder heads. Also, keep in mind that because the valve lift in diesel engines isn’t very high, the swirl starts taking place at about .001˝ of valve lift and continues to gross lift, which is about .004˝. So, we know the biggest differences will be that the diesel cylinder head is producing more swirl and doing it faster.
Why does this all really matter? Think of it this way, if swirl is helping to fill the ports of both the gasoline and diesel engines, why does the diesel demand more? In a gasoline engine, once the intake valve is closed and the swirling stops, the piston is starting to travel up the bore for the compression stroke. The mixture is just being compressed waiting to be ignited by the spark plug.
In a diesel engine, when the intake valve closes and the piston starts the compression stroke, the swirling continues! Yes, as the piston travels further up the bore, the air continues to swirl faster and faster in the cylinder and the piston bowl. It not only moves in a circular motion, but also a spiraling motion. The reason for this is to light off the diesel fuel.
Remember, diesel is injected into the cylinder in very fine atomized droplets, an action that occurs at approximately 4 degrees before top dead center. In a gasoline engine, on the compression stroke you have air and fuel being compressed in the cylinder. In a diesel, you just have air that is being compressed. So, when the piston is traveling up the bore in a diesel engine, cylinder pressure is increasing rapidly.
When diesel fuel enters the combustion chamber, it has to light off. The more the pressure rises, the harder it is for the diesel fuel to ignite. Pressure tends to keep the diesel droplets in a suspended state. So, the swirling motion lets the diesel droplets that enter the cylinder cling to the bowl and cylinder walls to aid in lighting the fuel off. This is why you hear the diesel make the “clacking” sound.
Most people think that port swirl in a diesel is used to aid in turbo lag. But actually, the turbo is used for the purposes of forced induction. This, in turn, puts more air in the cylinder. When swirl is taken away the power will go down.
One thing that has been incorporated in diesel engines is the use of pilot injection. In this case the piston travels up the bore and, before the normal injection takes place, a small amount of fuel is injected into the cylinder. This will start the light-off process faster. In turn it makes for easier cranking and quieter, more efficient operation.
If you look at a diesel from a performance standpoint, you can’t disturb the swirl of the port or the piston design. The manufacturers of these engines have gone through countless hours and testing with equipment we only wish we could use in order to design the port and piston combination that makes their engines most effective.
Robert McDonald is owner of Atlantic Engines in Granite Falls, NC, and specializes in high performance diesel and gasoline engines and cylinder heads for street, marine, dirt or drag racing. You may reach Robert at email@example.com.