EXHAUST MANIFOLD DESIGN FOR TURBOCHARGED ENGINES
The configuration of the exhaust manifolds used on turbocharged engines can have a significant effect on the performance of the engine. The turbine casing of the turbocharger has a relatively small throat area in its nozzle section in order to generate a high exhaust gas velocity at the entrance of the turbine wheel. This high entrance velocity is necessary to enable the turbine to generate the power needed to drive the compressor wheel. A typical entrance velocity triangle is illustrated below:
The small throat area of the turbine casing presents a restriction to the exhaust gas flow from the cylinders and results in a high pressure in the exhaust manifold ahead of the turbine casing. Since the engine pistons in 4-cycle engines must act against this pressure when evacuating the cylinders, the level of pressure in the exhaust manifold causes a parasitic loss in engine power.
If all the engine cylinders exhaust into a common manifold, the pressure in the manifold will remain at a high level for all the engine's pistons when they are pushing remaining exhaust gases out of the cylinders on their exhaust upstroke.
Thus, it is desirable to separate the exhaust manifold into several branches so that no successive exhaust pulse enters into a common branch. For example, in an in-line 6-cylinder engine that has a firing order of 1-5-3-6-2-4, it is advantageous to divide the manifold into two branches, allowing cylinders 1, 2 and 3 to exhaust into one branch and cylinders 4, 5 and 6 to exhaust into the other branch. This allows the pressure level from cylinder number 1 to fall to a low level before cylinder number 3 exhausts into that branch, etc. for each remaining cylinder. The result of this manifold division is a low average pressure in the manifold branches that reduces the pumping loss of the engine, increases power output, lowers fuel consumption, and reduces smoke on acceleration.
A schematic diagram that illustrates the exhaust pressure variation in a 6-cylinder engine divided manifold follows:
By separating the exhaust pulses by the manifold division, the pressure in each branch is allowed to fall to a low value before the next cylinder exhausts in that branch. The solid line in the above diagram represents the pressure in the 1-2-3 cylinder branch whereas the dotted line represents the pressure in the 4-5-6 cylinder branch. This manifold division is illustrated below with each branch connected to one opening in a divided turbine casing.
In a 4-cylinder engine with a firing order of 1-3-2-4, the cylinders should be divided with cylinders 1 and 2 in one branch, and cylinders 3 and 4 in another. This is schematically illustrated below:
In the case of a V-8 engine, the division of the manifolds becomes more complicated. There are a number of different firing orders that can be successfully used in 4-cycle V-8 engines. One commonly used firing order is 1-8-4-3-6-5-7-2. These cylinders should be divided into four branches using two turbochargers with divided turbine casings; one on each side of the engine. This can be illustrated as follows:
The above manifold division will work equally as well with several other firing orders. These are:
If peak engine performance is desired or required, then divided exhaust manifolds are an absolute necessity; either fabricated or cast. Compared with undivided manifolds, properly dividing the exhaust manifolds used on turbocharged engines will make a very noticeable improvement in engine and vehicle performance.