GB2426789A - Laser ignition system for an internal combustion engine - Google Patents

Abstract

A laser ignition system for an internal combustion engine comprises a laser head incorporating a laser medium 10, a first fully reflective mirror 12 and a Q-switch 16. A second partially reflective mirror 14 is mounted in a wall of a combustion chamber of the engine to emit laser light directly into the combustion chamber. An optic fibre 30 serves to guide light in both directions between the Q-switch 16 laser head and the second mirror 14 so as to form a part of the laser cavity and prolong the duration of the laser pulses used to ignite the charge in the combustion chamber. The laser ignition system is in three parts which can be mounted in different locations in a motor vehicle.

Description

IGNITION SYSTEM FOR AN INTERNAl1 COMBUSTION ENGINE The present invention

relates to a laser ignition system for an internal combustion engine.

It has already been proposed to use a pulsed laser instead of a spark to ignite the charge of an internal combustion. The use of a laser to ignite the mixture can result in improved combustion, one reason being that the laser light can be focused at a point within the combustion where it would not be possible to position the tip of a spark plug, thereby allowing the flame propagation to be optimized. Lasers are also believed to be better suited to operation at high speed, as they do not require time to charge a coil between sparks. Because laser ignition has been proposed in the prior art, it is not necessary within the present context to elaborate on the advantages that it offers over spark ignition.

It has been found that the quality of combustion is affected by the properties of the laser pulse used to initiate combustion. A parameter of the laser pulse that has been found to be of particular importance is the pulse duration, it being desirable for the duration of the pulse to be extended.

By way of background information, Figure 1 shows

schematically the essential components of a Q-switched pulsed Nd:YAG laser. The most important component is a laser medium which in the Figure 1 is formed by a Nd:YAG crystal rod 10. The crystal 10 lies between two mirrors 12 and 14 50 that light bouncing between the two mirrors passes through the ciystal rod, the mirrors defining between then a laser cavity having a length equal to the distance between the mirrors 12 and 14. The mirror 12 is a concave fully reflective mirror while the mirror 14 is a flat mirror that allows a small proportion of the light, about 10%, to pass through it while the remaining 90% is reflected. Between the crystal rod 10 and the mirror 14, there is arranged a so- called Q-switch 16, which is an optical switch such as a Fockels cell electro-optical modulator used in conjunction with a polarizer. In response to an applied electrical signal, the modulator can periodically interrupt the passage of light reflected back and forth between the mirrors and hence cavity oscillation.

The basis of lasing operation is stimulated emission of radiation. This radiation is emitted as photons of energy (and thus also of an associated wavelength) equal to the energy difference between two discrete energy levels of the active medium 10, these being referred to as the upper laser level and the lower laser level. The process of stimulated emission competes with the processes of spontaneous emission and absorption in the active medium. To amplify a beam of light by stimulated emission, it is necessary to increase the rate of stimulated emission in relation to the other two processes. For lasing action to occur, it is necessary to increase both the radiation density and population density of the upper level in relation to that of the lower level i.e. a "population inversion" must be created.

The mirrors 12 and 14 of the laser cavity provide optical feedback, to establish oscillations at the resonant optical frequency (determined by the mirror spacing, radius of curvature and radiation wavelength), but also to increase the stimulated emission cross-section i.e. to provide opportunities for those photons already generated to stimulate further emissions by reflecting them back into the medium. Thus, the coherent laser radiation builds up to an intense level by this avalanche effect, but kicked off by the weaker spontaneous emission of photons. Typically, the maximum power level and steady state beam envelope within the cavity are established within a few oscillations, that is to say passes of the cavity length.

Q-switching is a technique for obtaining short, intense bursts of oscillations from a laser. Single high power pulses can be obtained by introducing time-dependent or intensity_dependent losses into the laser cavity. If there is initially a high loss in the cavity, the gain due to the population inversion can reach a very high value without laser oscillation occurring. The high loss prevents laser action while energy is being pumped into the excited state of the medium. If, when a large population inversion is reached, the cavity loss is suddenly reduced (that is, the cavity Q is switched to a high value) laser oscillations will suddenly commence. On Q-switching, the threshold gain decreases immediately (to the normal associated with a cavity of high Q) while the actual gain remains high because of the large population inversion. Due to the large difference between the actual and threshold gain, the laser oscillations within the cavity build up very rapidly and all of the available energy is emitted in a single, large pulse.

This quickly depopulates the upper laser level to such an extent that the gain is reduced below threshold and the lasing action stops.

Q-switching results in a single pulse of great power, typically in the megawatt range, with pulse lengths of 10- lOOns. It is to be noted that, although there is a vast increase in the peak power of a Q-switched laser, the total energy emitted is less than in non-Q-switched operation due to the losses associated with the Q- switching mechanism.

Q-switching is carried out by placing a closed shutter device (the Qswitch) within the cavity, thereby effectively isolating the cavity from the active laser medium. After the laser has been pumped e.g. by the radiant energy of flash lamps, opening of the shutter restores the Q of the cavity.

There are two important requirements for effective Q- switching: 1) The rate of pumping must be faster than the spontaneous decay rate of the upper laser level, otherwise the upper level will empty more quickly than it can be filled which would mean a population inversion could not be achieved; ii) The Q-switch device must switch rapidly in comparison to the build up of oscillations, otherwise a more gradual build up will take place, resulting in a longer pulse length and reduced peak power. In practice, the Q-switch should operate in a time of less than ins.

The devices used for the Q-switch are usually either "active" types (such as a rotating mirror, an electro-optic modulator (Fockels Cell) or an acousto-optic modulator) or the "passive" type (such as a saturable absorber e.g. a bleachable dye) The operation time of the Q-switch, i.e. its total rise and fall time, does not determine the length of the output laser pulse from the cavity. The Q-switch does control when the output pulse starts, but the length of the output pulse depends in reality on how quickly the upper laser level is depleted to the point where the population inversion is below a threshold level required for laser oscillation. The pulse duration is thus more or less equivalent to the "cavity lifetime", which is proportional to the cavity transit time, i.e. the cavity length divided by the speed of light.

The pulse length is then given approximately by: ni rcav,iy = c(iii7) where n = mean refractive index of the medium; 1 = cavity length; c = speed of light in free space = 3 x 10-8 ms1; R = combined reflection coefficient of the cavity mirrors (usually = I(R1R2); R1 being the reflection coefficient of mirror M1 (usually = 1.0) and R2 the reflection coefficient of mirror M2 (usually = 0.9-0.95) Example: for a Nd:YAG rod, n = 1.82 for light having a wavelength of 1 micron, but for a cavity containing other internal components, the mean refractive index may be 1.0- 1.5; for a combined R = 0.9, cavity length of 30cm, the pulse duration would be around iOns.

It should be noted that, in principle, the value of R also affects pulse length, but to a lesser degree than one and, in practice, reducing R2 to values less than 0.9 would have a more significant effect on the population inversion build-up time.

It is clear from that above brief review of the operation of a Q-switched laser that in order to achieve extended laser pulse duration, it is necessary to extend the length of the laser cavity. Within the context of a motor vehicle, this creates packaging problem as there is no convenient location within an engine compartment for a unit large enough to accommodate an extended laser cavity.

The Journal of Cultural Heritage 4 (2003) 72s-76s contains a paper by R. Salimbeni et al in which it is proposed in a different context (the application of lasers in the conservation of works of art) to achieve laser pulses of prolonged duration by incorporating a coiled optic fibre as part of the light path between the two end mirrors of the laser cavity.

Figure 2 of the accompanying drawings shows the configuration suggested in the above paper. The components to 16 are the same as previously described. In addition, a focusing lens 18 and a coiled optic fibre 20 are interposed between the crystal rod 10 and the mirror 12.

There is no need for a lens at the left hand end, as viewed, of the optic fibre 20 as the curved mirror 12 acts to focus the reflected light back towards the end of the optic fibre 20.

Though the use of an optic fibre internal to the laser cavity allows the distance between the mirrors 12 and 14 to be reduced, and the length of the laser housing to be shortened, one still has to accommodate the coiled optic fibre 20 within the housing. As there are limits on the extent that the optic fibre can be bent, the packaging problem discussed above would still not be fully overcome by adopting the construction taught by Salimbeni. Furthermore, in using such a laser as an ignition source in an vehicle engine, one has still to guide the emitted laser pulses, represented by an arrow at the right of each of the drawings, to an engine combustion chamber. A safety problem then arises in the event of a breakage or misalignment of the light guide leading from the laser source to the combustion chamber as this would allow potentially dangerous laser pulses to be emitted.

With a view to mitigating the foregoing disadvantages, the present invention provides a laser ignition system for an internal combustion engine comprising a laser head incorporating a laser medium, a first fully reflective mirror and a Q-switch, a second partially reflective mirror for mounting in a wall of a combustion chamber of the engine to emit laser light directly into the combustion chamber, and an optic fibre acting to guide light in both directions between the laser head and the second mirror so as to form a part of the laser cavity.

In the present invention, the optic fibre needed to expose the charge within the combustion chamber to laser light is itself used to extend the length of the laser cavity in order to achieve a laser pulse of prolonged duration. The laser head itself be small and the desired laser pulse duration can be set by appropriate dimensioning of the length of the optic fibre.

Because the laser head can be positioned anywhere in a motor vehicle, it is possible use a long optic fibre without having to coil it tightly. Thus, for example, the laser head may be stored in the luggage compartment allowing the optic fibre to extend the full length of the vehicle without being coiled.

An important result of using the same optic fibre both to create and to guide the laser pulses to the combustion chamber is that in the event of it being damaged or misaligned it would prevent the laser pulses from being created and therefore the danger of escape of potentially harmful laser radiation from the ignition system is obviated.

The optic fibre needs to be associated at each end with suitable focusing optics to ensure that all the light resonating in the laser cavity passes through the optic fibre.

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which Figures 1 and 2 are, as previously described, schematic

diagrams of prior art Q-switched lasers, and

Figure 3 is a similar diagrams of a laser of the present invention.

The laser of the invention, as shown in Figure 3, differs from the known Q-switched laser described with reference to Figure 1, in that an optic fibre 30 and two lenses 32 and 34 are interposed between the Q-switch 16 and the mirror 14. The principle of operation is exactly the same as previously described except that the length of the laser cavity, as measured between the mirrors 12 and 14 has now been extended by the length of the optic fibre 30.

In this case, however, the various parts of the laser area not all contained in a common housing with the laser pulses emitted from one end of the housing. Instead, the laser is in three parts all of which can be mounted in different locations in a motor vehicle.

The head of the laser is composed of the mirror 12, the crystal rod or other laser medium 10 and the Q-switch 16.

This laser head, which can itself be fairly small, can if desired be mounted at the opposite end of the motor vehicle from the engine. The mirror 14 is mounted in a wall of the combustion chamber so that the laser light emitted through it enters the combustion chamber directly. The mirror can form part of a plug taking the place of a convention spark plug of a spark ignited engine. The connection between the plug and the laser head is effected by means of an optic fibre 30 with associated optics represented by two lenses 32 and 34.

The optic fibre 30 does not merely carry separately created laser pulses to the combustion chamber but itself forms part of the laser cavity in which the laser pulses are created, as previously described, by multiple reflections of light that is guided to pass through the crystal 10 and stimulate emission of light radiation with each pass. As a result, not only does the optic fibre 30 fulfil two functions but it also ensures that no stray laser light is ever emitted. Unless the optic fibre 30 completes the laser cavity, no laser light is produced in the first place.

In terms of packaging, the invention overcomes the problems of the prior art because of the small size of the laser head. This enables it to be positioned at will within the vehicle and allows the optic fibre to be of whatever length is necessary to achieve the desired pulse duration.

The optic fibre 30 does not need to be tightly coiled but it can bend sufficiently to allow for the fact that the mirror 14 will move with any rocking of the engine.

Claims (4)

- 10 - CLAIMS

1. A laser ignition system for an internal combustion engine comprising a laser head incorporating a laser medium, a first fully reflective mirror and a Q-switch, a second partially reflective mirror for mounting in a wall of a combustion chamber of the engine to emit laser light directly into the combustion chamber, and an optic fibre acting to guide light in both directions between the laser head and the second mirror so as to form a part of the laser cavity.

2. A laser ignition system as claimed in claim 1, wherein the optic fibre is be associated at each end with suitable focusing optics to ensure that all the light reflected by the first and second mirrors passes through the optic fibre.

3. A vehicle comprising a laser ignition system as claimed in claim 1 or 2, wherein the laser head and the second mirror are mounted at opposite ends of the vehicle to avoid or minimise the need to coil the optic fibre.

4. A laser ignition system substantially as herein described with reference to and as illustrated in Figure 3 of the accompanying drawings.