1. 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)

2. Lecture 1
Engine Terminology
Engine Classifications
Carburetors

3. Engine Terminology: Stroke and Displacement
amount of vertical travel of the piston from bottom dead center (BDC) to top dead center (TDC)
Displacement (D)
space displaced by the
piston during a stroke
TDC
BDC
D = (stroke)(p)(Bore)2/4
Bore

4. 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

5. Classification of Engines
External vs. Internal Combustion
Spark Ignition vs. Compression Ignition
Cylinder Configuration
Valve Location
2 Stroke or 4 Stroke

6. 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

7. 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

8. Engine Classification: Cylinder Configurations
In Line
(Automobile) V
(Automobile)
Horizontally Opposed (Subaru)
Opposed Piston (crankshafts geared together)
Radial (Aircraft)

9. Engine Classification: Valve Location
Most common: overhead-value or I-head
Intake
valve
Exhaust
valve

10. Engine Classification: 2 Stroke
Compression
(ports closed)
Air Taken Into
Crankcase
Combustion
(ports closed)
Exhaust
(intake port closed)
Scavenging
and Intake
(ports open)
Air compressed in crankcase

11. Engine Classification: 4 Stroke
Exhaust Valve 4 1
Intake Valve 2 3
Exhaust
Manifold
Intake
Manifold
Spark
Plug
Cylinder
Piston
Connecting
Rod
Crank
Crankcase
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
Compression Stroke
Both valves closed,
Fuel/air mixture is
compressed by rising
piston. Spark ignites
mixture near end of
stroke.
Exhaust Stroke
Exhaust valve open,
exhaust products are
displaced from cylinder.
Intake valve opens
near end of stroke.
Intake Stroke
Intake valve opens,
admitting fuel and air.
Exhaust valve closed
for most of stroke

12. Briggs Engine - Intake

13. Compression

14. Power Stroke

15. Exhaust Stroke

16. 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!

17. 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

18. Venturi-type Carburetor
Air/Fuel Mixture To Engine
Throttle Plate
Atomized Fuel
Valve Stem
Fuel Inlet
In 1797, Giovanni Battista Venturi, an Italian hydraulics engineer, observed that that as water flowed through a constriction in a pipe, its velocity increased and its pressure decreased
Float
Choke Plate
Venturi
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

19. Flo-Jet Carburetor Fuel tank is above carburetor
Fuel is fed directly to carburetor by gravity
Why the vent?

20. Flo-Jet Carburetor
Air-fuel
mixture
Fuel from tank
Air flow

21. Pulsa-Jet Carburetor
Incorporates a diaphragm type fuel pump and a constant level fuel chamber

22. 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

23. 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

24. 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

25. Name as many parts as you can
Parts of an IC Engine
Name as many parts as you can
Name: ________________
CROSS SECTION OF OVERHEAD VALVE FOUR CYCLE SI ENGINE