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ME240/107S: Engine Dissection

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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

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