GB2209797A - I.C. engine exhaust system - Google Patents

Abstract

An internal combustion engine exhaust system includes a variable-area valve 3 comprising a streamlined plug 10 cooperating with a fixed venturi and movable from a fully open position at high engine speed to progressively less open positions as engine speed falls, to enhance warm-up of exhaust catalyst in a canister 2. The plug 10 is moved by a lever 12 in response to change in engine speed. The plug and venturi may be made of an oxide or nitride ceramic or of a nickel based superalloy. <IMAGE>

Description

Internal combustion engine exhaust system This invention relates to internal combusion engine exhaust systems.

Exhaust pressure waves can significantly influence the performance of internal combustion engines, particularly those having a high specific power output. This arises from the fact that as the exhaust valves are opened a steep-fronted pressure wave in the escaping gas is commonly followed by a depression caused by reflections of the pressure wave from downstream features, or an open end, in the exhaust system. Such a depression, if appropriately phased in relation to the engine piston position and valve timing, can exert a strong scavenging effect on the cylinder contents, especially where a significant valve overlap is used, ie where the intake and exhaust valves operate so as to be simultaneously open for a significant period.

Such extractor-type exhaust systems, although invaluable for high-performance vehicles, adversely affect drivability and exhaust emission performance if deployed for normal use. The reason for this is that at low and medium engine speeds (where many vehicle engines spend most of their working life) the useful effects of exhaust pressure waves are accompanied by effects such as carry-over of fresh charge into the exhaust, reduced volumetric efficiency, and - in extreme cases - blow-back of partly burnt charge into the induction/fuel system.

These effects adversely affect the exhaust emission spectrum.

The present invention has as its objective an amelioration of the emissions problem by making use of a variable-area valve, in particular one comprising a streamlined plug movable into and out of engagement with a fixed venturi, from a fully open position at high engine speed to progressively less open positions as engine speed falls.

Both plug and venturi are preferably made of an oxide or nitride ceramic material, but may be of nickel based 'superalloy' material as an alternative.

In the fully open position of the valve there is minimal resistance to exhaust gas flow. At low and medium speeds the plug is very close to the venturi orifice, and provides both a partial throttling and a surface of strong reflection for pressure waves. The upstream portion of the exhaust system then functions as a multiple reflection semi-choked pipe. Partially plugging the exhaust system in this manner has a beneficial effect upon the function of catalytic converters.

In an exhaust system containing a platinum, rhodium or other catalyst to reduce emission of toxic gases, the variable-area valve is preferably situated immediately downstream of the catalyst. This has in practice the effect of substantially reducing the time that the catalyst needs in order to attain its effective temperature (above about 2500C) when the engine is started from cold, so that the sum of toxic emissions during warm-up (which is the worst period) is reduced because the hot gases have a longer residence time in the catalysor with effectively a greater number of gas passes.

An incidental effect is to reduce exhaust noise during low speed operation.

To bring about the change in position of the exhaust plug element within the valve body, a speed sensor is employed to relate engine revolutions per minute to a predetermined level of throttling. An actuator is then triggered to apply the necessary movement, usually remotely via rods, cables or a simple mechanical linkage.

The valve members are suitably contained in a temperatureresistant enclosure, typically a stainless steel canister or ceramic container, which may be clipped or bolted to other components in the exhaust line.

The invention is further illustrated with reference to the accompanying drawing, which is a longitudinal section of part of an exhaust system.

The system comprises the exhaust manifold 1 of an internal combustion engine, a canister 2 containing oxidising catalyst to reduce emission of toxic gases, a variable-area valve 3 and a pipe 4 leading to the tail of the exhaust. The arrows show the general direction of flow of the exhaust gases.

The variable-area valve 3 comprises a fixed venturi 7 and a streamlined plug 10 mounted on a rod 11 whose downstream end is pivotally attached to a lever 12 secured to and rotatable with a pin 13. The pin passes through the wall of the casing 14 to whose interior the venturi 7 is fixed.

Outside the casing, by means not shown, the pin is connected to the operating mechanism so as to respond to changes in engine speed as previously outlined, and thus to bring about sliding of plug 10 within venturi 7 and consequent change in area of the passage 15 forming the narrowest part of the venturi, through which exhaust gases must pass to reach the atmosphere.

Claims (4)

1. An internal combustion engine exhaust system which includes a variable-area valve comprising a streamlined plug movable into and out of engagement with a fixed venturi, from a fully open position at high engine speed to progressively less open positions as engine speed falls.

2. An exhaust system according to claim 1 which includes a catalyst to reduce emission of toxic gases, wherein said variable-area valve is situated immediately downstream of the catalyst, to enhance warm-up of the latter.

3. An exhaust system according to claim 1 or 2, wherein the plug and venturi valve members are of ceramic material.

4. An exhaust system according to claim 1, substantially as herein described with reference to the accompanying drawing.