This question comes up now and again answers are mixed and occasionally confused due to sources on the internet not making sense.
Basically this thread will describe what Twinport is, that is to say, just the Twinport technology, nothing else that is included on the new Twinport engines such as the enhanced EGR system, this is just about the Twinport mechanism, how it works and why it's used.
Background on emissions
The first point to note is that the engine has been designed to conform to emissions regulations and to be efficient and driveable. To ensure low emissions you want to reduce the amount of un-burnt fuel that goes through the exhaust. To ensure the maximum amount of fuel gets burnt it must have enough time in the cylinder to mix with the air, and it must have the right amount of air in the cylinder ideally a bit lean and with a good mixture composition. The amount of time is determined by engine speed, which is low for the emissions regulation checks. The volumetric efficiency of the air and the composition of the air-fuel mixture, is influenced heavily by inlet design. Now that you understand what is required to ensure low emissions you will have a better understanding of Twinport technology.
What is it in simple terms?
Twinport technology simply put involves partially blocking off one of the two inlet ports on each cylinder at low engine speeds. The 4 valve per cylinder engine design shows why there are two inlet ports.
How does it work ?
Twinport works by a spring loaded acctuator contracting at high rpm. At rest and low RPM the valve is left out. This acctuator controls (by way of a plastic rail) the 4 metal plates in each of the inlet port pairs.
My hand is controlling the spring loaded valve at this point, pushing it in moves the flaps up, letting it out let’s them fall to their default position.
Default Closed position at low RPM < ≈ 3000
The pressure difference between the cylinder and inlet tract is what causes the air to rush towards the cylinder. As the inlet valve opens there is a vacuum in the cylinder compared with the ambient pressure, this causes air to rush in, thus filling the cylinder. This is why the air tries to travel with a constant mass flow rate. Mass flow rate is the product of air density, velocity and the area through which the matter is flowing. The area has clearly decreased through blocking off one port. Therefore to maintain a constant mass flow rate of air the velocity must increase.
This faster moving air improves the low engine speed volumetric efficiency of the engine due to the increased inertia of the air (due to the increased velocity) resulting in greater amounts of air filling the cylinder after the piston has reached BDC. With the fixed cam timing on the engine after BDC as the piston rises air is forced out of the closing inlet valve. With increased air interia from the faster moving flow, air can continue to fill the cylinder or at least slow down the rejection of air from the cylinder, resulting in a greater amount of air in the cylinder during the compression phase.
This faster moving air induces turbulence as it travels through the inlet pipework. Extra turbulence is caused by the blocked port. This excited turbulent air is easier to mix with fuel particles. This results in more air and more fuel being mixed at the same time (homogeneous mxiture). The more turbulent tumble of the air fuel mixture inside the cylinder also promotes a more turbulent flame front upon ignition which also improves the burn phase of the fuel and subsequently reduces emissions.
This reduces emissions as un-burnt fuel containing hydrocarbons is not being wasted. Fuel economy increases, emissions drop and you can get a possible torque increase at low RPM compared to Non-Twinport. In this writers opinion it is other changes made to the newer engine through changed engine geometry, cam and head design which directly alter the peak bhp value.
This picture is just showing the path the block off plate will take. Note the holes in the plate are to ensure that the air does not get backed up . If it were completely solid it may result in the airflow of the system being slower than is required. The holes in the plates allow for excess which has gone down the blocked off channel to vent around it. A solid plate might stall the airflow and could slow the neighbouring airThe transition between this phase and the next “open” phase happens almost instantaneously it is only to show the mechanism.
Open – High RPM > ≈ 3000
At this engine speed the airflow is high enough to sustain proper air-fuel mixing, and any blockage reduces volumetric efficiency. This is combined with the fact that the engine speed is out of emissions testing regulations so it’s not required to provide perfect air-fuel ratio.