Tips for the operator

While cylinder outlet tem-perature is a central valuefrom which an enginebuilder can derive infor-

mation about component material tem-peratures, the quality of combustion andthe power output of the cylinder con-cerned, for the turbocharger manufacturer– and operator – it is the temperaturedirectly ahead of the turbocharger that isa decisive value.

But why should there, essentially, bea difference between these two tempera-tures? After all, only the engine’s usuallywell insulated exhaust pipes lie betweenthe exhaust valve ports of the cylinderhead and the turbocharger’s turbine inlet– and the nearest cylinder is often only afew centimeters from the turbocharger.

Well, there is a difference and this isexplained below. Suffice it to say at thispoint that the difference is considerableand the situation is counter-intuitive:measurements on engines in the fieldhave shown that the exhaust gasesdirectly ahead of the turbocharger canbe 100 – 140 °C above the temperatureindicated by the sensors engine buildersfit at the exhaust ports.

This temperature differential (�T) is animportant reference value for engineoperators. To get the important operat-ing values mentioned above, enginebuilders invariably fit temperature sen-sors at all the cylinders of their engines.By contrast, a temperature sensor is notalways fitted ahead of the turbocharger.

Wet CleaningThis situation can cause difficulties

when the engine operator wishes to car-ry out wet cleaning of the turbocharger’sturbine-side components. This is neces-sary to remove deposits left when theengine is burning fuels like marine diesel(MDO) or, especially, heavy fuel oils(HFO). Such deposits completely changethe carefully designed aerodynamic andthermodynamic behavior of the compo-nents in the exhaust gas path, in particu-lar at the nozzle ring, the turbine and tur-bine housings. In the case of lower qual-ity heavy fuels, these deposits can bevery hard to remove, especially in con-junction with exhaust gas temperaturesof above 520 °C.

The injecton of cold water causes asudden temperature change in the mate-rials of the turbine-side components andin most cases a noticeable rise inexhaust gas temperature. Injecting waterraises the density of the exhaust gasesand at the same time extracts heat fromthe exhaust gases, resulting in reducedenergy reaching the turbine. Both effectscause a decrease in turbocharger speedand hence lower charge air pressure,which in turn causes an increase inexhaust gas temperature. To maintainthe required engine output in certainapplications, e.g. baseload diesel powerplants, the loss of engine power is com-pensated by additional fuel injection.

Cylinder outlet tempera-ture vs turbocharger turbine inlet temperatureWhen wet cleaning turbocharger turbines on 4-stroke dieselengines it is important to maintain the correct turbine inlet temperature. But as Manfred Schumm explains, this can oftenonly be estimated.

1.0

0.9

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0 90 180 270 360

Degrees crankshaft angle

450 540 630 720

Valv

e lift

Exhaust

Intake

300 °

Valve timing example of a 4-stroke diesel engine.

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Tips for the operator

ABB Turbocharging specifies a seriesof parameters aimed at minimizing thesestresses during wet cleaning. Of these,limits for exhaust gas temperature aheadof the turbine, both before and duringwet cleaning, are the most important.

As we have seen, if the engine opera-tor uses the cylinder outlet temperatureto set the turbine inlet temperature, it ispossible that the actual temperatureahead of the turbocharger will be 100 to140 °C higher than assumed. The resultis the excessive thermal stresses men-tioned in the turbine housing and at thenozzle ring. And after only a few wetcleaning intervals performed at exces-sive exhaust gas temperature ahead ofthe turbine, the visible results could behairline cracks and even material parti-cles breaking off these components. Inaddition, the inadmissible stresses in -volved can cause distortion of flangesand hence lead to fluid leakages (gasand cleaning water).

How do these differences in the meas-ured temperatures occur?

A thermocouple in front of the turbineis permanently in the exhaust gas flow,and at constant engine load it is sur-rounded by gases with a more or lesshomogeneous temperature and flow. It isthus capable of measuring the real tur-bine inlet temperature.

By contrast, the thermocouple afterthe cylinder is subjected to changingconditions, principally due to the inter-

mittent flow of exhaust gases from thecylinder as the exhaust valve opens andcloses. It is thus not constantly sur-rounded by hot gas.

The cycle of a cylinder on a 4-strokeengine comprises 720 ° of crankshaftangle – i. e. two revolutions – anddepending on timings, the exhaust valvecan be open during about 300 of the720 °. This means the sensor is subjectedto hot gases for less than half the 4-strokes. No gas is flowing during theremaining period and the sensor coolsdown.

Another important factor is that valveoverlap is used to achieve “scavenging”of the cylinder – i. e. the inlet valve openswhile the exhaust valve is still open andmuch cooler charge air pushes theexhaust gases out of the cylinder. Theinevitable mixing of the air and theexhaust gases leads to further cooling ofthe temperature sensor.

The sensor after the cylinder thus experi-ences three different situations:1. Exhaust valve open:

hot gas is flowing past the sensor2. Exhaust and inlet valves both open:

a mixture of hot gases and relativelycool air is flowing past the sensor

3. Exhaust valve closed:the sensor is immersed in a relativelycool “scavenging pocket”

These variations combined with theinertia of the sensor during the rapid

temperature fluctuations and the con-struction of the sensor lead to a form of“averaging” of the temperature read-outseen by the engine end user.

Which proportion each of the threestates mentioned above contribute tothe overall conditions around the sensordepends on the position of the sensor,the engine’s valve timings, valve overlapand the scavenging gradient, i. e. the dif-ference between the charge air pressureand the pressure after the cylinder, whichin turn is influenced by the turbochargerspecification.

It also often occurs that the differentcylinder outlet temperatures deviate moreor less strongly from one another, whichmay derive from variations in combustionquality from cylinder to cylinder or inter-action between neighboring cylinders viathe exhaust gas collector system. This isinfluenced by the engine’s firing order(pressure wave from one cylinder coun-teracting the exhaust outflow fromanother).

Summary The quintessence is that cylinder out-

let temperature is not identical with tur-bocharger turbine inlet temperature andthe effects of wet cleaning turbochargerturbine side components at too high atemperature can be severely detrimental.

The engine end user should try to hitABB Turbocharging’s recommended tem-perature range based on turbochargerturbine inlet temperatures 100 to 140degree higher than cylinder outlet tem-perature.

Moreover, it may be necessary toadjust exhaust gas temperature by fur-ther reducing engine load after the startof wet cleaning due to the effects of thecleaning water described above.

The range 100 to 140 °C temperaturewe quote is based on experience fromthe field, gathered on several enginetypes. The band width of 40 degrees isderived from various parameters andinfluences. It is intended to serve as aguide for engine users for setting a suit-ably accurate exhaust gas temperatureahead of the turbocharger during wetcleaning when there is no temperaturesensor at the turbine inlet.

Only the well insulated exhaust gas collector (running along the center line of this vee engine) sepa-rates the engine’s cylinders from its turbocharger(s). Nonetheless, temperatures at the turbine inletcan be over 100 °C higher than at the exhaust ports.

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