Operational Theory of Compression Small Engines Ag 221
- Slides: 29
Operational Theory of Compression Small Engines Ag. 221
Objectives Identify basic terms and definitions associated with compression List individual engine parts related to compression Describe, compare and contrast compression ratios Calculate engine displacement Test engine compression
Four Strokes
Compression Parts Cylinder
Compression Parts Piston Rings Compression Ring Scraper ring Piston oil ring
Compression Parts Valve stem Valve face
Compression Parts Valve seats Valve guides Cylinder head
Compression Parts Spark plug Head gasket
Temperature Multi cylinder engines are liquid cooled and temperature remains constant
Temperature Single cylinder engines are air cooled Cooling Fins Blower Housing Flywheel
Operating Temperature is affected by: ambient temperature n Temperature of the surroundings n If it’s a hot day, mowers run hotter
Operating Temperature is affected by: operating speed n Engines operating at high speed build up high amounts of heat
Operating Temperature is affected by: load put on the engine n If the engine is running hard under load, cooling cannot always keep up
Tolerances and Clearances Tolerances are when you torque (tighten) a bolt to a certain measurement, allowing for function when the metal expands. If space is not allowed, excess friction will cause overheating or worse!!
Clearance is the distance by which one object avoids hitting another
Tolerances/Clearances Tolerances and clearances must compensate for the entire range of temperatures n n What happens to metal when it gets hot? What is the result if connecting rod/rod journal clearance is not accurate?
Compression Test Briggs and Stratton does not have compression measurements for single cylinder engines n Why? Simple compression test: n n Spin the flywheel backwards quickly If the flywheel rebounds on the compression stroke, the engine has enough compression
Valves Most important factor in compression; Valves must seal to maintain compression Valves operate under the most severe conditions within an engine, especially the exhaust valve
Valves Exhaust valve is exposed to temperatures that may reach 1200° F. (red hot) n Exhaust valve is made of specialized steel in order to withstand the corrosive action and high temperature of exhaust gases
Valves in an engine operating at 3000 rpms (revolutions per minute) will open and close in 1/50 of a second
Valves Intake valve is cooled by the incoming air/fuel mixture
Valves Cylinder head and piston are exposed to the same heat n n Cylinder head is cooled by air from the flywheel fan across the cooling fins Piston is cooled by oil in the crank case
Valve Failure How does valve failure in a V-8 compare to valve failure in a single cylinder engine? Single – loss of horsepower or a cessation of function (it stops running all together!!!) Multi – fractional reduction in horsepower
Displacement The volume that is displaced by the piston The longer the stroke and the greater the bore (diameter of the cylinder) the greater the displacement Displacement indicates the relative size of the engine; horsepower is directly proportional to displacement Displacement is calculated by: displacement = (bore diameter 2/4) X ¶ X stroke length
Compression Ratio Compression ratio gives an efficiency measurement (higher compression = higher efficiency) A compression ratio of 8: 1 n Volume in cylinder is 8 times larger when the piston is at the bottom than when it is at the top Briggs and Stratton engines operate at a compression ratio of 5 -6: 1. .
High Octane Fuel Higher compression requires higher octane Premium fuels are more difficult to ignite therefore higher compression is necessary High octane fuels used in lower compression engines will not have enough compression to burn all the fuel Briggs motors are ran on low octane fuel
Knocking Low octane fuel used in high compression engines Compression ignites the fuel and starts the piston down prior to spark plug spark When spark occurs, any remaining fuel in the cylinder is ignited causing a knocking sound
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