Patent GB-1496525-A (GB)
Title: Method of operating a four stroke internal combustion engine
IPC: F02M3/12
Filed: 1974-10-14
Abstract: 1496525 Spray carburetters, treating IC engine charges YAMAHA HATSUDOKI K K 14 Oct 1974 [26 June 1974 12 Aug 1974] 44377/74 Headings F1B and F1H A four-stroke I.C. engine having valve-controlled admission and exhaust ports and spark ignition is run on a lean mixture and when idling or running at low load, the air/fuel ratio of the mixture supplied to the engine is higher than 15 and ignition is timed to occur after T.D.C. In order to enable the engine to run satisfactorily at slow speed or light load on a weak mixture, the minimum open position of the throttle is arranged to be greater than normal, so that scavenging of the cylinders is improved by the increased gas flow through the engine which is permitted by the throttle valve position and the engine speed is reduced to a normal figure by retarding the ignition so that the spark occurs between 0-15 degrees after T.D.C. and preferably between 0-5 degrees after T.D.C. The constant vacuum carburetter shown in Fig. 10 has a piston 309 coupled to a diaphragm 305 and a passage 312 through the piston which communicates the vacuum downstream of the piston with the chamber on the upper side of the diaphragm. The main fuel supply circuit comprises taper needle 314, main jet 314a, float chamber 301, emulsion tube 318 and air bleed conduit 315 and jet 316. The idling circuit comprises air bleed jet 320, passage 321, emulsion tube 323, passageway 324 and discharge nozzles 33a close to the throttle valve 30b. In order to ensure a homogenuous mixture the diameter d o  of the idling air bleed jet 320, the maximum diameter d of the idling mixture passage 324 and the diameter D of carburetter bore should satisfy the relationships d o /D = 0À035 to 0À065 and d/D = 0À035 to 0À100. The above relationships are suitable for a carburetter supplying a single cylinder. When one carburetter supplies a number of cylinders, the diameter d is calculated from the expression d = #n(d-1À0)<SP>2</SP> + 1À0, where n is the number of cylinders and all the dimensions are in millimetres. Fig. 9 shows the starting carburetter, which provides a mixture having an air-fuel ratio which is close to stoichiometric. When a plunger 334 is lifted, air is drawn into passage 333 and fuel together with some entrained air which enters through conduit 331 is drawn through passage 337. The mixture enters the intake pipe through port 336. To improve vapourization of fuel droplets, a section of the intake pipe immediately upstream of the inlet port is maintained at a temperature between 90‹C and 135‹C, preferably at 110‹C. In a water-cooled engine this is achieved by spacing the intake pipe away from the watercooled parts of the engine and cooling it only by air. Fig. 6 shows a contact breaker assembly comprising a first set of points 251 and a second set of points 252 which are angularly displaced relative to the first set so that they open 10 to 15 degrees later than the first set. Switch contacts 272b and 272c are actuated by a lever 272 linked to the throttle valve 130, so that when the throttle is closed the contacts are also closed and the second set of breaker points are then connected in parallel with the first set, so that the spark does not occur until the second set of points opens, i.e. 10 to 15 degrees later. Exhaust gas recirculation is employed to suppress NO x  formation, by drawing exhaust gas through pipe 144a, Fig. 5, cooler 145, vacuumactuated valve 146 and pipe 144b. Valve 146 is moved towards the open position to an extent dependent on the depression in the intake pipe downstream of the throttle valve.