GB2353069A

GB2353069A - Introducing NOx into intake of i.c. engine to induce auto-ignition

Info

Publication number: GB2353069A
Application number: GB9919030A
Authority: GB
Inventors: Thomas Tsoi Hei Ma
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 1999-08-13
Filing date: 1999-08-13
Publication date: 2001-02-14
Also published as: GB9919030D0

Abstract

A spark-ignition i.c. engine has a NOx generator 14 arranged in the intake system 12 and a control system 18, 20 for selectively activating the NOx generator to introduce controlled quantities of NOx gases into the charge when it is desired to induce auto-ignition under part load. The NOx may be generated by a plasma discharge between a pair of electrodes connected to a high-frequency AC supply. The production of NOx may be enhanced by first increasing the concentration of oxygen in the air between the electrodes eg using molecular sieves or a selectively permeable membrane. The invention allows auto-ignition to be controlled by a high-speed closed-loop control system.

Description

2353069 ENGINE WITH CONTROLLED AUTO-IGNITION

Field of the invention

The present invention relates to a premixed charge internal combustion engine and in particular to such an engine having a system for inducing auto-ignition in a controlled manner during part-load operation.

Background of the invention

Spontaneous combustion, also termed compression ignition or auto-ignition, in spark ignited internal combustion engines is known and has generally been regarded as undesirable as it normally manifests itself as knock when the engine is operating under high load and as such is a cause of severe engine damage. Spontaneous combustion differs from normal combustion in that, in place of a controlled burning of the charge by flame propagation from a spark, there is a detonation when a large proportion of the charge effectively explodes and generates a very high combustion pressure.

More recently, it has been realised that though such spontaneous ignition should indeed be avoided under high load operating conditions, it has advantages if it can be induced at lower engine loads in that it produces cleaner exhaust gases (in particular very low Nox emissions) and allows leaner mixtures to be burnt completely. However, such combustion is difficult to control and various systems have been proposed to operate a spark ignition engine in the auto-ignition mode under predetermined part-load operating conditions.

Of course auto-ignition occurs all the time in compression ignition engines having a high compression ratio, such as diesel engine. In such engines, auto-ignition is controlled by not introducing fuel into the air charge until after the charge has been compressed, and the fuel is ignited spontaneously as soon as it is injected into the combustion chamber and is burnt at the rate of injection of the fuel injection equipment without forming a premixed charge.

Operating a diesel engine with a premixed charge is undesirable at high load as it triggers the whole charge to spontaneously ignite and produces excessively high combustion pressure causing severe engine damage. On the other hand, it has been realised that there are also advantages in using a premixed charge in a diesel engine at lower engine loads in that it produces cleaner exhaust gases especially for NOx and soot.

Known methods for controlling auto-ignition in a premixed charge engine operating at part load have included heating the intake air, varying the proportion of recirculated exhaust gas (EGR), or modifying the fuel/air ratio. Such methods however do not provide direct control of the timing of auto- ignition, because, depending on the charge composition and initial temperature, auto-ignition may occur at different times during the compression stroke and it may be too early or too late in the engine cycle for optimum thermal efficiency. For this reason, the load/speed range in which premixed charge compression ignition may be used is limited.

Object of the invention The present invention seeks to provide an engine control system that allows the timing of auto-ignition to be controlled with greater accuracy.

3 Summary of the invention

According to the present invention, there is provided an internal combustion engine having an NOx generating means arranged in the intake system of the engine and means for selectively activating the NOx generating means to introduce controlled quantities of NOx gases into the admitted intake charge when it is desired to induce auto-ignition of the charge.

NOx is normally produced by an engine under high load but it is not normally present to any significant extent in the intake charge. Such NOx as is present in the cylinder charge is normally part of the residual gases from a previous operating cycle and does not enter through the engine intake system. Recent evidence suggests that the presence of even a small quantity of NOx gases in the cylinder charge is a cause of engine knock in a spark ignition engine at high load as NOx gases act a strong oxidant that can trigger spontaneous combustion of the end gases.

While the current evidence was gathered during research to suppress knock in a spark ignition engine under high load, the present invention proposes the exact converse of intentionally introducing NOx into the intake charge in order to promote auto-ignition under part load.

NOx may conveniently be generated in the intake system by means by a plasma discharge between two electrodes connected to a high frequency power supply.

It has been reported that a thermal plasma containing a high concentration of NOx (up to 1000 ppm) can be produced by ionising intake air using an electric arc. Such generation of Nox has been proposed to assist in the burning 4 of soot in a diesel particulate trap at lower operating temperatures.

The production of NOx can be enhanced, if necessary, by first increasing the oxygen concentration of the air between the electrodes, for example by using a molecular sieve or a selectively permeable membrane to separate part of the nitrogen in the intake air into a different stream.

Though it is possible to generate a plasma discharge using a high voltage D.C. supply, it is preferred to use a high frequency A.C. supply as this allows a controlled plasma discharge to be sustained using lower voltages.

is To generate a plasma for producing NOx at a sufficiently high rate, it is necessary to be able to produce discharges at a high frequency. The high frequency is necessary so that at the time of each high voltage peak, the gases between the electrodes should still be ionised from the previous electrical cycle.

A volumetric plasma discharge system with this capability has already been proposed that relies on forming nanometre sized fibres on to the end of a piezoelectric crystal to make a high voltage transformer. By applying a

Claims

primary a.c. voltage of between 100V and 150V, it is claimed by the

manufacturers of the transformer that an output voltage of up to 20kV can be produced at up to 40 kHz to drive the discharge electrodes (see European Automotive Design, March 1999, Page 14).

The invention offers the important advantage of providing a variable parameter that can be used to control the timing of auto-ignition. This allows auto-ignition to be controlled by a high speed closed loop control system. Hence it is possible to measure the actual timing of autoignition and to compare it with the desired optimum timing S as determined by the engine management system to generate an error signal and the error signal can be used to control the rate at which NOx is produced by the plasma discharge generator in the intake system of the engine.

Brief description of the drawings The invention will now be described further, by way of example, with reference to the accompanying drawing, in which the single drawing shows a block schematic diagram of an engine embodying the present invention.

Detailed description of the preferred embodiments The single figure shows an engine 10 having an air intake pipe 30 that contains an NOx generator 14 and leads to the engine through the intake system 12 which typically includes an intake manifold, a butterfly throttle for regulating the intake air quantity and a mass air flow meter for measuring the intake air quantity. The engine also has an exhaust system 16 leading to an exhaust pipe 32.

The operation of the engine is controlled by a management system 20 that has various input lines 22 and output lines 24. The main operation of the management system will not be described in detail as it is generally conventional. Essentially, the management system 20 contains a micro computer that receives input signals from various sensors indicative of such parameters as engine speed, load, temperature and produces output signals that determine the fuel injection quantity and timing and the ignition timing. The control strategy of the engine and its various calibration settings are all incorporated in the program entered in the read only memory of the computer.

The engine management system 20 in the present invention is designed under part load conditions to induce 6 auto-ignition of the admitted intake charge. To this end, the management system may vary various known parameters that affect auto-ignition such as valve timing, EGR quantity, intake air temperature and fuel/air ratio. In addition, the management system 20 controls the NOx generator 14 to introduce NOx into the intake charge. The greater the amount of NOx, the greater the tendency for the charge to auto-ignite and therefore the earlier the onset of autoignition in the combustion cycle.

In the described preferred embodiment of the invention, the engine is fitted with an ignition sensor 18 that detects when the charge ignites. Ignition is accompanied by ionisation of the gases and the instant of ignition can be sensed by means of an ionisation sensor, which may for example be the spark plug or a separate pair of sensing electrodes. Of course ignition is also accompanied by a sharp rise in pressure which can be sensed using a pressure sensor. As a still further possibility, one may use an optical sensor. The actual ignition timing can now be fed as a further input parameter to the engine management system 20 and the latter can compute an error signal from the difference between this actual timing and the desired timing under the prevailing operating conditions. The management system can now advance or retard the instant of autoignition by controlling the NOx generator in the appropriate sense to reduce the computed error signal.

The efficiency of the NOx generator can be increased if necessary by increasing the oxygen concentration of the gases in the plasma discharge. This can suitably be achieved by using a selectively permeable membrane 40 or the like to divide the intake air into an oxygen rich and an oxygen depleted stream. The plasma discharge may then be placed in the oxygen rich stream and the two streams can be recombined in the intake system 12 before they enter the engine.

7 CLAIMS 1. An internal combustion engine having an NOx generating means arranged in the intake system of the engine and means for selectively activating the NOx generating means to introduce controlled quantities of NOx gases into the admitted intake charge when it is desired to induce auto-ignition of the charge.
2. An engine as claimed in claim 1, wherein the NOx generating means comprises two electrodes connected to a high frequency power supply to produce a plasma discharge between the electrodes.
3. An engine as claimed in claim 2, wherein in order to enhance the production of NOx the oxygen concentration of the air between the electrodes is increased.
4. An engine as claimed in claim 3, wherein the oxygen concentration of the air between the electrodes is increased by the use of a molecular sieve or a selectively permeable membrane to separate part of the nitrogen in the intake air into a different stream.
5. An engine as claimed in any preceding claim, further comprising a feedback control loop for optimising the quantity of NOx produced by the NOx generating means, the feedback loop including means for sensing the actual timing of auto-ignition, means for comparing the actual auto-ignition timing with the desired timing to produce a timing error signal and means for varying the quantity of NOx gases produced in proportion to the error signal and in a direction to reduce the error signal.
6. An engine constructed, arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawing.