Dual Fuel CNG-Diesel Heavy Duty Truck Engines with Optimum Speed Power Turbine

Abstract

The turbocharged direct injection lean burn Diesel engine is the most efficient engine now in production for transport applications with full load brake engine thermal efficiencies up to 40 to 45 % and reduced penalties in brake engine thermal efficiencies reducing the load by the quantity of fuel injected. The major downfalls of this engine are the carbon dioxide emissions, the depletion of fossil fuels using fossil Diesel, the energy security issues of using foreign fossil fuels in general, and finally the difficulty to meet future emission standards for soot, smoke, nitrogen oxides, carbon oxide and unburned hydrocarbons for the intrinsically “dirty” combustion of the fuel injected in liquid state and the lack of maturity the lean after treatment system. CNG is an alternative fuel with a better carbon to hydrogen ratio therefore permitting reduced carbon dioxide emissions. It is injected in gaseous form for a much cleaner combustion almost cancelling some of the emissions (even if unfortunately not all of them) of the Diesel and it permits a much better energy security within Australia. The paper discusses the best options currently available to convert Diesel engine platforms to CNG, with particular emphasis to the use of these CNG engines within Australia where the refuelling network is scarce. This option is determined in the dual fuel operation with a double injector design that couples a second CNG injector to the Diesel injector. This configuration permits the operation Diesel only and the operation Diesel pilot and CNG main depending on the availability of refuelling stations where the vehicle operates. Results of engine performance simulations are performed for a straight six cylinder 13 litres truck engine with a novel power turbine connected to the crankshaft through a constant variable transmission that may be by-passed when non helpful to increase the fuel economy of the vehicle or when damaging the performances of the after treatment system. The Diesel operation permits full load efficiencies of 45 % thanks to the power turbine arrangement. The Diesel-CNG operation permits slightly reduced full load efficiencies working at different air-to-fuel ratios with about same torque output as desired in the vehicle control.

F2012-A06-003