Review Paper on:

Optimisation of cold start behaviour of automotive catalysts using an electrically heated pre-catalyst

By:1) Soumyadeep Paul2) Sourabh Khandelwal

Mechanical Engineering1st year

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Aim of ResearchA study on improving the cold start behaviour using an Electrically Heated Pre-catalyst (EHC). The results of a systematic simulation study of all relevant geometrical parameters and of the appropriate heating strategy are presented.

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Carried Out By:1. T. KIRCHNER Institute for chemistry, Stuttgart University

Germany2. G. EIGENBERGER Institute for chemistry, Stuttgart University

Germany

Catalytic ConverterThere are millions of cars on the road in the world, and each one is a source of air pollution. Over the years, automakers have made many refinements to car engines and fuel systems One of these changes came about in 1975 with an interesting device called a catalytic converter.

A catalytic converter is a vehicle emissions control device which converts toxic by products of combustion in the exhaust of an internal combustion engine to less toxic substances by way of catalyzed chemical reactions.

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Mechanism: How A Converter Works

In chemistry, a catalyst is a substance that causes or accelerates a chemical reaction without itself being affected. Catalysts participate in the reactions, but are neither reactants nor products of the reaction they catalyze.

Tap pipe Emission:1. Water2. Carbon

Dioxide3. Nitrogen

Exhaust gas raw emission:1. HC2. CO3. NOX

Major Reactions:CO+1/2O2=CO2CH4+3O2=2CO2+3H2OCO+NOX=CO2+N2

In the catalytic converter, there are two different types of catalyst at work, a reduction catalyst and an oxidation catalyst.

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Reduction Catalyst

The reduction catalyst is the first stage of the catalytic converter. It uses platinum and rhodium to help reduce the NOx emissions. When an NO or NO2 molecule contacts the catalyst, the catalyst rips the nitrogen atom out of the molecule and holds on to it, freeing the oxygen in the form of O2. The nitrogen atoms bond with other nitrogen atoms that are also stuck to the catalyst, forming N2. For example:

2NO => N2 + O2 2NO2 => N2 + 2O2

Oxidation CatalystThe oxidation catalyst is the second stage of the catalytic converter. It reduces the unburned hydrocarbons and carbon monoxide by burning (oxidizing) them over a platinum and palladium catalyst. This catalyst aids the reaction of the CO and hydrocarbons with the remaining oxygen in the exhaust gas. For example:

2CO + O2 => 2CO2

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Cold StartAutomotive catalytic conveners require a minimum temperature of approx. 350 °C for proper combustion.

Before the invention of Electrically heated catalyst, the necessary heat for start-up was provided only by the hot exhaust gas.

Due to the heat capacity of the exhaust system it takes about 1 to 2 min after the start of the engine before this temperature level is reached.

The amount of emissions during this period represents a considerable fraction of the total amount during one test cycle. This phenomenon is known as cold start.

It is therefore necessary to considerably reduce the catalyst heat-up time.

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Electrically Heated Catalyst (EHC)Presently the most developed technique for reducing cold start consists of utilizing a small EHC (electrically heated catalyst) in front of the main catalyst.

It contains a metal support and is electrically heated by the car battery when the engine is started.

The heat of reaction set free at the pre-catalyst helps to ignite the main catalyst thereby increasing the temperature of conversion reactions.

The efficiency of this catalytic converter system is strongly influenced by certain design parameters and by the operation conditions of the EHC.

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Optimisation of Cold start using EHCThe following behaviour of the catalytic converter can be observed when simulating the cold-start with an electrically heated catalyst. Figure shows temperature and hydrocarbon concentration profiles at different time steps of the cold-start test cycle over the length of the exhaust pipe, which consists of a small EHC and a main catalyst.

When the engine is started (t--0 s) both the exhaust pipe and the catalysts are at ambient temperature (20 °C) and no hydrocarbons are converted at the catalyst.

The small EHC is heated to 400-500 °C in approx. 20 s, which yields a hydrocarbon conversion of approx. 40 % at the pre-catalyst.

The temperature of the exhaust gas stream, which passes the hot metallic monolith, is simultaneously raised to about 400 °C by the EHC.

The electrical heating plus the heat of reaction set free at the EHC helps to ignite the main catalyst and in this simulation run, total combustion of the hydrocarbons (HC) is reached after 54 s.

The use of this EHC reduces the HC-emissions by 64 % during the first 60 s.

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Optimum model of EHC

The goal of this optimization is to minimize the emissions or maximize the conversion of the pollutants in order to meet the stringent regulations of emissions.

In addition the maximum temperature of the EHC should not exceed 800 °C in order to prevent deterioration of catalyst activity.

From additional systematic studies the influence of the following parameters on the hydrocarbon emission during the initial 60 s is reported:

• EHC diameter and length (with constant volume of the EHC),• Thickness of the EHC monolithic wall,• Distribution of the electrical heating power over the length of the EHC.

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Dependence of HC emissions on EHC diameter and lengthVariation of diameter and length is however constrained by the assumption that the volume of the EHC is constant.

Experiments show that a flat disc monolith with bigger diameter is more preferred than that with a narrow diameter.

There are two viable reasons behind this:

1. This can be explained by the length dependent heat and mass transfer coefficients a z, Bz.j with a maximum at the catalyst entrance. So heat and mass convection takes place more effectively in this configuration.

2. The specified configuration also results in reduced external surface for heat losses to the surroundings.

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Dependence of HC emissions on wall thickness of EHC monolithThe dependence of the wall thickness on Cold-start emissions are quite simple. In order to minimize the thermal capacity the wall thickness of the EHC should always be kept at the lowest possible value.

If we unnecessarily increase the wall thickness the mass of the wall would increase leading to increase of the mass of the wall. Now,

(thermal capacity)=(mass) x (specific heat capacity)

So on increasing the thickness we are increasing the mass of the wall which would finally result in the increment of the thermal capacity of the wall.

If the thermal capacity is increased then the heat loss through the wall increases as well resulting in the decrease of efficiency of the EHC.

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Dependence of HC emissions on distribution of electrical heating power over the length of the catalyst Previously catalysts were designed to have uniform power distribution over the length of the monolith. But in the experiments performed a 10-15% reduction in emission levels were recorded on application of linearly dependent power distribution.

In case of linearly increasing power distribution a 10% reduction in emission was recorded. The hottest part of the catalyst is always at the end of the catalyst, because the face of the EHC is constantly cooled by the exhaust gas. Hence, the conversion of the pollutants always starts at the end of the EHC.

The increased heating at the end of the monolith causes the ignition temperature to be reached more quickly and maximum temperature of the surface is close to 800 °C. This elevated temperature of 800 °C however deteriorates the catalyst activity.

In case of linearly decreasing power distribution a 15% reduction in emission was recorded. The ignition temperature in this case is reached later because of the cooling effects of the exhaust gas stream at the face of the monolith. The heat of reaction set free at the front of the EHC helps to heat up the matrix of the catalyst and due to the convective heat transport of the exhaust gas; the EHC is warmed more uniformly as compared to case 1.

The maximum temperature attained is only approx. 650 °C, which means that thermal aging will be considerably reduced using this

heating strategy, The emissions are also lower than with uniform or increasing distribution of the heating power.

Uniform heating

Colloquium 12 Linearly increasing heating

Linearly decreasing

heating

Limitations of EHC Although the usefulness of Electrically preheated catalysts are immense and we can conclude that this technology has successfully optimised cold start of automobiles to a great extent, there still lies a number of limitations in this technology. They are listed as follows:

1. The maximum amount of electrical power available by the car battery is specified as a period of 20 s. considering the influence of the heating power on the cold-start emissions; it is obvious that the required heating power for reaching the ULEV-emission limit is approx. 1.3 kW. The car battery has to be charged almost to its maximum value. This shows that it is desirable to further improve the efficiency of the EHC system.

2. Another major limitation of the EHC technology is that the heating mechanism consumes energy and thus impacts fuel consumption.

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Future scopes for further development

Use of solar energy to generate electricity to heat the catalyst is a development area.

Modern EHCs also offer the opportunity to convert CO2-neutral energy generated by the alternator during deceleration into usable heat. This method of heating requires further research and can prove to be a very efficient method in the near future.

If electric power can be generated through brake energy recovery the heated catalytic converter would have no effect on fuel consumption at all.

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Solar Powered Car

Brake energy recovery system by Mazda

Conclusion One of the key features of modern emission control components is the rate at which they

convert harmful exhaust gases. Engine raw emissions have to be converted within a few seconds to prevent them from escaping into the environment untreated. This has led to the development of EHCs, the prime topic of this review paper.

Despite its age, the electrically heated catalyst is proving to be thoroughly modern because it represents an ideal supplement to the current vehicle concepts featuring energy recuperation and start-stop functioning.

By optimum coordination of the different functionalities and operating parameters it is also possible, with the electrically heated catalyst, to achieve effective thermal management even in terms of energy efficiency.

Compared to conventional engine-based catalyst heating the EHC shows many advantages such as local energy delivery, short response time and good control with independence from engine load point.

The optimization of this heating strategy and the investigation of potential further energy saving will

be the aim of future studies.

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Paper HighlightsThis review paper can be summarised to form the following paper highlights:

A catalytic converter is a vehicle emissions control device which converts toxic by products of combustion in the exhaust of an internal combustion engine to less toxic substances by way of catalyzed chemical reactions.

For a catalytic converter to work efficiently one of the basic requirements is to have an optimum temperature for its functioning. Especially during engine ignition the catalysts are not heated up for proper functioning (this particular behaviour is known as cold-start behaviour).

Presently the most developed technique for this purpose consists of utilizing a small EHC (~electrically heated catalyst) in front of the main catalyst. It contains a metal support and is electrically heated by the car battery when the engine is started. The heat of reaction set free at the pre-catalyst helps to ignite the main catalyst.

The favourable design of an EHC consists of :*large diameter of the EHC*low thickness of the monolithic wall*linearly decreasing power distribution over the length of the EHC

In order to decrease cold start emissions use of EHCs is the most efficient method. Emitec, a leading company in catalytic converter technology terms EHC to be the solution to future emission problems. It even claims that this particular technology is even capable of fulfilling more stringent emission laws in future.

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