Transcript Principles of the Compression Engine

Principles of the Compression
Engine
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
Typical diesel engine
SI Engine
with OHC
Air
cleaner
Carbure
tor
Camshaft
Rocker arm
Intake valve
Cam sprocket
Exhaust valve
Piston
Timing belt
Connecting
rod
Timing belt
tensor
Crankshaft
Crank sprocketOil pickup
Oil pump
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
Spark plug for SI engine
Fuel injector for CI engine
Valves
Clearance
volume
Top
Center
(TC)
Cylinder
wall
Stroke
Bottom
Center
(BC)
TC
0o
Crank shaft
q
270o
Piston
90o
180o
BC
• http://static.howstuffworks.com/flash/diesel.swf
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
Let’s take a closer look
The crankshaft and piston
Number of cylinders
Single-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 forces
associated with accelerating and decelerating piston) better than single
cylinder.
Types of injection
Glow plug
Orifice
-plate
Direct injection:
quiescent chamber
Direct injection:
swirl in chamber
Indirect injection: turbulent
and swirl pre-chamber
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
Any questions?