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 2

Carburetor

Ignition System

Drivetrain Components
ME240/107S: Product Dissection
Session 1: Carburetor and Ignition

In small groups, discuss your answers to the
following questions from Session 1
Carburetor:
 What
are the components of the Pulsa-Jet
Carburetor and how do they work?
 How
many jets are used? What is their function?
ME240/107S: Product Dissection
Session 1: Carburetor and Ignition
Ignition:
 What
is the purpose of the starter clutch?
 Why
is the flywheel comprised of different
materials? What are those materials?
 How
does the ignition system work?
 How
is ignition timing achieved in the engine?
ME240/107S: Product Dissection
Carburetor

Purpose of the carburetor is to produce a
mixture of fuel and air on which the engine
can operate

Uses a venturi nozzle to
reduce air pressure in
the carburetor to create
suction to “pull” fuel into air
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
Ignition System
Consists of:
 Spark
plug
 Armature
 Coil
(primary and
secondary)
 Flywheel
 Magnet
ME240/107S: Product Dissection
Ignition System - Electric Circuit

Uses a changing
magnetic field to
generate current in
primary and
secondary circuits
ME240/107S: Product Dissection
Ignition System - Magnetic Flux

As magnet approaches,
induces magnetic flux in
armature

Breaker points close
and current dissipates
through primary circuit
ME240/107S: Product Dissection
Ignition System - Spark Fires

After magnet rotates past
armature flux reverses
direction, and the breaker
points open

Change in magnetic flux
produces 170 volts in
primary circuit

Induces 10,000 volts in
secondary circuit, firing
spark plug
ME240/107S: Product Dissection
Ignition System - Breaker Point

Crankshaft rotation
causes mechanical
actuation of breaker
ME240/107S: Product Dissection
Ignition System - Magneto System

In Magnetron ignition
system, magnetic flux
induced by flywheel
magnet also actuates
primary circuit and
spark timing

Capacitor helps
regulate current (i.e.,
serves as condenser)
ME240/107S: Product Dissection
The Drivetrain

As a group, brainstorm as many drivetrain
components as you can. You have 3 min.
ME240/107S: Product Dissection
Drivetrain
Provide power via
rotating crankshaft
ME240/107S: Product Dissection
Drivetrain: Piston-Rod Assembly

Piston is connected
to the crankshaft by
the connecting rod

Vertical motion of
piston rotates the
crankshaft

What is purpose of
compression rings?
ME240/107S: Product Dissection
Drivetrain: Valves
Valves regulate air-fuel
flow mixture into and out
of piston cylinder
ME240/107S: Product Dissection
Drivetrain: L-Valve Actuation
Camshaft drives
actuation and
timing of intake
and exhaust
valves in L-head
configuration
ME240/107S: Product Dissection
Drivetrain: Valve Actuation


In overhead or I-head,
camshaft actuates a
tappet (lifter) which
forces a push rod
upward, causing the
rocker arm to rotate to
actuate the valves
What is the purpose
of the valve spring?
Rocker arm
Push rod
Tappet
ME240/107S: Product Dissection
4 Stroke Cycle
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
Parts of an
IC Engine
Name as many
parts as you can
Your name:_________________
CROSS SECTION OF OVERHEAD VALVE FOUR CYCLE SI ENGINE