Clean Engine Vehicle

The Clean Engine Vehicle project consisted in the conversion of an actual gasoline-powered engine to dedicated natural gas operation. This included the optimization of the compression ratio and the development and realization of a suitable catalyst converter and downsizing concept. The aim of the project was to reduce CO2 emissions in the New European Driving Cycle (NEDC) by 30% compared with a gasoline vehicle of similar performance. Additionally, the vehicle had to comply with the Euro-4 and SULEV emission standards (without deterioration factor).

Basic Vehicle and Test Setup

The basic project vehicle was a Volkswagen Polo (MY2000) with a curb weight of 1,020 kg and a naturally aspirated 4 cylinder, 1.0 liter, 2 valve gasoline engine with 37 kW at 5,000 rpm, and 86 Nm at 3,000 rpm. The original compression ratio was 10.7. The engine was equipped with manifold port injection (MPI), exhaust gas
recirculation (EGR), and a pre- and main catalytic converter. The vehicle was certified according to Euro-4.
The project included investigations on the engine and the chassis test bench. Because of the extremely low emission level, the dilution air of the CVS system was cleaned by additional air filters, dried by cooling down to 0°C for the elimination of water vapor and heated up to 23°C. In addition, the whole raw gas line until the mixing point in the CVS system was heated to 100°C to prevent the water from condensing. The response factor for CH4 of the propane-calibrated T.HC analyzer (FID) was 1.04. The CH4 emissions were measured with a methane calibrated GC-FID.

Increase in Compression Ratio

An increase in the compression ratio was implemented because of the higher octane number of natural gas. This results in a higher maximal compression pressure and temperature as well as because of the greater expansion at a lower exhaust gas temperature. The overall result is increased thermal efficiency or increased engine performance.
In a first step, the influence of the EGR rate, the ignition point, the engine load and speed on combustion was investigated. This was done by combustion, fuel consumption and exhaust gas analysis at some relevant operating points.
The following figure shows the simulated potential for the reduction in the brake-specific fuel consumption (bsfc) for constant engine speed at 2,000 rpm and a constant brake mean effective pressure (bmep) of 2 bar by varying the compression ratio and the position of combustion.

The simulation predicts the best brake-specific fuel consumption at a compression ratio of 13.5. At higher compression ratios, the pumping losses outweigh the advantages of the increased compression ratio. The  increase in the compression ratio at constant bmep shows a potential of about 2-3% compared to the basic compression ratio of 10.7 at a combustion center of 8° after TDC.
However, the highest possible compression ratio of the engine was defined by mechanical limitations at TDC.
The complete elimination of the combustion-chamber bowl resulted coincidentally in the targeted compression ratio of 13.5.

Downsizing

Using smaller but supercharged engines can improve the fuel economy of vehicles. Dedicated natural gas engines are much more suitable for supercharging than gasoline engines because of the high octane number of natural gas.
Because of the lower engine charge as a consequence of the injection of the fuel in gaseous form, a naturally aspirated natural gas engine shows a power and torque reduction of about 10 – 15 %. The basic 1.0 l gasoline engine in the vehicle running at 80 km/h in 5th gear had a torque reserve of 58 Nm. This value drops to 49 Nm when the same engine (with the basic compression ratio) is driven with natural gas.
In the project, it was possible to compensate the power losses by supercharging the 1.0 l engine so that it achieved the performance of a comparable 1.4 l engine. In this way, the torque reserve could actually be boosted beyond the original value. Simulations indicated that a torque reserve of about 80 Nm can be achieved at 80 km/h in 5th gear.

Engine Control System

The engine was equipped with a Bosch ME7 engine control system. This system transforms the position of the accelerator pedal to a desired value for the engine torque. All necessary engine parameters to achieve the desired torque are calculated by the ECU and adjusted accordingly. Transient procedures such as the intake manifold pressure behavior are simulated in models.
This allows controlled transient operations of the engine to be performed. This system is able to ensure comparably low engine-out exhaust emissions, especially in dynamic operation.
The engine control system was modified by Robert Bosch GmbH for dedicated natural gas operation and extended for the turbocharging with a boost controller.
Therefore, additional functions for the gas supply and injection had to be included and some gasoline-specific functions such as the evaporation control system or the gasoline wall wetting model could be removed.
After the realization of the final powertrain setup, the engine control system had to be applied on the engine and chassis test bench with regard to exhaust emissions, fuel consumption, engine performance and drivability.

Catalytic Converters

The original vehicle in gasoline operation was equipped with a metallic catalytic converter close to the engine (pre-catalytic converter) and an underfloor main catalytic converter. The original pre-catalytic converter was replaced in the project by a new one with a slightly higher volume, a higher cell density with lower wall thickness and a higher precious metal loading.
Because of the stable molecule structure of methane, the temperature of the catalytic converter is even more important for natural gas than for gasoline operation.
This two main differences (always λ=1, temperature of the catalytic converter) between gasoline and natural gas engines leads to other requirements for catalytic converters for natural gas engines, mainly regarding precious metal loading, oxygen storage capacity and placement.
The concept used in the Clean Engine Vehicle project consisted of a new pre-catalytic converter with a higher precious metal loading and the standard main catalyst.

Final Results

A CO2-reduction of 31%, compared to a gasoline vehicle with similar performance could be demonstrated in the European driving cycle.

The main CO2 advantage of about 20 % is given by the conversion to natural gas operation. The increase in the compression ratio and the realization of the downsizing concept resulted in a further CO2 reduction of about 10%.
The following tables show the final CO, T.HC, NMHC and NOx emissions compared with those of the gasoline engine in the NEDC and FTP-75 cycle.

Conclusion

Natural gas as a fuel has the potential to reduce environmental impact significantly and to increase powertrain efficiency, if appropriate technology is used.
Enhanced natural gas technology does not show the typical conflict of aims between CO2 emissions and pollutants as many gasoline and diesel technologies do.
The technologies needed for clean natural gas vehicles are available on the market. Similar CO2 emission and pollutant levels are achievable with gasoline and diesel vehicles, but only at much higher costs.
The emission reduction of natural gas vehicles compared with gasoline and diesel vehicles is even higher in real-world cycles than in the official cycles, because natural gas engines are always operated at a stoichiometric air to fuel ratio and the load in real world cycles is typically higher, resulting in higher catalyst temperatures.
While the SULEV regulations consider the different impact of the hydrocarbons regarding their ozone forming potential, the European regulations for passenger cars do not distinguish. A separation of CH4 and NMHC in the European passenger car regulations, as is already done for heavy duty applications, would simplify the development of natural gas vehicles without a significant ecological disadvantage.
The durability of the catalytic converter was not part of the current project, but will be investigated in a subsequent project.