Chapter2.4 Engine Measurement

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PISTON DISPLACEMENT

Engine displacement is the volume swept by all thepistons inside the cylinders of an internal combustionengine in a single movement from top dead centre (TDC)

to bottom dead centre (BDC). It is commonly specified in

cubic centimeters, litres, or cubic inches. The engine's displacement is frequently used in the

manufacturer's nomenclature. For instance, the BMW528 is a 5-series car with a 2.8 litre engine, andNissan's Teana 350JM is a car with a 3498 cc

(213.5 cubic inch displacement ). Motorcycles are oftenlabeled this way. However this can be misleading. Forinstance, the current BMW 335i only has a 3.0 litreengine.


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Piston-Engine Geometries
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PISTON DISPLACEMENT

Example: The 427 Chevy V8 bore is4.312 inches, and the stroke is 3.65

The math looks like this:

427 Chevy V8


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PISTON DISPLACEMENT

Example: The inline 4 piston engine, bore is

102.5 mm, and the stroke is 88.9 mm.

Find the piston displacement (swept volume)

of the cylinder (cc).

Find the Total displacement of the engine.

S1 = 733.566 cubic centimeter

S4 = 2,934.265 cubic centimeter

= 3.0 cc


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Motorcycle Bore StrokeB/Sratio

Type of bike

Ducati 999R 104 58.8 1.76 Sportbike

Honda CBR 600 F4 67 42.5 1.57 SportbikeYamaha V Maxscooter

76 66 1.15 Cruiser

Honda Goldwing 74 71 1.04 Touring

Vespa PX 150 57 58 0.98 Commuter

Effect of Bore-Stroke ratio on engineoperating speed
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Compression Ratio

Compression ratios do not tell us the horse-power of an engine.

They do have a meaning as regards the efficiency of an engine.

Generally, the higher the compression ratio, the greater the efficiency.

However, as compression ratios areincreased, the loads and stresses upon engine parts

become more severe.


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The compression ratio of an internal-combustion engine or external

combustion engine is a value that represents the ratio of the volume

of its combustion chamber; from its largest capacity to its smallest

capacity. It is a fundamental specification for many common

combustion engines.

It compares two volumes in the cylinder. One is swept volume plus

clearance volume. Thats the volume above top dead center. The

other is the clearance volume only. Putting these volumes into a ratio

gives us the compression ratio - 6 to 1.

In a piston engine it is the ratio between the volume of the cylinderand combustion chamber when the piston is at the bottom of its

stroke, and the volume of the combustion chamber when the piston

is at the top of its stroke.[1]


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Picture a cylinder with the piston at the bottom of its stroke

containing 1000 cc of air.

When the piston has moved up to the top of its stroke inside thecylinder, and the remaining volume inside the head or combustion

chamber has been reduced to 100 cc, then the compression ratio

would be proportionally described as 1000:100, or with fractional

reduction, a 10:1 compression ratio.

A high compression ratio is desirable because it allows an engine

to extract more mechanical energy from a given mass of air-fuel

mixture due to its higher thermal efficiency. High ratios place the

available oxygen and fuel molecules into a reduced space along

with the adiabatic heat of compression - causing better mixing and

evaporation of the fuel droplets. Thus they allow increased powerat the moment of ignition and the extraction of more useful work

from that power by expanding the hot gas to a greater degree.


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Higher compression ratios will however make gasoline engines

subject to engine knocking, also known as detonation and this canreduce an engine's efficiency or even physically damage it.

Diesel engines on the other hand operate on the principle of

compression ignition, so that a fuel which resists autoignition will

cause late ignition which will also lead to engine knock.


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Typical compression ratios

Petrol/gasoline engineDue to pinging(detonation), the CR in a gasoline/petrol

powered engine will usually not be much higher than 10:1,

although some production automotive engines built for high-

performance from 1955-1972 had compression ratios as

high as 12.5:1, which could run safely on the high-octaneleaded gasolinethen available.

A technique used to prevent the onset of knock is the high

"swirl" engine that forces the intake charge to adopt a very

fast circular rotation in the cylinder during compression that

provides quicker and more complete combustion. Recently,with the addition of variable valve timing and knock sensors

to delay ignition timing, it is possible to manufacture gasoline

engines with compression ratios of over 11:1 that can use 87

MON (octane rating) fuel.


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Petrol/gasoline engine with pressure-charging

In aturbocharged or supercharged gasoline engine, the CR is

customarily built at 9:1 or lower.

Petrol/gasoline engine for racing

Motorcycle racing engines can use compression ratios as

high as 14:1, and it is not uncommon to find motorcycles

with compression ratios above 12.0:1 designed for 86 or 87

octane fuel.

Racing engines burningmethanol and ethanol often exceed a

CR of 15:1. Consumers may note that "gasohol", or 90%

gasoline with 10% ethanol gives a higher octane rating(knock suppression).


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Gas-fueled engine

In engines running exclusively on LPG or CNG, the CR may behigher, due to the higher octane rating of these fuels.

Diesel engine

In an auto-ignition diesel engine, (no electrical sparking plug--

the hot air of compression lights the injected fuel) the CR willcustomarily exceed 14:1. Ratios over 22:1 are common. The

appropriate compression ratio depends on the design of the

cylinder head. The figure is usually between 14:1 and 16:1

for direct injection engines and between 18:1 and 20:1 for

indirect injection engines.


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Chapter2.4 Engine Measurement

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