Approaches to Meeting Fluctuating Natural Gas Quality in Large Bore Engine Applications

Abstract

State-of-the-art large bore gas engines for power generation are traditionally designed for long-term operation at high load. They run at high power density and high engine efficiency, which is guaranteed as long as the engine boundary conditions, e.g., fuel gas quality, stay within certain limits. A growing share of gases from alternative sources such as biomass, hydrogen from power-to-gas technologies or LNG will be found in the pipeline grids of the future. This is further promoted by the harmonization process for gas qualities in Europe. The European standard EN 16726 as well as EASEE Gas Directive set rather wide limits for relative density, calorific value and methane number. For both engine manufacturers and operators, new challenges arise that have to be met by improving operating strategies.

Operating a stationary large engine with gaseous fuels – spark or direct ignited (diesel pilot) – leads to the problem of auto-ignition. The detection of knocking combustion is an important measure for the active prevention of engine damage. Different regimes during the combustion process result in a diversification of the cylinder pressure signal as well as the acceleration sensor signal. This requires different strategies for the distinct identification of knocking combustion. The methods shown are mainly based on the use of the cylinder pressure signal and focus on knock detection, detection of knock onset and evaluation of knock intensity.

If knocking combustion is detected, the engine control system has to react appropriately in order to ensure stable and safe engine operation. Here one can distinguish between approaches that adjust soft parameters of state-of-the-art engines (ignition timing or mixture quality), and approaches that need additional hardware and their functionality to be realized (variable intake valve timing or variable compression ratio). It is shown how these approaches to knock control impact engine performance.

Glossary

bp

Index for a bandpass filtered value

BFG

Blast furnace gas

BTF

Biomass to fuel

CR

Geometric compression ratio

DFKI

Dual fuel knock indicator

EAR

Excess air ratio

ETA_eff

Engine break efficiency

IT

Ignition timing

IVC

Intake valve closing angle

KRT

Knock related threshold

MAT

Manifold air temperature

MBF_50%

Crank angle at which 50% of fuel mass is burned

MN

Methane number

NG

Natural gas

P_RAIL

Rail pressure

SE

Signal energy

SOC

Start of combustion

SOI

Start of injection

TDC

Top dead center

T_knock

Temperature of the unburned zone where knocking occurs

T_unb

Temperature of the unburned zone

φ_Diesel

Energetic amount of diesel