|Consumption measurements of the dual-fuel natural gas-diesel engine as a function of speed and load. Source: Ott et al. Click to enlarge.|
Researchers at ETH Zürich have developed a light-duty hybrid electric vehicle featuring a dual-fuel natural gas-diesel engine. In an open access paper in the journal Energies, they report that CO2 emissions can be reduced to as low as 43g/km in a subcompact car configuration using the dual-fuel hybrid electric powertrain on the New European Driving Cycle (NEDC). The dual-fuel engine can also serve in a non-hybrid application.
In the dual-fuel engine, the natural gas (usually a fuel for spark-ignited engines) is injected into the intake manifold of a compression-ignition diesel engine. Instead of using a spark plug for ignition, the premixed air-gas mixture is ignited with a small amount of directly injected diesel fuel. This approach—already being successfully commercialized in heavy-duty engines by Westport Innovations (e.g., earlier post) and Clean Air Power (e.g., earlier post)—delivers the high performance and efficiency of diesel with lower CO2 and tailpipe emissions.
The team started with a standard 2.0L Volkswagen TDI diesel equipped with a common-rail diesel injection system, turbocharger with variable turbine geometry, and a high pressure exhaust gas recirculation system. Compression ratio is 16.5:1. They then installed a port-fuel injection system for gaseous fuels; a low-pressure exhaust gas recirculation system; and cylinder-pressure sensors in all four cylinders.
Combustion phasing and combustion noise in the dual-fuel engine are very sensitive to the start of injection; the start of combustion depends on the injection time and on the ignition delay of the diesel fuel. The ignition delay is mainly dependent on the chemical reaction kinetics of the diesel fuel. This process is very sensitive to small changes in pressure, temperature or cylinder charge composition.
To overcome this problem and to ensure a stable engine operation also during transients,the ETH Zürich team used feedback control; the start of injection and the duration of injection of the diesel fuel are controlled based on the measured cylinder pressure.
In the study, they compared a baseline diesel configuration; a dual-fuel natural gas-diesel engine configuration; and a full parallel hybrid configuration (20 kW traction motor) with a sufficiently high hybridization ratio to exploit the potential of hybridization.
Stationary engine measurements demonstrated that the natural gas-diesel engine reaches efficiencies as high as 39.5% without any need for lean NOx aftertreatment. NOx emissions are below 75 ppm for all operating points where the engine is operated lean. The soot concentration measured is below 3 mg/m3 for all operating points measured.
The gas ratio is low for low loads, while at higher loads, the gas ratio is increasing and reaches a peak value above 98%. At medium to high loads the engine is operated stoichiometrically, which enables the use of a three-way catalytic converter. The high efficiency in combination with a high gas ratio leads to CO2 reductions of up to 22% with respect to the base diesel engine.
Using hardware-in-the-loop experiments of a non-hybrid configuration on the the Worldwide harmonized Light vehicles Test Procedures (WLTP), they also demonstrated that transient operation of the dual-fuel natural gas-diesel engine is also possible.
Vehicle emulation experiments for hybrid-electric vehicles in combination with the dual-fuel natural gas-diesel engine show that very low levels of CO2 are achieved. Three vehicles (subcompact, compact, full-size) are investigated on two different driving cycles (NEDC, WLTP). The CO2 emissions vary from 43.0 g/km to 77.7 g/km. The low CO2 levels are due to the high efficiency of the engine and the high gas ratio. The average efficiency of the internal combustion engine varies from 34.9% to 36.4%. The energetic gas ratio lies between 89.6% and 94.1% for the vehicles and driving cycles investigated.
Vehicle emulation experiments for a non-hybrid vehicle equipped with a dual-fuel natural gas-Diesel engine show that transient operation of the engine is also possible. The natural gas-Diesel engine can thus also be used in a non-hybrid vehicle. However, the benefit of hybridization is especially large for the natural gas-Diesel engine. Hybridization drastically reduces the CO2 emissions of a vehicle with a natural gas-diesel engine by two effects: First it increases the average engine efficiency by avoiding engine operation at low load and low efficiency. Second it increases the gas ratio by avoiding low-load operation, where the gas ratio is low. The resulting CO2 emissions on the various driving cycles are compared with simulation results for the base Diesel engine. The CO2 reduction of the gas Diesel-engine is between 13.1% and 18.4% for the hybrid-electric vehicles and between 2.4% and 9.0% for the conventional vehicle.—Ott et al.
One of the issues with which the team is currently grappling is the temperature in the catalytic converter, which needs to reach at least 300 degrees.
At the moment, we are concentrating particularly on the temperature in the catalytic converter. Our combustion engine converts heat energy into mechanical energy with such efficiency that the exhaust gas is not warm enough to create sufficient heat, particularly after start-up.—Tobias Ott, lead author and doctoral student in Professor Lino Guzzella’s research group
The researchers want to solve the problem by modified control of the engine during the warm-up.
Co-author Christopher Onder believes that the natural gas-diesel engine can be produced in series production in five years if the team can find an industrial partner who can take charge of developing a prototype. The researchers believe that the success of their engine depends critically on its production costs. They stress that their solution may not be cheap, but it is comparatively cost-effective. Because their concept is based on technology that already exists, it can be implemented quickly and could function as a bridging technology for the next 10 to 20 years. The researchers say they are already engaged in negotiations with a car manufacturer.
The project was supported by the Competence Center Energy and Mobility (CCEM) and by the Swiss Federal Office of Energy.
Ott T, Onder C, Guzzella L (2013) Hybrid-Electric Vehicle with Natural Gas-Diesel Engine. Energies 6: 3571-3592, doi: 10.3390/en6073571