Engine Systems

Introduction Engines permeate our society. Labor costs for small engine are over $50.00 an hour. To operate a small engine efficiency you must be able to troubleshoot engine problems. To be a good troubleshooter you must know the name and function of all of the engine parts.

Five (5) Engine Systems All engine parts and functions can be divided into five (5) systems. Compression Fuel Electrical Cooling Lubrication If an engine will not start, which one of the five system(s) is most likely the problem? fuel If an engine has a lot of operating time, is hard to start and doesn’t seem to be producing as much power as it once did, which one of the five system(s) is most likely the problem?

Compression System Parts The compression system includes all of the parts that create, contain and manage the engine compression. Valves Valve springs Connecting rod Crankshaft Gaskets Block Piston Piston rings Cylinder head Bearings Parts

Compression Problems Two possible problems: Inadequate compression Symptoms Cause(s) Leaks Poor starting Less power Excessive compression Symptoms Cause(s) Harder to crank Detonation Preignition Carbon buildup in combustion chamber

Detonation An undesirable engine condition in which pockets of fuel start to burn at about the same time as the spark plug fires. Multiple pressure fronts collide. Sometimes called knocking, spark knock or pinging. Causes large pressure differentials in the combustion chamber. Will cause engine damage.

How do you prevent detonation? Causes Increased compression High temperatures Lean fuel/air mixture Advanced ignition timing Low octane fuels How do you prevent detonation? Remove the causes

Preignition Fuel starts to burn before the spark plug fires. Increases the peak combustion pressure in the cylinder. Increases internal temperature. Decreases engine performance and produces and audible pinging or knocking sound in the engine. Will cause engine pistons, connecting rods, crankshafts and other compression system parts to fail.

How do you prevent preignition? Causes An overheated spark plug Glowing carbon deposits Over heated exhaust valve A sharp edge in the combustion chamber or on top of the piston Sharp edges on valves that were reground improperly A lean fuel mixture. Remove the cause(s) How do you prevent preignition?

Piston Rings Compression Wiper ring Oil ring Subject to to greatest amount of chemical corrosion and highest temperatures. Transfers 70% of combustion heat from piston to cylinder walls. Compression Ring Wiper Ring Oil Ring Wiper ring Meters oil film on cylinder walls Must be installed correctly. Oil ring Constructed of two thin rails with holes or slots cut in-between. Has the highest pressure against the cylinder wall of the three rings.

Crankcase Breather Maintains pressure in the crankcase at less than ambient pressure to assist in the control of oil consumption. Excessive blow by renders the breather useless. Old engines vent to the atmosphere. New engines vent to the carburetor. Should the crankcase breather be removed during regular servicing? No

Compression Release Compression release systems are used to decrease effort required to start engine. Holds the exhaust valve slightly open during starting, and then allows it to fully close once engine starts. Compression release pin

Fuel System

Introduction The function of the fuel system includes: storing metering atomizing vaporizing mixing fuel and air delivering fuel air mixture to intake manifold What symptom(s) will an engine exhibit if the fuel system has failed? Failure in any part of the system results in an engine that is hard to start and/or will not produce rated power.

Engines Fuels Common Small Engine Fuels include: Gasoline Diesel LPG LNG CNG Why are so many different types of fuel available? Each type has advantages and disadvantages

Fuel system—cont. Fuel system parts: Supply (tank) Lines Shut off valves Filter Pump Carburetor What is the function of each part?

Fuel system—cont. Additional facts about the fuel system: Fuel must be clean and good quality Air fuel mixture must be richer for starting Excess fuel washes lubrication off of the cylinder walls and accelerates wear. Excess fuel dilutes the oil and accelerates wear. Air fuel mixture that is too lean will cause preignition

Volatility Volatility is the propensity of a liquid to become a vapor. The volatility of gasoline changes with the seasons. Low Volatility (summer) High Volatility (winter) Poor cold weather operation Poor hot weather operation Spark plug deposit buildup Vapor lock Combustion chamber deposit buildup Poor fuel economy Poor cold starting Excessive fuel evaporation Fuel bubbles develop in system. What is vapor lock?

What is required to vaporize a liquid fuel? Vaporization Vaporization is the process converting a liquid to a vapor. Fuel must be vaporized because liquid fuel will not burn. The rate and efficiency of vaporization is improved when the liquid is reduced to small droplets (atomized). What is required to vaporize a liquid fuel? Heat

Carburetor

Identify the parts on the carburetor and explain their function(s)? “Carburetor: the engine component that provides the required air-fuel mixture to the combustion chamber based on engine speed and load.” Newer carburetor Identify the parts on the carburetor and explain their function(s)? A B C What is different? A. Crankcase breather tube attachment Attach crankcase breather to carburetor B. Low speed adjustment screw Adjust low speed air-fuel mixture C. Throttle Set engine speed D. Idle speed adjustment screw Set idle speed E. Float bowel Fuel reservoir for carburetor F. High speed adjustment screw High speed fuel adjustment G. Fuel inlet Fuel line attaches to carburetor H. Choke Enrich fuel for starting D H E G F

Four Fluid Principles Fluids flow from areas of high pressure to areas of low pressure. When there is no pressure difference--there is no fluid flow. Fluids exert pressure of the same value throughout a system. Fluid flow in a carburetor utilizes Bernoulli’s principle. Air flowing through a narrowed portion of a tube (Venturi) increases in velocity and decreases in pressure. Airplane wing Name another application of Bernoulli’s principle.

Carburetor Operating Principles A carburetor is a tube attached to the intake port of the engine and open to the atmosphere. On the intake stroke a volume with little to no pressure develops in the combustion chamber. Atmospheric pressure outside the engine--14.7 psi Low pressure in the combustion chamber--0 to slight vacuum. Result: air wants to flows from outside of to inside the engine. What can reduce the amount or rate of air flow? Clogged air filter Obstruction in carburetor Stuck choke valve

Carburetor Operating Principles As the air flows through the carburetor, the fuel is metered, atomized and vaporized. To have available fuel, the carburetor must have a source of fuel. In the float type carburetor this source is the fuel bowel. Why does the carburetor have an air vent to the fuel bowel?

Carburetor--Venturi A fluid will not flow without a pressure difference A pressure difference is needed to cause the fuel to flow from the fuel bowel into the air stream. This is accomplished using a Venturi and a tube connecting the mouth of the venture to the fuel bowel. Add a jet to the emulsion tube and you have a functioning carburetor. It will function correctly as long as it has a constant load and constant speed. How many engines operate at a constant load and constant speed? Very few.

Carburetor-Throttle To adjust the rate of fuel flow a throttle is used. When the throttle is in the closed position there is minimum air flow through the carburetor. Low air flow = less pressure difference between the venture and fuel bowel less pressure difference = less fuel flow less fuel flow = less speed. Engine speed increases Open throttle = more air flow = greater pressure difference = greater fuel flow What happens when the throttle is opened? Why?

Throttle--cont. When the throttle is in the wide open position, there is maximum air flow through the carburetor. Maximum air flow = maximum pressure difference between Venturi and fuel bowel Maximum pressure difference = maximum fuel flow Maximum fuel flow = maximum speed & power. A carburetor with this design would function well under varying loads and speeds, What controls the position of the throttle? Governor and governor spring. Older carburetors used a needle valve to adjust the fuel flow into the emulsion tube. Newer carburetors use a fixed orifice.

Carburetor-Choke Engines require a richer mixture for starting. This is accomplished by a choke or primer bulb. When the choke is closed the maximum pressure difference occurs in the Venturi. Maximum pressure = maximum fuel flow. Once engine starts the choke must be opened to prevent the engine from running too rich. A primer bulb is used on some engines.

Carburetor-Choke—cont. Briggs and Stratton newest engine uses a thermostatically controlled choke. An air vane is attached to the choke lever and the lever on the choke control. As the bimetallic strip in the thermostat heats up it moves the lever and the air vane opens the choke.

Carburetor-Idle Circuit The addition of a choke/primer improved engine starting, but this carburetor still has a problem if the engine needs to run at idle. When the throttle is in the idle position, almost closed, the area with greatest restriction, and greatest pressure difference, moves from the Venturi to the area between the throttle plate and the carburetor wall. No low pressure area in the Venturi means no fuel flow. This problem was solved with the addition of an idle circuit and idle needle valve. No Many new walk behind mowers are single speed. They are either running at full governed speed or off? Will these carburetors have an idle circuit.

Carburetor-Float To have constant fuel flow with constant pressure difference, the lift, distance from the top of the fuel to the top of the main nozzle, must remain the same. A constant level of fuel is maintained in the fuel bowel by the float, float needle valve and float needle valve seat.

Carburetor-Additional Features Several additional features have been tried/added to improve carburetor performance. Air bleeds Fixed jets Transition ports Pilot jets

Three Types of Briggs & Stratton Carburetors Vacu-jet Carburetor mounted on top of fuel tank. Must use shallow fuel tank because the main jet extends from the Venturi to the bottom of the fuel tank. As the level of fuel in the tank changes, the fuel-air ratio changes. Not included in latest B & S repair manual.

Three Types of Briggs & Stratton Carburetors--cont. Pulsa-jet Carburetor mounted on top of fuel tank. Carburetor includes a fuel pump that elevates fuel from the main tank to the secondary tank. The combination of excess fuel pump capacity and secondary tank drain keeps the fuel level in the secondary tank at a constant level. Not included in latest B & S repair manual.

Pulsa Jet Parts Fuel pump Primary fuel tube Primary fuel tank Primary fuel tube check valve Fuel screens Secondary fuel tube check valve Secondary fuel tank Secondary fuel tube Choke High speed needle valve Air horn (inlet)

Three Types of Briggs & Stratton Carburetors--cont. Flow-jet Different types and sizes are used. Most popular on modern engines. All use a fuel bowel and float system to maintain a consistent supply of fuel.

Governor System

Introduction The function of the governor system is to maintain the desired engine speed regardless of engine load. The governor is attached to the throttle on the carburetor and supplies a force that attempts to close the throttle. The governor spring is attached to the governor linkage and applies a force that attempts to open the throttle. A constant engine speed means these two forces are balanced.

Mechanical Governor The weights are rotated by the governor gear which meshes with the crankshaft gear. As the governor spins the governor weights move out from the center shaft. The weights are mounted on a lever arm that pushes the governor shaft up as the weights move out. The higher the speed the greater the force produced. What will be the effect if the governor doesn’t work, or because of slack in the linkage the throttle plate doesn’t move? Engine goes to maximum rpm and something breaks.

Mechanical Governor The governor spring is constantly trying to open the throttle and the governor is always trying to close the throttle.

Electrical System

Introduction Electricity is a predictable force, yet it is often challenging to service electrical systems because it can not been seen and there is the concern of electrical shock. Because almost all small engine electrical systems operate on 12 volts, the danger of severe electrical shock is reduced.

Electrical Terms Before attempting to understand small engine electrical systems, it is important to know the terms and parts associated with electricity and the electrical systems. Electricity Conductor Electron Free Electron Voltage Load Current Direct Current Alternating Current Polarity Amperes Resistance Short circuit Series Circuits Parallel Circuits Ohm’s Law Magnetism Induction Solenoid Diodes Voltage Regulator Battery Primary winding Secondary winding Condenser

Resistance Resistance is opposition to the flow of electrons. All circuit components have some resistance. Forcing electricity through a resistance uses energy. The energy is lost as heat. Resistance is measured in units of Ohms (). The amount of current flow and resistance in a circuit determines the wire size for the circuit. Wire Size and Resistance AWG Number Diameter /1000 ft (68 oF) 12 80.8 1.6 14 64.1 2.5 16 50.8 4.0 18 40.3 6.4 20 32 10.2 22 25.35 16.2

Magnetism “Magnetism is an atomic level force derived from the atomic structure and motion of certain orbiting electrons.” A Magnet field is an area of magnetic force created and defined by lines of magnetic flux surrounding a material in three dimensions. Magnetic flux: invisible lines of force in a magnetic field. Magnet: a material that attracts iron, cobalt or nickel and produces a magnetic field. Permanent Temporary

Induction Induction: the production of voltage and current by the proximity and motion of a magnetic field or electric charge. With a conductor, either current, a magnetic field or motion can be produced as long as the other two are present. Magnetic field: When electricity passes through a conductor it forms a magnetic field around the conductor. Current: When a conductor passes through a magnetic field or when magnetic field moves and/or varies in strength around a conductor, electrons are made to flow. A current is induced in the conductor.

Small Gas Engine Electrical Systems Small engines may have one or more of five (5) electrical systems. Charging Ignition Starting Accessories Safety

1. Charging System Charging systems produces electrical to operate accessories and the replace electrical energy taken from a battery. Two different systems can be used. Generator Alternator Generator produces DC. Alternator produces AC. When DC is needed the current is converted, rectified. Some small engines use a stationary coil close to the flywheel. When the flywheel magnets pass by the coil they induce a current in the coil. Other systems use stationary magnets and a rotating coil. Conductors are sized for circuits with low current flow.

Charging System--cont. The components of a charging system may include: Coil Magnets Voltage regulator Rectifier Switches Conductors

2. Ignition System The ignition system provides a high voltage spark in the combustion chamber at the proper time. Two types of ignition systems Battery Magneto Battery systems transforms the battery voltage and fires the spark plug at the correct time. Magneto systems must produce the current, transform the voltage and time the spark plug. Most small engines use the magneto system Two types of magneto systems: Breaker point ignition Solid state (electronic) ignition

2. Ignition System-cont. Breaker point ignition Solid state ignition Older system. Most manufacturers have replaced them with solid state. Uses a set of points to break the primary circuit. Solid state ignition Uses a transistor to break the primary circuit.

Magneto Ignition System Parts Magnets Points (Breaker point only) Trigger coil Conductors Spark plug Condenser (Breaker point only) Lamination stack Primary winding Secondary winding

Magneto Ignition system As magnets in flywheel rotate past the magneto, the points close. The magnetic flux of the magnets in the flywheel induces a current in the primary coil. With current flowing in the primary circuit, a magnetic field develops around the primary coil. This magnetic field also surrounds the secondary coil. As the flywheel continues to rotate the breaker points open.

Magneto Ignition System- Firing Spark Plug When the breaker points open the magnetic field produced by the current in the primary winding collapses. The collapsing magnetic field flows across the secondary coil which induces a current in the secondary coil. Because there is a 60:1 ratio of windings in the two coils, the voltage is transformed to the 10,000 and 15,000 volts needed to fire the spark plug.

Magneto Ignition system As long as the flywheel is rotating and the ignition switch is on, the spark plug fires every time the magnets move past the magneto.

3. Starting System The purpose of the starting system is to use energy to turn the engine until it starts. System components may include: Electrical source Starting motor Conductors Ignition switch Solenoid switch Two primary electrical systems. Single switch Solenoid

Staring Systems--cont. Single Switch For systems with a single switch the switch must be able to switch the current for the starting motor. Requires a heavy duty switch because starter motors drawn a lot of current. Solenoid In this system the ignition switch only switches the current that powers the solenoid. The solenoid has heavy duty contacts for switching the current to the starting motor.

4. Accessories Small engines are used on machines that may require electricity to operate accessories. Accessories may include: Lights Electrical clutches Electrical lift systems Radio, etc. The conductors must be sized for the electrical load. Each separate circuit should have overload protection.

5. Safety It is common for small engines to be used on machines that may have one or more electrical safety systems. These systems are usually designed to stop the engine when activated. The electrical system is used because that is the easiest way to automate an engine stopping system. Safety systems can include: Low oil switch Seat switch Anti after fire solenoid Deck switch Transmission switch Tilt switch

Questions