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PISTON ENGINES Part 1 Introduction
Introduction In 1903, the Wright brothers made history with the first powered aeroplane that could carry a man. Their flying machine was powered by a piston engine – and today, a century later, piston engines are still used, in hundreds of thousands of aircraft, all over the world. 1903 1955 1991 1935 1916
External Combustion Combustion Air Piston Steam Solid Fuel There are many types of piston engine where solid fuel is burnt externally in a fire box, to turn water into steam, which is piped to the engine to drive the pistons. But these engines are much too heavy for aviation.
External Combustion Combustion Air Connecting Rod Steam Crank Crank Crank Driven Wheel Solid Fuel Even though this type of engine is heavy, it is efficient; that is, more of the energy developed (power) is available for driving the vehicle through the connecting rod, crank and drive wheels, compared to other forms of power generation. Piston
Bottom Dead Centre (BDC). Internal Combustion The combustion process takes place inside the engine. But there are some similarities between the two types of engine. Top Dead Centre (TDC). With the advent of flight, power supplies had to be more efficient - from smaller and lighter engines. Which is where the Internal Combustion Engine came to the fore. Connecting Rod Piston Crank Driven Wheel Crankshaft
The piston engine and a bicycle pump are basically similar, in that both have a cylinder, inside which is a plunger or piston. Pull the piston back to force air in. Then push the piston up the cylinder, to push the air out. to push the air out. Air forced out Basics Mechanical Arrangement A Bicycle Pump Air forced in in A Piston Engine CYLINDER PISTON Compressed charge of fuel and air Expanding gases force piston down CYLINDER PISTON
Basics Mechanical Arrangement Leg force in direction ‘A’ gives rotational force in direction ‘B’ on the crank Hand force in direction ‘A’ gives a rotational force in direction ‘B’ to the nut BA In both cases Linear Force has brought about rotation. A B Crank
Basics Mechanical Arrangement Cylinder Piston Crank Shaft A piston engine has a crankshaft, which works the same way as the bicycle pedals and crank. The force on the bicycle pedal from leg muscles is equivalent to the force supplied by burning fuel and air. A B Crank Fuel and Air
Basics Mechanical Arrangement Main Bearings Offset Bearing Counter Balance Weights Cylinder Piston Connecting Rod Crank Shaft ‘Big-end’ Gudgeon Pin The crankshaft is made-up of various parts. The piston connects to the crankshaft by a connecting (con) rod. The piston is attached to the conrod by a ‘Gudgeon’ pin. And the conrod itself is attached to the crankshaft by the ‘Big-end’ bearing. Fuel and Air
The piston movement now causes the crankshaft to rotate, so we can use the crankshaft rotation to drive a propeller, or the wheels of a car. The rotating crankshaft also provides a means of returning the piston to its starting point. So the piston produces linear thrusting movements time after time. Basics Mechanical Arrangement Main Bearings Offset Bearing Counter Balance Weights Cylinder Piston Connecting Rod Crank Shaft ‘Big-end’ Gudgeon Pin Fuel and Air
Basics Mechanical Arrangement FRONT Crankshaft Timing Belt & Water Pump driving pulley location Flywheel attachment flange REAR The crankshaft is mounted in the engine via the main bearings; on the rotational centre of the crankshaft. At the rear there is a mounting flange for a flywheel. The timing gear and ancillaries (water pump and alternator) are driven from the front of the crankshaft, which is where the aircraft propeller is mounted.
Basics Mechanical Arrangement FRONT Crankshaft Pistons1 2 3 4 Timing Belt & Water Pump driving pulley location Flywheel attachment flange REAR The piston big ends are then attached to the crankshaft. They are attached on the ‘cranked’ offsets – hence the name crankshaft. The offsets are the equivalent to the pedals on a bicycle.
Basics Mechanical Arrangement Major Engine Assemblies Flywheel Pistons and Crankshaft Sump (Oil Tank) Cylinder Block Cylinder Head Assembly Valve Gear ‘Rocker Box’ Cover The Cylinder Head Assembly houses the valve gear mechanism and the top of the cylinders. The spark plugs, inlet and exhaust manifolds are bolted to the head. The Cylinder Block houses the cylinders and the pistons. Also the oil pump and filter (inside), and externally the water pump, and the alternator. The Rocker Box Cover shields the valve mechanism and contains the lubricating oil. The Sump keeps the oil in and can double as an oil tank.
Valve Operation Mechanical Arrangement The Exhaust Valve needs to be open, to allow the burnt gases out The Inlet Valve needs to be open, to allow the fuel/air mixture in Inlet Valve Exhaust Valve In order to compress the fuel air mixture in each cylinder, first it has to be able to get in, then it has to be sealed in or compression cannot take place. This is done through the operation of inlet and exhaust valves.
Valve Operation Push Rod Mechanism Mechanical Arrangement Inlet Valve Open Rocker Rocker Shaft Return Spring Valve Seat Push Rod Cam Shaft Valve Lift Inlet Valve Closed The valve opening motion comes from a rotating cam shaft. A cam has a raised portion that lifts the cam follower or push rod, which either operates a rocking lever or the cam bears directly on the top end of the valve stem, to open the valve. When the valve is forced open, the return spring is compressed, so when the cam rotates to a none raised section, the spring pushes the valve closed.
Valve Operation Mechanical Arrangement Overhead Cam No Rocker Overhead Cam and Rocker Cam Shaft Return Spring Valve Rocker Arm The less parts there are, the more accurate the work with opening and closing times, and the more efficient and the more efficient the engine. Some engines feature a hydraulic system for valve opening (closing via a return spring), but driven by a cam. Some manufacturers now fit VARIABLE valve timing; VARIABLE valve timing; to ensure that the valves operate at the optimum point at all rpm values to gain maximum efficiency from a piston engine.
Valve Operation The Cam Drive Mechanical Arrangement Modern Engines use a toothed rubber belt to drive the cam shafts Cam Drive Mechanism Pistons and Crank Shaft Twin Cam System Chain Tensioners
Check of Understanding What, essentially, is used to drive a propellor? The piston The con rod The crankshaft The push rod
Attaching the big-end to the piston Attaching the piston to the conrod Attaching the crankshaft to the big-end Attaching the conrod to the crankshaft What is the purpose of a gudgeon pin in a piston engine? Check of Understanding
Y W Z X In the diagram below which arrow points to the crankshaft? Check of Understanding
The push rod The cam shaft The cam drive The rocker leg In the diagram below, what is the item indicated called? Check of Understanding
The cam shaft The return spring The cylinder Expanding gases What is used to close the exhaust and inlet valves on a piston? Check of Understanding
The cylinder head assembly The cam shaft The sump The cylinder block In an engine, what are the spark plugs, inlet and exhaust manifolds bolted to? Check of Understanding
The flywheel flange The cam drive mechanism The cylinder head assembly The main bearings Via what means is the crankshaft mounted in the engine? Check of Understanding