ch 2 pistons, rings, pin

8/9/2019 CH 2 Pistons, Rings, Pin

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PISTONS

SI ENGINE


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Piston structure


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SI engine piston head designs


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Increasing specific power output results in highermechanical load on the piston group as a consequenceof higher cylinder pressures and/or engine speeds.With increasing specific power output, the piston thermalloading also rises. Therefore the selection of material mustbe carefully carried out, and due consideration given tothe respective thermal limits. Appropriate countermeasurescan then be applied as necessary.These load trends, together with the demand for reduced

oscillating masses, constitute one of the key challenges forpiston producers .


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Cylinder pressures for SI engines


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Lightweight Construction of SI pistons


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The lightweight construction diagram shows the so called

apparent density of the piston (k factor), plotted versus therelative compression height. There is virtually a linearrelationship between k factor and relative compressionheight for pistons based on the same design principle.In the direction towards low compression heights, thepiston design is limited by the space needed for the ringzone and conrod clearance as well as increasing piston

crown stresses. The level of tolerable crown stresses mayby increased by material substitution. At a constantcompression height, the reduction in weight is constrained

by the minimum wall thicknesses that can be implementedwith the material aluminum (elasticity) and the wallthicknesses needed to absorb lateral piston forces.


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The red arrow in the diagram characterizes the successfulimplementation of light weight design in the last decade. Itis obvious that piston weight for port fuel injected (PFI)engines could be reduced continuously in spite ofincreasing power density. This was only possible with thesupport of advanced analytical design tools (e.g. FEA) andoptimizations of materials and the casting process.

The piston weight cannot be independent of the load level.The piston lightweight level attainable today is shown,using the example of two implemented development

projects with extremely different load levels, by points 1:(55 kW/l; 7.5 MPa) and 2: (72 kW/l; 11.0 MPa). The areabetween these two points is therefore representative of thestatus reached today.


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The selection of the piston design principle (forged piston

monolithic core, or cast piston multi-part core, in somecases combined with cast recess behind the ring zone), andof the piston material depends on the specific requirements

and the dimensional specifications for the respectiveapplication.The decrease in oscillating masses leads to improvementsin engine friction and noise excitation. The demand for lowerpiston weight is often also justified by the higher enginespeed needed to boost power output. Weight reductionstarting from todays already low values typically leads tomore flexible components. A more flexible piston, however,may well lead to increased subjective engine noise. In thiscase an optimum design is the only way to prevent

undesirable side effects of lightweight construction.


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Today, most pistons are made of the high-temperature alloyKS1295 to protect the first groove and to withstand theresulting high crown and boss loads.

Microwelding in the First Ring GrooveRising groove temperatures caused by reduced top landheight, deep valve pockets and a general increase in powerdensity, in combination with reduced oil consumption, make

measures to protect the first ring groove from microweldingof aluminium from the groove onto the first ring increasinglynecessary. As the countermeasures on the ring have so farnot yielded a reliable result, problem solving efforts have tobe concentrated on the piston. Next Figure shows resultsfrom studies of various piston and groove materials inengines with respect to their microwelding tendency in thefirst groove.


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Influence of different groove materials and groove protectionmeasures in a 60h microwelding test


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Piston for V8 engine, material KS1295, locally limited protection offirst groove byhard anodizing, with Ferrocoat layer


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Weight-optimized ring carrier piston, top land height 3.2 mm


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Cooling-gallery piston for a supercharged SI engine


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High power density piston for SI engine


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Demands for a drastic reduction of fuel consumption,

together with severe competition from diesel engines, haveboosted efforts for the development of new combustionmethods for spark ignition engines.

In Europe, DI spark-ignition engines have been developedto commercial maturity during the last few years.

These engines are based on either wall formed stratifiedcharge or homogeneous combustion concepts.

In addition, efforts for the combination of these twoconcepts with turbo-charging and the development ofspray-formed combustion processes are under way . Inaddition to reduced fuel consumption, these activities areaimed at an engine power increase.


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One way to reduce fuel consumption of a DI spark-ignitionengine is the operation with stratified charge so that

throttling can be avoided over a wide range of themap.Other methods of optimizing consumption consist of utilizing

the inside cooling effect so that the compression ratio canbe increased and the efficiency is improved, together withthe resulting downsizing effect in combination with turbo-charging.

Pistons developed for stratified charge operation usuallyhave a complex piston crown geometry.

As a result of the deeper bowl compared with current spark-ignition engines, along with the required conrod clearance,extremely low compression heights cannot be achieved inpistons for DI spark-ignition engines.


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Pistons for SI engine DI


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Moreover, the requirement to match the piston surfacecontour outside the combustion chamber bowl as much as

possible to the shape of the cylinder head involves anaccumulation of material above the piston pin boss.

These two constraints have led to the situation that, despiteall efforts, such pistons were until now normally heavier

than comparable conventional pistons.


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LiteKS piston design for DI spark-ignition engine, newly developedcasting technology


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Using the LiteKS (patent pending) piston design, this trendcan be broken.

With the LiteKS piston design (previous Figure), it ispossible to implement cast recesses in the ring zone area,which as far as possible match the material accumulations

above the piston pin axis so that weight disadvantages arecompensated. In addition, the comfort features (defined skirtelasticity distribution), without scuff sensitivity are maintainedin the LiteKS piston.


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In addition, the comfort features (defined skirt elasticitydistribution), without scuff sensitivity are maintained inthe LiteKS piston.

In this way, weight savings of 20% compared withconventional lightweight piston designs and significant

fatigue strength improvements in the highly loaded pistonareas have already been achieved.

Further weight reductions of the piston group by acombination of the LiteKS piston with a tapered conrodand optimum piston pin dimensions are increasingly beingimplemented.


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The so-called apparent density of the piston (k factor),is

There is virtually a linear relationship between the k factorand the relative compression height for pistons based on

the same design principle. In the direction towards lowcompression heights, the piston design is limited by thespace required for the ring zone and conrod clearance aswell as increasing crown stresses. The level of tolerablecrown stresses can be increased by material substitution.


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The weight of DI spark-ignition pistons is significantly

higher today than that of recent developments for port-fuel-injected (PFI) spark-ignition engines.

With the use of LiteKS piston technology, even pistonsthat would be substantially heavier than average due totheir combustion chamber design, can achieve the weightlevel of current production pistons for PFI engines.


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The combustion chamber related mass accumulation in thepiston crown and the resulting high centre of gravityincreases the risk of piston noise. Therefore, special attentionmust be paid to good skirt guidance (length).

Newly developed wear resistant LofriKS plastic skirtcoatings allow a permanently low assembly clearance to be

achieved.The influence of different radii on the crack formation wasstudied. The result of such studies is that the optimumcompromise between the sharp-edged design desirable forcombustion reasons and the rounding requiredfor strength is R = 1 mm, both in terms of thermal shock

resistance and (tensile) stress load.


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By reducing the top land height to a dimension that can

reasonably be expected for the future, the groovetemperature level was raised and then set to the valuerequired for the various tests by adjusting the ignition

timing. The degree of groove damage is assessed on thebasis of the extent and depth of the microwelded zones.The individual columns represent the results of the fourcylinders of an engine.

By weld hardening or hard anodizing of the first groove, theload tolerance can be increased further. The measures

shown here are applied in series production and have beentried and tested for many years.


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After completing the groove anodizing development forgasoline engines to allow the process to beline-integratedwith optimum low groove roughness, a reduction of thezone to be anodized (selective groove coating) wasaccomplished by further development efforts. Anodizing of

the piston crown in conjunction with the groove consumeselectrolyte, and it offers no benefit in this area. Theprocess applied by KS offers optimum conditions for

screening the zones not to be coated.Local anodizing can be easily combined with plasticcoatings of the piston skirt or iron coating of the otherpiston surfaces. An iron coat on the piston crown, forexample, may be retained as an effective layer forprotection against detonation damage


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Two stroke engine

?


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CI ENGINE


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The diesel engine has continued its advance in thepassenger car sector in Europe.

This situation is not least due to the great progress achievedin diesel development with respect to the power, dynamicsand ride comfort of the vehicles during the past few years. To

days diesel vehicles play a significant role in the reduction offleet fuel consumption, leading to a significant decrease inCO2 greenhouse gas emissions. The increased number ofdiesel registrations, however, also has an effect on NOx andespecially on particle emissions. It is anticipated that dieselparticle filters will be used more widely.


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Compared with pistons for commercial vehicle engines,

passenger car engine pistons are lightweight and possessa low construction height.The specific power output has in the meantime reached

approx. 50 kW/l to 60 kW/l. These engines usually have 4valves per cylinder and a combustion bowl locatedcentrally in the piston. The combustion pressure of suchengines reaches peak values between 160 bar and180 bar. Pistons for these loads are currently equippedwith a rotationally symmetrical cooling gallery. For thereinforcement of the top ring groove, the well-proven

niresist ring carrier is the best alternative, consideringboth function and cost-effectiveness .


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Development TrendsMost direct injection diesel engines for passenger cars dueto be launched in series production around the year 2004will reach a specific power output of up to about 65 kW/l.The increased engine power output is accompanied by a

rise in peak cylinder pressure to values of 200 bar.These engines are characterized by 4 valves per cylinder,a combustion bowl located centrally in the piston and high-

pressure injection systems (third generationcommon rail, unit injection), with good controllability of theinjection process by means of electronic controls for pre-injection and post-injection.This development of course has an influence on thecombustion bowl geometry.


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Euro 4 bowls tend to be shallower and have a larger diameter

than theirpredecessors (Fig. 1). This means for the piston areduction of the distance between the combustion bowl andring carrier. The effects of this trend on the piston thermalloading are shown in Fig. 2. This represents the temperaturesat the bowl edge and behind the first ring groove, which arecritical for function and durability, as a function of the specificpower output.Despite the cooling gallery, the bowl edge temperaturereaches critical values for aluminium piston alloys of over 400C. The top ring groove temperature can rise to over 300 C,

which also has an adverse effect on top ring function due tohigh carbonization.


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Figure 1: Design of combustion bowls


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Figure 2: Piston temperatures as a function of the specificengine power output


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As a solution to this problem, offers cooling galleries withvariable cross-section. This development was aimed at betterpiston cooling whilst also reducing weight andachieving higher component strength.

What may seem contradictory at first glance was implementedas follows: to achieve a higher cooling efficiency, the cooling

gallery must be positioned as close to the first groove andbowl edge as possible. Considering the stress values of thecooling cavity around the entire circumference, it was foundthat there are critical values towards the bowl over the pin axis,but low stress values perpendicular to the pin axis.


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The cooling gallery was then optimised by targeted materialdistribution in such a way (Fig. 3) that the stress peaks arebalanced.Maintaining its proven salt core technology, the coolinggallery is optimized with respect to shape and position andcan be produced cost-effectively.

Figure 3: Cooling gallery with variable cross-section


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As inclined oil jets are employed in most engines, the impactpoint of the oil stream on the piston under crown depends on

the stroke position and thus on the crank angle.

In order to improve the cooling oil flow, the inlet opening was

designed as a slot, and a deflector (jet splitter) was positionedon the crown side in such a way that the oil jet is divertedcircumferentially in a targeted way and an even,controlled volumetric flow is obtained (Fig. 4).

When the oil jet no longer enters the inlet opening as thestroke progresses, it cools the piston interior and additionally

supplies oil to the pin / pin bore / conrod small end system.This combined gallery and under crown cooling has been wellproven in many production applications.


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Figure 4: Impact point of the oil jet on the piston under crown


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When the oil jet no longer enters the inlet opening as thestroke progresses, it cools the piston interior and additionallysupplies oil to the pin / pin bore / conrod small end system.This combined gallery and under crown cooling has been wellproven in many production applications.

A comparison with the standard cooling gallery shows that thevariable cooling gallery offers temperatures that are approx.10 C lower at the bowl edge and approx. 15 C lower at the

top ring groove.


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Fig. 5: Comparison of cooling gallery temperatures


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Surface plot of the temperature distribution.


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PISTON RINGS


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Compression rings


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Oil Control Rings


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PISTON PIN


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ch 2 pistons, rings, pin

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