IC engines 2

IC engines 2

1. differences in SI and CI

   • Spark ignition
     • air/fuel mixture drawn in
     • lightweight, possible to have higher RPM
     • fuel must not be volatile at low temp to reduce knocking
   • Compressed ingnition
     • only air drawn in intake stroke
     • heavy, lower RPM
     • fuel must be volatile at low temp to esure spontaneous ignition
     • no throttle required in diesel
     • lower fuel consumption

2. two- stroke engine

   • intake opens fuel oil air mix is taken in, while this is happening the previous intake is in the chamber compressing and is ignited
     • combustion expands gases pushing down the piston providing work, until exhaust valve opens and combusted mix is let out. as piston goes down furtehr intake opens and new fuel oil air mix goes into the chamber
   • Advantages
     • work is generated every 2 strokes not every 4, so power/weight ratio much better
     • simple design.
   • Disadvantages
     • oil is needed in the mixture to lubricate the crankshaft, but this has to be burned in the chamber (a lot of oil wasted)
     • incomplete scavenging : not all of exhaust gas leaves chamber before next intake, not good for combustion.
     • excesive scavenging:???

3. rotary engine

   • rotary: compact design
   • works at high speed due to the smothness of the motion
   • limited commercial success
   • verrrry high tolerances needed to keep the wankl chambers separate
   • low compression ratio

4. Air intake process

   • naturally aspirated: the intake is just normal air at atmospheric pressure
   • supercharger: compresses the intake air before it goes into the chamber to get more power out of the engine. the power to run the supercharger comes from the crankshaft or another mechanical form
     • if the supercharger's power is supplied by a turbine at the exhaust of the the engine, the supercharger is known as a turbosupercharger or turbocharger.
       • not a parasitic load in the direct sense but the turbine causes back-pressure which makes it harder for the exhaust gas to escape (possibly causing incomplete scavenging)
         • the pressure in exhaust is not much more than atmospheric so not big pressure dif across turbine.
         • turbine speed needs to be very fast to produce enough power, reliability of turbocharger can be a problem.
     • by using power from another part of the engine the supercharger is parasitic, so will take some work out of the system.

5. method of fuel input

   • in SI engines the fuel/air ratio are mixed prior to entry in cylinder, and are roughly constant for proper combustion
     • the mixing historically was done using a carbeurator but nowadays most cars have electronic fuel injection systems