Power plants – Internal combustion engine with treatment or handling of... – Methods
Patent
1993-12-10
1995-06-20
Lazarus, Ira S.
Power plants
Internal combustion engine with treatment or handling of...
Methods
60286, 60289, 60307, F01N 320
Patent
active
054252334
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a method of operating an internal combustion engine having an afterburner to reduce emissions during cold starts.
BACKGROUND OF THE INVENTION
An exhaust catalytic converter only performs its task of reducing the unburnt hydrocarbons, carbon monoxide and oxides of nitrogen content of the exhaust gases after it has reached a critical temperature, termed the light-off temperature, which is between 300.degree. C. and 400.degree. C. During cold starts, it is important to minimise the time taken for the catalyst to reach this temperature, more especially since the emission test drive cycles which are laid down by various legislations all include a cold start.
Various solutions have already been put forward to enable the light-off time to be reduced. The simplest solution is to place the catalyst very near the engine so that it is heated by the exhaust gases before these have been cooled by the exhaust system. This method of mounting a catalyst, usually termed close-coupling, creates problems when the engine is running under high speed and high load conditions. Under such conditions, the exhaust gas temperature can exceed 850.degree. C., which is enough to cause permanent damage to the catalyst. It is therefore preferred not to provide a close-coupled catalyst but to use one mounted some distance away from the engine, normally termed an under-body catalyst. Such mounting is safe for high speed and high load operation but exacerbates the warm-up problem because the exhaust gases are cooled before reaching the catalyst during the start-up phase.
To speed up the warming of a catalytic converter, an external heat supply has been proposed, including electric heaters and microwave heaters. These proposals have involved significant additional cost and complexity, more especially when it is appreciated that the power requirement is of the order of 2 to 3 kilowatts, which with a 12 volts supply calls for a current of 166 to 250 amps.
It has also been proposed to use chemical energy to reduce light-off time by injecting fuel into the exhaust pipe and igniting it. The complexity in this case is that petrol/air mixtures do not always ignite reliably when diluted with exhaust gases from the engine and if they should fail to do so they aggravate the problem by cooling the catalyst and by dramatically increasing the hydrocarbon emissions in the exhaust. There are further complexities imposed by the need to ensure safety, it being inherently dangerous to provide a fuel line opening into a hot exhaust pipe.
A still further proposal has been the use of the so-called thermal reactor in which air is injected into the exhaust stream close to the exhaust port to intercept the exhaust gases while they are still hot. If the mixture is set slightly rich, the combustion reaction continues in the exhaust gases, albeit at a reduce rate, and this raises the temperature of the exhaust system to reduce the light-off time of the catalytic converter. Though this proposal works, the benefits one achieves by it are only of limited value. Typically, the light-off time would be reduced to around two minutes, which still falls short of enabling the more exacting permitted legal emission levels to be met.
A still further proposal has been to use an afterburner. The engine is once again run with a rich mixture and fresh air is added to the exhaust gas stream but this time the mixture is ignited, for example by a spark to burn within a chamber arranged immediately upstream of the converter.
It is important to differentiate between the reaction initiated by ignition in an afterburner and the reaction which normally takes place on the surface of a catalytic converter. In an afterburner, there is created a luminous open flame which propagates through the gases and is not confined to a surface. The ignition can be initiated by a spark, a pilot flame or indeed by a heated catalytic element. Once ignited the flame is not confined to the igniter and the gases burn as they would in an unconfined space.
REFERENCES:
patent: 3744249 (1973-07-01), August
patent: 3745983 (1973-07-01), Sweeney
patent: 3911676 (1975-10-01), Jensen
patent: 4098078 (1978-07-01), Laurent
Collings Nicholas
Ma Thomas T.
Ford Motor Company
Lazarus Ira S.
Malleck Joseph W.
May Roger L.
O'Connor Daniel J.
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