Engine Dissection - Penn State Mechanical and Nuclear

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ME240/107S: Product Dissection
ME240/107S: Engine Dissection
You are dissecting a 3.5 HP
single cylinder, 4 cycle
engine, made by Briggs &
Stratton in Milwaukee, WI
These engines are typically
used in lawn mowers, snow
blowers, go-carts, etc
(ref. 2, Used by permission of Briggs and Statton, ©1992, all rights reserved)
ME240/107S: Product Dissection
Lecture 1

Engine Terminology

Engine Classifications

Carburetors
ME240/107S: Product Dissection
Engine Terminology:
Stroke and Displacement

Stroke
 amount
of vertical travel of the piston from bottom
dead center (BDC) to top dead center (TDC)

TDC
Displacement (D)
 space
displaced by the
piston during a stroke
D=
BDC
(stroke)(p)(Bore)2/4
Bore
ME240/107S: Product Dissection
Engine Terminology:
Compression Ratio

Compression ratio (CR):
 ratio
of total volume to the volume of the
combustion chamber
 spark ignition engines have CR = 7-12
CR = (C + D)/C
where C = volume of
combustion
chamber
D = displacement
ME240/107S: Product Dissection
Classification of Engines

External vs. Internal Combustion

Spark Ignition vs. Compression Ignition

Cylinder Configuration

Valve Location

2 Stroke or 4 Stroke
ME240/107S: Product Dissection
Engine Classification:
External vs. Internal Combustion

External combustion
 combustion
of an air-fuel mixture transfers
heat to a second fluid which becomes the
motive (working) fluid that produces power
 E.g., steam driven engine

Internal combustion
 the
products of combustion are the motive
fluid
ME240/107S: Product Dissection
Engine Classification:
Spark vs. Compression Ignition

Spark ignition (SI) engines
a
compressed, homogeneous air-fuel mixture
(15:1 ratio of air to fuel by mass) is ignited
using a spark

Compression ignition (CI) engines
 rapid
compression of air to a high pressure
raises the temperature so that fuel, when
delivered into combustion chamber,
spontaneously ignites without need for a spark
 often referred to as a Diesel engine
ME240/107S: Product Dissection
Engine Classification:
Cylinder Configurations
In Line
(Automobile)
V
(Automobile)
Horizontally Opposed
(Subaru)
Radial (Aircraft)
Opposed Piston
(crankshafts
geared together)
ME240/107S: Product Dissection
Engine Classification:
Valve Location

Most common: overhead-value or I-head
Intake
valve
Exhaust
valve
ME240/107S: Product Dissection
Engine Classification:
2 Stroke
Compression
(ports closed)
Air Taken Into
Crankcase
Exhaust
Combustion
(ports closed) (intake port closed)
Air compressed in crankcase
Scavenging
and Intake
(ports open)
ME240/107S: Product Dissection
Engine Classification: 4 Stroke
1
Intake Valve
Intake
Manifold
Cylinder
2
Exhaust Valve
Exhaust
Manifold
3
4
Spark
Plug
Piston
Connecting
Rod
Intake Stroke
Intake valve opens,
admitting fuel and air.
Exhaust valve closed
for most of stroke
Crank
Crankcase
Compression Stroke
Both valves closed,
Fuel/air mixture is
compressed by rising
piston. Spark ignites
mixture near end of
stroke.
Power Stroke
Fuel-air mixture burns,
increasing temperature
and pressure, expansion
of combustion gases
drives piston down. Both
valves closed - exhaust
valve opens near end
of stroke
Exhaust Stroke
Exhaust valve open,
exhaust products are
displaced from cylinder.
Intake valve opens
near end of stroke.
ME240/107S: Product Dissection
Briggs Engine - Intake
ME240/107S: Product Dissection
Compression
ME240/107S: Product Dissection
Power Stroke
ME240/107S: Product Dissection
Exhaust Stroke
ME240/107S: Product Dissection
Carburetors

Purpose of the carburetor is to produce a
mixture of fuel and air on which the engine
can operate

Must produce economical fuel consumption
and smooth engine operation over a wide
range of speeds

Requires complicated device rather than a
simple mixing valve; price is very important!
ME240/107S: Product Dissection
Venturi (nozzle)

Use force of atmospheric
pressure and artificially
created low pressure area
to mix fuel and air

Use a venturi nozzle to
lower air pressure in
carburetor to create
suction to “pull” fuel into air
Venturi (nozzle)
Bernoulli Principle: P+1/2 V2 = Constant
ME240/107S: Product Dissection
Venturi-type Carburetor
Air/Fuel Mixture To Engine
Throttle Plate
Atomized Fuel
Valve Stem
Fuel Inlet
Float
Venturi
Choke Plate
Bowl
Constant level is
maintained in bowl as float moves down,
valve stem moves
down, allowing more
fuel into bowl, float
moves up and closes
valve
Fuel
Nozzle
Inlet Air
Metering Orifice
Ref. Obert
ME240/107S: Product Dissection
Flo-Jet Carburetor

Fuel tank is above
carburetor

Fuel is fed directly to
carburetor by gravity

Why the vent?
ME240/107S: Product Dissection
Flo-Jet Carburetor
Air-fuel
mixture
Fuel from tank
Air flow
ME240/107S: Product Dissection
Pulsa-Jet Carburetor

Incorporates a diaphragm type fuel pump
and a constant level fuel chamber
ME240/107S: Product Dissection
Pulsa-Jet Carburetor Operation

Intake stroke of piston
creates a vacuum in
carburetor elbow

Pulls cap A and pump
diaphragm B inward and
compresses spring

Vacuum thus created on
“cover side” of diaphragm
pulls fuel up suction pipe S
into intake valve D
ME240/107S: Product Dissection
Pulsa-Jet Carburetor Operation

When engine intake stroke
is complete, spring C
pushes plunger A outward

Gasoline in pocket above
diaphragm to close inlet
valve D and open
discharge valve E

Fuel is then pumped into
fuel cup F
ME240/107S: Product Dissection
Pulsa-Jet Carburetor Operation

Venturi in carburetor is
connected to intake pipe
I which draws gasoline
from fuel cup F

Process is repeated on
the next stroke, keeping
the fuel cup full

Since fuel cup level is
constant, engine gets
constant air-fuel ratio
ME240/107S: Product Dissection
Parts of an
IC Engine
Name as many
parts as you can
Name: ________________
CROSS SECTION OF OVERHEAD VALVE FOUR CYCLE SI ENGINE

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