Exhaust valves don’t have to be as large as intake valves because the gas movement does not rely on the suction created by the downwards-moving piston. Combustion pressure, as well as pressure generated by the upwards-moving pistons, help to drive the spent gases out.
The speed of sound in air at under-bonnet temperature is only about 330 m/s (1188 km/h), whereas sound travels through the hot exhaust gas at about 518 m/s (1865 km/h). Consequently, an exhaust pressure pulse, which moves at the speed of sound, is about 57% faster than the intake pulse.
When an exhaust valve opens at the end of the power stroke it releases a high-density compression wave into the exhaust system. This travels down the exhaust pipe and when it reaches the open end it is diffused into the atmosphere. This creates an area of low pressure that now travels back to the exhaust valve at the same speed. When this pulse reaches the exhaust valve it is reflected back, but with diminished amplitude (wave height). This sequence repeats itself with the pressure waves getting weaker and weaker, until the next time the exhaust valve opens, when the process is repeated.
Exhaust pipe lengths are usually chosen to get maximum gain from the pulsing waves. The first reflected negative wave should reach the exhaust valve when the piston has just passed top dead centre near the end of the exhaust period. (The exhaust valve should still be open.) This creates the most favourable condition for the exhaust gases to be completely scavenged. This so-called ram-charging effect is often employed on modern exhaust systems.
In any particular car the pipe length is fixed so that maximum gain occurs only at a certain engine speed. At the other speeds the reflected negative pulse will impinge on a closed, or nearly closed, valve and hence be unable to help with the scavenging process.
It has been discovered that connecting together those cylinders that don’t fire immediately after each other into common downpipes will result in a useful gain – if the lengths are suitably chosen. Connecting the pipes in this way, we get what is known as interference working of the pulses. Most modern high-performance engine use interference working.
Another method is to connect all the (equal length) pipes together at some calculated point by means of a collector box into a common downpipe. This is known as independence working.
Usually these downpipes are also connected along the way to the back of the car, and this will slow the pulse timing down further, to say nothing of the pulse effect of other cylinders. lf properly designed, this complicated interplay of pulses has the effect of working together at certain engine speeds to give a boost to the breathing, thereby benefiting the average cylinder pressure and hence the torque, resulting in an increase in power
Couple this to a similar pulse effect in the intake manifold, and it’s possible to begin to appreciate why many modern cars have hills and valleys in their torque curves. At some engine speeds the lengths work together and at other speeds they conflict.
ANALYSING THE EXHAUST SOUND
For many years I used to think that the combustion process inside an engine causes the noise that you can hear coming out of the exhaust pipe. Later, I discovered that the noise is generated by the sudden release of a compressed mixture of spent gasses issuing from the rapidly-opening exhaust valves. If you open any cylinder full of compressed gas you will hear a similar but more continuous noise.
Once the exhaust gas has been liberated from the cylinder, the next challenge is to quieten it down.
Sound is a transmission of energy through a medium in the form of waves that consist of a number of so-called sine waves superimposed onto one another. When you play a musical instrument the sound you hear consists of a superposition of sine waves of different frequencies. This is measured in cycles per second where one cycle/second is one Hertz (Hz). For example, when a violin plays the middle C note, the strings generate vibrations in the air at about 262 Hz, which is called the fundamental or basic frequency because it determines the pitch of the note.
This pure note is modified by superimposed waves of other frequencies and amplitudes (wave heights), which are called harmonics because they have frequencies that are multiples of the pure notes. In this case, the harmonics have frequencies of 2 x 262 = 524 Hz; 3 x 262 = 786 Hz; 4 x 262 = 1 048 Hz and so on, with the sound getting weaker all the time. These harmonics are responsible for the tone, or harmonic content, of the particular instrument, i.e. they are the reason why a violin and a trumpet playing the same note sound totally different.
What determines the basic frequency of an exhaust system? It is simply the frequency with which the exhaust valves open, so that this frequency increases with a rise in engine speed.
A silencer is device that not only reduces the intensity of the sound, but also gives a different proportion to the harmonics by filtering some of them out, so it changes the tone.