April 17, 2024
Motorcycle Engine Design

Engine Design & Basic Theory Part 4 (The Top End)

On this part 4 of the engine design and basic theory we are going to cover the top end which is everything that fits above the crankcase assembly. These are parts that control the flow of gases in and out of the engine and turn combustion energy to the piston motion movement.

  • cylinders and cylinder heads
    • found the cylinder block are machined hole (bore) where the piston moves and up and down
    • most engines use in the cylinder block pressed-in liners made of steel alloy. If liner becomes damaged it can be repaired by re-bore
    • some use plated bores but are more expensive
    • cylinder head holds the sparkplug and valves
  • pistons
    • disc fitted into a hollow cylinder and used to transmit motion
    • piston pins connects the piston to the connection rod.
  • valves
    • engines need some way of allowing fueld and air mixture in the cylinder, sealing it off and expelling which is what the valves and camshaft do.
    • every cylinder on a four -stroke has at least one intake and exhaust valve
    • today all valves are located in the cylinder head – the OHV design short for overhead valve means the valves are located above the piston
    • old designs had the valve in the cylinder block and the cylinder head only the spark plug ( these were called flatheads – made famous by Harley Davidson)
  • cams ( for over-head cam designs)
    • camshaft has several designs, but it is used to control the closing and opening of the valves.
    • common design is to use two concentric springs to close the valve.
    • when a single cam is used to open the intake and exhaust (SOHC)
    • if engine is too wide, two cams to control each intake and exhaust (DOHC)
    • engines with cams located at the bottom of the crankcase are called pushrod engines.

Hope this shed some basic understanding of the top end of engine. Next will discuss the last part of this engine series which is the engine layout.

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