THERMODYNAMIC & SCAVENGING LAYOUT OF MODERN LOW-SPEED TWO-STROKE MARINE DIESEL ENGINE

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THERMODYNAMIC & SCAVENGING LAYOUT

The optimisation of the engine efficiency has always been one of the most important targets of the internal combustion engine developer. Thus, the engine development starts with the thermodynamic layout.

THERMODYNAMIC & SCAVENGING LAYOUT PROCESS

The thermodynamic & scavenging layout determines fundamental dimensions of engine such as the shape of the combustion chamber, size & location of the inlet ports, turbocharger selection, cooler sizing, valve and injection timing, etc. The most important steps in this development are described below:

  • Firstly, a semi-empirical, basic layout takes place based on assumed inlet port and exhaust valve dimensions, firing pressure, scavenge pressure and valve timing. These parameters are often taken from the engines with similar stroke-to bore ratios
  • In parallel, derating strategies are developed by applying state of the art tuning methodologies
  • A zero-dimensional process calculation is then made for the complete rating field, which gives as output the first approximation of the cylinder pressure, exhaust flows, temperatures, injection & valve timing, etc. This is used as input for layout of powertrain, hot parts, fuel injection & hydraulics, scavenge system.
  • In an iterative process, with the above as input, the first dimensioning of the scavenge system can be made, which is in turn used as an input for the first engine outlines and hot parts. The combustion chamber is thence drafted in CAD
  • Subsequently, CFD simulations of the scavenging and combustion processes are carried out for a single-cylinder geometry. The injection parameters and injector geometry are optimized and an atomizer pre-selection is made in order to keep the combustion chamber temperatures within the required limits and optimize the SFC/NOx trade-off.
  • The Turbocharger(TC) assignment, auxiliary blower selection & Scavenge Air Cooler (SAC) calculations are done & optimised in an iterative process in parallel with the scavenge system design
  • The latter is subsequently checked with a stress calculation, taking into account the temperature, pressure and weight of the parts
  • After the design is finalised, the tuning is approved on the prototype test engine where appropriate pressure & temperature measurements are made. These measurements are used to check the process calculation and ensure that the boundary conditions have been correct.

The process simulation tools used throughout this process have been developed & maintained over decades & are specifically optimised for the application on large marine 2-stroke engines.

CONCEPT, DESIGN AND COMPONENT CHOICES

Throughout the thermodynamic & scavenging layout, the engine designer has several concept, design and component choices to make, some of the significant ones are listed below:

  • Maximum firing pressure: Normally higher firing pressures lead to higher efficiency at the expense of higher NOx emissions & higher loading on crosshead & crank pin bearings, combustion chamber parts & engine structure
  • Compression ratio: higher compression ratio leads to increased thermal efficiency, limited by heat and scavenging losses, tribology aspects and increasing NOx emissions due to higher combustion temperatures
  • TC-selection: The required volume for the flow of the TC is given by the scavenging demand, whereas the desired scavenging pressure results from the desired firing pressure, design criteria for pressure gradients and the given compression ratio. A range of TCs is thus chosen to fulfil these requirements & optimize efficiency in different parts of the rating field
  • SAC layout: The SAC is laid out for efficient cooling in order to minimise pressure losses and increase overall engine efficiency while minimising the volume for cost and space reasons. The scavenging air temperature is limited by the temperature of the available cooling fluids
  • Inlet port position & height: A higher port height leads to higher scavenging efficiency but to lower effective compression ratio (need to open exhaust valve earlier) and hence reduction of engine work
  • Exhaust valve diameter: A larger valve improves the scavenging process whereas a smaller diameter reduces the hydraulic forces to open it and the cost of the expensive valve stem
  • Exhaust duct angle: Normally as small(parallel to liner) as possible to decrease scavenging back pressure, albeit a trade-off exists with overall engine height & position/stability of exhaust manifold
  • Exhaust valve and injection timing: The exhaust valve and injection timing is used to control the combustion phasing and to adjust the effective compression ratio. Electronic engines provide the possibility of Miller timing & sequential injection to optimize efficiency & control emissions