The present invention concerns an internal combustion engine comprising at least one cylinder and at least one laser light source for time-controlled externally supplied ignition.
Internal combustion engines of that kind are known in various different configurations. In the case of Otto cycle engines a homogeneous fuel-air mixture (a variation in the fuel-air ratio λ over the combustion chamber of less than 10%) is fired by way of an ignition system. That usually involves a spark plug or a laser light source, the use of laser light sources being advantageous for various reasons. For example when a laser light source is used adjustment of the electrode spacing which is necessary in the case of spark plugs after a given period of operation is no longer required. In addition with laser ignition, instead of spark ignition by way of spark plugs, it is also possible to reliably fire a very lean fuel-air mixture. Otto cycle engines can be for example in the form of a carburettor Otto cycle engine, an injection Otto cycle engine or a gas Otto cycle engine, wherein the latter is operated with a fuel which is gaseous in the normal condition.
In a further large group of internal combustion engines involving externally supplied ignition the fuel-air mixture in the combustion chamber is non-homogeneous. That is the case for example with an internal combustion engine with charge stratification in which a richer fuel-air mixture is provided around the ignition point, than in the rest of the combustion chamber. That results in an improvement in ignition certainty.
If combustion is initiated only at one point in the combustion chamber, the duration of the combustion process is dependent on the speed of propagation of the flame front in the combustion chamber. That also applies when using a laser light source for externally implemented ignition.
The presence of at least two and ideally a plurality of ignition points in the combustion chamber in comparison therewith has the advantage of enhanced ignition certainty and a reduction in the overall combustion duration. It is to be noted in that respect that these advantageous effects occur to their full extent only when there are a plurality of real focal points at which the light emanating from the laser light source is focussed on to finally many individual points. An internal combustion engine having a device for focussing a laser beam on to at least two different focal points is to be found for example in AT 5307 U1. In that case the laser beam from the laser light source is either reciprocated between two ignition locations by an electrically shiftable mirror or simultaneous focussing of the laser beam on to two different focal points is achieved by a convergent lens with a special grind.
JP 63173852 A discloses a combustion chamber in which a laser beam is shifted to and fro between two focal points by a rotating prism.
In JP 58074875 A the production of two simultaneous firing locations is implemented by the arrangement of two mirrors in the combustion chamber.
In comparison therewith, in a situation involving only partial focussing of the light (blurring of the light intensity on to a line-shaped region), as is provided for example in what is referred to as laser cavity ignition, that procedure involves a local intensity peak which is several orders of magnitude less than when there are a plurality of real focal points.