Improving the Efficiency of LPG Compression Ignition Engines for Passenger Cars through Waste Heat Recovery

Abstract

The turbocharged direct injection lean burn Diesel engine is the most efficient now in production for transport applications with full load brake efficiencies up to 40 to 45% and reduced penalties in brake efficiencies reducing the load by the quantity of fuel injected. The secrets of this engine's performances are the high compression ratio and the lean bulk combustion mostly diffusion controlled in addition to the partial recovery of the exhaust energy to boost the charging efficiency. The major downfalls of this engine are the carbon dioxide emissions and 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 combustion of the fuel injected in liquid state and the lack of maturity the lean after treatment system. LPG is an alternative fuel with a better carbon to hydrogen ratio therefore permitting reduced carbon dioxide emissions. It flashes immediately to gaseous form even if injected in liquid state 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 presents a passenger car diesel engines converted to LPG. In this engine the efficiency is then improved recovering the waste heat. This recovery has impacts on both the in cylinder fuel conversion efficiency and the efficiency of the after treatment. Results of engine performance simulations are performed for a in-line four cylinder 1.6 litres LPG CI passenger car engine with a power turbine following the turbine of the turbocharger or an heat exchanger to recover the exhaust (and other) waste heat and compared with the experimental results for the diesel without waste heat recovery