In engine emission reduction technology primary measures denotes modifications to the basic, fully integrated features of any diesel engine – fuel injection, turbocharging, governing, valve timing, etc. Their variation influences the combustion process and hence power output, fuel consumption and the formation of noxious exhaust emissions. Part and parcel of the engine as delivered, combating emissions using primary measures involves neither additional work for the engine operator nor the use of additional equipment and consumables.
A major emphasis of all combustion engine emissions legislation is oxides of nitrogen (NOx). Since temperature peaks during combustion are responsible for over 90% of NOx formation, MAN Diesel’s work with primary measures targets the elimination of such areas in the combustion chamber without incurring fuel consumption penalties or, if possible, at improved fuel efficiency.
Hence, in recent years, on engines with progressively lower NOX emissions, MAN Diesel has adopted a range of interacting primary measures targeting the combination of cooler combustion per se, making temperatures in the combustion chamber more homogeneous and cooling the combustion air entering the cylinder. To improve fuel air mixing the measures include improved re-entrant piston bowls; low swirl inlet ports; higher compression ratios; higher fuel injection pressures and improved injector nozzle spray patterns. Measures to reduce combustion temperatures comprise revised fuel injection timing, improved charge-air cooling and the combination of revised “Miller Cycle” valve timing and high efficiency, high pressure turbocharging.
Central to MAN Diesel’s latest, very substantial reductions in NOx combined with improved power output and fuel consumption are further improvements in the interaction of separate measures thanks to advanced electronic control, and a further intensification of the “Miller Cycle” made possible by the introduction of two-stage turbocharging giving pressure ratios up to 7:1.
The Miller Cycle involves the early closure of the inlet valve, causing the air entering the cylinder to expand and cool and hence reduce temperature peaks during combustion. However, the shorter inlet valve opening period would mean a lower mass of air combustion air entering the cylinder and hence reduced engine power and torque. To counter this effect, higher pressure turbocharging ensures that an equal – or in the case of MAN Diesel’s new technology package greater – amount of air can enter the cylinder in the shorter time available. During trials using an intensive Miller Cycle under full load conditions and turbocharger pressure ratios of 6.5 to 7, MAN Diesel has recorded reductions in NOx of over 30%, reductions in fuel consumption as great as 8% and a 15% increase in specific power output.
To cover a full spectrum of applications, MAN Diesel is developing two versions of the Miller Cycle engine based on differing technology packages. A version with fixed Miller valve timing, variable two-stage turbocharging and conventional fuel injection will cover baseload power generation applications. A variable Miller Cycle version with variable valve timing targets variable load (load following) applications, e.g. power supply to rolling mills or other variable load consumers or marine power generation or marine propulsion, and employs variable high pressure, two-stage turbocharging complemented by common rail fuel injection and variable inlet valve timing. The latter is necessary because at low loads, the shortened inlet valve timings in the intensive Miller Cycle can lead to increased smoke emissions.
The technology packages are based entirely on in-house developed technology at MAN Diesel. Unique among builders of medium speed engines, MAN Diesel has all the strategic elements of engine development in-house, including large exhaust turbochargers, fuel injection equipment, electronic hardware and software for control, monitoring and diagnosis of engines and driven equipment and has now developed its own variable valve timing system.
All these technologies come together in the variable Miller Cycle package, where essentially, MAN Diesel seeks to exploit the enhanced levels of control over diesel combustion possible using its advanced SaCoSone, microprocessor safety and control system to manage common rail (CR) fuel injection, two-stage high pressure turbocharging with variable turbine area (VTA) technology and variable valve timing (VVT).
In the high pressure two-stage turbocharging system, two turbochargers are arranged in tandem with an intermediate charge air cooler. The second, smaller turbocharger is fitted with the VTA control system to enable precise control of the charge air output. VTA denotes “variable turbine area” control, a system which achieves regulation of turbocharger compressor output by means of turbocharger nozzle rings equipped with adjustable vanes instead of the fixed-vane rings used in MAN Diesel’s standard TCA (axial) and TCR (radial) turbochargers. Adjusting of the vane pitch regulates the pressure of the exhaust gases impinging on the turbine to vary the output of the turbocharger compressor. This gives the capability to optimise charge air delivery at all points on the engines performance map, i.e. it allows the quantity of charge air to be more precisely matched to the quantity of injected fuel to the benefit of emissions, fuel consumption and engine response to load changes.
Giving a similar level of parameter setting flexibility on the fuel injection side is MAN Diesel’s common rail fuel injection system, which allows injection pressure, duration and timing to be freely selected at all engine speeds and loads.
Completing the package is MAN Diesel’s electronically controlled, in-house developed VVT variable valve timing system, employing cam followers on eccentric shafts to enable load dependent valve timing and hence vary the intensity of the Miller Cycle, particularly to suppress engine smoke at part load.
This combination of technologies - the variable Miller Cycle - has been tested with excellent results extensively on the inline, six cylinder version of MAN Diesel’s newest four-stroke engine, the all-electronic 32/44CR. As well as reductions in NOx emissions of over 30% and a maximum of 8% reduction in specific fuel oil consumption (SFOC), the greater availability of combustion air in absolute terms has also allowed a considerable increase in cylinder output from 560 to 640 kW on the 6L32/44CR test engine. In addition, in part load operation, the VVT system has enabled smoke emissions comparable with those of the standard 32/44CR engine with single stage turbocharging – i.e. well within the visibility boundary.
The favourable fuel consumption values are attributable to the increased air-fuel ratios above 20% rated output and the high efficiency levels of the two-stage charging system, MAN Diesel states.
Download pictures from our picture gallery