Evaporation-Cooled Manifold for Spark-Ignition Engines
When the RDE method for testing the exhaust emissions produced under real driving conditions comes into force, spark-ignition engines will have to operate with a stoichiometric air-fuel ratio at every engine operating point. However, emissions of hydrocarbons and carbon monoxide currently exceed these limits, particularly at high load. A new cooling system by IAV reduces the exhaust temperature at engine high loads so that the engine runs permanently with an air-fuel ratio of one.
The new RDE test method merely states route requirements and ambient temperatures in a relatively wide window for valid test drives in order to take due account of the many possible operating conditions in real driving situations. In terms of calibration, this means that the spark-ignition engine has to comply with the emission limit values at all conceivable operating points. “It is very difficult to achieve this, for example, at high load on the highway”, explains Thomas Arnold, team manager for combustion engine advance engineering at IAV. “The exhaust gas gets too hot for the exhaust turbocharger and catalyst, which could be damaged or destroyed.”
The current solution consists in injecting additional fuel that is not ignited but evaporates, thus cooling the exhaust gas. But the result is unacceptably high hydrocarbon and carbon monoxide emissions. IAV has therefore developed a new evaporationcooled exhaust manifold that keeps the exhaust gas temperature on an optimum level for the exhaust turbocharger and catalyst.
Evaporation for extremely high heat flow
The solution consists primarily of a double-wall manifold: the exhaust gas flows through the inside, and a mixture of water and ethanol as coolant flows through the outer channel. The heat input makes the mixture evaporate in the manifold: it subsequently condenses again in a condenser.
“The condenser can be relatively small because we’re condensing vapor, not cooling liquid”, says Arnold. “The cooling circuit therefore only has to convert a small amount of mass: around two percent compared to a liquid-cooled system.”
As a result, a pump with an output of just 15 watt is completely adequate. IAV has been working on the new cooling system since 2014 in the framework of a proprietary development project. The system has already proven its capacity on the test bench and attracted great interest from customers. “Our solution is practically package-neutral”, says Arnold. “The new manifold is scarcely bigger than a conventional cast manifold. The 12-volt pump is about as big as a small can of coke and the coolant tank with about half a liter volume has the dimensions of approximately two Tetra packs.” What’s more, the system needs no maintenance, quite in contrast to its main rival: in solutions with water injection, the driver has to constantly top up the water.
All emission limits were met
“Using a 1.4-liter turbo engine, we have demonstrated that we can operate with an air-fuel ratio of one at all operating points with the new cooling system, and still comply with the emission limits”, concludes Arnold. “Customers particularly like the fact that this is a separate circuit so that the engine doesn’t have to be modified.” The concept could even be extended to the whole engine cooling system in future. Arnold and his team are already working on it.
The article was published in automotion 02/2019, the automotive engineering magazine of IAV. Here you can order the autmotion free of charge.