When talking about engines, the size of the engine or its "displacement" is often mentioned. What is displacement? It is the volume of air that the engine consumes or pumps through itself every two revolutions. Why two revolutions of the crank? Because it takes two revolutions of the crankshaft to complete the 4-stroke combustion cycle for all of the engine's cylinders. This applies to all four-stroke engines (fours, straight sixes, V6s, V8s, V10s, V12s and V16s). Engine displacement is typically stated as either "Cubic Inches of Displacement" (CID) or in Liters. Auto manufacturers will typically round off engine displacement in their promotional literature to the nearest cubic inch or two-decimal liter value. For example, a late model 5.7L Dodge Hemi engine actually displaces 5654 cubic centimeters (cc) or 345 CID. Another example would be a Corvette 6.2L engine that actually displaces 6162 cc or 376 CID. Engine Displacement Table of Liters to Cubic Inches: 1.0L = 61.0 CID 1.5L = 91.5 CID 2.0L = 122.0 CID 2.5L = 152.6 CID 3.0L = 183.1 CID 3.5L = 213.6 CID 4.0L = 244.1 CID 4.5L = 274.6 CID 5.0L = 305.1 CID 5.5L = 335.6 CID 6.0L = 366.1 CID 6.5L = 396.6 CID 7.0L = 427.2 CID 7.5L = 457.7 CID 8.0L = 488.2 CID Engine Displacement Table of Cubic Inches to Liters: 100 CID = 1.6L 150 CID = 2.5L 200 CID = 3.3L 250 CID = 4.1L 300 CID = 4.9L 350 CID = 5.7L 400 CID = 6.6L 450 CID = 7.4L What Determines Engine Displacement? Engine displacement is the volume of each cylinder times the number of cylinders. The volume of each cylinder is determined by the "bore" (width) of the cylinder, and the "stroke" (the distance the piston travels up and down the cylinder). The distance traveled by the piston is determined by the "throw" or offset of each connecting rod journal on the crankshaft. If the crank journal offset is 4 inches from the dead center of the crank, the piston will travel up and down four inches every revolution of the crankshaft. Bore and stroke determine engine displacement. How to Measure Engine Displacement The basic formula is: Engine Displacement = 0.7854 x (bore diameter x bore diameter) x stroke x number of cylinders Basically you calculate the volume of each cylinder, then multiply by the number of cylinders. Bore and stroke dimensions can be measured in inches or millimeters, then you do the math to determine the number of cubic inches, cubic centimeters or liters. Or, use a conversion table to convert from liters to cubic inches or vice versa. How to Measure Bore Bore width or diameter can be measured with a caliper, bore gauge, tape measure or ruler (the caliper or bore gauge will be a lot more accurate than a tape measure or ruler!). How to Measure Stroke Stroke can be measured by turning the crankshaft until a piston is at Top Dead Center (TDC), which is as high as it will go in its bore. You can then use a dial indicator, tape measure or ruler to measure how far the piston moves down when the crank is rotated to move the piston to Bottom Dead Center (BDC). If an engine is assembled and you want to determine its stroke, remove a spark plug and use a small piece of stiff wire or a plastic straw to "feel" how far down the piston travels from TDC to BDC. Use a marker to mark position of the wire or straw when the piston is at TDC, and then again when the piston reaches BDC. Then measure the distance between the two marks to see how far the piston traveled. Cylinder Volume Test Another method for measuring engine displacement on an assembled engine is a cylinder volume test: Remove a spark plug and rotate the crank until the piston is at TDC. Rotate the crank 180 degrees to move the piston down to BDC Pour thin oil into the cylinder through the spark plug hole until the cylinder is full. Slowly rotate the crank by hand to force the oil back out of the spark plug hole into a container so you can measure the volume of oil displaced by the cylinder. Then multiply by the volume of oil by the number of cylinders to determine engine displacement. Can You Determine Engine Displacement By Looking at an Engine? It is hard to judge a book by its cover, but you can determine the displacement IF the engine is original, unmodified and you can read the engine serial number on the block or the engine VIN code on the vehicle identification plate. You can also Google the year/make/model of the vehicle to see what engine sizes were available for that application. If only one size engine was offered, its that size engine. If there were option engines such as a four, V6 or V8, just count the spark plugs to figure out which engine it is. On applications where the same block may be used for different displacements (such as older Chevy small block and big block V8s), bores and strokes can vary quite a bit. A big block Chevy might be a 396, 402, 427, 454 or something else if the engine has been bored or fitted with a different crank. The external appearance of the engine and serial number on the block may be no help if an engine has been modified. Someone selling a used engine might also claim the engine is not what it actually is, so you may have to use a cylinder volume test to accurately determine engine displacement. Why Engine Displacement Important Engine displacement is simply a way of comparing engine sizes. Generally speaking, more displacement means more horsepower and torque because a larger engine is capable of pumping and burning more air/fuel mixture in its cylinders. Even so, there is no direct correlation between engine displacement and horsepower because a lot of variables affect how much power any engine of a given size will actually produce. An engine's power output depends on its "volumetric efficiency" and "thermal efficiency" as well as its horsepower and torque curves across its RPM range. Volumetric Efficiency Volumetric Efficiency (VE) is how efficiently the engine breathes as it pumps air through itself. Volumetric efficiency typically ranges from 80 percent to nearly 100 percent. Engines with three or four valves per cylinder typically flow air better than those with two valves per cylinder, so they are usually have better volumetric efficiency numbers especially at higher engine speeds. An engine with two valves per cylinder will typically achieve a VE of 80 to 85 percent. An engine with four valves per cylinder will do better with a VE of 85 to 90 percent. An engine with four valves per cylinder and variable valve timing can often reach VEs in the 95 to 100 percent range. In modified naturally aspirated street performance and race engines, VE may exceed 100 percent and be as high as 115 to 120 percent. Turbocharged and supercharged engines create boost pressure to force even more air into an engine, allowing it to breathe at volumetric efficiencies that far exceed 100 percent. The higher the boost pressure, the higher the volumetric efficiency. A turbo that delivers 8 to 10 PSI of boost pressure may increase an engine's volumetric efficiency as much as 140 to 160 percent. The formula for calculating the volumetric efficiency of a naturally aspirated engine is: VE = (CFM x 3,456) divided by (CID x RPM) CFM is how much air is flowing through the engine in cubic feet per minute. This can be measured with special airflow equipment on a dynamometer or estimated (see formula below). CID is Cubic Inch displacement, and RPM is Revolutions Per Minute. To estimate how much air is flowing through an engine, use the following formula: Estimated engine airflow in CFM = (RPM x displacement) divided by 3456 For stock street engines, multiply the estimated engine airflow in CFM by 0.85 For a naturally aspirated race engine, multiply the estimated engine airflow in CFM by 1.1. Thermal Efficiency Thermal Efficiency (TE) is how much useful power the engine makes from a given quantity of fuel burned in a cylinder. Internal combustion engines are not very efficient and typically wasting almost two-thirds of the heat energy produced by every combustion cycle. Nearly one-third of the heat energy produced during combustion goes out the tailpipe as hot exhaust. Another third of the heat energy is absorbed by the engine itself and is carried away by the cooling system to the radiator. That leaves only about a third of the energy to push the pistons down and drive the vehicle forward. Diesel engines are more thermally efficient than gasoline engines because of their much higher Compression Ratio (16 to 1 or higher for a diesel compared to 10 or 11 to 1 for most late model gasoline engines). Higher compression ratios reduce heat losses in the combustion chamber for better fuel efficiency, power and fuel economy. However, late model Gasoline Direct Injection (GDI) engines also have higher compression ratios (some as high as 14 to 1) which makes their thermal efficiency almost as good as a diesel. Engine Displacement and Power How much power an engine of a given displacement will actually generate depends on many variables, including the design of the cylinder heads and their flow characteristics, the size and number of valves per cylinder, camshaft valve lift, duration and overlap, cam timing, spark timing, the type of carburetion or fuel injection (port injection or direct injection), the air/fuel ratio at part and full throttle, the design of the intake and exhaust manifolds, engine compression ratio and type of fuel (gasoline, alcohol, gas/ethanol blends, race gas, diesel, propane or natural gas). Consequently, a "naturally aspirated" (not turbocharged or supercharged) 350 CID V8 might make anywhere from 250 to 450 peak horsepower depending on how all of these variables affect volumetric and thermal efficiency. Turbocharging increases volumetric efficiency to boost power. Boosting Airflow Increases an Engine's Effective Displacement In a boosted engine (one with a turbocharger or supercharger), additional air can be forced under pressure into the engine on demand. This trick makes a small displacement engine breathe and produ…

Fonte: AA1Car.com