The Intake and Exhaust Systems - EngineKnowHow

The Intake System

In a spark ignition engine the intake system is typically composed of an air filter, throttle, inlet manifold, inlet ports and the intake valves.  If the engine is boosted there will also be a compressor and an intercooler located between the air filter and throttle.  A compression ignition engine operates without the throttle in the intake system, instead controlling load by the amount of fuel injected into the cylinder.

Source: Porsche - "Der neue V8 Turbomotor von Porsche", Wiener Motorensymposium, (J. Kerner, T. Günther, M. Werner, A. Kronich, M. Krämer, A. Kramer, C. Pleuß, W. Liesen, S. König, F. Maier)

Engines which run an external Exhaust Gas Recirculation (EGR) system will typically require the installation of a throttle in the intake system to create a low pressure system to draw the external EGR from the exhaust system into the intake system.


The plot below shows the air consumption of three different engines under full load conditions showing how much air moves through the intake system.  A measure of how effectively the air travels from the atmosphere and fills the cylinder under full load conditions is the atmospheric volumetric efficiency.

The Exhaust System

In an internal combustion engine, the exhaust system is typically composed of the exhaust valves, exhaust ports, exhaust manifold, catalytic converter and a muffler system.  If the engine is turbocharged then a turbine will be located after the exhaust manifold and before the catalytic converter.

Source: Renault - "Downsizing a Small Displacement Spark Ignition Engine by Turbo Charging", Aachener Motoren- und Fahrzeugkolloquium, (V. Martin-Arcas, E. Jean, L. Kermanac’h, T. Mansion, M. Marti Moreno)


Diesel engines, owing to the higher soot and NOx emissions, could have a Diesel Particulate Filter (DPF) for the reduction of soot emissions located after the catalytic converter.  Additionally, a Selective Catalytic Reducer (SCR) or a Lean NOx Trap can be located after the DPF for the control of NOx emissions.

Source: Honda - "Efficiency Optimisation of SCR Coated Particle Filters", Aachener Motoren- und Fahrzeugkolloquium, (M Fischer, H Kojima, T Mito, N Ohya, H Haga, N Sato, C Beidl)

Engines which run an external EGR system will extract the exhaust from pre-turbocharger, known as high pressure EGR, or pre-catalytic converter, known as low pressure EGR.  Between the intake and exhaust systems there will be a cooler for cooling the exhaust prior to entering the intake system and a throttle valve to control the level of EGR removed from the exhaust system.


The plots below show the temperatures and pressures at different stages in the exhaust system for a V6, turbocharged petrol engine at full load.

Note that the between 1000 – 2250rpm, the temperatures exiting the catalytic converter is greater than the turbocharger in temperature.  This indicates that a high level of reactants were entering the catalytic converter (due to boosting and the valve timing strategy) and reacting within the catalytic converter and thereby generating heat.