Starting Air Overlap Period during which starting air

Starting Air Overlap Period during which starting air is admitted into two cylinders at the same time, to ensure prompt starting from any position. Ü Overlap is normally 15 O to 25 O Ü Overlap depends on type of engine – 2 stroke or 4 stroke number of cylinders time of opening of v/v – earlier opening, greater overlap exhaust v/v opening timing

Starting Angle The starting angle and the period of starting air opening is governed by three main factors: Ü Firing interval of the engine Ü Opening of starting air valve Starting air v/v should open after firing dead centre to give positive turning moment in the correct direction. Some v/vs are arranged to open even 10 O before TDC Ü Closing of starting air valve Starting air valve must close before exhausting commences Period of starting air opening = Overlap + Firing interval

Effect of Engine Type on Overlap 2 -stroke engines have closer firing angle and are easier to start as compared to 4 -stroke engine with same number of cyl. 2 -stroke 6 cylinder engine Period of starting air admittance = 360/6 + 25 = 85 O 4 -stroke 6 cylinder engine Period of starting air admittance = 720/6 + 25 = 145 O For highly turbocharged engine with early exhaust valve opening (75 O before BDC), achieving overlap is problem

No of Cyl & its effect on Overlap Ü More the number of cylinders, smaller will be the firing crank angle Ü Will result in easy starting Ü Greater overlap ( 25 O ) Ü Shorter will be period of starting air supply to individual cylinder Ü Air can be supplied during the most effective part of stroke for greater turning moment

Effect of Exh Timing on Overlap Ü Exhaust timing put a limit on valve overlap Ü Starting air supply is cut off 10 O before exhaust valve opens Ü In highly supercharged engines exh v/v opens at 75 O before BDC as compared to 40 O Ü Results in small overlap & shorter starting air period