The Piston Engine Flashcards by Tom Gilmour

The Otto cycle stages

Induction stroke

Compression stroke

Power stroke

Exhaust stroke

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Induction stroke

As the piston goes down - creates low pressure - draws air in

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Compression stroke

As the cylinder head goes up - air compresses and creates heat

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Power stroke

The ignition of the fuel and air mix = rapid expansion

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Exhaust stroke

Exhaust leans the cylinder

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Why is more efficient at high altitude

Easier to push the end gasses out - less atmospheric pressure acting against it

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Bottom dead centre

Lowest point the cylinder gets to

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Top dead centre

Highest point the cylinder gets to

Crank and connected rod are perfectly aligned

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Inline engine

Cylinders are aligned in a line on 1 crank shaft

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Inverted inline engine

Turned upside down - could get puddles of oil in each of the cylinders

Pulling through get rid of these puddles by turning the engine before starting

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Radial engine

Engine cylinders are arranged in a circle - equal cooling throughout - more surface area

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Horizontally opposed

Cylinders on each side

Shorter engine - better cooling - no hydro locking

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The crank case

2 parts bolted together - crank in the middle

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Main journal

Main crankshaft centre

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Big end journal

To the side of the main crankshaft centre - where the cylinders are joined onto it

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Stroke

Full distance through which the piston moves

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Throw

Half the stroke

From the centreline of the main journal to the centreline of the big end journal

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Connecting rods

Connects piston to the crank shaft

Generally made of steel

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Gudgen pin

Pin through the cylinder head which connects the connector rod to the cylinder head

Cylinder head is generally made of

Aluminium alloy

Piston rings

Will expand and contract to create a gas type seal
1st and 2nd = compression rings

3rd = oil control rings

Normally made of carbon steel

How the valves operate

Cam shaft pushes the hydraulic tappet/ cam follower which pushes the push rod which moves the rocker arm to push the valve tip which pushes the valve open and close using the spring

Camshaft

Moves at half the speed of the crank

Regulates the movement of the rocker - opening and closing the inlet and outtake valves

Inlet and exhaust valves

Inlet is slightly wider than the exhaust

Exhaust valves are hollow and filled with sodium

Boor area

Area on top of the cylinder

Swept volume

The amount of volume displaced by the piston during a stroke

Clearance volume

Residual volume left at the top of the cylinder at TDC

Compression ratio

= total volume (stoke + clearance) / clearance

Multi-cylinder stroke sequence

1-3-4-2

1 horsepower =

33000 ft lbs/min Based on 33lbs being lifted through 100 ft

How to measure brake-horsepower

Torque x rpm Pony brake - clamped to the shaft BHP = indicated HP - friction HP

Indicated horsepower

= PLANE/33000(ft lbs/min) ``` P= indicated mean effective power L= length of the stroke A = area of the piston crown (pxa = force) N = number of power strokes/min E = number of cylinders ```

Friction HP

Horsepower taken from the total HP for other things e.g air con

Why does power decrease with altitude

Less pressure = less air in the piston = less fuel burn = less power However at altitude it is easier for the inlet and outlier valve to work so it reduces the power loss - improves scavenging

Efficiency equation piston engine

Ratio of work done by a measure of energy it gives Thermal efficiency = work out (BHP)/ work contained within the fuel

% of the fuel which gets turned into stuff

30% into work 40% in the exhaust 25% In the cooling system 5% on friction

Mechanical efficiency

Ratio of BHP to IHP Percentage of power in the engine to turn the prop Typically 80 to 85%

Volumetric efficiency

Ratio of volume of change drawn into the cylinder on the induction stroke compared to the swept volume Indication of how well the engine is breathing Typically 85%

Theoretical cycle

All events happen at TDC or BDC

Induction - practical cycle

Opens the net valve early - ensures the valve is fully open by TDC Valve lead

Compression

Inlet valve closes late - after BDC - allows the momentum of the incoming mixture to increase the the mas of the induced charge Valve lag

Ignition advanced - practical cycle

Takes a finite time to ignite and fir the flame to expand across the crown Spark is initiated early to achieve max pressure at 10 degrees after TDC

Power - practical cycle

Exhaust valve opens early By the time the piston has passed 90 degrees on the power stroke most of the pressure energy has been expended Exhaust valve opens early - prevents back pressure which resists upwards movement of the piston

Exhaust - pratical cycle

Valve closes late Remains open after TDC - momentum of the moving exhaust gasses removes the last of the burned gasses - allows more space for incoming air/fuel mix

Valve overlap - practical cycle

A period where both valves are partially open Exhaust gasses flowing out helps the air/fuel mix to flowing in Improves old metric efficiency Mass of the induced charge is increased