Principles of the Compression

busyicicleMécanique

22 févr. 2014 (il y a 3 années et 7 mois)

110 vue(s)

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

Camshaft

Intake valve

Rocker arm

Piston

Connecting
rod

Crankshaft

Oil pump

Exhaust valve

Carbure
tor

Crank sprocket

Oil pickup

Timing belt

Cam sprocket

Air
cleaner

Timing belt

tensor

Definitions


Temperature /
o
C, F, K


Volume /cm
3

or cc


Pressure / atm, psi, Pa



See also glossary of terms

What is diesel?


Crude oil fraction after gasoline (petrol)


C
9
H
20



C
12
H
26
nonane


dodecane


Exact composition governed by various
standards

The Chemistry


Diesel is burnt:


C
n
H
2n+2

+ O
2

H
2
O + CO
2
+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

90
o

180
o

BC

TC

0
o

270
o

q

Spark plug for SI engine

Fuel injector for CI engine

Top
Center

(TC)

Bottom

Center

(BC)

Valves

Clearance

volume

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,
540
o
C)


Fuel injected as piston
reaches top of stroke


Engine does work on
the gases during this
stroke


3. Power Stroke


Fuel/air m
ixture burns
to form hot gases
(2000
o
C, 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.


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

Any questions?