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The closed-loop threeway catalyst on the European market has considerably reduced the emissions of passenger cars in an absolute sense. The question arose if gaseous fuels still have a real advantage. To answer this question an extensive programme was set up to compare gasoline and diesel with LPG and natural gas. Not only the regulated components, but also a whole range of unregulated components were measured. The resulting data were grouped into environmental aspects, such as direct local toxicity, longterm toxicity, smog, acidification and global warming. The conclusion is that gaseous fuels still hold an advantage over liquid fuels.

The lengthy experience of TNO with the application of gaseous fuels in engines is discussed. The emphasis is on emissions and efficiency of optimal gaseous fuelled engines in comparison to systems with partial diesel fuel replacement. In spark ignition operation (100% diesel fuel replacement) lean-burn and stoichiometric (electronic control and 3-way catalyst) concepts have been developed. In the optimization mathematical modelling of combustion and flow phenomena is used in combination with engine test bed work. Essential new hardware including micro-electronic control systems is developed. It has been demonstrated on test bed with CNG and LPG that the US HD Diesel Standards of 1994 can be met in the diesel transient cycle. Sufficient lifetime of the system has yet to be demonstrated however.