PISTON ENGINES Part 1 Introduction

Introduction In 1903, the Wright brothers made history 1916 1935 1955 1991 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.

External Combustion Steam Combustion Air Solid Fuel Piston 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 Steam Combustion Air Solid Fuel Piston Connecting Rod Crank Driven Wheel 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.

Internal Combustion Piston Connecting Rod Crank Crankshaft 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. The combustion process takes place inside the engine. But there are some similarities between the two types of engine. Piston Top Dead Centre (TDC). Connecting Rod Crankshaft Crank Driven Wheel Bottom Dead Centre (BDC).

Mechanical Arrangement Basics 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. Air forced out PISTON Compressed charge of fuel and air Expanding gases force piston down Air forced in CYLINDER PISTON CYLINDER A Bicycle Pump A Piston Engine

Mechanical Arrangement Basics B Leg force in direction ‘A’ gives rotational force in direction ‘B’ on the crank A Hand force in direction ‘A’ gives a rotational force in direction ‘B’ to the nut A B Crank In both cases Linear Force has brought about rotation.

Mechanical Arrangement Basics 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. Fuel and Air Cylinder A Piston B Crank Crank Shaft

Mechanical Arrangement Basics 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 Cylinder Piston Gudgeon Pin Main Bearings Connecting Rod Offset Bearing ‘Big-end’ Crank Shaft Counter Balance Weights

Mechanical Arrangement Basics 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. Fuel and Air Cylinder Piston Gudgeon Pin Main Bearings Connecting Rod Offset Bearing ‘Big-end’ Crank Shaft Counter Balance Weights

Mechanical Arrangement Basics 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. Flywheel attachment flange Timing Belt & Water Pump driving pulley location REAR Main Bearings Crankshaft FRONT

Mechanical Arrangement Basics 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. 3 Pistons 2 4 1 Flywheel attachment flange Timing Belt & Water Pump driving pulley location REAR Main Bearings Crankshaft FRONT

Mechanical Arrangement Basics Major Engine Assemblies 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. ‘Rocker Box’ Cover Valve Gear Cylinder Head Assembly Cylinder Block Flywheel Pistons and Crankshaft Sump (Oil Tank)

Mechanical Arrangement Valve Operation 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. Inlet Valve Exhaust Valve The Inlet Valve needs to be open, to allow the fuel/air mixture in The Exhaust Valve needs to be open, to allow the burnt gases out

Mechanical Arrangement Valve Operation Push Rod Mechanism 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. Rocker Rocker Shaft Return Spring Valve Seat Inlet Valve Closed Inlet Valve Open Valve Lift Push Rod Cam Shaft

Mechanical Arrangement Valve Operation Rocker Arm The less parts there are, the more accurate the work with opening and closing times, 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; to ensure that the valves operate at the optimum point at all rpm values to gain maximum efficiency from a piston engine. Cam Shaft Return Spring Valve Overhead Cam and Rocker Overhead Cam No Rocker

Mechanical Arrangement Valve Operation The Cam Drive Modern Engines use a toothed rubber belt to drive the cam shafts Twin Cam System Chain Tensioners Pistons and Crank Shaft Cam Drive Mechanism

Check of Understanding What, essentially, is used to drive a propellor? The con rod The push rod The crankshaft The piston

Check of Understanding What is the purpose of a gudgeon pin in a piston engine? Attaching the piston to the conrod Attaching the conrod to the crankshaft Attaching the crankshaft to the big-end Attaching the big-end to the piston

Check of Understanding In the diagram below which arrow points to the crankshaft? W X Y Z

Check of Understanding In the diagram below, what is the item indicated called? The cam shaft The rocker leg The push rod The cam drive

Check of Understanding What is used to close the exhaust and inlet valves on a piston? The return spring Expanding gases The cylinder The cam shaft

Check of Understanding In an engine, what are the spark plugs, inlet and exhaust manifolds bolted to? The cam shaft The cylinder block The cylinder head assembly The sump

Check of Understanding Via what means is the crankshaft mounted in the engine? The cam drive mechanism The main bearings The cylinder head assembly The flywheel flange

PISTON ENGINES End of Presentation