Small Engines Compression System Compression System Components Function

Small Engines Compression System

Compression System Components • Function – Components direct, contain, and compress the airfuel mixture, and then discharge the gases

Compression Ratios • Most small engines operate with a compression ratio between 6: 1 to 8. 5: 1

Changes in the Charge • Air-fuel mixture enters the combustion chamber the gas in the mixture is closer to a liquid state. • As the piston moves from BDC to TDC the airfuel mixture is compressed changing the mix to a gaseous state

Changes in the charge • Heat is produced as a byproduct • Larger droplets break down into smaller droplets increasing the total surface area, exposing more liquid gasoline to the air.

Ignition • The energy required to initiate combustion is provided by spark plug. • This is achieved by creating a spark that jumps across the gap in the electrode end of the spark plug.

Compression Problems • Two types of problems with compression • Excessive Compression • Inadequate compression

Inadequate Compression • Most common cause is from leaks • If the leak occurs from the exhaust valve this causes an increase in dilution of the air to fuel mixture

Maverick Air • This undesirable air entering the combustion chamber is called “Maverick Air” • Maverick air could potentially come from a leaking valve • Entering through the exhaust valve from the muffler or bypassing the carburetor and entering through the intake valve.

Excessive Compression • Can lead to detonation and pre ignition of the air-fuel charge • Detonation is the condition of a significant portion of the charge spontaneously combusting before the spark induced ignition • Also known as “knocking” or “pinging”

Detonation • Occurs when the compression ration is increased • This can be caused by carbon build up and deposits • Can lead to potential failures of the piston connecting rod or occasionally crankshaft failures

Pre-Ignition • Caused by a small portion of the combustion chamber becoming excessively heated • Or can also be caused by a particle in the combustion chamber igniting the charge as it enters the combustion chamber. • Can also lead to part failure in the engine.

Valve Dynamics • Most valves in a small engine operate in a circular motion. • This has an positive effect on the overall lifespan of the valve • Also helps by causing a mild scraping action that helps clean the face of the valve

Valve Dynamics • This rotation can be achieved by specially designed cam shafts or other features such as valve springs that create torque on the valve stem

Measurements • Many different measurements are often needed to make sure that parts on a small engine are not becoming excessively worn out. • Two tools are often used in small engine measurements • The Dial Caliper and the Micrometers

Dial Caliper

Dial Caliper • Used to make precise measurements • Several types of measurements can be made with the dial caliper • Outside diameter, length or height, or thickness of a part

Dial Calipers • Used to make precise measurements up to 1000 th of an inch • To make a measurement set the caliper to zero then adjust the tool until the measuring contacts come into contact with the part that you wish to measure

Reading a Dial Caliper • Start by reading the measurement given on the Bar • The bar is given in increments of inches and tenths of inches • Continue by then reading the dial • The dial is given in increments of thousands of an inch

• To get the overall measurement add the measurement from the bar to the measurement from the dial to get the total measurement.

Micrometer

Micrometer • Twist the micrometer tightening knob clockwise until the anvil and spindle are tight against the part being measured • Read the whole numbers on the sleeve first followed by the corresponding numbers on the thimbles.