At the recent SAE 2010 World Congress, researchers from the Institute of Internal Combustion Engines and Automotive Engineering (IVK) University of Stuttgart and FKFS (Forschungsinstitut für Kraftfahrwesen und Fahrzeugmotoren Stuttgart) presented a paper describing the optimization of a natural-gas engine used in a parallel hybrid powertrain through the use of EGR and an oversized turbocharger with active waste gate (WG) control.
Since the end of 2006, Opel, Robert Bosch, IVK and FKFS have been working on a natural gas hybrid vehicle in a project supported by the German Federal Ministry of Economics and technology. The primary goal is developing a prototype of a CNG-fueled parallel hybrid based on an Opel Astra Caravan with less than 90 g CO2/km emissions.
In recent years two methods, namely downsizing and downspeeding, were established on the market to reduce CO2 emissions while maintaining the driving performance of engines with identical effective power but a larger displacement. Characteristic for both methods is the shift of engine operating points towards higher loads and therefore to areas of better brake specific fuel consumption.
Using a hybrid power train layout additional possibilities for turbocharged ICE are given to enhance the engine efficiency over its whole range of operation. Active-WG control is used for improvements at all engine speeds (< 5500 rpm). Further optimization of operating points at low engine speeds (< 4000 rpm) and loads (IMEP (< 13.5 bar) is achieved by applying high- or low-pressure exhaust-gas recirculation loops.
Operation at high loads (13.5 < IMEP < 22.5 bar) can be revised by the substitution of the turbocharger by an enlarged one. If packaging problems have to be solved, the application of a wider turbine-neck cross-section will also be possible. This keeps the turbine blades identical.
Basically the underlying principle of all described methods is the minimization of the pumping mean effective pressure (PMEP) in order to increase the engine’s efficiency.
—Boland et al.
In the project, the base 4-cylinder SI engine was replaced by a turbocharged 1.0-liter, 3-cylinder based on the Opel Ecotec Compact run on natural gas; a 25 kW electrical machine; and an additional clutch between the two.
By adding an exhaust gas turbocharger to the base 1.0L engine, the team boosted rated torque from 80 N·m in gasoline mode to 160 N·m in natural gas mode; rated power was boosted from 44 kW to 71 kW. The compression ratio was increased from 10.1 to 11.5.
To handle the increased mechanical and thermal loads, the team made a number of modifications to the engine, including the replacement of the cast pistons of the standard engine with pressed pistons which were geometrically modified to increase the compression ratio. Splash oil cooling was provided in the engine block to compensate for the increased introduction of heat into the piston crown, and to reduce NOx emissions.
The high specific engine output, the splash-oil cooling and the turbocharger generated a significant increase in the introduction of heat into the engine oil; an additional oil-to-water heat exchanger was installed for this reason.
The cylinder head largely conforms to the base engine, with the exception of asymmetrically designed intake ports to support the high EGR rate while simultaneously reducing engine-out NOx. One of the intake ports was designed as a swirl duct, the other as a fill duct. Compared with the standard cylinder head, it was possible to increase the swirl level in the combustion chamber.
Exhaust-gas turbocharging was selected to realize the highest levels of efficiency. The exhaust manifold and upstream catalytic converter was replaced by a manifold with a turbocharger flange. To utilize the exhaust-gas dynamics in a 3-cylinder engine, the cross sections (damping volumes) of the exhaust manifold were minimized. Boost pressure is controlled by a pulse-width-modulated activation of the overpressure actuated turbine bypass (waste gate).
Starting out from the original engine application (variant I) the carbon-dioxide emissions are lowered by 1.9% by the use of an oversize turbocharger (variant II). In combination with the Active-WG strategy (variant III) a further lowering by 1.0% is successful. The greatest reduction potential (4.5%) is provided by a combination of all the methods examined (variant IV). Here it was possible to lower the carbon-dioxide emissions below the requested 90 g/km in the NEDC.
—Boland et al.
Daniel Boland et al. (2010) Optimization of a CNG Driven SI Engine Within a Parallel Hybrid Power Train by Using EGR and an Oversized Turbocharger with Active-WG Control (SAE 2010-01-0820)