Exhaust system

An exhaust system is usually piping used to guide reaction exhaust gases away from a controlled combustion inside an engine or stove. The entire system conveys burnt gases from the engine and includes one or more exhaust pipes. Depending on the overall system design, the exhaust gas may flow through one or more of:

  • Cylinder head and exhaust manifold
  • A turbocharger to increase engine power.
  • A catalytic converter to reduce air pollution.
  • A muffler (North America) / silencer (UK/India), to reduce noise.

Exhaust manifold (chrome plated) on a car engine

Muffler and tailpipe on a car

Design criteria

Exhaust system of the Opel Corsa B 1.2 petrol

An exhaust pipe must be carefully designed to carry toxic and/or noxious gases away from the users of the machine. Indoor generatorsand furnaces can quickly fill an enclosed space with poisonous exhaust gases such as hydrocarbons, carbon monoxide and nitrogen oxides,[1] if they are not properly vented to the outdoors. Also, the gases from most types of machines are very hot; the pipe must be heat-resistant, and it must not pass through or near anything that can burn or can be damaged by heat. A chimney serves as an exhaust pipe in a stationary structure. For the internal combustion engine it is important to have the exhaust system “tuned” (refer to tuned exhaust) for optimal efficiency. Also this should meet the regulation norms maintained in each country. In China, China 5; In European countries, EURO 5; In India, BS-4, etc.,

In the vehicle type category, the passenger cars segment is highly lucrative and is estimated to reach a significant market value by the end of the year of assessment. This segment is the fastest growing and is projected to grow at a value CAGR of 4.2% throughout the period of forecast 2017-2027. [2]

Motorcycles

Exhaust piping and silencer (muffler) on a Ducati Monster motorcycle

In most motorcycles all or most of the exhaust system is visible and may be chrome plated as a display feature. Aftermarket exhausts may be made from steel, aluminium, titanium, or carbon fiber.

Motorcycle exhausts come in many varieties depending on the type of engine and its intended use. A twin cylinder may flow its exhaust into separate exhaust sections, such as seen in the Kawasaki EX250 (also known as the Ninja 250 in the US, or the GPX 250). Or, they may flow into a single exhaust section known as a two-into-one (2-1). Larger engines that come with 4 cylinders, such as Japanese super-sport or super-bikes (such the Kawasaki ZX series, Honda‘s CBR series, Yamaha‘s YZF series, also known as R6 and R1, and Suzuki‘s GSX-R series) often come with a twin exhaust system. A “full system” may be bought as an aftermarket accessory, also called a 4-2-1 or 4-1, depending on its layout. In the past, these bikes would come standard with a single exhaust, as seen on the Kawasaki ZX-6R 2000 and 2001 models. However, EU noise and pollution regulations have generally stopped this practice, forcing companies to use other methods to increase performance of the motorcycle.

A 2-stroke exhaust can be made to be quieter than 70dBa at 6000 rpm without sacrificing any power gains. The latest generations of 4-stroke street motorcycle engines that can come close to a 2T in redline RPM but still having the extra weight and inertia of cams and valves make the differences in being “cleaner burning” negligible, with the decreased engine service life to be expected, while also having the sound of a 4T exhaust carrying for miles in many cases. Additionally, “the nature of the beast” is that a 4T exhaust note above 6000 rpm is unduly harsh to the human ear.

Trucks

In many trucks / lorries all or most of the exhaust system is visible. Often in such trucks the silencer is surrounded by a perforated metal sheath to avoid people getting burnt touching the hot silencer. This sheath may be chrome plated as a display feature. Part of the pipe between the engine and the silencer is often flexible metal industrial ducting, which helps to avoid vibration from the engine being transferred into the exhaust system. Sometimes a large diesel exhaust pipe is vertical, to blow the hot noxious gas well away from people; in such cases the end of the exhaust pipe often has a hinged metal flap to stop debris, birds and rainwater from falling inside.

Two-stroke engines

In a two-stroke engine, such as that used on dirt bikes, a bulge in the exhaust pipe known as an expansion chamber uses the pressure of the exhaust to create a pump that squeezes more air and fuel into the cylinder during the intake stroke. This provides greater power and fuel efficiency. See Kadenacy effect.

Marine engines

With an onboard diesel or petrol (gasoline) engine below-decks on marine vessels:-

  • Lagging the exhaust pipe stops it from overheating the engine room where people must work to service the engine.
  • Feeding water into the exhaust pipe cools the exhaust gas and thus lessens the back-pressure at the engine’s cylinders. Often in marine service the exhaust manifold is integral with a heat exchanger which allows sea water to cool a closed system of fresh water circulating within the engine.

Outboard motors

In outboard motors the exhaust system is usually a vertical passage through the engine structure and to reduce out-of-water noise blows out underwater, sometimes through the middle of the propeller.

Terminology

Manifold or header

Aftermarket exhaust manifold

In most production engines, the manifold is an assembly designed to collect the exhaust gas from two or more cylinders into one pipe. Manifolds are often made of cast iron in stock production cars, and may have material-saving design features such as to use the least metal, to occupy the least space necessary, or have the lowest production cost. These design restrictions often result in a design that is cost effective but that does not do the most efficient job of venting the gases from the engine. Inefficiencies generally occur due to the nature of the combustion engine and its cylinders. Since cylinders fire at different times, exhaust leaves them at different times, and pressure waves from gas emerging from one cylinder might not be completely vacated through the exhaust system when another comes. This creates a back pressure and restriction in the engine’s exhaust system that can restrict the engine’s true performance possibilities. In Australia, the pipe of the exhaust system which attaches to the exhaust manifold is called the ‘engine pipe’ and the pipe emitting gases to ambient air called the ‘tail pipe’.

Regardless of the negative attributes focused upon by potential sellers of steel tube exhaust outlet configurations, engineers who design engine components choose conventional cast iron exhaust manifolds can similarly list positive attributes, such as an array of heat management properties and superior longevity than any other type of exhaust outlet design. For the average consumer, having trouble with an exhaust outlet system may qualify as ‘poorer performance’.

header (sometimes called set of extractors in Australia) is a manifold specifically designed for performance.[3] During design, engineers create a manifold without regard to weight or cost but instead for optimal flow of the exhaust gases. This design results in a header that is more efficient at scavenging the exhaust from the cylinders. Headers are generally circular steel tubing with bends and folds calculated to make the paths from each cylinder’s exhaust port to the common outlet all equal length, and joined at narrow angles to encourage pressure waves to flow through the outlet, and not back towards other cylinders. In a set of tuned headers the pipe lengths are carefully calculated to enhance exhaust flow in a particular engine revolutions per minute range.

Headers are generally made by aftermarket automotive companies, but sometimes can be bought from the high-performance parts department at car dealerships. Generally, most car performance enthusiasts buy aftermarket headers made by companies solely focused on producing reliable, cost-effective well-designed headers specifically for their car. Headers can also be custom designed by a custom shop. Due to the advanced materials that some aftermarket headers are made of, this can be expensive. Luckily, an exhaust system can be custom built for any car, and generally is not specific to the car’s motor or design except for needing to properly connect solidly to the engine. This is usually accomplished by correct sizing in the design stage, and selecting a proper gasket type and size for the engine.

Header-back

The Header-back (or header back) is the part of the exhaust system from the outlet of the header to the final vent to open air — everything from the header back. Header-back systems are generally produced as aftermarket performance systems for cars without turbochargers.

Turbo-back

The Turbo-back (or turbo back) is the part of the exhaust system from the outlet of a turbocharger to the final vent to open air. Turbo-back systems are generally produced as aftermarket performance systems for cars with turbochargers. Some turbo-back (and header-back) systems replace stock catalytic converters with others having less flow restriction.

With or without catalytic converter

Some systems (including in former time all systems) (sometimes nowadays called catless or de-cat) eliminate the catalytic converter. It is illegal and is against Federal Law in the United States and other countries to not have a catalytic converter.[4][5] Converters may not be removed from a vehicle that is used only for “off-road” driving in the United States.[6] The main purpose of a catalytic converter on an automobile is to reduce harmful emissions of hydrocarbons, carbon monoxide and nitrogen oxides into the atmosphere.[1] They work by transforming the polluted exhaust components into water and carbon dioxide.[7]

Cat-back

Cat-back (also cat back and catback) refers to the portion of the exhaust system from the outlet of the catalytic converter to the final vent to open air. This generally includes the pipe from the converter to the muffler, the muffler, and the final length of pipe to open air.

Cat-back exhaust systems generally use larger diameter pipe than the stock system. The mufflers included in these kits are often glasspacks, to reduce back pressure. If the system is engineered more for show than functionality, it may be tuned to enhance the lower sounds that are lacking from high-RPM low-displacement engines.

Tailpipe and exhaust

The Corvette C7 has four tailpipes

With trucks, sometimes the silencer is crossways under the front of the cab and its tailpipe blows sideways to the offside (right side if driving on the left, left side if driving on the right). The side of a passenger car on which the exhaust exits beneath the rear bumper usually indicates the market for which the vehicle was designed, i.e. Japanese (and some older British) vehicles have exhausts on the right so they are furthest from the curb in countries which drive on the left, while European vehicles have exhausts on the left.[citation needed]

The end of the final length of exhaust pipe where it vents to open air, generally the only visible part of the exhaust system part on a vehicle, often ends with just a straight or angled cut, but may include a fancy tip. The tip is sometimes chromed. It is often of larger pipe than the rest of the exhaust system. This produces a final reduction in pressure, and sometimes used to enhance the appearance of the car.

In the late 1950s in the United States manufacturers had a fashion in car styling to form the rear bumper with a hole at each end through which the exhaust would pass. Two outlets symbolized V-8 power, and only the most expensive cars (Cadillac, Lincoln, Imperial, Packard) were fitted with this design. One justification for this was that luxury cars in those days had such a long rear overhang that the exhaust pipe scraped the ground when the car traversed ramps. The fashion disappeared after customers noted that the rear end of the car, being a low-pressure area, collected soot from the exhaust and its acidic content ate into the chrome-plated rear bumper.[citation needed]

When a bus, truck or tractor or excavator has a vertical exhaust pipe (called stacks or pipes behind the cab), sometimes the end is curved, or has a hinged cover flap which the gas flow blows out of the way, to try to prevent foreign objects (including droppings from a bird perching on the exhaust pipe when the vehicle is not being used) getting inside the exhaust pipe.

In some trucks, when the silencer (muffler) is front-to-back under the chassis, the end of the tailpipe turns 90° and blows downwards. That protects anyone near a stationary truck from getting a direct blast of the exhaust gas, but often raises dust when the truck is driving on a dry dusty unmade surface such as on a building site.

Lake pipes

A consequence of the problematic nature in adaptation of large diameter exhaust tubing to the undercarriage of ladder-frame or body-on-frame chassis architecture vehicles with altered geometry suspensions, lake pipes evolved to become a front-engined vehicle exhaust archetype crafted by specialty motor sport engine specialists of the 1930s, 40s and 50s, whose preoccupation was optimization of the acoustic effect associated with high output internal combustion engines. The name is derived from their use on the vast, empty dry lake beds northeast of Los Angeles County, where engine specialists of yore custom crafted, interchanged and evaluated one-piece header manifolds of various mil thicknesses, a function of temperature, humidity, elevation and climate they anticipated.[citation needed]

No intrinsic performance gain to be derived, per se, lake pipes evolved a function of practicality. Common instances, their manifolds routed straight out the front wheel-whels posing an asphyxiation axiom to the race driver, “lake pipes” were fashioned, extending from the header flange along the rocker panels, bottom-side of the vehicle, beneath the doors, thus allowing (1) suspension tuners a lower ride height sufficient for land speed record attempts, and (2) engine tuners ease and flexibility of interchanging different exhaust manifolds without hoisting the vehicle, thus precluding having to wrench undercarriage of the vehicle.

Body-on-frame chassis architecture ceding to superleggera, unit-body and monocoque archetypes, in tandem with smog abatement legislation rendered lake pipes, as a bona fide performance prerequisite, obsolete. No meaningful performance gain to be had for contemporary vehicles, lake pipes persist into the 21st century as a superfluous, retrograde aesthetic, usually chrome plated with various options, allowing the driver to control whether exhaust gas is routed the standard exhaust system, or through lake pipes, which are commonly fashioned by laker caps which, affixed by fasteners at the terminal end of exhaust tips, serve to (1) “cap” the exhaust system when not in use, and/or (2) signal authorities that the presence of lake pipes is merely cosmetic.

Exhaust system tuning

Aftermarket exhaust system including headers and a white plasma-sprayed ceramic coating

Many automotive companies offer aftermarket exhaust system upgrades as a subcategory of engine tuning. This is often fairly expensive as it usually includes replacing the entire exhaust manifold or other large components. These upgrades however can significantly improve engine performance and do this through means of two main principles:

  • By reducing the exhaust back pressure, engine power is increased in four-stroke engines
  • By reducing the amount of heat from the exhaust being lost into the underbonnet area. This reduces the underbonnet temperature and consequently lowers the intake manifold temperature, increasing power. This also has positive side effect of preventing heat-sensitive components from being damaged. Furthermore, keeping the heat in the exhaust gases speeds these up, therefore reducing back pressure as well.

Back pressure is most commonly reduced by replacing exhaust manifolds with headers, which have smoother bends and normally wider pipe diameters.

Exhaust heat management helps in reducing the amount of exhaust heat radiated out from the exhaust pipe and components. One dominant solution to aftermarket upgraders is the use of a ceramic coating applied via thermal spraying. This not only reduces heat loss and lessens back pressure, but also provides an effective way to protect the exhaust system from wear and tear, thermal degradation and corrosion.

References

  1. a b “VP Exhaust”. VP Exhaust. Retrieved 2014-04-20.
  2. “Automotive Exhaust Systems Market By 2027”. Future Market Insights. 2018-01-25.
  3. How Headers Work How Headers Improve Engine Performance
  4. “Environmental Protection Agency” (PDF). US Government Printing Office. Retrieved 17 March 2017.
  5. Under federal law, catalytic converters may not be removed and replaced with “converter replacement pipes’ by any person. The 1990 Clean Air Act Amendments even prohibit private individuals from installing “converter replacement pipes” on their own vehicles. Anyone who installs such pipes would violate section 203(a)(3)(A) and (B) of the Clean Air Act (Act) In addition to Federal law, forty-five out of the fifty States also have statutes or regulations which prohibit tampering with the pollution control equipment on motor vehicles or driving or selling such vehicles. Thus, vehicle owners who tamper with their own vehicles may be subject to substantial penalties under both Federal and State law.
  6. “Fact Sheet: Exhaust Svstem Repair Guidelines” (PDF). United States Environmental Protection Agency. 13 March 1991. Retrieved 17 March 2017.
  7. Volkmann, Jörg (2007). “2”. Exhaust systems’ models investigation by theoretical group methods. Göttingen Cuvillier. pp. 13–34. ISBN 9783867274227. Retrieved 17 March 2017.
Tuned exhaust A tuned exhaust system is an exhaust system for an internal combustion engine which improves its efficiency by using precise geometry to reflect the pressure waves from the exhaust valve or port back to the valve or port at a particular time in the cycle. Ferrari V10 engine showing one of its two tuned extractor manifolds Two-stroke engines Yasuni aftermarket motor scooterexhaust system. The exhaust passes first through the expansion chamber at the bottom and then exits through the muffler above it. A conceptual animation of a two-stroke engine with a tuned exhaust system using an expansion chamber. Exhaust gases are in grey, fuel/air mixture is green. In practice the fuel/air mix is unlikely to progress as far down the exhaust pipe as shown. In many two-stroke engines, the exhaust port is opened and closed directly by the position of the piston rather than by a separate valve, which restricts the timing of its operation; Typically, the port remains...
Expansion chamber On a two-stroke engine, an expansion chamber or tuned pipe is a tuned exhaust system used to enhance its poweroutput by improving its volumetric efficiency. Scooter exhaust with expansion chamber and silencer History Direct comparison between different types of exhausts for the two-stroke engine, on the left you can see the engine and its exhaust, in the center the progression curves of the pressures (effective pressure in atmospheres) to the exhaust port (detection area highlighted in red), on the right the power curves of the various drains. A) Traditional discharge with constant section B) Discharge with divergent section C) Resonant expansion chamber with expansion chamber, in the power graph the influence of the exhaust back pressure valve is also highlighted Expansion chambers were invented and successfully manufactured by Limbach, a German engineer, in 1938, to economize fuel in two stroke engines. Germany was running short of petrol, which was at ...
Exhaust manifold In automotive engineering, an exhaust manifold collects the exhaust gases from multiple cylinders into one pipe. The word manifoldcomes from the Old English word manigfeald (from the Anglo-Saxon manig  and feald ) and refers to the folding together of multiple inputs and outputs (in contrast, an inlet or intake manifold supplies air to the cylinders). Exhaust manifolds are generally simple cast iron or stainless steel units which collect engine exhaust gas from multiple cylinders and deliver it to the exhaust pipe. For many engines, there are aftermarket tubular exhaust manifolds known as headers in American English, as extractor manifolds in British and Australian English, and simply as "tubular manifolds" in British English.These consist of individual exhaust headpipes for each cylinder, which then usually converge into one tube called a collector. Headers that do not have collectors are called zoomie headers. The most common types of aftermarket headers are made of mil...
Exhaust gas Exhaust gas or flue gas is emitted as a result of the combustion of fuels such as natural gas, gasoline, petrol, biodiesel blends, diesel fuel, fuel oil, or coal. According to the type of engine, it is discharged into the atmosphere through an exhaust pipe, flue gas stack, or propelling nozzle. It often disperses downwind in a pattern called an exhaust plume. It is a major component of motor vehicle emissions (and from stationary internal combustion engines), which can also include: Crankcase blow-by Evaporation of unused gasoline Motor vehicle emissions contribute to air pollution and are a major ingredient in the creation of smog in some large cities. A 2013 study by MIT indicates that 53,000 early deaths occur per year in the United States alone because of vehicle emissions. According to another study from the same university, traffic fumes alone cause the death of 5,000 people every year just in the United Kingdom. This diesel-powered truck emits an exhaust gas...
Diesel particulate filter A diesel particulate filter (DPF) is a device designed to remove diesel particulate matter or soot from the exhaust gas of a diesel engine. A diesel particulate filter (top left) in a Peugeot Off-road - DPF Installation Mode of action Wall-flow diesel particulate filters usually remove 85% or more of the soot, and under certain conditions can attain soot removal efficiencies approaching 100%. Some filters are single-use, intended for disposal and replacement once full of accumulated ash. Others are designed to burn off the accumulated particulate either passively through the use of a catalyst or by active means such as a fuel burner which heats the filter to soot combustion temperatures. This is accomplished by engine programming to run (when the filter is full) in a manner that elevates exhaust temperature, in conjunction with an extra fuel injector in the exhaust stream that injects fuel to react with a catalyst element to burn off accumulated soot in ...