Audi T-CNG – A New Dimension in Driving with Natural Gas

There are many good reasons to use natural gas as a fuel for cars. Besides the very clean exhaust, the potential for reducing greenhouse gases is currently most prominent. The low carbon content in natural gas, which is principally made up of methane (CH4), give a CO2 advantage of around 20 percent compared to petrol.

The low carbon content in natural gas, which is principally made up of methane (CH4), give a CO2 advantage of around 20 percent compared to petrol.

An important factor in the environmental consideration is the option of refuelling with specially prepared biogas. This improves the balance of climate-relevant emissions even further. In Switzerland and Sweden, much of the network of CNG filling stations (compressed natural gas) is made up of biogas filling stations. In Germany and Austria, this regenerative fuel has started being fed into the natural gas network. From an environmental point of view, this is an interesting path, as biogas, like other second generation biological fuels, are produced using entire plants and agricultural waste. This means that biogas scores highly in the important criterion energy yield per hectare and in its environmental- and CO2 balance.

Because of the national desire to make other energy sources available in addition to mineral oil, further political support in the form of tax benefits can be expected for natural gas and biogas. Customers can benefit from this, for they can expect lower fuel costs compared to petrol or diesel vehicles. In Germany, the lowest taxation is on natural gas, and this has been fixed until 2018. This means that the effective fuel costs for a CNG-powered vehicle are less than 50 percent of those of a petrol car and around 30 percent less than those of a diesel.

Looked at today, there are still large gaps in the provision of natural gas – the filling station infrastructure is not yet adequate. That applies to Germany and to other markets that are relevant to CNG, for example Italy, Sweden and Switzerland. However, great increases in the density and quality of the network are expected over the next two to three years. This includes the building of around 150 new natural gas filling stations directly on the motorways in Germany. This is an important prerequisite for the success of CNG-powered vehicles.

The thin network of filling stations is a major problem as many vehicles that run on natural gas only have a relatively short range. Even when it is compressed under a pressure of 200 bar, the energy density of natural gas is less than that of petrol by a factor of 5. Yet an acceptable range is an important factor in the success of a CNG-powered vehicle. Low fuel costs alone are not enough for real commercial success.

Existing models – to date predominantly small delivery vehicles and MPVs – are able to house a relatively large tank due to their sheer size, so that they can achieve an acceptable range. However, their driving performance is usually anything but sporty. Other CNG-powered cars have had space and versatility compromised by the tanks fitted in the luggage compartment.

Audi customers expect more. They expect their vehicle to have a supreme performance without noticeable functional limitations in terms of space. And of course they expect a long range. For this reason, Audi is intensively working on implementing a concept that satisfies these demands. Providing the density of filling stations is sufficient, Audi is ready to put gas-powered vehicles into series production as soon as customers demand it.

The Audi A5 2.0 T-CNG, Audi's concept vehicle in the natural gas segment, makes no compromises and achieves important technological progress. New generations of lightweight pressure tanks and pressure regulators, plus the outstanding technical basis of the new TFSI engine generation and the package benefits of the new Audi A4 and A5 generation make it possible to realise a concept that meets the exacting demands of an Audi customer.

The gas tanks: 130 l volume for 21 kg
The four pressure tanks on board the A5 have a total volume of 130 litres. That is the equivalent of 21 kg of CNG. The cylindrical tanks incorporate state-of-the-art lightweight construction technology, using fibre-composite materials and aluminium. This enables the additional weight of the car (compared to a comparable petrol-engine vehicle) to be reduced to less than 90 kg – significantly less than current natural gas vehicles. With a range of 420 km running on CNG, the Audi A5 2.0 T-CNG offers a level that could previously only be achieved by a handful of MPVs.

The configuration of the Audi technology, with its front-wheel drive vehicles, creates space because it does not need a propeller shaft or rear axle differential. A larger middle silencer and an accordingly smaller rear silencer provide additional space in the rear underfloor area. The petrol tank has been reduced in size to 14 litres – still enough for a range of 180 km.

To ensure that the natural gas is used to great efficiency, the Audi A5 2.0 T-CNG is powered by a direct injection petrol engine with the latest TFSI technology. Based on the current EA888 series, its capacity has been increased from 1.8 to 2.0 litres. The bivalent four-cylinder engine has the same output in both fuel modes – petrol and gas: 120 kW/163 bhp.
Accelerating from 0 to 100 km/h in 8.9 seconds and with a top speed of 228 km/h, the Audi A5 2.0T-CNG is as sporty as you would expect from an Audi. Despite the low-rpm configuration of the sixth gear, it accelerates from 80 to 120 km/h in just 13.1 seconds in the elasticity test. Its full-load characteristic reaches its peak torque of 260 Nm at just 1500 rpm. This enables the engine to develop stately pulling power from very low revs. Its CO2 balance is also excellent –around 20% lower that the already highly efficient TFSI petrol engines, the T-CNG's figures really are exceptionally low.
Configuring the engine for natural gas operation meant a number of modifications. The engine management system was extended by additional functions for controlling the CNG injectors and for the changeover and safety functions. The intake manifolds were modified accordingly, and to ensure the engine retains its long service life, especially high-quality valve rings were used. The coating of the catalytic converter was also modified to take the different exhaust composition into account.

In contrast to many competitors, Audi also decided to make systematic use of the very high knock resistance of CNG fuel to improve the efficiency of the engine. Here, the 2.0 T-CNG was able to benefit from a typical FSI characteristic: the high compression ratio.

Competition CNG engines currently available on the market work in petrol mode with intake manifold injection. These engines can only achieve a compression ratio of about 9.1:1 in petrol mode, if it is turbocharged. However, a compression ratio of at least 12.1:1 is optimal for operation with high-octane natural gas. This difference ultimately leads to a compromise, with the potential efficiency in gas mode remaining unused. Such limitations are not encountered on the Audi concept. The engine is able to run with a compression ratio of 12:1. This means that greater efficiency petrol is also possible in petrol mode.

Peak torque in just 1.2 seconds
During the development of the 2.0 T-CNG, the response of the turbocharger posed a great challenge due to the need for very high charge pressure in natural gas mode. In conjunction with the configuration of the overall concept, the choice of a suitable turbocharger made a competitive, dynamic response possible. With complex fine tuning, it was able to achieve the high standard of the new 1.8 TFSI. The full-load jump to maximum torque is dynamic, completed in little more than a second. In everyday customer use, this means a spontaneous reaction from the engine to every movement of the throttle.

With the new Audi A5 2.0 T-CNG, Audi has demonstrated that it also possesses technical competence in the field of natural gas/biogas, and that it can set new standards with systematic concepts and innovative technologies. In a typically Audi manner, the A5 T-CNG combines emotional sportiness with great customer benefit in matters of efficiency and economy. Not least for these reasons did Audi choose the elegant form of the Audi A5 to realise the new concept.

Fuente: Fourtitude
Fecha: 24 de Julio 2008

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