Audi extended its range of natural-gas-fueled g-tron models just a few weeks ago (earlier post) with the new A4 Avant g-tron and A5 Sportback g-tron. The A3 Sportback g-tron has been on the market since 2014.
The new A4 Avant g-tron and A5 Sportback g-tron are powered by a 2.0 TFSI engine operating according to the efficient, Audi-optimized B-cycle (Budack-cycle, earlier post) combustion principle. Whereas the traditional Miller cycle closes the intake valves just before the end of the intake stroke, the B-cycle closes the intake valves much earlier. This results in longer effective combustion as well as faster air flow for the incoming gases, which improves the mixing of the fuel and air.
Overall, the engineers used 29 new components and materials, and adjusted the power unit so that it also runs optimally on gas. The pistons and valves have been specially modified and allow for an optimal compression ratio.
The turbo engine produces 125 kW (170 hp). Its maximum torque of 270 N·m (199.1 lb-ft) is available at 1,650 rpm. An electronic controller reduces the high pressure of the gas flowing from the tank from as much as 200 bar to a working pressure of 5 to 10 bar in the engine. This operation is performed dynamically and precisely in response to the power requested by the driver.
The correct pressure is always present in the gas line and at the injector valves – low pressure for efficient driving in the lower speed range, and higher pressure for more power and torque.
The engine was tested in extreme temperature conditions and had to withstand minus 35 degrees in Scandinavia. Additional features were developed to protect the components required for CNG operation from cold temperatures. These prevent the gas valves from freezing up, for example.
To ensure smooth running in high summertime temperatures, the developers made modifications especially to components for the gasoline mode. For example, they came up with a stainless steel valve to protect the high-pressure injection valve for gasoline. The component may heat up to as much as 400 ˚C as a result of the CNG combustion process.
In the NEDC, the Audi A4 Avant g-tron with optional S tronic consumes just 3.8 kilograms of CNG per 100 kilometers, corresponding to CO2 emissions of 102 grams per kilometer (164.2 g/mi). In gasoline mode, the figures are 5.5 liters per 100 kilometers (42.8 mpg US) and 126 grams of CO2 per kilometer (202.8 g/mi).
The figures for the A5 Sportback g-tron with S tronic are identical in CNG mode. When running on gasoline, it consumes 5.6 liters per 100 kilometers (42.0 mpg US) and emits 126 grams of CO2 per kilometer (202.8 g/mi). Both models accelerate from 0 to 100 km/h (62.1 mph) in 8.4 seconds. The A4 Avant g-tron reaches a top speed of 221 km/h (137.3 mph), the A5 Sportback g-tron 224 km/h (139.2 mph).
The overall range of the new midsize models is 950 kilometers (590.3 mi)—including up to 500 kilometers (310.7 mi) on CNG alone. This fuel is stored in four tanks that are distributed space-efficiently through the rearward structure. There is also a gasoline tank with a capacity of 25 liters (6.6 US gal). Fuel costs compared with an equivalent gasoline engine are much lower, at around €4 per 100 kilometers (62.1 mi) (Germany, as at: September 2017). The lower CO2 emissions also mean owners pay less in motor vehicle tax.
e-gas. The g-tron models are especially eco-friendly when running on Audi e-gas. Compared to a gasoline-powered car in the same performance class, their CO2 emissions are 80% lower. That means no extra costs for the customer: As the standard offering, Audi will supply the green fuel Audi e-gas for three years for all g-tron customers who place an order by May 31, 2018.
Customers can fill up their g-tron model at any CNG filling station and pay the regular price. By feeding the computed volume of Audi e-gas into the natural gas grid, Audi works behind the scenes to ensure the green benefits of the program, including the corresponding reduction in CO2 emissions.
Audi produces the synthetic e-gas using renewable energy from water and CO2 or from organic residual materials such as straw and plant clippings. This process takes place in its own power-to-gas facility in Werlte in Lower Saxony (Emsland), among other venues. During its production, Audi e-gas binds exactly the amount of CO2 emitted by the car.
In the CNG mode, the clean drive in the g-tron models already undercuts the limit values of future emission standards. Independent institutes have confirmed that. For us, g-tron with e-gas is more than merely a bridging technology, it is a fixed element in our future strategy on alternative drives.—Peter Mertens, Member of the Board of Management of Audi AG for Technical Development