Four Stroke Engine Cycles and Timing

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FOUR STROKE ENGINES

CYCLES AND TIMING

MEHTAP DOAN HARUN KANCA FERHAT KALKAN EREN NDER FATH MEHMET KALKAN


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FOUR STROKE DESEL

ENGNESThe medium speed four stroke trunk piston engines can be found on most medium tolarge merchant vessels even if the main engine is either a steam turbine or a two strokecrosshead engine. In these cases it will often be found that the electrical power issupplied by alternators driven by medium speed four stoke engines.

They are the favored method of propulsion on ships where head room is a minimum,for instance, on ferries and passenger vessels, and where, as is the current trend forthese ships, diesel electric propulsion is utilized. Diesel electric propulsion allows the

engines to be placed wherever is most suitable, as they no longer have to be alignedwith reduction gearing and shafting as is the case with conventional installations.

Generally, medium speed engines run at between 250-850 rpm. Above this range theyare defined as high speed engines. Although not as powerful as their two strokecrosshead cousins, the largest four stroke engines are delivering just over 2000 kw percylinder. Advances in design and materials have led to an increase in efficiency,together with an increase in turbocharger pressure rations which allow a greaterquantity of fuel to be burnt per cycle. Medium speed engines have a higher power toweight ratio than the slow speed two strokes, but due to the higher speeds tends to

have reduced maintenance intervals.The name Trunk Piston refers to the piston skirt or trunk. The purpose of the skirt ortrunk in four stroke cycle engines is to act in a similar manner to a crosshead. It takesthe thrust caused by connecting rod angularity and transmits it to the side of thecylinder liner, in the same way as the crosshead slipper transmits the thrust to thecrosshead guide. With such engines, which are termed trunk piston engines, the engineheight is considerably reduced compared with that of a crosshead engine of similarpower and speed. The engine manufacturing costs are also reduced. It means of coursethat there is no separation between the crank case and the liner and piston. This has its

disadvantages, especially when considering the choice of lubricating oils when burninghigh sulphur residual fuels


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Four stroke engine

1.INTAKE2.COMPRESSION

3.POWER

4.EXHAUST


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INTAKE STROKE:

As the piston moves upward and approaches to the top dead center (TDC), as measuredby crankshaft rotation, the camshaft lobe starts to lift the cam follower. This causes thepushrod to move upward and pivots the rocker arm on the rocker arm shaft. As the valve

lash is taken up, the rocker arm pushes the intake valve downward and the valve starts toopen. The intake stroke now starts while the exhaust valve is still open. The flow of theexhaust gases will have created a low pressure condition within the cylinder and will help pullin the fresh air charge as shown in figure 7.3. The piston continues its upward travel throughtop dead center (TDC) while fresh air enters and exhaust gases leave. After top dead center(ATDC), the camshaft exhaust lobe rotates so that the exhaust valve will start to close. Thisis accomplished through the valve spring, which was compressed when the valve wasopened, forcing the rocker arm and cam follower back against the cam lobe as it rotates. Thetime frame during which both the intake and exhaust valves are open is called valve overlapand is necessary to allow the fresh air to help scavenge (remove) the spent exhaust gasesand cool the cylinder. In most engines, 30 to 50 times cylinder volume is scavenged throughthe cylinder during overlap. This excess cool air also provides the necessary cooling effecton the engine parts.

As the piston passes TDC and moves downward and, like a pump, creates a vacuum thatdraws fresh air through the intake valve into the combustion chamber. Once the pistonreaches the bottom of the cylinder, the intake valve closes, trapping the air inside thecylinder.


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COMPRESSION STROKE

After bottom dead center (ABDC), the intake valve starts to close. At this point the air charge is at

normal pressure (14.7 psia=1 atm) and ambient air temperature.

The piston has traveled about half of its stroke, thus reducing the volume in the cylinder by half. The

temperature has now doubled to ~70 C and pressure is 34 psia. According to the laws of physics, whenair is compressed, temperature increases.

At about 43 BTDC the piston has traveled upward 3.062 inches of its stroke and the volume is once

again halved. Consequently, the temperature again doubles to about 320F and pressure is ~85 psia.

.When the piston has traveled to 3.530 inches ~640 F and pressure 277 psia. When the piston have

traveled to 3.757 iches of each stroke, or the volume is again halved, the temperature climbs to 1280F

and pressure reaches 742 psia. With the piston area of 9.616 in the pressure in the cylinder is exerting a

force approximately 7135 lb. or3-1/2 tons of force.

The above numbers are ideal and provide a good exp. of what is occurring in an engine during

compression. In an actual engine, pressures reach only about 690 psia. .This is primarily to the heat loss

to the surrounding engine parts.


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POWER STROKEFuel in a liquid state is injected into the cylinder at a precise time and rateto ensure that the combustion pressure is forced on the piston neither tooearly nor too late. The fuel enters the cylinder where the heated compressedair is present; however, it will only burn when it is in a vaporized state(attained through the addition of heat to cause vaporization) and intimatelymixed a supply of oxygen. The first minute droplets of fuel enter thecombustion chamber and are quickly vaporized. The vaporization of fuelcauses the air surrounding the fuel to cool and it requires time for the air toreheat sufficiently to ignite the vaporized fuel. But once ignition has started,

the additional heat from combustion helps to further vaporize the new fuelentering the chamber, as long as oxygen is present. Fuel injection starts forexample at 28 C BTDC and ends at 3C ATDC; therefore, fuel is injected fora duration of 31C.

Both valves are closed, and the fresh air charge has been compressed. Thefuel has been injected and is starting to burn. After the piston passes TDC,heat is rapidly released by the ignition of the fuel, causing a rise in cylinderpressure. Combustion temperatures are around 2336F . This rise in pressure

forces the piston downward and increases the force on the crankshaft for thepower stroke as illustrated in. The energy generated by the combustionprocess isnt all harnessed. In a two stroke diesel engine only about % 38 ofthe generated power is harnessed to do work, about %30 is wasted in theform of heat rejected to the cooling system, and about % 32 in the form ofheat is rejected out the exhaust. In comparison four stroke diesel engine hasa thermal distribution of % 42 converted to useful work, % 28 heat rejectedto the cooling system, and % 30 heat rejected out the exhaust.


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EXHAUST STROKE

As the piston approaches for example 48C BBDC, the camof the exhaust lobe starts to force the follower upward,

causing the exhaust valve to lift off its seat.

The exhaust gasses start to flow out the exhaust valve due tocylinder pressure and into the exhaust manifold. After

passing BDC, the piston moves upward and accelerates to itsmaximum speed at 63 BTDC. From this point on the pistonis decelerating. As the piston speed slows down, the velocityof the gasses flowing out of the cylinder creates a pressure

slightly lower than atmospheric pressure. At 28 C BTDC,the intake valve opens and the cycle starts again.


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TIMING

In its original configuration, the four-stroke engine relies entirely on the piston's motion to draw in fuel and air (Naturally Aspirated

Engine), and to force out the exhaust gasses. As the piston descends on the intake (inlet) stroke, a partial vacuum is created within the

cylinder which draws in the air/fuel mixture. The intake valve then closes, the piston ascends, and the mixture is compressed and ignited,causing the piston to descend again. As the exhaust valve opens, the piston ascends once more and forces the exhaust gases out. This was

the technique used in early four-stroke engines. It was soon discovered, however, that at rotational speeds approaching 100 revolutions

per minute (RPM) or greater, the exhaust gasses could not change direction quickly enough to exit past the exhaust valve by

thepiston'smotion alone.

At high rotational speeds, consistent flow through the intake and exhaust ports is maintained by allowing the intake and exhaust valves to

be open simultaneously at top dead center (known as valve overlap). The momentum of the exhausting gas maintains the outward flow

and creates a suction effect on the cylinder known as scavenging, helping to draw the intake charge into the cylinder. In order to retain

efficiency, however, the exhaust valve must be closed soon enough so that too much fuel/air mixture from the intake port is not drawn

into the engine's exhaust, wasting fuel. In a high-power situation such as racing, where high engine speeds and forced induction arecommon, this wasted fuel charge can serve to cool the exhaust valve and prevent detonation.

After ignition of the fuel/air charge, as the piston approaches bottom dead center, combustion slows. Just before the charge is finished

combusting, the exhaust valve is opened at approximately twenty degrees of crankshaft rotation before bottom dead center. This allows

the still-expanding gasses inside the cylinder to push out through the exhaust port, starting exhaust flow and giving the exhaust flow

momentum. Though a small amount of force is lost through the exhaust port that could be driving the piston, the force that the piston

must exert on the gasses to exhaust them from the cylinder is reduced, resulting in increased efficiency.

Exhaust systems in many situations are a compromise between cost of production, optimum flow, low emissions, and low noise levels.

Restrictions in an exhaust system, including emissions equipment, mufflers, and simple exhaust tubing can restrict proper exhaust flow.

In multi-cylinder applications, in which many cylinders share a common exhaust pipe, pressure waves created by cylinders exhaustinggas can impede flow of exhaust from other cylinders. Since this prevents exhaust gas from exiting the cylinder, the overlap of the intake

valve can result in reversion, when exhaust gas enters the intake port. The internal pressure problems due to a multi-cylinder engine

sharing a common intake plenum can be overcome by using a carburetor or injector for each cylinder.

Accomplishing maximum volumetric efficiency for a given engine is not a formulaic process. Variables such as flow rates , overlap,

valve lift, porting specifications and the location of valve events create a large set of variables. Different intake and exhaust equipment is

tested at different speeds and loads, and the end result is usually a compromise between power, emissions, and cost, except in situations

where maximum power is desired regardless of cost or emissions (such as racing.)


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Four Stroke Engine Cycles and Timing

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