Study explores impact of alcohol-gasoline blends with early inlet valve closing at low and moderate loads on EGR tolerance
A team from Brunel University, MAHLE Powertrain and University College London studied the combined effects of different inlet valve operating strategies on combustion, performance and emissions with different ethanol and 1-butanol blends with gasoline in a single-cylinder spark-ignition research engine equipped with a fully variable valvetrain. Their paper is published in the journal Fuel.
The focus was to better quantify the effects of alcohol content and Early Inlet Valve Closing (EIVC) operation on EGR tolerance under the lowest speed-load conditions typically encountered (e.g., engine idle) while also quantifying the changes in optimum valvetrain settings at moderate speeds and loads where the effects of varying EGR tolerance were less dominant.
Variable valve timing is an established technique for improving the fuel economy of a gasoline engine via several mechanisms. There is growing interest in fully variable valvetrain systems for further reduction in throttling losses via load control directly and the inlet valves, and increased thermal efficiency through a greater effective expansion ratio.
While numerous fully variable valvetrain strategies are possible depending upon the application, other studies have highlighted the use of Early Intake Valve Closing (EIVC) as a favorable strategy at part load, the authors noted.
The researchers also noted the significant global interest in alcohol fuels for SI engines.
Compared to gasoline, the lower alcohols exhibit high latent heat of vaporisation and anti-knock rating, which makes them attractive for use in future “downsized” highly boosted SI engines that endure significantly higher peak in-cylinder pressures and temperatures. Such downsizing may help offset the low energy density of ethanol, while problems with cold star due to reduced volatility can also be reduced via, for example, advanced DI operating strategies. However, gasoline-ethanol blends are known to exhibit azeotropic behaviour, with profound effects on the vaporisation and thermodynamic properties of the blend.
...Elsewhere, “higher” alcohols such as propanol, butanol and pentanol have also been considered for automotive use. From a thermodynamic stance the higher alcohols generally exhibit favorable calorific value (and hence better volumetric fuel consumption), better water tolerance, volatility control and lower Reid vapour pressure. However, the increased molecular carbon chain reduces the available hydrogen bonding effect and benefits in latent heat (charge cooling) and hence knock resistance are typically reduced.—Cairns et al.
The study explores the interaction of these two technology threads—i.e., VVT and alcohol biofuels.
The worst case engine operating condition for SI engine EGR tolerance is normally engine idle, where the low speed and low load lead to minimum in-cylinder gas pressures and temperatures, reduce in-cylinder charge motion levels, decreased burning velocities and relatively high cylinder wall heat transfer losses.
In the testing, the team set the engine to EIVC operation at idle and then attempted to increase the valve overlap until the acceptable combustion stability limit was approached.
The researchers used a naturally aspirated single cylinder four-valve per cylinder spark ignition research engine. The cylinder head included a prototype mechanical variable valvetrain assembly, fitted to both the inlet and exhaust. The system is an evolution of MAHLE’s Variable Lift and Duration (VLD) mechanism. The system is based on a shaft-in-shaft cam operating principle. The opening and closing control cams act in tandem to produce a mean cam (and valve) lift curve. By advancing the phasing of the inner shaft relative to the outer shaft, the closing of the valve is advanced, allowing reduced lift and duration.
The phasing of the closing control cam was controlled with a prototype wide range hydraulic cam phaser (VLD Phase). To achieve fully variable valvetrain operation, a second hydraulic VVT Phaser was fixed to the other end of the entire camshaft assembly.
Fuels tested included a commercial UK pump-grade unleaded gasoline (95 RON) as a baseline; iso-octane (i100), ethanol (E100); 1-butanol (Bu100), and 25% splash blends of ethanol and 1-butanol with gasoline and the iso-octane.
The experiments involved study of EIVC at varied degrees of valve overlap. The inlet valvetrain was variable in duration/lift and timing, whereas the exhaust valve actuation was only variable in timing.
Among the findings were that:
During warm idle operation, high ethanol content fuels allowed significant improvement in tolerance to internally recycled burned gases. At lowest load conditions, this allows higher valve lifts to be used, with reduced throttling locally at the inlet valves, further small fuel savings and reductions in engine-out emissions of NOx.
For example, when using E100, the overlap was increased from 15° to 33° crank, resulting in a small additional fuel consumption savings of ~2.7% compared to EIVC-only operation.
The authors attributed this to the faster laminar burning velocities of ethanol and faster rates of mass burning.
1-butanol content, regardless of the percentage of the blend, appears not to influence EGR tolerance.
Use of high ethanol content fuels at idle enabled moderate reductions (up to 20%) in engine-out NOx, primarily associated with significant increase in the residual mass tolerated.
For all ethanol fuels, increasing the valve overlap reduced the combustion efficiency (qualitative reduction of ~1.7% for E100, for example.
At moderate speeds and loads, where throttling losses were less, the valvetrain could still be used to attain additional thermal efficiency improvements including reduced compression losses and further expansion work for all fuels.
However, a trade-off with increased pumping losses during the exhaust stroke was apparent, with the throttling moved from the inlet to the exhaust valves at the most retarded valve timings studied.
For all fuel blends, variable valve timing alone offered the greatest NOx reduction potential at moderate loads, insinuating the ability to operate variable valve timing with and without early intake valve closing may offer one viable path to meeting future engine emissions targets.
Alasdair Cairns, Hua Zhao, Alan Todd, Pavlos Aleiferis (2013) A study of mechanical variable valve operation with gasoline–alcohol fuels in a spark ignition engine, Fuel, Volume 106, Pages 802-813 doi: 10.1016/j.fuel.2012.10.041