Ducati have been focusing on taming the GP12’s aggressive power delivery since last season when Rossi climbed aboard the GP11 beast and complained constantly for Ducati to make the bike easier to ride and more competitive in MotoGP.
MotoGP electronics are receiving a lot of press attention these day’s, however one area that has been overlooked recently in the media is the tuning affect of the exhaust system on power delivery.
We often hear about how exhaust systems are tuned to maximise power. Exhaust tuning can also be used to move the power curve peaks around, tuning for better low, mid range or maximum power.
How does this actually work?
Well the exhaust system has a number of jobs.
The basic requirement is to scavenge or empty the cylinder of exhaust gases before the new inlet charge enters for the next combustion cycle.
When the exhaust valve opens and blow down occurs, exhaust gas particles and pressure waves are created in the exhaust system.
The exhaust gas particles, described as a ‘slug’ by the great Australian Engineer Phil Irving, travel along the exhaust pipe at around speeds of 100 metres per second or 350 km/hr.
The pressure waves, which are also created on blow down, travel back and forward through the exhaust gases at the speed of sound which is around 350 metres per second. However the speed of sound changes with temperature and temperatures in the exhaust system can be 700 degrees C which speed up the pressure waves to over 500 metres per second or nearly 2000 km/hr!
To get an understanding of the pressure wave speeds, Engineers log the exhaust gas temperature or EGT data by way of thermocouple sensors in the exhaust system, normally on the dyno and sometimes on the track.
Exhaust gas particles also have weight or mass, and once this mass is moving its called momentum. Momentum is defined as mass in motion. This exhaust gas particle ‘slug’s’ momentum will flow even after the exhaust valve is closed and creates a vacuum or negative pressure that can be used to suck exhaust gases from the cylinder.
Also when the exhaust valve opens, burnt gas explodes from the cylinder and a positive pressure wave is created which travels out the exhaust system. When this wave reaches a sudden change in area, such as a merge, collector branch or the atmosphere, a negative or suction wave is created which travels back up the exhaust system to the exhaust valve. Timing of this negative or vacuum wave at the exhaust valve is critical as it effectively draws out exhaust gas from the cylinder and assists some of the fresh charge entering the engine on valve overlap, when both the intake and exhaust valves are open simultaneously.
Exhaust tuning can dramatically improve the engines volumetric efficiency, which is defined as the mass of air supplied through the intake valve during the intake period, by comparison with a mass required to perfectly fill the swept volume. An example of increased volumetric efficiency would be a MotoGP cylinder of 250cc squeezing in 255cc of inlet charge, the exhaust system tuning having a supercharging effect.
The MotoGP Engine Engineers conundrum is to design the exhaust system length and size to enable this suction wave to arrive at the exhaust valve at the correct moment which will maximise the efficiency of the engine.
As a rule of thumb, the header pipe diameters control the exhaust gas velocity. A larger pipe will reduce the velocity and smaller pipe will have the opposite affect. This diameter has a large affect on where in the rpm range peak torque will take place. Changing the exhaust lengths and junction positions has an effect of pivoting the torque curve around the peak torque point. Longer pipes tending to improve low and mid range whilst shorter pipes have the opposite affect improving top end.
The next time you look at a MotoGP exhaust system you can now visualise the symphony of exhaust gas particle slugs mixed with positive and negative pressure wave’s fanging up and down inside those pipes.
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