I just came across this info which is interesting to say the least:
"Toyota started development of CNG powered vehicles back in 1984 and now has a range of commercial vehicles which use CNG. Toyota has also just released a CNG-powered, four-cylinder Carnry for limited volume sales to fleets in the USA with volumes planned to increase over the next two years based on continued research and development into the viability of CNG. "
5S-FNE ENGINE
The 5S-FNE engine is a new engine that uses CNG (Compressed Natural Gas) as fuel, which has been developed based on the 5S-FE gasoline engine. The main component of natural gas is methane (CH4), which has fewer carbous (C) than gasoline (mean molecular formula: C7.5H13.4). Therefore, the amount of CO2 discharged by this engine is approximately 70% that of the gasoline engine. Furthermore, this engine achieves low-emission operation by precisely controlled air-fuel ratio and using special catalysts for the CNG application. However, because the fuel is in the gaseous form, its volumetric efficiency is low, causing lower power output if this fuel is used in the base engine.
Therefore, the 5S-FNE engine has adopted the following features: a high compression ratio, the intake valves with early closed timing, the intake and exhaust valves with increased lift, a low back pressure muffler made possible by the 2-way exhaust system, injectors for gaseous fuel, and a catalytic converter designed exclusively for CNG application. At the same time, precision air-fuel ratio control is effected through the use of the air-fuel ratio sensor* and the heater oxygen sensor, thus achieving the level of performance that is equivalent to the base engine. Moreover, this engine realizes extremely low emissions and restoring engine torque."
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