Intake and Exhaust System Architecture Part 1

Moving on from the basic engine geometry we can begin to calculate intake and exhaust architecture; Now to appeal again to all I have changed the engine from a Formula 1 engine to a MotoGP engine of 2011 Regulations (800cc), feel free to carry out the following on a 1000cc engine of 2012 on, but remember bore cannot exceed 81mm alternatively continue with the F1 engine geometry calculated in the cylinder blog.

The MotoGP cylinder architecture is;

Engine Specification
Bore	79.6mm
Stroke	40mm
S:B Ratio	0.503
mps	25m/s
Max RPM	18750RPM

Intake system;

The whole concept of the inlet port is to deliver the correct amount of charge in the cylinder at the critical time (Inlet valve closed (IVC)). So mass or speed of the intake charge will only matter at a critical stage in the filling procedure just as it will on the exhaust side during scavenging. The length of the overall intake system will affect the filling procedure due to rarefaction and pressure wave tuning the same as the overall diameter affects CFM and velocity.

This is confirmed by Heisler;

 ‘When the engine is running, a column of air moves through the induction tract passageway from the point of entry to the inlet port and valve and then into the cylinder. Every time the inlet valve opens, the reduction in cylinder pressure produces a negative pressure-wave which travels (at the speed of sound) through the column of air from the back of the inlet valve to the open atmospheric end of the tract. Immediately this pressure-wave pulse reaches the atmosphere, rarefaction occurs. Instantly the surrounding air rushes in to fill this depression. As a result, a reflected positive pressure-wave is produced that travels back to the inlet port’ [1]

Heisler goes on to say;

‘good manifold design are as follows ……5 to provide the smallest possible induction tract diameter that will maintain adequate air velocity at low speed without impeding volumetric efficiency in the upper speed range’ [1]

Blair writes:

‘Without the reflection of pressure waves, such as found in normal acoustic analysis, volumetric efficiency and thus power would be greatly affected’ [2]

The length of the intake system should be calculated by taking into account the speed of sound and the RPM that the engine will operate whilst taking into consideration the time in crankshaft degrees it will take for a pulse to travel the length of the system and back to the atmosphere. The degrees of crankshaft rotation during this time can be between eighty and ninety degrees dependant on manifold design so an array of differing durations need to be calculated and later optimised for the final engine. C is the speed of sound in air (343m/s) and N is the operating engine speed you wish to optimise the intake for. In this case peak RPM and power (18750RPM);

As can be seen in figure 2.2 the diameter of the intake system is directly related to the engine’s piston area and the these diameters a, b, c, d and e (fig 2.3) can be calculated as follows;

Exhaust Port;

The length of the exhaust system can be calculated as follows;

 

The Inlet Valve to Exhaust Valve Ratio for a high performance engine is given as 1.2 [3] and therefore the exhaust valve can be calculated as;

30.8/1.2 = 25.6mm

The exhaust system is also directly related to the piston area and as such the exhaust primary (the area where the exhaust manifold meets the cylinder head) can also be calculated;

‘The optimum value of the ratio of exhaust primary cross section area to piston area is 0.287.’ [3] thus;

[1]       Heinz Heisler. Advanced Engine Technology. Butterworth Heinemann ISBN 0-340-56822-4, 1995 Updated 2003.

[2]      Gordon P. Blair. Design and Simulation of Four Stroke Engines. Published by Society of Automotive Engineers, 1999. ISBN 0-7680-0440-3

[3]      G Cantore and E Mattarelli. Similarity Rules and Parametric Design of Four Stroke Moto GP Engines. 2004 SAE Technical Paper 2004-01-3560.