COMBUSTION CHAMBER

Abstract: A combustion chamber (12) formed in the crown (10) of a piston for an indirect injection diesel engine is described the combustion chamber (12) having an elongate entry channel (14) adjacent the gas efflux of a precombustion chamber, the channel leading into four branches (24,26,28,30) which generate vortices in the burning charge and which vortices are coherent and mutually self-reinforcing.

Description: The present invention relates to a combustion chamber formed in the crown of a piston for an indirect injection (IDI) diesel engine. In IDI diesel engines combustion of the charge occurs partly in a precombustion chamber, which is frequently situated in the cylinder head, and partly in a combustion chamber formed between the piston crown and the cylinder head surface. The shape of the latter chamber is generally formed mainly in the piston crown. The objective of the piston combustion chamber is to generate high turbulence to effect complete combustion of the fuel charge. Turbulence is generated by the shaped piston crown by redirection of the burning charge which is issuing from the orifice of the precombustion chamber. The turbulence may be in the form of vortices which are intended to expand and fill the whole combustion chamber formed in the cylinder during the power stroke of the engine. Typical piston combustion chambers which has been used in the past are shown in US 2113711 of Ricardo, US 3924580 of Taira et al and US 4063537 of Lampredi. Such combustion chambers have not been completely effective in burning the entire fuel charge as is evidenced by deposits formed on the piston crown in areas which have either low turbulence or lower temperatures or both. The consequence of inefficient combustion is increased emissions from the engine and a decay in thermal efficiency. In FR-A-1344892 there is described, and illustrated in Figure 10, a crown, for a piston for an indirect injection diesel engine, having formed therein a combustion chamber comprising a central circular transfer cavity to co-operate with an associated pre-combustion chamber, and the central transfer cavity is linked to four angularly distributed, circular secondary cavities by four channels. According to the present invention a piston for an indirect injection diesel engine has a combustion chamber formed in the crown of the piston having an entry channel to be adjacent to the gas efflux of a co-operating precombustion chamber, the channel leading into two diverging primary branches thereof, and each of the two primary branches is split into a pair of diverging, and arcuate, secondary branches, and the arrangement is such that each of the four arcuate secondary branches is to generate a vortex in the burning charge, and the four vortices so generated are coherent and mutually self-reinforcing. It is believed that the four vortices which are generated by the combustion chamber of a piston in accordance with the present invention serve to more efficiently sweep up small droplets of fresh charge which are prematurely ejected from the precombustion chamber prior to being ignited. The turbulence generated by the combustion chamber of a piston in accordance with the present invention caused these droplets to be brought back into a zone more suitable for combustion, such as adjacent the entry channel from the precombustion chamber. These small droplets in prior art combustion chambers have not been able to assume the swirl movement generated by the combustion chamber and have therefore failed to become completely atomised. The combustion chamber of a piston according to the present invention is able to sweep up such ejected fuel droplets in the much stronger and self reinforcing vortices which are formed. It has been found with the combustion chamber of a piston according to the present invention that fuel consumption, smoke emission and power output are all significantly improved at higher engine speeds. In order that the present invention may be more fully understood, an example will now be described by way of illustration only with reference to the accompanying drawings, of which: Figure 1 shows a plan form of a combustion chamber for a piston according to the present invention; Figure 2 shows a section through the line AA of Figure 1; Figures 3, 4 and 5 show plan forms of prior art combustion chambers having super-imposed thereon a schematic representation of the gas flow in such combustion chambers, and Figure 6 which shows the gas flow in the combustion chamber of Figure 1. Referring now to Figures 1 and 2 and where the crown of a piston is designated generally at 10. A combustion chamber 12 is formed in the top surface of the piston either by casting or machining. The combustion chamber 12 comprises an elongate entry channel 14 having a closed end 16 adjacent the gas efflux (not shown) of a precombustion chamber (not shown) in the co-operating cylinder head (not shown). Remote from the closed end 16, the entry channel 14 is split into two diverging primary branches 18 and 20 leaving a projecting nose piece 22 opposite the channel 14. Each of the two primary branches 18 and 20 is split into a pair of diverging and arcuate, secondary branches, respectively, 24, 26 and 28, 30. Between the constituent branches of each pair of secondary branches 24, 26 and 28, 30 there is a nose piece, respectively, 32 and 34. Each secondary branch 24, 26, 28 and 30 has a ramp 36 at the end, remote from the associated primary branch, respectively, 18 and 20, to divert gases in an upwardly direction towards the cylinder head (not shown). The secondary branches 24, 26, 28 and 30 are, for example, part circular in form. Depending upon the specific engine design these secondary branches may be of the same or different circumferential extent and may be of the same or differing curvatures. Similarly, the width and side profile or each primary and secondary branch may differ in order to optimise combustion with regard to the particular characteristics of the engine in question. The prior art combustion chambers shown in Figures 3; 4 and 5 generate swirl of the burning fuel charge in the direction shown by the broad arrows. The vortices, however, tend to be localised in the combustion chamber as the piston descends and fail to completely atomise small fuel droplets (indicated by the narrow curved arrows) which are prematurely ejected from the precombustion chamber. Under such conditions it is inevitable that a certain percentage of fuel, prematurely ejected from the precombustion chamber and excluded from the area of the vortices indicated by the broad arrows, fails to burn completely. Inefficient fuel combustion is evidenced by the presence of carbonaceous deposits in the shaded areas 40. As indicated in Figure 6, the combustion chamber of a piston according to the present invention generates high turbulence by vortices which expand to fill the whole area of the piston crown as the piston descends on the power stroke. Because of the directions of circular motion which the gases are forced to adopt by the primary branches 18, 20, the arcuate secondary branches 24, 26, 28 and 30 and the nose pieces 22, 32 and 34 the vortices are self-reinforcing or self-exciting because at the regions where they touch the gas flows have the same direction. Pistons having the form of combustion chamber described have been run in engines for extended periods and shown no localised carbonaceous deposits. The pistons were fitted to a 1724cc, four cylinder diesel engine which was originally fitted with pistons having a crown combustion chamber similar to that shown in Figure 4. Comparative testing has shown a reduction of 26% in smoke emission at 4000rev/min, an increase in power of 3.5% at 4300rev/min and a reduction of 3.5% in fuel consumption at 4400 rev/min. The performance of the combustion chamber of a piston according to the present invention was significantly better in all respects at engine speeds above 3700 rev/min whilst below this engine speed the performances were virtually the same.

Claims: A piston for an indirect injection diesel engine has a combustion chamber (12) formed in the crown (10) of the piston, the chamber having an entry channel (14) to be adjacent to the gas efflux of a co-operating precombustion chamber, characterised by the channel leading into two diverging primary branches (18 and 20) thereof, and each of the two primary branches is split into a pair of diverging, and arcuate secondary branches (24 and 26, and 28 and 30), and the arrangement is such that each of the four arcuate secondary branches (24, 26, 28, 30) is to generate a vortex in the burning charge, and the four vortices so generated are coherent and mutually self-reinforcing. A piston according to Claim 1 characterised in that there are a primary branch and two arcuate secondary branches each side of a line which bisects the entry channel of the combustion chamber. A piston according to Claim 1 or Claim 2 characterised in that each arcuate secondary branch (24, 26, 28, 30) is terminated with an inclined ramp (36). A piston according to any one preceding claim characterised in that between the two primary branches (18, 20) there is a nose piece (22) to aid directing of gas flow during combustion.

Inventors: ONGETTA, RENATO
Applicants: BORGO-NOVA SPA
Application year: 1989
Main classification: F02F3
IPC codes: F02F3, F02B19, F02B31, F02B3, F02B23