Marine diesel engines run on the diesel cycle. With the introduction of natural gas as a marine fuel, Otto cycle is also employed in some engines. Explain the difference between the two cycles and elaborate on the suitability of natural gas as a fuel in such engines. (16)
Structured for DG Shipping MEO Class II examination scoring criteria.
The Otto cycle achieves combustion at approximately constant volume, while the diesel cycle achieves combustion at approximately constant pressure. In the ideal air-standard Otto cycle, the working fluid (air) is compressed adiabatically, heat is added at constant volume (representing a very fast burn at or very near TDC), the gas expands adiabatically and heat is rejected at constant volume. Because heat addition is considered instantaneous at TDC, the peak pressure is high but the process is approximated as constant volume. The Otto engine is a spark-ignition (SI) or pre-mixed-charge engine: a homogeneous air-fuel mixture is compressed and then ignited by a spark or by a small pilot (in lean-burn gas engines). The compression ratio is limited by knock (about 8-14) because the pre-mixed charge self-ignites.
The diesel cycle adds heat at constant pressure: air is compressed adiabatically to a high compression ratio, fuel is injected and burns progressively at approximately constant pressure as the piston descends, then adiabatic expansion. It is compression-ignition (CI): only air is compressed, fuel is injected near TDC and self-ignites. The compression ratio is high (12-25) because there is no pre-mixed charge to knock, so thermal efficiency is higher.
In the real engines both approach the dual (limited pressure) cycle where heat addition is partly at constant volume and partly at constant pressure.
Efficiency comparison at the same compression ratio: Otto is more efficient; but the marine diesel achieves a much higher compression ratio, hence much higher overall efficiency.
Natural gas (methane, CH4) has a high octane rating (resistance to knock), making it very suitable for the Otto (SI/lean-burn gas) cycle, where a lean homogeneous methane-air mixture is compressed and ignited by a small pilot diesel (about 2-5% of energy) or a spark. Because it is a lean premixed charge, combustion temperatures and NOx are low, and the fuel burns cleanly with very low particulates/smoke. Natural gas is now used in dual-fuel and dedicated gas engines (e.g. LNG-fuelled engines on LNG carriers, gas-electric vessels) using the Otto cycle in gas mode.
However, natural gas is less suitable for a pure diesel (constant pressure, or conventional CI) cycle because methane has very high auto-ignition temperature and poor ignitability (low cetane number), so it would not self-ignite reliably under compression; hence gas engines use the Otto cycle (pilot ignited) rather than pure diesel combustion. Challenges for gas: low energy density (stored as LNG at -163 deg C in cryogenic tanks, or compressed), methane slip (unburned methane emitted), methane's high global warming potential, the need for gas handling/safety systems, and knock control. Benefits: much lower SOx (essentially zero sulphur), lower NOx (lean burn), lower CO2 (per unit energy ~20-25% lower than diesel), lower particulates, lower operating cost where gas is cheap.
So diesel (CI) engines use the diesel/dual cycle; gas (SI) engines use the Otto cycle - the choice is dictated by the fuels' ignition and knocking properties.