The typical natural gas engine is essentially a modified gasoline engine with slightly different ignition mapping to account for natural gas’ lower energy content.
Natural gas engine conversions are fairly commonplace in places where natural gas is abundant [and cheap!], but efficiency often suffers due to poor programming. For example, in Perú, where the average price of GNV [Gas Naturál Vehicular] is the equivalent to $1.40/gal, it makes much more economic sense than sticking with 90-Octane Gasoline, averaging $4.54/gal.
Unfortunately, since Perú has lax [nonexistent?] emissions laws, I would estimate that most of the converted vehicles are running with the check-engine light on, a sure indicator of poor [nonexistent?] programming of the engine to run on natural gas, as well as very poor efficiency. Vehicles specifically designed for multiple fuels, such as Ford Focus FFV or Audi Sportback G-Tron, require extra sensors and more complex programming to be able to switch between fuels.
Natural gas engines that are specifically designed for natural gas, such as the Mercedes Benz B200 CNG, don’t require programming as complicated, but still require a spark, just like a typical gasoline engine. Researchers at the Institute for Dynamic Systems and Control, at ETH [Eidgenössische Technische Hochschule] Zurich, Switzerland, found their most success in converting a typical Volkswagen diesel engine to run natural gas.
The new natural gas engine, instead of using a spark to ignite the fuel, as in gasoline engines, uses a small amount of diesel fuel to ignite, on the heat of compression, the diesel and the natural gas fuel mixture. The fuel mixture is just 10% diesel, the other 90% being compressed natural gas.
The approach isn’t new, having been in use in stationary applications for years, such as in generators and stationary machinery, but vehicle demands on an engine are highly variable in speed and power output. ETH Zurich researcher Tobias Ott explains, “In a vehicle, the engine speed and load change constantly, which means the engine system is far more complicated.”
Once researchers were able to get the engine programmed properly to run on a 90% natural gas mixture, they were able to achieve an efficiency of 39.6% [Today’s best engines are less than 25% efficient.] Interestingly, because the new natural gas engine is so efficient, that is, it doesn’t waste that much heat, it doesn’t put out enough heat in the exhaust to run the catalytic converter. The catalytic converter only starts functioning once it hits about 300°C [572°F].
Once they figure out how to heat up the catalytic converter and finalize programming, the ETH Zurich natural gas engine could see production within a few years.
Image © ETH Zurich