Cold start Combustion

The main converter, which is located downstream of the EHC, heats to functional temperature much more quickly because of catalytic combustion of exhaust gases that would otherwise pass unconverted through the catalyst during the cold start period. The EHC theoretical power required for a reference case (161) was 1600 watts to heat an EHC to 400°C in 15 s in order to initiate the catalytic reactions and obtain the resultant exotherm of the chemical energy contained in the exhaust. Demonstrations have been made of energy requirements of 15—20 Wh and 2 to 3 kW of power (160,161). Such systems have achieved nonmethane HC emissions below the California ULEV standard of 0.025 g/km. The principal issues of the EHC are system durabihty, battery life, system complexity, and cost (137,162—168). 

Exhaust system The engine operating mode controls the tailpipe emissions of hydrocarbons (HC) and carbon monoxide (CO). Over 80% of HC and CO emissions are generated during cold-start and warm-up due to incomplete combustion. Fuel vaporization and fuel/ air mixing are important factors in achieving thorough combustion of the hydrocarbons. 

Cylinder and piston ring wear have been observed when using alcohol fuels. Metal loss could be due to wear caused by removal of the lubricating oil film by liquid alcohol during cold starting or by the corrosive action of formic acid or other acids formed during the combustion process. Use of more corrosion- and wear-resistant metal alloys in engine construction have helped resolve this problem. 

One of the earliest reported uses was for the improvement of the cold starting properties of internal combustion engines, the polonium being incorporated into the sparking plug electrode alloy (38), but its effectiveness for this purpose has been disputed (40) and a health hazard would certainly arise from the burning off of the polonium from the electrode and its discharge into the air. 

However, diesel engines and some gasoline engines are operated under lean-burn conditions, where the oxygen is fed in excess, i.e., 10-20% more than is required to meet the stoichiometry for combustion of the fuel [132,489]. Gold catalysts have therefore been examined for their potential in low-temperature activity to combat cold-start emission problems and removal of NOj, from lean-burn engines [202]. 

Three-way catalyses (TWC) require a minimum temperature of approx. 3500C for proper catalytic combustion. Due to the heat capacity of the exhaust system it takes about 1 min after engine start until this temperature level is reached if the catalyst is only heated by the exhaust gas. The amount of toxics produced during this cold-start period presents a considerable fraction of the total amount during one test cycle [1]. Due to more stringent legal purification requirements several concepts were developed to reduce the catalyst heat up time. Presently the main approaches to lower the cold-start emissions are the use of an electrically heated catalyst (EHC) [2], a burner heated catalyst (BHC) [3, 4] and hydrocarbon adsorber systems [5, 61. 

Within the scope of this study an irmovative concept is developed, where the light-off of the monolith is induced by an exothermic reaction at the catalytic surface (combustion heated catalyst, CHC). It will be shown that in comparison with the other concepts the cold-start emissions with the CHC-concept are lowest level and almost independent of local deactivation of the catalyst. 

The CHC-concept shows the lowest HC-emissions in comparison to the other cold-start concepts. This is due to the fast and direct heating of the catalytic surface where the hydrogen combustion only takes place at active parts of the catalyst. 

The time between the beginning of fuel injection and the start of combustion is called ignition delay . As stated earlier, higher cetane number fuels result in shorter ignition delays, providing improved combustion, lower combustion noise, easier cold starting, faster warm-up, less smoke, and in many engines, reduction of... 

Now, an RFR appears to be useful both for wider application of natural gas and for startup emission treatment. Cold start of internal combustion engines results in the release of large amounts of hydrocarbons that cannot be treated by a converter because it has not yet been heated up to operating temperatures. Future regulations in USA, Europe and Japan will require abatement of these start-up emissions. 

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