Someone wanting to buy an environmentally friendly car is faced with conflicting information. Apart from information on the number one, the Toyota Prius–a hybrid car–the advice contained on the new Energy Label from the Federal Office of Energy and that on the VCS Car Environmental List is contradictory. On the Energy Label diesel engines turns up trumps; on the VCS Car Environmental List, it's exclusively petrol engines. The reason of the contradiction are the different assessment criteria. While the VCS rates consumption, pollutant and noise emission, the Energy Label looks at CO2 emission. Today's diesel engines emit around 15 per cent less CO2 than comparable petrol engines. This explains the good marks achieved by diesel-driven vehicles on the Energy Label. This is why the car industry and some politicians want to promote cars driven on diesel.

Strict regulation hurting diesel

Konstantinos Boulouchos, Professor at the Institute of Energy Technology at ETH Zurich and head of the Aerothermochemistry and Combustion Systems Laboratory, is skeptical as far as the political establishment's promotion of diesel is concerned. "The increasingly strict regulations on polluting emissions are becoming a problem for diesel engines," says Boulouchos. True, the relatively high levels of nitrogen oxides and particle emission can be reduced using appropriate technology. But allied to this, CO2 emission increased by a few per cent. Boulouchos does the sums: "The particle filter increases consumption by two per cent and dealing with nitrogen oxides adds up to a further five per cent". This lowers the CO2 advantage of diesel engines by five to ten per cent. Nonetheless, Europe's motor industries are banking on diesel. The EU has reached an agreement with car producers to reduce average emission of CO2 by 140 grams per kilometer by 2008/09 (1).

Natural gas massively reduces emissions

Whether the calculations of the motor industry with regard to diesel-driven vehicles will eventually add up is still an open question. Because experts start from the principle that progress will be made in the coming years above all on petrol engines, especially owing to improved injection technology and variable valve timing. Konstantinos Boulouchos counts on a reduction of CO2 emission by seven to eight per cent–even without hybrid technology–for petrol engines. This would bring them down to the level of diesel engines. Notwithstanding, Boulouchos considers it would be worthwhile to invest in the development of cars powered by natural gas. The results of the CEV (Clean Engine Vehicle), a joint project from Empa and ETH Zurich, are impressive. Researchers have been able to show that extremely low levels of pollution and CO2 emissions can be achieved with such engines. Polluting particles were lower than the strict Californian limits for Super-Ultra-Low-Emissions-Vehicles (SULEV) and CO2 emissions were 30 per cent lower.

Potential not exhausted

For Christian Bach, head of the Department of Combustion Engines at Empa in Dübendorf, gas engines not only have the potential to emit lower levels of CO2 but also to become very clean at relatively low costs. At present this potential was not exhausted because the engine technology and catalytic converters used today were actually developed for petrol engines. Bach: "The exhaust fumes of a gas engine are different from those of a petrol engine. This is why the gas engine needs a differently constructed catalytic converter".

Konstantinos Boulouchos, Professor at the Institute of Energy Technology at ETH Zurich and head of the Aerothermochemistry and Combustion Systems Laboratory, is skeptical with regard to the political establishment's championing of diesel engines (Picture: L. Denzler).

Suitable catalytic converters were still in the early stages of development; for this Empa is collaborating with VW. The advantage of a gas engine is that it does not produce very high cold-start emissions. With petrol engines, on the other hand, a considerable proportion of the emissions ensue during the first few minutes of the ride because the catalytic converter is still cold and not yet working to optimum capacity.

Downsizing the engines

A reduction of CO2 emissions by 30 per cent has been achieved by increasing the efficiency of the engine. Thanks to turbo charging, and a boost-control process developed at the ETH Institute of Automatic Control and System Dynamics, it had become possible to shrink the cylinder capacity without loss of efficiency. The smaller engine is thus as powerful as the original but uses less fuel.

Engineers call this downsizing. According to Christian Bach, the savings potential for petrol engines lies somewhere around five per cent; for gas engines–because they are particularly suited to downsizing–emissions could be reduced by ten per cent. The objective of a new Empa/ETH project, to be launched in summer, is to optimise the gas engine even further and reduce its CO2 emissions by 40 per cent compared to a petrol-driven engine. The basic idea is to build a small electro engine as a complement to the gas engine in order to ensure enough power when the engine is started or pushed to its limits. This would mean that the gas engine could be slimmed down yet again and fuel consumption further reduced. The team of researchers will be working closely with the R&D departments of Volkswagen and Bosch.

Bi-Fuel cars as a provisional solution

Gas driven vehicles could play an important part in climate protection and cleaner air. The EU Commission aims to achieve a proportion of 10 per cent by 2020. This goal makes sense to Christian Bach. In Switzerland he foresees their use above all in cities and their agglomerations, some of which suffer from high levels of pollution and, moreover, already have access to the gas grid. Until such time as garages and petrol pump networks are installed and optimised gas-driven vehicles arrive on the market, so-called bi-fuel cars are a sensible interim solution. Such bi-fuel vehicles would run on either gas or petrol. However, the disadvantage, according to Bach, is that bi-fuel versions would not get the full benefit of the advantages of a gas engine.

The optimal use of a range of various energy sources in transportation does not only depend on the most efficient propulsion technologies. Attention must also be paid to the production of the fuel used to power a vehicle, both with regard to its transport and losses. For example, should natural gas, which is composed mainly of methane, end up in the atmosphere instead of in the fuel tank, this would have serious consequences because the greenhouse effect of methane is even greater than that of CO2.