Conversion of Diesel Engines for CNG Fuel Operation

Abstract

The objective of converting a diesel engine to neat CNG operation is manifold. To achieve very low emissions without complex equipment, use alternative gaseous fuel available in countries which have only expensive access to liquid fuel and not the least emit relatively lower carbon dioxide. The lean burn combustion strategy has been the darling some extreme designers claiming improved efficiency and reduced nitric oxides. Though the improvements could be shown in the laboratory in practical conditions, the difference was either negligible or counterproductive. In addition, treatment of NO_x in lean conditions was a big challenge. Therefore, almost all the designers have switched to stoichiometric combustion. Transient control of stoichiometry is easier when port or manifold injection is practiced instead of carburation. Again, high voltage electronic ignition system with smart spark timing management, goes a long way in controlling combustion and emissions. Long life spark plugs of high heat rating with integrated ignition coil not only simplifies the design but also enhances the voltage at the spark plug by reducing losses. The engine modifications by redesigning the piston, cylinder head, exhaust valve system and manifolds, and introduction of gas and ignition system, cooling system and turbocharger are carried out meticulously. The vehicle is adapted for the CNG system consisting of the storage cylinders, high pressure tubes, pressure regulator and gas filter. The thermal efficiency and emissions of the engine is improved by reduced throttle losses and using a simplified EGR circuit. This chapter will provide the general guidelines for converting a diesel compression ignition engine to spark ignited compressed natural gas engine. An example is provided to convert a diesel engine to a state-of-the-art natural gas engine with computer controlled multipoint injection.

Appendix

A table below at a typical point of 146.5 kW at 2400 rpm demonstrates the success of the thermodynamic model for a turbocharged engine model using simple Vibe parameters derived from a similar naturally aspirated engine. At, Boost pressure = 1.61 bar, Lambda = 1, Bar Pressure = 750 mm of Hg

	Experimental	Simulation
Power (kW)	146	146
Torque (Nm)	582	582
BSFC (g/kWh)	207	205
Peak pressure (bar)	100	97
Air flow (kg/h)	525	514
Fuel flow (kg/h)	31	30
EGR flow (kg/h)	38	42
Exhaust temp before turbine (°C)	768	760
Exhaust temp after turbine (°C)	683	664
NOx (ppm)	1866	1851