Power2 Two stage Turbocharging for Marine Diesel and Gas engines: First engine with Power2 reaches market

ABB Turbocharging, based in Baden Switzerland recently launched its Power2 two stage turbocharging system and already the first engine to use the technology is commercially available.

Power2 technology is capable of producing pressure ratios as high as 8 and above and it is already widely accepted as a major enabling technology of the strong Miller Cycles which can substantially reduce NOx emissions on diesel engines while realising potentials for enhanced  power densities and fuel efficiencies. On gas engines Power2 is also seen as a Miller enabler, but in early applications a major focus is employing the high order of charge air pressures Power2 produces to increase power density and efficiency.

First commercial application

Indeed, with its Power2 technology ABB Turbocharging has the distinction of equipping the first commercial application of a large bore gas engine with two stage turbocharging. At the time of writing a 24 cylinder GE Jenbacher type J624 large bore spark ignited gas engine with Power2 two stage turbocharging  is due to be commissioned in a stationary cogeneration application in the Netherlands.

The 24 cylinder J624 with two stage turbocharging achieves a rated output of 4.4 MW compared to the 4 MW of its predecessor with conventional turbocharging and offers an efficiency increase of about 2% while, like modern lean burn gas engines,  undercutting IMO Tier III limits on NOx by a wide margin. Moreover, the higher level of charge air pressure from the ABB two stage turbocharging system helps the engine maintain rated outputs in climates with high ambient temperatures and humidity and on gases of variable calorific value.

At the present stage of development, these enhancements are based on pressure ratios well over 6 bar, enabling the GE engine to run at mean effective pressures above 22 bar and giving the scope for the future application of advanced Miller Cycles on the GE gas engine. For its part, the high turbocharging efficiency of ABB two stage turbocharging contributes substantially to the J624 gas engine’s increased overall efficiency.

Power2 marine gas engine perspectives

Significantly, Power2’s potential for substantially improving gas engine performance has been demonstrated “in the iron” at a time when natural gas is increasingly being employed to fuel for marine engines. Its use is especially prevalent in gas rich and environmentally aware countries like Norway, where both LNG and CNG fuel a number of ferries and vessels used in the offshore sector. Building on this start, engines fuelled on natural gas are also being strongly considered as a method of both complying with strict NOx emissions coming into force in the middle of the present decade and more imminent regulations limiting or banning the use of ship’s diesel powered onboard generator sets to cover the “hotel load” when vessels are in port (cold ironing).

Power2 marine diesel perspectives

As stated, Power2 enables the benefits of Miller Cycles to be applied to diesel engines. On 4 stroke diesels it has been demonstrated that strong Power2 can produce turbocharger pressure ratios high enough to achieve very strong Miller Cycles capable of achieving high, double digit NOx reduction percentages on 4-stroke medium-speed diesel engines. Calculations for Power2’s potential made by ABB Turbocharging’s development engineers using advance simulation software indicate that, for example, the 80 % reduction in NOx values via-à-vis IMO Tier I required under IMO Tier III for ships operating in designated Emissions Control Areas (ECAs), are already a realistic prospect.

The basic technology

The Power2 two stage turbocharging system consists of two turbochargers of different, tuned frame sizes connected in tandem on the compressor side via an intermediate air cooler – cooling the compressed air issuing from the first turbocharger means the second turbocharger needs to do less work and can be more compact. Development is already well underway, both on turbocharger test beds at the ABB Turbocharging technical centre in Baden and on two and four stroke engine from major engine builders.

Taming the trade-off

By assisting engine builders to attain strong Miller Cycles, Power2 is instrumental in alleviating one of the hitherto most intractable constraints in the search for lower NOx emissions without incurring penalties on the fuel efficiency side: the compromise between NOx emissions and specific fuel consumption (SFC).

The so-called “NOx-SFC Trade-off” reflects the fact that NOx formation reduces with lower combustion temperatures while engine fuel efficiency increases with higher combustion temperatures. Hence, in the early days of emissions reduction on diesel engines, a widespread measure was to reduce combustion temperatures by retarding fuel injection to reduce the rate of heat released from the fuel. Thus a fuel consumption penalty was incurred in the interests of lower NOx emissions.

However, while the trade-off will always be a fact of every engine developer’s life, the findings of ABB and its development partners show that using a combination of much higher turbocharging pressures, variable valve timing, advanced fuel injection technology (e.g. common rail), and electronic control, the Miller Cycle is capable of shifting this compromise between NOx emissions and specific fuel consumption values into a new, far lower range. Moreover, as stated, a further effect of the higher turbocharging pressures used is a useful increase in engine power density.

Miller Cycle

These assertions are explained by examination of the Miller Cycle and its effect. The term denotes an ingenious method of cooling an engine’s combustion air and so eliminating the high temperature peaks in the engine’s combustion chamber which are responsible for over 90 % of NOx formation.

On 4-stroke engines, the cooling effect is achieved by shortening the opening period of the inlet valve and so reducing the time during which air can enter the cylinder on the engine’s induction stroke. The earlier end of induction promotes expansion, and hence cooling as the air continues to expand. On 2-stroke engines, where the timing of air induction is a function of the piston passing fixed inlet ports in the cylinder wall, a similar effect can be achieved by varying the closure of the exhaust valve.

In both cases, however, without turbocharging countermeasures, a shorter period for air induction would mean only a reduced mass of air could enter the combustion chamber and engine power output and response to load changes would suffer. Hence, higher turbocharging pressures are used to compensate the shorter time for induction, allowing an equal – or even greater – mass of combustion air to be forced into the cylinder in the briefer period available. In this way engine power characteristics can be maintained – and bettered – while still achieving significant reductions in NOx formation due to lower combustion chamber temperatures. Significantly, this does not affect the quality of combustion and thus hence need not impair fuel consumption.

VCM for variable Miller

The strong Miller Cycle described above assumes an engine operating at its best point for power and/or fuel consumption and/or emissions. In the case of a diesel powering a ship this is the so called “maximum continuous rating” (MCR). ?