Exhaust review
In goes the good air, out goes the bad—or maybe it’s just the opposite. now, it’s exhaust system fundamentals
Suppose we review all this stuff for a minute. Thus far, we’ve examined the passing of air and fuel into an engine’s combustion chamber(s) and what takes place during combustion. There has also been some discussion about the different types of engines, all of which “burn” air/fuel mixtures for the production of horsepower. But up to this point we’ve only gotten to the stage where power has been developed, with no regard as to what happens to “spent” air/fuel mixtures and what influence this combustion residue may have on subsequent fresh air/ fuel mixtures.
For purposes of discussion, let’s break the subject of exhaust systems into two parts: (1) muffling devices and (2) exhaust manifolding ahead of the mufflers. Each of these categories will be discussed with some expansion of the exhaust manifolding part, especially since there is a variety of types of exhaust manifolds
(headers) that relate to different types of engine performance.
In order to understand more easily what follows, suppose we introduce some terms and definitions that will crop up in the story. There’s even a chance you’ll see one or more of ‘em in the question and answer section this month. Well, at least they could be in the question section. The first term is backpressure. Since we know exhaust gas is trying to leave a cylinder and reach the outside world, any resistance to this movement might be called backpressure. Restricted exhaust systems (for example) could cause unwanted backpressure. But there are times when this might be helpful. More on this in a minute.
Blowdown is a term that has nothing to do with the three little pigs. Suppose we relate this to how efficiently a cylinder is cleaned of exhaust gas before fresh air and fuel enter. More technically, blowdown includes the amount of work required to be performed by the piston to rid a cylinder of exhaust gas. You didn’t know the piston did any of this, right? Well, think of it this way. If the exhaust valve was opened exactly at bottom dead center piston position, the piston is going to be working against higher cylinder pressure during the remainder of the exhaust cycle than if the valve had been opened sooner. However, if the exhaust valve is unseated too early, some usable cylinder pressure is going to be blown right out the exhaust system—and some power loss will result. So one of the critical factors in obtaining good cylinder blowdown is proper exhaust valve opening points (relative to piston position). A good compromise is to open the exhaust valve somewhere in the 40°-70° before bottom dead center piston position (at the end of the power stroke). You might check out all this verbal footwork by examination of the illustration. Pictures sure help, don’t they?
A. Three designs are common to mufflers. Note how each tends to diminish the strength (amplitude) of the exhaust pulse or pressure disturbances as passage through the muffler is accomplished. Noise reduction and low backpressure characterize most good muffler designs. B. The relationship between exhaust gas flow velocity, engine rpm and torque production is critical with respect to where resonant (or “critical”) flow velocity is produced. Almost irrespective of engine size or parts mix, when this critical flow velocity is achieved, an “exhaust system torque peak” is produced.