The Piston Engine Flashcards
The Otto cycle stages
Induction stroke
Compression stroke
Power stroke
Exhaust stroke
Induction stroke
As the piston goes down - creates low pressure - draws air in
Compression stroke
As the cylinder head goes up - air compresses and creates heat
Power stroke
The ignition of the fuel and air mix = rapid expansion
Exhaust stroke
Exhaust leans the cylinder
Why is more efficient at high altitude
Easier to push the end gasses out - less atmospheric pressure acting against it
Bottom dead centre
Lowest point the cylinder gets to
Top dead centre
Highest point the cylinder gets to
Crank and connected rod are perfectly aligned
Inline engine
Cylinders are aligned in a line on 1 crank shaft
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
Radial engine
Engine cylinders are arranged in a circle - equal cooling throughout - more surface area
Horizontally opposed
Cylinders on each side
Shorter engine - better cooling - no hydro locking
The crank case
2 parts bolted together - crank in the middle
Main journal
Main crankshaft centre
Big end journal
To the side of the main crankshaft centre - where the cylinders are joined onto it
Stroke
Full distance through which the piston moves
Throw
Half the stroke
From the centreline of the main journal to the centreline of the big end journal
Connecting rods
Connects piston to the crank shaft
Generally made of steel
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
