Gas Flow Inertia
Valve overlap. (Stay with us. This all gets back to the exhaust system in a couple of paragraphs.) For optimum engine performance, it has long since been found that intake and exhaust valves should not be opened and/or closed exactly at top and bottom dead center. Gas flow inertia (resistance to movement and changes in flow speed) is low at low rpm and high at higher engine speeds. For this reason, valve overlap periods are typically short for low-rpm and longer for high-rpm operation. Specifically, overlap is the period of time (measured in crankshaft degrees of rotation) at the end of the exhaust stroke and beginning of the intake stroke when both valves are still off their respective seats. Note the illustration showing this period. Exhaust systems, almost regardless of design, are capable of “seeing” overlap periods and are affected by the amount and timing of the overlap period.
Exhaust gas dilution. Let’s call this
the presence of combustion residue (exhaust gas) in fresh air/fuel mixtures. For example, if the cylinder was not efficiently blown down, some amount of exhaust gas might mix with incoming air/fuel mixtures. This would reduce the amount of heat available during the next combustion process, thereby decreasing fuel economy and chopping some feel-able performance from the engine. However, it’s possible to reduce the amount of heat (oxides of nitrogen) during combustion by diluting the fresh charges. You know of this as exhaust gas recirculation (EGR). And maybe you now know some of its other effects besides emissions reduction.
Resonant frequency of exhaust gas flow. This can get a little technically muddy. But for the time being, let’s say this is the rate of exhaust gas flow which provides the most efficient passage of gas through the system.
We might also call the resonant frequency of the exhaust system its critical flow rate (in feet per second). Since exhaust gas flow velocity is primarily, governed by engine speed and piston displacement, the higher the rpm (or larger the piston displacement) the faster the rate of gas flow.
We’d like to emphasize that this area of discussion is particularly important to the overall efficiency of a given exhaust system. It is also the area of study which reveals those engine speeds at which torque peaks (or gains) can be expected—or even designed into the engine for which known ranges of rpm will be used. Critical exhaust gas flow velocities are usually found at or near peak torque points in the rpm range. At engine speeds beyond such a torque peak, gas flow velocities may increase, but the ability of the system to operate in a resonant condition diminishes.
C. Valve overlap period begins when the intake valve starts to open and ends when the exhaust valve closes. Pressure conditions in the cylinder during this time can be “seen” by both intake and exhaust systems. D. Changing the length of a section of primary exhaust pipe tends to “rock” the torque curve about that rpm point already established by the i.d. of the pipe. Amounts of torque produced above and below the peak change, but the peak point does not.