This second step of development of the PC2.6 B engine has been backed-up by extensive tests performed on a three-cylinder engine during 1500 hours.
The severised endurance test program used, lasting 500 running hours, comprised:
･ 2000 cycles of a low cycle fatigue test from idle speed to overload and back to idle. The duration of each load step has been chosen in order to provide enough time to reach stabilised engine parts temperatures.
・3.5 × 106 high cycle fatigue cycles at overload.
Two such tests have been performed with respectively 10 and 15% overload.
The PC2.6 B complies with the IMO limit value of NOx emissions without compromising the specific fuel consumption (177 g/kWh + 5% tolerance, without driven pumps).
The first five 16 cylinders engines have been delivered mid-1999 in a power plant in Honduras, and have clocked over 6000 running hours in May 2000.
3. LATEST PA6 ENGINE DEVELOPMENT
The PA6 engines have been used in all types of civil applications, moreover their high power density made them perfectly well suited for navies. Thus, over one third of the engines produced have been devoted to this market segment.
During the last decade a new market has developed quite rapidly: high speed ferries. Those ships, of either catamaran or mono-hull types cruising at speeds between 35 and 45 knots, require high power propulsion systems with low weight and bulk. Therefore only diesel engines with weight/power ratios limited to 5 kg/kW can economically compete with gas turbines.
To develop such an engine, use has been made of the 280 mm bore PA6 B engine which stroke has been enlarged from 290 (former PA6 standard) to 330 mm.
While choosing the power target and corresponding running parameters, following considerations have been taken into account:
･ the high speed ferries are heavy duty applications, as the engines are usually used at high loads (close to MCR) for relatively short trips.
･ the past experience of the high power ratings of the PA6 B STC (two stage turbocharged) developed in the 70's for navies has to be used (Fig.3)
･ the sequential turbocharging system previously developed and in operation on PA6 standard engines for navies should be incorporated in this new design.
The specific power of 405 kW/cyl at 1050 rpm which has been retained allows to get, with 20 cylinders, the most powerful (8.1 MW) high speed ferry engine available today on the market, while keeping a quite conservative value of 22.7 bar bmep to ensure low maintenance costs and high reliability.
This new PA6 engine version, running exclusively on marine diesel oil or IF 30 with 10% power derating, features following new or modified components:
Cylinder head
As for the PC2.6 B, the increased piston speed (11.5 m/s) required a new cylinder head with reduced air and gas flow resistance. The flow coefficients have been increased by 48 and 62% respectively for inlet and exhaust ports. A reinforced fire plate, of the so-called bore-cooling type, is used to allow higher combustion pressures (160 bar).
Connecting rod
The choice of a straight cut connecting rod has been made in order to avoid recurrence of some problems occasionally experienced on bevel cut connecting rods like fretting in the serrations for instance.
Moreover, the adoption of piston cooling by oil jets avoids the need of oil drillings in the connecting rod and allows to use ungrooved big end bearing shells with following advantages:
- increased bearing load capacity,
- no risk of so-called cam-wear on the crankpin
- no risk of cavitation erosion in the shells.
Crankshaft
The main journal and crankpin diameters have been both increased by 20 mm. This results in lower bearing loads and crankshaft stresses as well as higher torsional natural frequencies.
The induction hardening of the main journals and crankpins has been replaced by nitriding, including the junction fillets to the web in order to increase fatigue limits. The 20-cylinder engine crankshaft design has been checked in collaboration with AVL through an enhanced dynamic analysis using F.E.M.
Sequential turbocharging (STC)
The specific performances of turbocharged diesel engines have been continuously increased during the last few decades, mainly through higher boost air pressures delivered by the turbochargers.
This has been accompanied by increased difficulties to match the turbocharger performance to the engine needs at all running conditions between idle and full load. In fact, when the turbochargers are well matched at nominal engine speed and power, the turbine area becomes too large to make the best use of the exhaust gas energy at reduced engine speeds. Then, the amount of air required to properly burn the fuel is limited, resulting in poor engine torque ability and degraded combustion with detrimental consequences on fuel consumption, smoke emissions, combustion and exhaust gas temperatures.
This problem can be solved by the use of sequential turbocharging based on the principle of reducing the number of turbochargers in operation as the engine speed and load are dropped. Thus, the speed of the turbochargers remaining in operation is increased, resulting in significantly larger air quantities delivered to the engine.
S.E.M.T. Pielstick has applied this idea to the PA6 engine in a very simple way, using only two turbochargers, one of them being switched off below approximately 50% of nominal power [2]. This is achieved simply by closing two flap valves located at the compressor outlet and at the turbine inlet of one of the two turbochargers.
