engine 4 stroke

8/14/2019 Engine 4 stroke

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Engine Parts

Stroke Cycle

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Intake Stroke

Stroke Cycle



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Piston is going down so the volume of the cylinder is getting larger Intake valve is open. A vacuum, or pressure of less than atmospheric is created in the cylinder Atmospheric pressure forces air into the cylinder. 14.7 psi at sea level.

Fuel is mixed with the air, and the cylinder is filled with the mixture of air and fuel.

Compression Stroke

Stroke Cycle



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Piston is going up so the volume of the cylinder is getting smaller. Both valves are closed. Air/fuel mixture is trapped in the cylinder. Pressure is increasing. Mixture is compressed into approximately 1/8 it's original volume. 150 psi built up. As the mixture is compressed, it heats up.

Power Stroke

Stroke Cycle



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Piston is going down. Volume of the cylinder is increasing. Both valves closed. Spark plug fires, causing compressed air/fuel mixture to burn. The flame front travels across the combustion chamber. The burning fuel creates tremendous pressure which forces the piston down. turning thecrankshaft and producing power.

This is not an explosion, but a smooth, controlled burn.

Exhaust Stroke

Stroke Cycle



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Piston is going up, so the volume of the cylinder is getting smaller. Piston pushes the burned exhaust gasses out past the open exhaust valve, and out through thexhaust system and into the atmosphere. Both valves are open for a short period of time at TDC. This is called "Valve Overlap"

The intake stroke of the new cycle follows this exhaust stroke.

or more info on the four stroke cycle engine, check this out.....

tp://www.autoshop-online.com/auto101/eng.html

Engine terms

Top Dead Center (TDC) - Point of uppermost travel of the piston and crankshaft.Bottom Dead Center (BDC) - Point of lowermost travel of the piston and crankshaft.Bore - Diameter of the cylinder.Stroke - Distance the piston travels on a stroke, or the distance from TDC to BDC. The stroke is twthe distance of the throw.Throw - The distance from the center of the main journal of the crankshaft to its rod journal. Thethrow is one half the distance of the stroke.

Compression Ratio

- is the amount the mixture is squeezed on the compression stroke.

-all other things being equal, the more the fuel/air mixture is compressed, the more power the engine

Stroke Cycle

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oduces.

As the mixture is compressed, it becomes hotter. If it becomes hotter than the flash point of thfuel, it explodes prematurely, and engine damage results. This is called pre-ignition, or detonatiomore commonly, knock, or ping. High NOx emissions result from the combustion temperature being too high, and therefore theNitrogen in the air, oxidizes, or burns.

ompression Ratios of Different Engines:

: 1 Vintage cars -because of poor quality of fuel, and hand crank starting system.

: 1 - 10 : 1 Modern Production Cars -higher compression ratios with computerized knock sensornition systems.

p to 12.5 : 1 Muscle Cars from the late 1960's -as low emissions became more important,ompression ratios were reduced, to reduce NOx.

p to 17 : 1 Race Cars - because they use better quality, high octane gasoline, or alcohol; and becausstrictor plates in the intake system, they are able to use higher compression ratios.

p to 25 : 1 Diesel Engines - diesels take only air in, on the intake stroke, and use the highmperature of the air on compression stroke to ignite the fuel which is injected directly into

ombustion chamber.

Engine Size ( cubic displacement)

What is it?

- The total amount of fuel / air mixture the engine is theoreticallycapable of taking in during one

Stroke Cycle



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ycle.

- The volume of all the cylinders, but not the combustion chambers, put together.

- Measured in Cubic Inches (cu. in.) in imperial measurement, or in Cubic Centimeters ( c.c.'s ) or L) in metric measurement.

The amount of power an engine produces, is a direct function of how much fuel the engine burnse larger the cylinders are, the more fuel and air they can take in, and therefore burn. One engine that i

wice as big as another, does not necessarily have twice as much power though, because it also has m

ore internal friction, or drag. It does, however use twice as much fuel.

Engine Performance

To get more performance from an engine, you have to burn more fuel. You must have air, specificallyxygen, to support the combustion. The fuel is no problem. If that was all there was to it, we could put auge fuel pump in the car that would pump the tank dry in seconds. The problem is with getting enoughto support the combustion. Air - fuel ratio is expressed as ratio by weight, and is around 15 : 1. If we

xpressed it by volume ( air weighs a lot less than gas ), it would be around 10,000 parts of air to one pael.

How do engine parts affect performance?

Air Cleaner: A less restrictive air cleaner, or no air cleaner and a velocity stack, or an air cleaner wtakes in cooler, denser air, will increase the amount of air entering the engine.

1.

4 Barrel Carburetor or larger throttle body: Reduces restriction at high speed. A carburetor, bydefinition, has a restriction, ( the venturi ) and therefore, theoretically, has more power than acarburated engine.

2.

Intake Manifold: Less restrictive intake manifolds reduce restriction at high RPM, but can hurt lowRPM power by reducing turbulence, and therefore reducing mixing of the air and fuel, and therefo

reducing vaporization of the gas, and hurting low speed power.

3.

Big Valve Heads: Again reduces restriction, but only at high RPM.4.

Free Flowing Exhaust: Headers, free flowing catalytic converters, and large pipe, dual exhaust,increase breathing by reducing exhaust back pressure.

5.

Camshaft: A long duration, long valve overlap, high lift camshaft, will increase high RPM power, bwill reduce power at low RPM.

6.

R.P.M.: The higher the speed the engine runs at, the more power strokes per minute it will have, atherefore more power. Balancing the reciprocating and rotating assembly allows the engine to turhigher RPM safely without blowing up.

7.

Superchargers and Turbochargers: Rather than depending on atmospheric pressure to fill thecylinders with fuel and air, a blower forces it in under pressure. This greatly increases power.

8.

Nitrous Oxide: Nitrous is an oxidizer, not a fuel. It is 35% oxygen instead of 20%, as air is. Becaushigher in O2 than air, more fuel can be burned. Unfortunately, it can only be used for short periodstime, because the pistons would melt if it was used longer.

9.

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Mechanical Reality

Any point in the operation of the engine can be described as the number of degrees before, or after, Tead Center ( TDC ), or Bottom Dead Center ( BDC ).

Because of the speed the engine runs at, we can't just slam the valves open and closed at Top Deadenter and Bottom Dead Center. The valve train parts just wouldn't last. They must be opened and closlatively gently, over a few degrees of crankshaft rotation.

he crankshaft describes a circle. We call it rotary motion. The piston moves up and down, or back andrth, or in and out. We call it reciprocating motion. When the piston and crankshaft are at BDC, and to sser degree, TDC; the piston moves very little, for a relatively large amount of crankshaft travel. In fac

bout 45 degrees before and after BDC, and for about 25 degrees before and after TDC, the piston bareove at all; so we can use that time to gradually open and close the valves.

We think of air as being nothing. Air is not nothing. It has weight. It has mass; and therefore, it hasertia. If you get it moving, it wants to keep moving. ( Newton's third law; a body in motion, tends to staotion, etc.) Air moving into the engine at high RPM is moving at as much as 150 M.P.H. or more, so eough the piston has stopped moving down on the intake stroke at BDC, the air, because of it's ownertia, tends to keep filling the cylinder with fuel and air. If the intake valve is left open longer, even afte

e piston is coming back up on the compression stroke, more air and fuel will enter the cylinder, and wll get more power at high RPM. The longer we leave the intake valve open, the more high RPM power et. Leaving the intake valve open longer, hurts low RPM power, though, because we don't get the 'ramfect', we get at high RPM, and the piston starts to force mixture back out through the still open intake

alve, ruining engine vacuum, causing rough idle, killing low RPM power, and hurting engine emission

Stroke Cycle



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n the above engine, the intake valve starts to open at 5 degrees Before TDC ( BTDC ); it is open all the

ay round until 45 degrees after BDC ( ABDC ), for a total duration of 230 degrees. The longer we leave alve open for, the higher the speed the engine is designed to run at. This engine is an extremely low sngine.

The compression stroke starts with the closing of the intake valve at 45 degrees ABDC and concludeth the firing of the spark plug just before TDC. The duration of the compression stroke is around 130

egrees. It really doesn't take long to compress the mixture. We're not really moving anything; we're jusompressing what's already in there.

The power stroke starts with the firing of the spark plug, and ends with the opening of the exhaust vareally doesn't take much time for the fuel to burn, and push the piston down. By about 100 degrees ATere is really no point in leaving the valve shut. All the energy is gone out of the burning fuel, and the

ylinder is now so large that cylinder pressure is now very low. In the above engine, the power stroke isbout 140 degrees of crankshaft travel long.

The exhaust stroke starts with the opening of the exhaust valve at 45 degrees BBDC, lasts through 1egrees from BDC to TDC, and lasts until 5 degrees ATDC; for a total duration of 230 degrees.

If you do the math, one cycle in this engine lasts through 730 degrees of crankshaft rotation. This isecause both valves are open for a short period of time at TDC between the exhaust and intake strokeshis is called valve overlap. The longer the valve overlap is, the rougher the idle, and the higher the spee engine makes its power at.

For all intents and purposes, we will call the cycle 2 revolutions of the crankshaft, or 720 degrees lonhe power stroke is the only stroke where power is actually added to the engine. All other strokes takeower from the engine to make them happen. In a single cylinder engine, the only thing which keeps thngine running through its non-power strokes, is the inertia of the flywheel. It is 720 degrees between

Stroke Cycle



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ower strokes in a single cylinder engine. This makes a single cylinder engine rough running especiallyle speed; and the larger the displacement, the larger the piston, and the more it vibrates. By taking the

ame displacement, splitting it up into two cylinders, we can make it only 360 degrees between powerrokes. This makes the engine run much smoother. A four cylinder engine has 180 degrees between prokes, which makes it smoother still. Remember the engine in our example above? The power stroke

30 degrees long. An eight cylinder engine has only 90 degrees between power strokes, so there is no tthe operation of the engine when there isn't a power stroke happening somewhere in the engine. Theeater the number of cylinders, the smoother running the engine will be.

f all the different cylinder arrangements, the least efficient has to be the inline arrangement. They are

Stroke Cycle



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latively long, and large, heavy counterweights must be used on the crankshaft, to off-set the weight ostons, and keep the engine from vibrating.

Vee- type cylinder arrangements make for a much shorter engine and therefore, a shorter, stifferankshaft. They don't need as heavy a counterweights on the crankshaft because the weight of one bapistons partially off-sets the weight of the other bank.

Stroke Cycle



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The flat, or horizontally opposed arrangement, is the most efficient of all. It has a short stiff crankshand the weight of one bank of pistons totally off-sets the weight of the other bank. This engine is said toherently balanced. Because of its efficiency, and light weight, the horizontally opposed engine is usedmost exclusively in light aircraft. While light weight is important in cars, it is even more important inrcraft. Lighter weight means more payload, longer range, and more performance. Larger aircraft, not such today (because of the use of gas turbine engines) as in the past, used radial engines with up to th

anks of nine cylinders, or twenty seven cylinders, with up to 3500 cu.in. and 3000 horsepower.

.so, you think you know it all? ...take the following self test

Stroke Cycle



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Stroke Cycle



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op

Stroke Cycle



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New Test

Stroke Cycle
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engine 4 stroke

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