The exhaust manifold is bolted to the cylinder head or onto the exhaust ports.
On passenger vehicles the exhaust manifold is usually made of cast iron.
Sometimes there is a separate passage for each exhaust port.
The length of the passages in the exhaust manifold should be designed so that pulses of exhaust gases from one cylinder assist the flow of gases from another.
It has large tubular sections, to improve gas flow, And no sharp bends to slow the gases down.
As well, the exhaust manifold must be shaped to allow for the location of the engine, and the bodywork of the vehicle. Some vehicles use heat shields to protect nearby components and the passenger compartment from the heat radiated from the exhaust components.
The engine pipe is attached to the outlet of the exhaust manifold. It can be designed to branch in various ways. On this engine, the exhaust gas from 3 cylinders is fed into each engine pipe.
The system needs flexible mountings to allow for engine movement, and prevent noise and vibration being transmitted into the vehicle body.
On some front-wheel drive systems, the exhaust system is designed to be strong enough to form part of the engine mounting system.
Rubber mountings are popular because of rubber’s natural dampening effect.
They also allow for thermal expansion during use.
Some exhaust systems have extractor pipes, sometimes called “headers” - one pipe for each exhaust port. They are designed with sweeps that stop restriction.
The extractor pipes also provide a scavenging effect - helping remove exhaust gases from the cylinders. The outgoing pulse from one cylinder is timed to arrive at the junction at exactly the right time to help draw out the pulse from another cylinder.
They are widely used on high performance vehicles.
The muffler is located in the exhaust system between the exhaust manifold and the exhaust outlet.
It is usually made of sheet steel, coated with aluminum to reduce corrosion. Some are made of stainless steel.
A muffler contains perforated pipes, baffles and resonance chambers.
Many also contain sound-absorbing material such as fiberglass or wire wool.
The muffler slows down the gases and breaks up the pulsating sound waves, and so reduces the noise.
It must cause as little restriction as possible. Poor design can cause excessive back-pressure that will slow down the escape of the exhaust gases and reduce engine performance.
Some mufflers combine baffles and pipes to change the flow of gases without restricting them. Gases enter through the inlet and must reverse their direction of flow before they exit through the outlet. This is called a reverse-flow muffler.
Some mufflers use double outer-skins to minimize heat and noise transmission.
Some exhaust systems use a resonator as well as a muffler. It looks like a muffler but it usually has a straight-through design and it contains sound absorbing material. It’s designed to remove types of sound that mufflers can’t remove.
A catalytic converter is used to convert unacceptable exhaust pollutants, such as carbon monoxide, certain hydrocarbons and oxides of nitrogen into less dangerous substances.
3-way converters convert hydrocarbons and carbon monoxide to water and carbon dioxide. They convert the oxides of nitrogen back into nitrogen and oxygen.
Older catalytic converters converted hydrocarbons and carbon monoxide to water and carbon dioxide, but were not able to convert the oxides of nitrogen.
A catalytic converter fits into an exhaust system like a muffler. It is located close to the exhaust manifold so that it can reach its operating temperature as soon as possible.
Leaded fuel must not be used in an engine with a catalytic converter because lead will contaminate the catalyst and prevent it doing its job.
It operates by starting and then maintaining a chemical reaction in the exhaust gases.
It usually operates at higher temperatures than a muffler.
There is a flexible connection between an engine pipe and an intermediate pipe. It is used close to the engine.
Its main functions are to allow engine movement and reduce vibration without passing it along the exhaust - especially in front wheel drive vehicles.
It also helps with the alignment of the pipes.
In the pursuit of getting the most power from an engine new technologies are always being tried. High performance developments are being continuously incorporated into on-road vehicles as efficiency gains are sought and one technology that is now becoming commonplace is the use of metallic ceramic coatings.
Engine components are made from dissimilar metals and due to metallurgical differences of the components heat is absorbed and dissipated different rates.
Since temperature control - heat resistance, lubrication - friction reduction, and wear protection are key to the protection of internal and external components it is hardly surprising that anything that contributes to wear protection significantly contributes to the horsepower and performance characteristics of a particular engine.
Performance engine builders have been refining such horsepower gains through the use of ceramic coatings as a means of wear protection.
Ceramic coatings are being used as a barrier between dissimilar metals to reduce friction, which in turn reduces wear in internal engine components.
By applying ceramic coatings to the dissimilar metal components, it allows them to interface with one another more uniformly and compatibly.
The most common applications for ceramic coatings are on the exhaust system, intake manifolds, and exhaust headers.
When ceramic thermal barrier coatings are applied to exhaust manifolds or headers, they provide two advantages.
Firstly, they protect the headers from rust and corrosion; and,
Secondly, they reduce heat loss which can be translated into high engine output.
If the exhaust headers are internally coated, the hot exhaust gases travel at a higher velocity with less turbulence due to smoother surface inside the system.
Internally, ceramic-coating the cylinder head's combustion chamber and exhaust ports has the effect of creating a faster, hotter burn during the power stroke and help scavenge gases from the cylinder at a quicker rate.
The coating of these passages also assists in creating a thermal transfer from the hot gases to the cylinder head itself.
Externally, the use of an oil-shedding coating in the the valve train location area or valley as it is sometimes referred to can assist the speed of the oil used to lubricate the valve train in it's return to the sump.
The use of a thermal dispersant coating on the cylinder head's external surface assist in cooling the cylinder head.
Valve springs can also be coated with an oil-shedding ceramic to aid in the oil return to the sump and whilst the Camshaft bearing surfaces should not be treated, the rest of the camshaft can be coated with a dry film lubricant.
The crankshaft and connecting rods can also be coated with the oil-shedding coating to cut "parasitic" drag.
The primary benefits of using Metallic Ceramic Coatings are:
- A metallic ceramic coating when applied to metal surfaces, protects against Rust and Corrosion and extends component life.
- As a thermal barrier a ceramic coating enhances engine performance and significantly reduces engine compartment and component temperature.
- A ceramic coating will not chip, crack, or peel. It can survives bending and thermal shock.
- A ceramic coating is highly resistant to corrosives.
- A ceramic coating can survive base metal temperatures up to 2000 degrees F or 1100 degrees C
- A ceramic coating can come in a range of surface finishes and colors to suit a particular application.
- A ceramic coating is easily cleaned.
The function of a vehicle’s muffler is to minimize the sounds coming from the vehicle exhaust system. These sounds originate from the combustion process within the engine.
Exhaust noise pollution becomes an issue as:
- Vehicle systems become generally quieter
- The number of vehicles on our roads increase
To understand the operation of modern exhaust noise reduction systems, it is helpful to understand what sound is:
- We sense sound from our eardrum within the ear.
- The eardrum is made to move by variations in air pressure.
- Variations in air pressure can be created when a force is placed upon an object.
- An example would be clapping your hands. As the two hands collide they push the air surrounding them, away.
- This creates a moving wave of air pressure or a sound wave.
- This sound wave moves your eardrum, which is interpreted as sound, by the brain.
Noise cancellation is a system that prevents the sound waves leaving the exhaust system by canceling them out inside the muffler.
These systems create gas pressures that are equal in force but opposite in direction to the noise source.
- These generated pressures are known as anti-noise.
- Any remaining sound is referred to as residual noise.
Anti-noise pressures can be generated by various means, such methods include:
Baffles and chambers
This relies upon variations in exhaust back- pressure to cancel pressure variations in exhaust flow. This system is only effective over a limited range of engine speeds and loads.
Variable flow exhaust
A moveable valve fitted within the exhaust system is used to change the amount of exhaust back-pressure. At higher engine speeds when exhaust noise levels are unacceptable the valve is closed, thus reducing the bore of the exhaust. This creates greater back-pressure and noise cancellation is the result. The valve can be operated by
- Pneumatics - exhaust gas pressure
- Electronics - a computer
When a variable flow exhaust is added to the baffle and chamber system, quieter noise emissions are the result. This is because the system can partially respond to changes in engine speed and load.
Electronic mufflers
Any restriction to exhaust flow in the exhaust system creates back-pressure. Whilst some back-pressure can be beneficial, excessive back-pressure reduces volumetric efficiency. This in turn reduces engine efficiency.
Electronic mufflers are designed to produce anti noise without restricting exhaust flow. This computer-controlled system uses a microphone to detect the sound waves produced within the exhaust system. As the exhaust gas leaves the tail pipe, computer driven loudspeakers are operated to generate the correct amount of anti-noise.
The result is a virtually silent exhaust without generating additional and unwanted back-pressure across all engine operating conditions. This increases fuel economy and reduces exhaust emissions.
Thermal expansion
Thermal expansion refers to the way some materials expand when they're heated. The same amount of heating can produce different amounts of expansion in different substances.
Components that are subject to heating have to allow for this expansion in their design. An engine exhaust system is subjected to very hot exhaust gases so its components expand and contract as they heat and cool. Mountings in the system are designed to allow this to happen.
Superchargers
Power is produced when a mixture of air and fuel is burned inside an engine cylinder. If more air is forced into the cylinder, then more fuel can be burned and more power produced with each stroke.
A supercharger compresses the air intake to above atmospheric pressure which increases the inlet air density to the engine.
Naturally-aspirated engines operate with uncompressed air at atmospheric pressure, 14.7 pounds per square inch, or 1 Bar. When the cylinder intake valve opens, atmospheric pressure pushes air into the cylinder as the piston is lowered. When the exhaust valve opens, the piston pushes the exhaust gases out into the exhaust system, again at normal atmospheric pressure. Since both the intake and exhaust ends of the system are at the same air pressure, there is no natural flow of air through the system. In such engines, valve timing, camshaft timing & exhaust sizing are critical to getting the maximum power output.
In a supercharged system, there is a greater air mass flowrate, that is, a higher density and speed of air flow. Air pressure is increased by the compressor on the way in to the engine, more power is produced by combustion, and the exhaust gases exit much more rapidly, making the timings and exhaust sizing less important. Although some of the extra power produced must be used to drive the supercharger, the net result is more total power from the system. The supercharger includes a bypass valve system which allows the supercharger to 'idle' when 'high power' is not required, turning off the pressure and allowing the engine to run as a naturally aspirated engine. The bypass valve can be mounted remotely, or directly onto the intake port.
A turbocharger is a forced induction system that uses wasted kinetic energy from the exhaust gases to increase the intake pressure. Like superchargers, turbochargers increase the amount of air that flows into the engine, but they have a negative effect on the flow of air out of the engine.
This means that for maximum power output, valves, cam timing, and exhaust system design are more important in turbocharged systems than in supercharged systems.
Intercoolers
A turbocharger or supercharger is used to increase the volume of air in the engine cylinder by compressing the air above atmospheric pressure. More air in a cylinder means more oxygen molecules available for combustion, which means more fuel can be completely burned, giving greater power output.
However, when air is compressed, it heats up, which causes the volume of the gas to increase, and that lowers the air density. Hot air under pressure in a cylinder contains fewer oxygen molecules than cooler air at the same pressure in the same volume.
The purpose of an intercooler is to reduce the intake air temperature up to 390 degrees Fahrenheit, or 200 degrees Celsius, before it enters the intake manifold. This increases the density of the pressurized air and improves engine efficiency.
The intercooler works at its most efficient when the turbo boost exceeds 15lbs or 100Kpa. At lower pressures, there are still enough oxygen molecules in the air to provide for complete combustion of the air/fuel mixture.
Most intercoolers operate on an air-to-air principle, by feeding compressed air from the turbocharger through the intercooler and into the intake manifold. The intercooler works like a radiator. Inside it, the air passes through small tubes with thin fins attached. The heated compressed air flowing through the intercooler heats up the fins and the tubes, and as the vehicle moves forward, the cool outside air flowing across the fins pulls heat away from the tubes and fins. This heat transfer occurs constantly during engine operation.
Some larger engine applications have a liquid operated intercooler. In this system the air is fed through small tubes in a heat exchanger, and the vehicle coolant absorbs the heat and transfers it to the engine cooling system.
Frequency
Sound travels through the air by producing pressure waves - areas of high pressure and areas of low pressure. The rate at which these waves reach our ears is called frequency. It is measured in cycles per second, where a cycle is the distance between waves, or wavelength. The higher the frequency, the higher the pitch of the sound.
The range of human hearing is approximately from 20 cycles per second, to 20,000 cycles per second. An engine produces sounds across a wide range of frequencies. The mufflers and resonators in the engine exhaust system reduce these sounds to an acceptable level.
Back-pressure
Back-pressure in an exhaust system refers to a build-up of pressure in the system that interferes with the outward flow of exhaust gases. This area of high pressure acts as a kind of wall to stop gas flow.
It can be caused by a blockage in a muffler or a similar restriction.
