Because of the rapid combustion of fuel in reciprocating piston internal combustion engines loud noise tends to be generated. In order to quiet the operation of engines exhaust mufflers are required, but the effectiveness of mufflers depends greatly on their design and application.
Flow and Backpressure
Exhaust Gas Scavenging
The simplest way to quiet an engine is just to restrict the exhaust outlet to create backpressure. The problem with this of course is that the performance of the engine is then reduced considerably. A better way to quiet an engine is the use of multiple chambers to baffle the sound waves without creating backpressure. Any muffler system of course is going to create some small amount of backpressure, but a well designed muffler system is able to do an enormous amount of sound deadening without creating more than the just the slightest amount of backpressure. The easiest way to get a muffler system to dramatically quiet an engine without restricting flow or hurting performance is simply to make the muffler very large. Big mufflers with many chambers and large passages for the exhaust gases are able to quiet a wide range of frequencies of noise without creating backpressure even when the engine is running at maximum power output. Smaller size mufflers can also be made to quite effectively quiet an engine without creating back pressure, but small size mufflers must be carefully designed for a specific application in order to work well.
The quietest reciprocating piston internal combustion engines tend to be full flame front travel gasoline engines running at low power output and moderate engine speeds. Just what engine speed is quietest for a full flame front travel engine depends on the bore and stroke dimensions of the engine as well as the flame front travel speed of the fuel. Where this gets confusing is that even full flame front travel engines tend to be more efficient when running more slowly than the speed at which they are quietest or with more spark advance than where they are quietest. A full flame front travel engine running as quietly as it can tends to be dirty and inefficient because the flame front does not have a chance to travel out to the "far corners" of the combustion chamber before the piston has receded so far that the temperature and pressure have dropped off to the point where combustion is no longer possible. Getting a full flame front travel engine to run as efficiently and cleanly as it can requires that the fuel be burned quite early, and this leads to harsh and loud operation as well as increased engine wear. Muffling a full flame front travel engine is rather straight forward since it will normally operate over only a rater narrow range of engine speeds.
Diesel engines tend to be the most difficult to muffle since they can run over extremely wide ranges of engine speeds. The saving grace for diesel engines is that they can be made to run with the fuel burning smoothly over the correct length of time for a particular engine speed, and this tends to make them rather quiet. A good running diesel engine can be fairly quiet with no muffler at all, particularly if the engine speed is low to moderate. What tends to make diesel engines loud is when the injection system is not capable of matching the injection start timing and the injection flow rate to the requirements of the engine over the entire range of operable engine speeds. When an inline injected engine runs with excessively early start timing at low engine speed loud clanking noises will be generated that cannot be controlled even with good muffler systems. High power output turbocharged diesel engines with fixed injection flow rates also tend to be rather loud, even if the injection start timing never becomes excessively early. This harsh loudness caused by a high injection flow rate can however be controlled with a good muffler system. For more on traditional injection system limitationssee Diesel Basics.
Gasoline engines running in late compression ignition mode are extremely loud with no exhaust muffler system, but can be made to be rather quiet with an effective muffler installed. Even when gasoline engines run at excessively low engine speeds in late compression ignition mode mufflers can quite effectively control sound emissions. The best example of this are the extremely effective muffler systems found on many automotive gasoline engines which are able to almost totally eliminate the loud and harsh sounds of late compression ignition all the way down to 2,000RPM. Gasoline engines running in late compression ignition mode at 2,000 to 3,000RPM are still harsh and loud, but with a good muffler system not much of that noise is coming out the exhaust outlet. Up at 3,000 to about 8,000RPM gasoline engines are also extremely loud with no muffler system, but can be made to run rather quietly with an effective muffler system. At very high engine speeds muffler systems are still effective at reducing noise, but the higher frequency noise is more easily transferred through thin sheet metal exhaust manifolds and tail pipes before ever reaching the muffler. The scream of a gasoline engine at 8,000 to 12,000RPM certainly can be quieted, but more than just a good muffler is required to attain low noise levels. A cooling jacket goes a long way to quieting high speed gasoline engines, and air cooled engines are notoriously loud at anywhere from 4,000RPM up. For more on why gasoline engines have such a hard time running at low to moderate engine speedssee Combustion Properties of Fuel.
There are two things that exhaust gas scavenging can accomplish. One is to compensate for the small amount of backpressure generated by most muffler systems. Really effective exhaust gas scavenging can actually suck a vacuum on the engine both reducing the amount of hot exhaust gases remaining in the cylinder and in some instances drawing more intake air into the engine. Particularly on normally aspirated two stroke engines exhaust gas scavenging is of great importance, but even four stroke engines can be made to run with volumetric efficiencies slightly greater than 100%. Normally aspirated four stroke engines that attain greater than 100% volumetric efficiency at some engine speed generally also are benefiting from tuned intake runner length and diameter, which can force more intake air through the intake valves over some narrow range of engine speeds. Because the valves are open only small amounts during split overlap there is just not much opportunity for exhaust gas scavenging to actually draw more intake air into a four stroke engine. On a two stroke engine exhaust gas scavenging does not allow greater than 100% volumetric efficiency, but rather it is a case of exhaust gas scavenging being of critical importance in getting a full intake charge into the cylinder. Without exhaust gas scavenging a two stroke does not attain anywhere near 100% volumetric efficiency.
Where exhaust gas scavenging relates to the noise that a gasoline engine produces is that powerful exhaust gas scavenging can more easily be made to work at higher engine speeds. Down at 3,500 to 5,000RPM tuned length headers coming into a collector on a multiple cylinder engine certainly can be made to generate some useful exhaust gas scavenging, but exhaust gas scavenging works much better up at higher engine speeds. What is going on is that the fuel is burning so early down at 3,500 to 5,000RPM that a large portion of the energy of combustion is being transferred to the cooling jacket. At higher engine speed of 6,000 to 8,000RPM where gasoline engines run well in late compression ignition mode not only is the energy of combustion better able to drive the piston down, but there is more heat left over in the expanding exhaust to drive exhaust gas scavenging. Where this gets confusing is that exhaust gas scavenging just keeps working better and better up to higher engine speeds even though gasoline engines probably do not need to spin faster than about 7,000RPM to run at their best. The reason that exhaust gas scavenging just keeps working better and better at higher engine speeds is that more and more energy is available in the hot exhaust gases at higher engine speeds. Just what speed it is that gasoline engines will run well up to at the latest possible time of late compression ignition is not entirely clear, but it seems to be at least about 10,000RPM. At significantly higher engine speeds where making the time of late compression ignition earlier than the latest possible time actually yields higher efficiency exhaust gas scavenging still continues to work better and better. How ever fast a gasoline engine can be made to spin exhaust gas scavenging can be made to work better at higher engine speeds simply because there is more energy available in the hot exhaust gases.
Because exhaust gas scavenging is driven by the expansion of hot exhaust gases in the first section of header pipe and at the junction between header pipes on multiple cylinder engines the muffler system is actually completely separate and not directly related to exhaust gas scavenging. As long as the muffler system does not produce more than a certain small amount of back pressure it does not interfere with exhaust scavenging in any way. There has been a widespread misconception that exhaust gas scavenging somehow cannot work with a quiet muffler, but this is just not the case. The origin of this misconception has to do with the fact that adding an overly restrictive muffler will indeed both quiet an engine considerably and interfere with exhaust gas scavenging. A good muffler system that relies on multiple properly sized and shaped chambers connected by large and non-restrictive passages does not produce much back pressure and does not interfere with what ever exhaust gas scavenging a particular header system might be capable of producing. The only caveat on this is that the best possible functioning exhaust systems are designed to work with the very small amount of back pressure produced by a functional muffler system. In some extreme cases removing this very small amount of back pressure will actually interfere slightly with the operation of the exhaust system. Normally though back pressure is a very simple concept, less is better. Reducing back pressure nearly always increases power output and widens the range of operable engine speeds.