GB2353822A

GB2353822A - Injecting atomic nitrogen into i.c. engine combustion chamber to reduce NOx

Info

Publication number: GB2353822A
Application number: GB9920907A
Authority: GB
Inventors: Thomas Tsoi Hei Ma
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 1999-09-04
Filing date: 1999-09-04
Publication date: 2001-03-07
Also published as: GB9920907D0

Abstract

An internal combustion engine is disclosed in which NOx present in the residual gases is neutralised at the commencement of each engine cycle to reduce the tendency of the engine to knock. The NOx is neutralised by injecting a stream of atomic nitrogen into the residual gases in the combustion chamber 10 at the start of each intake stroke, at least when the engine is operating under high load. The atomic nitrogen is produced by a spark discharge between an electrode 31 and a tube 30 through which flows a stream of ambient air, from which oxygen has been removed by molecular sieves 34, 34'.

Description

2353822 Internal Combustion Engine The present invention relates to a

spark ignition internal combustion engine and seeks to reduce the tendency 5 for such an engine to knock when operating under high load.

According to the present invention in its broadest aspect, there is provided an internal combustion engine having means for injecting a stream of atomic nitrogen into the residual gases in the combustion chamber at the start of each intake stroke, at least when the engine is operating under high load.

It has recently been discovered that the presence of NOx in the combustible charge of a spark ignition engine increases the tendency for the engine to knock (See SAE 19 9 9- 01-057 2). This paper describes how knock can be provoked by intentionally metering NOx into the intake charge. When an engine operates under high load, there is usually NOx present in the exhaust gases and while these exhaust gases are mostly discharged to ambient through the exhaust system, there will be NOx present in the residual charge that will increase the tendency of the engine to knock in the next cycle.

It has also been demonstrated that NOx in the exhaust gases can be neutralised by injecting atomic nitrogen into the exhaust gases (see SAE 982515). If molecular nitrogen is exposed to a spark discharge, atomic nitrogen is formed and the atoms can survive in this form for 1 to 3 ms. If such nitrogen atoms should react during this time with NOx, the NOx is reduced to nitrogen and oxygen is given off. Though this reaction is reversible, at lower temperature (less than typically 2500C in the presence of 15% oxygen) it proceeds predominantly in the direction of reduction of NOx to nitrogen.

The present invention is predicated on the realisation that one can reduce the tendency for an engine to knock by injecting atomic nitrogen not into the exhaust gases in the exhaust system but into the residual gases in the combustion chamber, in order to remove any NOx present in the residual gases from the previous combustion cycle. As it is only the NOx present in the residual gases that needs to be neutralised, the flow of nitrogen atoms that is required is relatively small and readily achievable by means of a spark discharge in the engine intake system.

The effectiveness of a spark to atomise nitrogen is dependent upon the purity of the nitrogen. In particular, the presence of oxygen, C02 and water vapour are all known to interfere with the dissociation of nitrogen molecules into their atoms. It is not possible merely to ionise the intake air as this would produce more NOx instead of neutralising any NOx present in the residual gases. It is therefore necessary either to use a pressurised nitrogen supply or more preferably provide means such as a molecular sieve for removing most of the oxygen from the air before it is subjected to a spark discharge.

Ideally, the atomic nitrogen should be injected directly into the residual gases in the combustion chamber on account of the short lifetime of the atomic nitrogen. However, it is more convenient to inject the atomic nitrogen into the intake port at a location and at an instant that allow the nitrogen atoms to reach the NOx in the residual charge before they revert to nitrogen molecules.

Brief description of the drawings

The invention will now be described further, by way of example, with reference to the accompanying drawing which is a schematic section of an engine combustion chamber with an intake system embodying the present invention.

3 In the drawing there is shown a variable volume combustion chamber 10 defined by a piston 14 that reciprocates within a cylinder bore 12. Air and fuel are admitted into the combustion chamber 10 past an intake valve 26 through an intake port 16. The trapped charge is ignited by a spark plug 20 and subsequently the exhaust gases are discharged past an exhaust valve 28 into an exhaust port 18. Such an engine is of course entirely conventional and need not therefore be described in greater detail.

The combustion chamber 10 always contains some residual gases from the previous cycle at the commencement of the intake stroke of the next cycle. These residual gases have the same composition as the discharged exhaust gases and may contain NOx especially when the engine is operating under high load. Because it has been discovered that the presence of even a small quantity of NOx gases in the combustion charge provokes knock, the present invention takes special steps as now to be described to neutralise any NOx that may be present in the residual gases.

The principle used to neutralise the NOx content of the residual gases is to inject a stream of atomic nitrogen into the combustion chamber at the start of the intake stroke.

The atomic nitrogen is generated as will be described below by the components designated 31 to 38. The atomic nitrogen only survives in this state for a very short time and for this reason it should ideally be injected as soon as the exhaust valve closes. As this is expensive to achieve, the described embodiment adopts the less costly option of introducing the atomic nitrogen into the intake port immediately next to the intake valve 26 to be admitted into the combustion chamber 10 as soon as the intake valve 26 opens.

The atomic nitrogen is produced by means of a spark discharge between an high tension electrode 31 and an earthed tube 30 through which a stream of nitrogen gas is aspirated into the intake port 16. The nitrogen gas is ionised into atomic nitrogen and ejected as a jet or a spray into the intake charge. 5 It is important that the nitrogen gas should be depleted of oxygen and this may be achieved by drawing air from an inlet 38 and past the valve 32 through a molecular sieve 34 which preferentially adsorbs the oxygen.

Eventually such a molecular sieve will be saturated with oxygen and will cease to be effective. To purge the oxygen out of the molecular sieve 34, it may be heated by a heater 36 that surrounds the molecular sieve. Suitable molecular sieves using carbon are described in the prior art, see for example US-A-4925461. As nitrogen cannot be supplied by the molecular sieve during the periods that it is being purged, a continuous supply of nitrogen is achieved in the illustrated embodiment by switching using the valves 32 and 32' between two essentially identical molecular sieves 34 and 34'. Like components in the two sieves have been allocated like reference numerals with the addition of a prime.

Claims

1. An internal combustion engine having means for injecting a stream of atomic nitrogen into the residual gases in the combustion chamber at the start of each intake stroke, at least when the engine is operating under high load.

2. An engine as claimed in claim 1, wherein the atomic nitrogen is injected into the intake port close to the intake valve to reach the residual gases while still in atomic form.

3. An engine as claimed in claim 1 or 2, wherein the means for injecting a stream of atomic nitrogen comprises means for supplying a stream of nitrogen gas and exposing the nitrogen gas stream to a spark discharge.

4. An engine as claimed in claim 3, wherein the means for supplying a stream of nitrogen comprises two molecular sieves for preferentially absorbing oxygen from an air stream, means for heating the molecular sieves to purge them of adsorbed oxygen and valves connecting the molecular sieve to the intake port, the valves and heating means being operated in alternation to connect an active molecular sieve to the intake port while isolating the molecular sieve undergoing purging from the intake port.

5. An engine constructed and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawing.