MILLER CYCLE

       A high turbocharger efficiency contributes to reduced carbon dioxide emissions by improving the engine efficiency. The same turbocharger, combined with a ‘smart’ turbocharging system able to guarantee an optimum air/fuel ratio under all conditions, would contribute even more to the control of soot emissions. NOx is produced during combustion at very high temperatures—something which can hardly be influenced by changes in the turbocharging system since the flame temperature depends on local conditions in the cylinder and not on the global mean air/fuel ratio.

Hence efforts were put-in to make the combustion chamber efficient in reducing combustion air temperature which consequently reduces NOx emissions. Thus, a joint development programme involving both the turbocharging system and the engine, aimed at reducing the temperatures of the working cycle in the cylinders.

            The first idea was turbocooling, in which the charge air is cooled in a process that makes use of a special turbocharger. If the pre-compressed air is further compressed in a second-stage compressor, then cooled and expanded through a turbine, very low temperatures can be obtained at the cylinder inlet. First evaluations revealed that the available turbocharger efficiencies for this process were not high enough for reasonable engine efficiencies, ABB Turbo Systems reports.

The Miller cycle promises much better results.

MILLER CYCLE

The idea is similar to that on which turbocooling is based. The charge air is compressed to a pressure higher than that needed for the engine cycle, but filling of the cylinders is reduced by suitable timing of the inlet valve. Thus, the expansion of the air and the consequent cooling take place in the cylinders and not in a turbine. The Miller cycle was initially used to increase the power density of some engines (see Niigata engines).

Reducing the temperature of the charge allows the power of a given engine to be increased without making any major changes to the cylinder unit. When the temperature is lower at the beginning of the cycle the air density is increased without a change in pressure (the mechanical limit of the engine is shifted to a higher power). At the same time, the thermal load limit shifts due to the lower mean

temperatures of the cycle.

Promising results were obtained on an engine in which the Miller cycle was used to reduce the cycle temperatures at constant power for a reduction in NOx formation during combustion: a 10 per cent reduction at full load was achieved, while fuel consumption was improved by around 1 per cent. This was mainly due to the fact that with the Miller cycle—at the same cylinder pressure level—the heat losses are reduced due to the air/fuel ratio being slightly higher, and the temperatures lower.

Thus, the reduced temperature of combustion will reduce NOX emissions.

HOW

            During the intake cycle of the combustion, the inlet valve is closed before its normal closing time. This expands the air and helps in reducing the temperature.

            Imagine, the piston is moving from TDC to BDC with its inlet valve open. The air draws in as the piston goes down towards BDC. Consider that I closed inlet valve 20degrees before BDC, the further movement of the piston for this 20degrees will expand the air. This expansion reduces the air temperature which is already drawn in.

FUEL INJECTOR

The fuel valve design was changed a few years ago from a conventional type to the mini-sac type. The aim was to reduce the sac volume in the fuel nozzle and curb dripping, thus improving combustion; introducing the mini-sac valves reduced the sac volume to approximately one third of the original (see the chapter on Fuel Injection). To  improve combustion even further, however, a new slide-type valve was introduced for all large bore engines, completely eliminating the sac volume (see the chapter on Exhaust Emissions and Control). A significant improvement in combustion is accompanied by reduced NOx, smoke and particulate emissions. The reduced particulates also improve the cylinder condition, and the wear rates of cylinder liner, piston rings and ring grooves are also generally lower with slide-type fuel valves. Slide-type valves were introduced on the K98MC engine from the beginning, their positive effect confirmed on the testbed. With fuel nozzles optimized with respect to heat distribution and specific fuel consumption, the NOx emission values associated with this engine are described as very satisfactory.

The increased mean effective pressure ratings of modern engines require increased flow areas throughout the fuel valve which, in turn, leads to increased sac volumes in the fuel nozzle itself and a higher risk of after-dripping. Consequently, more fuel from the sac volume may enter the combustion chamber and contribute to the emission of smoke and unburned hydrocarbons as well as to increased deposits in the combustion chamber. The relatively large sac volume in a standard design fuel nozzle thus has a negative influence on the formation of soot particles and hydrocarbons. The so-called mini-sacfuel valve introduced by MAN B&W Diesel incorporates a conventional conical spindle seat as well as a slide inside the fuel nozzle. The mini-sac leaves the flow conditions in the vicinity of the nozzle holes similar to the flow conditions in the conventional fuel nozzle. But its much reduced sac volume—only about 15 per cent that of the conventional fuel valve—has demonstrated a positive influence on the cleanliness of the combustion chamber and exhaust gas outlet ducts. Such valves also reduce the formation of NOx during combustion. A new type of fuel valve—essentially eliminating the sac volume— was subsequently developed and introduced by MAN B&W Diesel as standard to its larger low speed engines (Figure 3.9). The main advantages of this slide-type fuel valve are reduced emissions of NOx, CO, smoke and unburned hydrocarbons as well as significantly fewer deposits inside the engine. A positive effect on the cylinder condition in general is reported. Applying slide fuel valves to a 12K90MC containership engine yielded a 40 per cent reduction in smoke (BSN10) compared with the minisac valved engine, while hydrocarbons and CO were  reduced by 33 per cent and 42 per cent, respectively, albeit from a low level. NOx was reduced by 14 per cent, while the fuel consumption remained virtually unchanged and with a slight reduction at part load.

CRANKSHAFT

Is either fully built(all individual and later shrink fit), semi built(with one crank throw forged or cast as a single unit), fully forged

      C/shaft is usually cast and for large engines is forged from 0.4% carbon steel with Nitralloy( 1.5% CR, 1% Al and 0.2%MO). Molybdenum refines the grain structure. The C/shaft is heated in Ammonia for 4days and the nitrogen in ammonia dissociates and combines with CR and AL to form hard nitrates on surface.

 Fold forged method using hydraulics is used to forge a large single billet with its ends flattened. care is taken that it defect free at the bends and cooling is done at a controlled rate.  Usually forged to get continuous grain structure and we achieve no sub-surface defects due to repeated heating and pressing. 

Identical crank thros are then shrink fitted with the main journals at desired angles. Usually, cold shrinking is preferred over hot shrinking as hot shrinking gives residual stresses and oxidation products are formed in btw the interface. Shrink fit allowance is usu 1/570 - 1/660 of journal dia so that it provides necessary radial pressure. The thickness can be reduced as it is forged but the area around the interference fit is increased as there are large tensile hoop stress present in material after shrink fit.

By virtue of its geometry, c/shafts are subjected to fatigue stresses of combined gas load, bending and torsional stresses. The junction bte the pin and web is where the highest likely possiblity of repetition and reversal of stresses.

To minimise the stress, a fillet (usu 5% of journal dia) passing into the web so as not to disturb the bearing surface area of the pin.

   The fillets are cold rolled or strain hardened to 220BHN, to induce compressive stress, again to increase fatigue resistance.

The running surfaces are induction hardened. flame hardened to abt 480BHN.

C/shaft is subjected to -

- tensile and compressice stresses at TDC and BDC due to gas load

- bending stresses due to gas combustion load at different crank angles 

- Gas load also imparts flexing in and out of the webs leading to axial vibration.

- torsional stresses due to varying periodic firing impulses from the different units which twist the shaft at varying torque

        Moreover, these all are cyclic and varying leading to fatigue stresses.

RTA Design Features

The RTA design benefited from principles proven in earlier generations of Sulzer R-type engines. The key elements are:

-       A sturdy engine structure designed for low stresses and small deflections comprises a bedplate, columns and cylinder block pretensioned by vertical tie rods.

-       The single-wall bedplate has an integrated thrust block and incorporates standardized large surface main bearing shells. The robust A-shaped columns are assembled with stiffening plates or are of monobloc design. The single cast iron cylinder jackets are bolted together to form a rigid cylinder block (multi-cylinder jacket units for smaller bore engines).

-        Lamellar cast iron, bore-cooled cylinder liners with back-pressure timed, load-dependent cylinder lubrication.

-       Solid, forged bore-cooled cylinder covers with one large central exhaust valve arranged in a bolted-on valve cage; the valve is made from a heat- and corrosion-resistant material and its seat ring is bore-cooled.

-       Semi-built crankshaft divided into two parts for larger bore engines with a large number of cylinders.

-       Running gear comprising

connecting rod,

crosshead pin with very large surface crosshead bearing shells (with high pressure   lubrication) and

double-guided slippers,

piston rod and bore cooled piston crown using oil cooling.

short piston skirts.

 All combustion chamber components are bore cooled, a traditional feature of Sulzer engines fostering optimum surface temperatures and preventing high temperature corrosion due to high temperatures on one side and sulphuric acid corrosion due to too low temperatures on the other.

Comfortable working conditions for the exhaust valve are promoted by: hydraulic operation with controlled valve landing speed; air spring; full rotational symmetry of the valve seat, yielding well-balanced thermal and mechanical stresses and deformations of valve and valve seat, as well as uniform seating; extremely low and even temperatures in valve seat areas due to efficient bore cooling; valve rotation by simple vane impeller; valve actuation free from lateral forces, with axial symmetry; and simple guide bushes sealed by pressurized air. The low exhaust valve seating face temperature reportedly secures an ample safety margin to avoid corrosive attack from vanadium/ sodium compounds under all conditions. Efficient valve cooling is given by intimate contact with the bore-cooled seat, together with the appropriate excess air ratio in the cylinder. The specific design features of the valve assembly are also said to deter the build-up of seat deposits, seat distortion, misalignment and other factors which may accelerate seat damage.

-       Camshaft gear drive housed in a special double column or integrated into a monobloc column, placed at the driving end or in the centre of the engine for larger bore models with a large number of cylinders.

-       Balancer gear can be mounted on larger bore engines, when required, to counter second-order couples for four-, five- and sixcylinder models, and combined first- and second-order couples for four-cylinder models.

-       A compact integral axial detuner can be incorporated, if required, in the free end of the engine bedplate.

-       The fuel injection pump and exhaust valve actuator are combined in common units for each two cylinders.

The camshaft-driven injection pump with double valve-controlled variable injection timing delivers fuel to multiple uncooled injectors. The camshaftdriven actuators impart hydraulic drive to the single central exhaust valve working against an air spring.

-       Constant pressure turbocharging is based on high efficiency uncooled turbochargers; auxiliary blowers support uniflow scavenging during low load operation. In-service cleaning of the charge air coolers is possible. A standard optional three-stage charge air cooler unit can be specified for heat recovery.

RTA DESIGN DEVELOPMENTS

The reported benefits of the triple-valve configuration are a more uniform temperature distribution around the principal combustion space components (cylinder cover, liner and piston crown) at the increased maximum combustion pressures, along with even lower temperatures despite the higher loads. Three fuel valves also foster significantly lower exhaust valve and valve seat temperatures. Other spin-offs from the research engine included a modified cylinder liner bore-cooling geometry whose tangential outlets of the bores aim for optimum distribution of wall temperatures and thermal strains at higher specific loads. The geometry of the oil cooling arrangements of the piston crown was also modified to maintain an optimum temperature distribution. The good piston running behaviour was maintained by retaining established features of the RTA design: multilevel cylinder lubrication; die-casting technology for cylinder liners; and temperature-optimized cylinder liners. Advances in materials technology in terms of wear resistance have permitted engines to run at higher liner surface temperatures. This, in turn, allows a safe margin to be maintained above the increased dew point temperature and thus avoiding corrosive wear. Some refinements were introduced, however, to match the new