The new exhaust developed by Yamaha is getting a lot of attention. But few elements have as big an impact on the design of a motorcycle and its engine as the exhaust system. From the junction of the cylinder head to the throttle outlet, the gently curved shapes help define the character of our motorcycles and their engines, and the shapes are far from purely aesthetic.
Exhaust gases contain useful energy in the form of velocity, and smooth shapes minimize the loss of this energy (whereas, on the contrary, abrupt changes in direction disrupt this flow). This is why its curvature must be as uniform as possible.
At full throttle, during the explosion of the gas mixture in the combustion chamber, the pressure at the piston head reaches approximately 50 bars. The gases expand and this pressure drops quickly as the piston moves down, transmitting power to it. When the exhaust valves begin to open, the pressure resulting from the expansion of the gases in the combustion chamber is “only” 5 bars.
But then, given that the atmospheric pressure is around 1 bar, why not continue to use all the energy of the gases to push the piston back? It would be possible, but we don't do it because the exhaust valves have to start opening before Bottom Dead Center, and the energy is used more efficiently in another way: in an exhaust tube optimized, where exhaust energy is used as sound waves.
Positive (pressure) and negative (suction) waves, traveling in properly sized conduits, can be used to assist the movement of the piston to remove spent exhaust gases and fill the cylinder with fuel. fresh air.
The basic rule in exhaust tube sizing is: wherever the duct increases in cross-section, an exhaust pressure pulse expands, thereby radiating/reflecting a negative wave upstream. Wherever the conduit decreases in surface area, it retains the impulse, causing a positive wave to be reflected.
Principles of an exhaust for a 4-stroke engine
In a 4-stroke engine, when the exhaust valves begin to open, a pulse of exhaust pressure enters the manifold of that cylinder. When it reaches a point of enlargement of the conduit it returns a negative pressure wave towards the motor. The manifold length is properly sized so that this negative wave arrives at the cylinder during valve crossover – the period around top dead center at the end of the exhaust stroke, when the exhaust valves are not yet all the way to closed but the intake valves have already started to open. This negative wave enters the cylinder, first extracting inert exhaust gas from the combustion chamber above the piston, then entering the intake system to help promote intake flow into the cylinder, even before the piston has started to descend. This increases torque by preventing dilution of the fresh charge by remaining exhaust gases, as well as giving a head start to the intake process.
Because the negative and positive waves alternate in the exhaust tube, at a lower speed, it is no longer a negative wave which enters the combustion chamber when the valves cross: it is a positive wave which represses the exhaust gas into the combustion chamber, through the intake valves and into the intake system. This, by diluting the fresh load that the piston is about to absorb, causes a reduction in engine torque.
And the 2T then?
Two-stroke engine exhausts are completely different. They start with a manifold, just like that of a four-stroke, but that's about the only resemblance. The collector is followed by a divergent cone and a larger volume expansion chamber, after which the diameter rapidly narrows into a counter cone.
When a two-stroke piston, descending on its power stroke, begins to show the exhaust port in the cylinder wall, a pressure pulse is released into the manifold. When this wave reaches the largest diameter, in the relaxation zone, it is a negative wave which is reflected towards the cylinder. There, the low pressure helps remove the exhaust gases from the cylinder and helps the fresh charge enter through two or more transfers.
This diagram shows the pressure waves experienced by the exhaust gases in a expansion pot
As the cylinder fills with air, some of this fresh mixture begins to be lost through the still open exhaust port. In the exhaust tube, the exhaust pulse has passed through the center section of the pipe and is now entering the counter cone, reflecting a positive wave toward the exhaust port. This positive wave arrives just in time to put the charge of fresh air that has begun to escape back into the cylinder.
The exhaust pipe designer's job is to create a wide enough engine torque zone that the rider, using the gearbox, can keep the engine in that zone most of the time. This is the goal of Yamaha's new exhaust: to allow riders to have higher torque at mid-range to help exit corners and try to limit their power deficit on the straights.
