Swirl, or rotation of the air charge, is increasingly popular as an aid to fuel distribution in Diesel engines, but as yet little is on record as to the principles of its production and use. On certain types of engine it is possible to mount a “swirl meter” or freely rotating vane within the combustion chamber, and thus by motoring the engine to obtain a measure of the swirl rate. From such experiments and from theoretical considerations, one can draw certain conclusions, and although these are directly established only as regards one particular combustion system, indirect evidence suggests that they have a much wider application. The most important conclusion is this, that with the combustion system in question there is an optimum “swirl ratio,” or number of revolutions of the vane per minute divided by the number of revolutions of the engine per minute which gives the best performance, whatever the engine size, speed, or load, and which is, in fact, characteristic of that combustion system. It seems probable that the same rule applies to other swirl systems. In most cases the “swirl ratio“ is roughly constant over a wide speed range. This means that if this ratio is made correct at one speed, it will not be far wrong at any speed within the working range, a fact which accounts for the popularity of swirl type engines in vehicle and other variable-speed services. The primary effect of swirl is that of winnowing the fuel spray, blowing away fine spray and burnt gas, and giving the heavier particles access to fresh air. There are, however, important secondary effects of which the causes are not known with certainty.