Principles of the Compression Engine. What is a compression engine? Needs no spark to ignite...
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Transcript of Principles of the Compression Engine. What is a compression engine? Needs no spark to ignite...
What is a compression engine?
• Needs no spark to ignite fuel/air mixture as in a gasoline spark ignition (SI) engine
• Combustion occurs spontaneously due to compression
• All diesel engines are compression ignition (CI) engines
SI Engine with OHC
Camshaft
Intake valve
Rocker arm
Piston
Connecting rod
Crankshaft
Oil pump
Exhaust valve
Carburetor
Crank sprocketOil pickup
Timing belt
Cam sprocket
Air cleaner
Timing belttensor
Definitions
• Temperature /oC, F, K
• Volume /cm3 or cc
• Pressure / atm, psi, Pa
• See also glossary of terms
What is diesel?
• Crude oil fraction after gasoline (petrol)
• C9H20 – C12H26 nonane – dodecane
• Exact composition governed by various standards
The Chemistry
• Diesel is burnt:
CnH2n+2 + O2 H2O + CO2 +HEAT
• Inefficient combustion– CO, carbon monoxide– C, soot, diesel particulates– PCAs, benzene etc.
4-stroke operating cycle
• http://static.howstuffworks.com/flash/diesel.swf
Crank shaft
90o
180o
BC
TC0o
270o
Spark plug for SI engineFuel injector for CI engine
Top Center(TC)
BottomCenter(BC)
Valves
Clearancevolume
Cylinder wall
Piston
Stroke
1. Induction Stroke
• Engine pulls piston out of cylinder
• Low pressure inside cylinder (< 1 atm.)
• Atmospheric pressure pushes air into cylinder
• Engine does work on the gases during this stroke
2. Compression stroke
• Engine pushes piston into cylinder
• Air is compressed to high pressure and temperature (700psi, 540oC)
• Fuel injected as piston reaches top of stroke
• Engine does work on the gases during this stroke
3. Power Stroke
• Fuel/air mixture burns to form hot gases (2000oC, 1000psi)
• Gases push piston out of cylinder
• Gases expand to lower pressure and temperature
• Gases do work on engine during this stroke
4. Exhaust stroke
• Engine pushes piston into cylinder
• High pressure inside cylinder
• Pressure pushes burned gases out of cylinder
• Engine does work on the gases during this stroke
4-stroke cycle
• http://static.howstuffworks.com/flash/diesel.swf
Number of cylindersSingle-cylinder
…engine gives one power stroke per crank revolution (2 stroke) or two revolutions (4 stroke). The torque pulses are widely spaced, and engine vibration and smoothness are significant problems.Used in small engine applications where engine size is more important
Multi-cylinder
…engines spread out the displacement volume amongst multiple smaller cylinders. Increased frequency of power strokes produces smoother torque characteristics. Engine balance (inertia forcesassociated with accelerating and decelerating piston) better than singlecylinder.
Direct injection:quiescent chamber
Direct injection:swirl in chamber Indirect injection: turbulent
and swirl pre-chamber
Orifice -plate
Glow plug
Types of injection
Critical factors
• Compression must be high enough– Batteries– Worn piston/barrel– Valve seating
• Fuel and air must mix thoroughly– Injectors working correctly
• Glow plugs must work properly where fitted