CNG and Downsizing—Over 30 percent lower CO2 emissions

The combination of downsizing and running on compressed natural gas (CNG) provides the potential to drastically reduce CO2 emissions. MAHLE has analyzed the potential and associated challenges using a reconfigured downsizing engine both with engine testing and in a test vehicle. With an optimized engine design, CO2 emissions were up to 31 percent lower.

CNG-powered vehicles from various manufacturers have been on the market for many years now. Their engines are originally designed to run on conventional fuel and therefore utilize the majority of the CO2 potential purely by substituting fuel—which can improve emissions by up to 24 percent. Further optimization with CNG-specific development is faced with the still relatively low number of CNG vehicles produced.

MAHLE has now investigated the potential for further reducing CO2 emissions with a combination of downsizing and CNG. In a first step, the well-known MAHLE downsizing engine— a three-cylinder, single-stage turbocharged gasoline engine with direct injection and 1.2 L displacement—has been adapted for dual operation with natural gas and gasoline. To this end, the compression ratio was increased to 12.5:1 and an additional CNG infeed was implemented in the intake manifold. The engine was then tested and applied on the engine test bench, and integrated into a test vehicle and measured. The goal of the first step was to demonstrate the potential and limits of the bivalent variant, determine potential optimization focus, and identify the associated challenges. The results form the basis for the second step, in which an optimized, monovalent design and the application of the necessary engine systems and components will be implemented.

The lower susceptibility of CNG to knocking in comparison with gasoline is fundamentally favorable for the downsizing approach: an earlier ignition point is possible, as is an increase in the compression ratio. The pressure and temperature loads on engine components, however, also increase. In order to fulfill peak performance requirements, the components must be adapted to the changed conditions and the necessary boundary conditions must be precisely understood. In order to ensure that the engine components can meet the extreme requirements for pressure resistance and thermal diffusity, MAHLE has planned to use the following technologies for the monovalent variant:

  • Pistons: the EVOTEC® RSC combines high stability and optimal cooling due to its cast-in ring carrier and cooling channel.
  • Ring pack: optimized for friction losses and wear, with three-piece oil control ring
  • Connecting rod: keystone connecting rod with pressed-in bushing for better dissipation of the higher surface pressure
  • Valves: sodium-cooled TopTherm® valves on the intake and exhaust sides for increased temperature resistance, with a reinforced bottom plate
  • Piston pins: pins with DLC coating
  • Bearings: polymer-coated conrod and main bearings

The results of MAHLE engine tests show three operating ranges in which the performance of the CNG downsizing engine is currently limited and can be significantly improved with targeted measures.

In the low speed range, the engine has performance issues in natural gas mode in comparison with modern gasoline engines with direct injection. The reason is that when running on natural gas with intake pipe infeed, a purging combustion process (“scavenging”)—i.e., a partial overlap of the opening times of the intake and exhaust valves in order to improve the responsiveness of the turbocharger—is not possible. Natural gas also displaces part of the incoming air mass and thus reduces cylinder charging. This deficit can be offset, for example, by using a turbocharger with variable turbine geometry in conjunction with CNG intake pipe infeed. Alternatively, direct CNG infeed can be used in combination with an optimized wastegate turbocharger to improve performance in the speed range that is crucial in real driving operation. MAHLE will apply and evaluate both variants in the test vehicle.

The middle speed range is primarily dominated by the limit on peak cylinder pressure and thus by the limits of component specifications. Starting at 125 bar, peak pressures of up to 185 bar must be withstood, which is possible using the listed MAHLE engine components.

The performance issues in the top speed range are primarily attributable to the thermal loads on injector gaskets for direct injection and on cylinder components. If the engine is made exclusively monovalent—i.e., running only on CNG with no gasoline direct injection—and the engine components are optimized accordingly, then the operating range relevant to rated power can be increased significantly.

Calculations and simulations indicate that the required peak pressure resistance of 185 bar and higher temperature limits are possible with improvements to the basic engine listed above. This also means that specific power output of 110 kW/L and nominal torque of 270 Nm (above 1,600 rpm) are possible in monovalent CNG operation with the MAHLE downsizing engine. A family van with a curb weight of 1,700 kg can thus achieve CO2 emissions of about 106 g/km, putting it nearly 31 percent below the current gasoline variant with comparable driving performance.

  • The well-known MAHLE downsizing engine has been adapted for dual operation with natural gas and gasoline. The engine was then tested and applied on the engine test bench, and integrated into a test vehicle and measured.
  • These results form the basis for the next step, wherein an optimized, monovalent design is implemented with optimized engine systems and components.
  • Calculations and simulations have already shown that the required peak pressure resistance of 185 bar and higher temperature limits are possible with improvements to the basic engine.
  • For the downsizing engine, a specific output of 110 kilowatts per liter and nominal torque of 270 Newton meters are possible in monovalent CNG operation.
  • With an optimized engine design, CO2 emissions in the test vehicle are up to 31 percent lower.