Engine Terminology Engine Measurement Lesson 8 March 2008

Engine Terminology Engine Measurement Lesson 8 March 2008

Engine Size Measurement l. Engine size is determined by the cylinder diameter, the amount of piston travel (stroke) and the number of cylinders.

Bore and Stroke

Cylinder Bore l. Diameter of the engine cylinder l. Measured across the cylinder, parallel with the top of the block l. Cylinder bores vary in size, but typically range from 3– 4" (75– 100 mm)

Piston Stroke l. Distance the piston moves from top dead center (TDC) to bottom dead center (BDC) l. Crankshaft journal offset (throw) controls the piston stroke l. Stroke varies from about 3– 4" (75– 100 mm)

Changing Bore Size What happens if the engine is bored? Definition: Boring l. Material is moved from the cylinder walls and a larger piston is installed Displacement & compression ratio are both increased when bored (AKA “bored out”) How would the displacement of a stock 6 cylinder engine with a bore of 4. 000” and a stroke of 3. 000” change if it were bored 0. 060”?

Changing Length of Stroke What happens if the engine is stroked? Definition: Stroked l. Changing crankshaft) the crankshaft offset (installing a new Displacement & compression ratio are both increased when stroked How would the displacement of a stock 6 cylinder engine with a bore of 4. 000” and a stroke of 3. 000” change if it were stroked 0. 125”?

Change of Bore and Stroke What happens if the engine is bored & stroked? l. Changing the crankshaft offset and boring the cylinder How would the displacement of a stock 6 cylinder engine with a bore of 4. 000” and a stroke of 3. 000” change if it were stroked 0. 125” and the bore 0. 060”?

Oversquare and Undersquare

How to increase engine output n n How to increase engine output Increase volumetric efficiency n Better gas flow n Forced induction n Altered valve timing Improve engine efficiency n Better combustion efficiency n Lower internal friction n Reduce heat loss n Better fuel

Piston Displacement l. Volume the piston displaces as it travels from BDC to TDC Found by comparing cylinder diameter and piston stroke l. Piston displacement formula: piston = displacement bore squared 3. 14 stroke 4 OR piston = displacement bore 0. 7854 stroke

Example of Piston Displacement Formula If an engine has a bore of 4" and a stroke of 3", what is it’s piston displacement? piston displacement = (42) 3. 14 3 4 16 3. 14 3 4 150. 72 4 = 37. 68 (cu. in. ) OR 4 x. 7854 x 3 = 37. 69 (cu. in. )

Volumetric Efficiency l. Ratio of air drawn into the cylinder and the maximum amount of air that could enter the cylinder • Simply put, it is the ability of the engine to breath in on the Intake Stroke Formula: Volumetric Efficiency = actual volume of air taken into each cylinder volume of the cylinder

Factors that impact Volumetric Efficiency Engines are capable of only 80– 90% volumetric efficiency. Restrictions in the ports and around the valves limit airflow. Can you name some of the factors that would impede air flow into and engine?

Is ‘volumetric efficiency’ the same as cylinder volume? 8 to fill BDC 1 to fill TDC

Swept and Clearance Volume

Compression Ratio Compares the cylinder volume with the piston at TDC to the cylinder volume with the piston at BDC Ratio of swept volume V 1 divided by combustion chamber volume V 2 n

Compression Ratio – Gas Engine

Compression Ratio – Diesel Engine

High Compression Ratio Pros and Cons Higher compression gives greater combustion efficiency, up to a point n Efficiency goes down after a certain point due to need to retard ignition timing to avoid knock n

Volumetric Efficiency and Compression Ratio Relationship Volumetric efficiency is: l. Ratio of air drawn into the cylinder (by the intake stroke) and the maximum amount of air that could enter the cylinder (restricted by design of intake passages, size of valve openings, valve timing and overlap ). Compression ratio is: • Ratio between the cylinder volume when the piston is at TDC compared to when it is at BDC.

Compression Pressure Checking the Seals How is it measured? What unit of measurement is used? (Answers on next slide) l. Amount of pressure in the cylinder on the compression stroke l. Measured l. Gasoline in pounds per square inch (psi) or kilopascals (k. Pa) engine compression pressure: l 130– 180 l. Diesel psi (900– 1200 k. Pa) engine compression pressure: l 250– 400 psi (1700– 2800 k. Pa)

Measuring Combustion Pressure l. Used to measure compression pressure l. Using a compression gauge: lscrew the gauge into the spark plug, injector, or glow plug hole lcrank lnote l. Gauge the engine compression gauge readings are a good indicator of engine mechanical condition (engine seal)

Brake Mean Effective Pressure Not actually a measure of pressure, just expressed in pressure units (N/m 2 or psi) n A measure of work output from an engine n n BMEP = Horsepower_____ (Swept volume Vs) ( rpm/120)

Examples of Engine Displacement If one piston displaces 25 cu. in. and the engine has four cylinders, what is the engine displacement? 25 cu. in. 4 = 100 cu. in. If one piston displaces 500 cc and the engine has six cylinders, what is the engine displacement? 500 cc 6 = 3000 cc = 3. 0 L

Conversion of Measurement 61. 02 cubic inches = 1 litre From litres cubic inches 5. 0 L x 61. 02 = 305 CID From cubic inches litres 305 / 61. 02 = 5. 0 L 1 L = 1000 cc 1 cu. in. =16. 39 cc

Engine Displacement l. Volume displaced by all the pistons in an engine lpiston displacement multiplied by the number of cylinders l. Units of engine displacement: lcubic inch displacement (CID) bore lliters bore(L) 0. 7854 stroke x cylinder number