SCC Engine Workings



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Workings of the SCC engine step by step

  • Expansion stroke

  • 1. The air/fuel mixture burns. The combustion heat causes the pressure of the gas mixture to rise, which presses the piston downwards.
  • Exhaust stroke

  • 2. The exhaust valves open when the piston has reached the bottom of its stroke. Most of the exhaust gases are discharged through the exhaust ports due to the pressure difference between the interior of the cylinder and the outside of the gas ports. This takes place during a short period when the piston is at the bottom dead centre. The remainder of the exhaust gases is discharged through the exhaust ports as the piston moves up.

    3. Just before the piston reaches top dead centre, petrol is injected into the cylinder through the spark plug injector. The inlet valves open at the same time. Exhaust gases mixed with petrol are discharged through both the exhaust and the inlet ports. The prevailing operating conditions determine the exact length of time during which the opening of the exhaust and inlet valves overlaps (and thus the proportion of exhaust gases that will remain in the combustion chamber during combustion). As a general rule, the proportion of exhaust gases is higher at low load than it is at high load.



  • Intake stroke

  • 4. The piston moves downwards. The exhaust and inlet valves are open. The mixture of exhaust gases and petrol is drawn back from the exhaust ports into the cylinder. A large proportion of the exhaust gas/petrol mixture flows up into the inlet ports.

    5. The piston continues on its downward travel. The exhaust valves close but the inlet valves remain open, and part of the exhaust gas/petrol mixture that flowed up into the inlet manifold is drawn back into the cylinder.

    6. The piston approaches bottom dead centre. All of the exhaust gas/petrol mixture has now been drawn back into the cylinder, and during the final phase of the inlet stroke, the air needed for combustion is drawn in (14.6 parts of air for every part of fuel).



  • Compression stroke

  • 7. The inlet valves close. The piston moves upward, and the mixture of exhaust gases, air and petrol is compressed. About half-way up the compression stroke (about 45 degrees of crankshaft rotation) and before the spark has ignited the air/petrol mixture, the spark plug injector delivers a blast of air into the cylinder. The air blast creates the turbulence needed to facilitate combustion and shorten the combustion time.

    8. Just before the piston has reached top dead centre, a spark from the electrode of the spark plug injector ignites the air/petrol mixture, and the next expansion stroke begins. The exact instant of ignition is determined by the prevailing operating conditions. Depending on when the ignition instant occurs, the spark is fired either to the fixed electrode across a gap of 3.5 mm or to the electrode in the piston. The spark follows the laws of physics and is fired to the piston as soon as the piston electrode is closer than 3.5 mm to the centre electrode. As a general rule, the spark is fired to the fixed electrode at low load and to the piston electrode at high load.


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