Exhaust manifold

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Single pipe exhaust manifold

The exhaust manifold is a component of the exhaust system of internal combustion engines . Its job is to collect the exhaust gases emitted by the cylinders of the engine and convey them to the exhaust. The exhaust manifold gets its name from its usually curved shape. This arises primarily from the fact that the manifold has to divert the exhaust gases from mostly several horizontal cylinder openings of the engine into the exhaust system running further down. Shape details can also be influenced by fluid mechanics considerations.

Construction and functioning

The exhaust manifold must discharge the exhaust gases as quickly as possible and prevent them from flowing back into the cylinders. In a well-designed manifold, the pressure front of the exhaust gas flowing out leads to a negative pressure that supports the exhaust gas output of the following cylinder in a certain speed range. In this sense, the manifold is a particularly important element in two-stroke engines : Together with the exhaust pipe, it ensures that pressure oscillations build up in the exhaust system, which - again limited to a certain speed range - lead to a reloading effect. But even with four-stroke engines , the pressure oscillations in an exhaust system with a cleverly designed exhaust manifold can result in increased engine performance. The material used for exhaust manifolds is traditionally alloyed cast iron , which can withstand the high temperatures of the exhaust gases (over 900 ° C). Increasingly, however, lighter and more compact manifolds made of stainless steel are used. Components of such modern elbows are manufactured using hydroforming from one to two millimeters thick, and since 2002 also from 0.8 millimeter thick tubes. Both ferritic (e.g. 1.4509) and austenitic steels (e.g. 1.4301, 1.4828, 1.4845) are used. Built manifolds are u. U. more expensive than cast manifolds. However, they have emission advantages (see below) and the additional costs can usually be compensated by reducing the precious metal content in the catalytic converter. The greatest challenge in the development of manifolds is taking into account the temperature expansion and the associated stress distribution in the manifold. These tensions are usually so great that they destroy the manifold after a finite number of heating-cooling cycles . Nevertheless, manifolds are being developed today that can withstand several thousand such cycles and, accordingly, several hundred thousand kilometers. In the first generation of built manifolds, cracks still regularly appeared, which led to expensive repairs.

Construction variants

In addition to pipe length, pipe diameter and curvature, design variables are also the material (smooth surfaces reduce the friction between the exhaust gas flow and the manifold wall) and the order in which the exhaust lines of the individual cylinders are brought together. To optimize the engine's performance, four-stroke engines often use so-called 4-in-2 exhaust manifolds, which initially only combine two exhaust lines. Similar arrangements can also be found in engines with more than four cylinders.

So-called power manifolds for motorcycles are made of stainless steel, have an enlarged diameter and are intended to increase engine performance through improved exhaust gas dynamics. Their counterparts in the automotive sector are manifolds , which are installed as part of vehicle tuning , in some high-quality sports cars, but also ex works.

Originally, exhaust manifolds were always separate components with the advantage of being easy to replace. In the meantime (2017), the integrated exhaust manifold has established itself particularly for three-cylinder engines . These exhaust manifolds are built into the engine's cylinder head and integrated into the cooling system. By cooling the manifold, on the one hand, the engine reaches its operating temperature more quickly (the cooling water heats up more quickly), on the other hand, the temperature of the exhaust gases drops in stationary operation.

Exhaust manifold and environmental protection

The legal limitation of pollutant emissions stimulated the development of thin-walled manifolds in the mid-1980s, as manifolds with a lower thermal mass mean that the catalyst located behind them reaches its operating temperature more quickly after the engine has been cold-started . Alternatively, so-called LSI bends (LSI = air gap insulated) can be installed. Here a manifold housing is built around the actual manifold with a gap of a few millimeters; Thanks to the insulating effect of the air gap, this ensures low heat losses in the exhaust gas on the way to the catalytic converter and thus also ensures that the catalytic converter heats up quickly after the engine has been cold-started. Despite cooling the exhaust gases, engines with integrated exhaust manifolds apparently do not slow down the response of the catalytic converter: The reduction in the surface area of ​​the compact, integrated manifold can still ensure that the catalytic converter operating temperature is reached quickly during a cold start. Cooling the exhaust gases also increases fuel efficiency when the engine is running at full load: it means that enrichment can be dispensed with when the engine is high.

Individual evidence

  1. Fred Schäfer, Richard van Basshuysen (ed.): Handbook internal combustion engine . 2nd Edition. Springer Vieweg, Braunschweig / Wiesbaden 2002, ISBN 3-528-13933-1 , pp. 301 .
  2. Konrad Reif (Ed.): Exhaust technology for internal combustion engines . Springer Vieweg, Wiesbaden 2015, ISBN 3-658-09521-0 , p. 91 ( limited preview in Google Book search).
  3. cf. Functions and variants
  4. Fred Schäfer, Richard van Basshuysen (ed.): Handbook internal combustion engine . 8th edition. Springer Vieweg, Wiesbaden 2017, ISBN 3-658-10901-7 , p. 380 ( limited preview in Google Book search).
  5. Wolfgang Siebenpfeiffer (Ed.): Energy-efficient drive technologies . Springer Vieweg, Wiesbaden 2013, ISBN 978-3-658-00789-8 , p. 91 ( limited preview in Google Book search).