|Pneumatic hybrid engine on the test bench. Photo: P. Rüegg / ETH Zurich. Click to enlarge.
Researchers at ETH Zürich are developing a pneumatic hybrid engine—an internal combustion engine connected to a compressed air tank instead of a battery system. A member of the ETH research team, which is led by Lino Guzzella, Professor of Thermotronics, will present a paper on their work at the upcoming SAE World Congress 2009 in Detroit in April—one of a number to be presented on the topic there.
The pneumatic hybrid engine, which follows the downsizing and supercharging paradigm, offers a fuel-saving potential that is almost equal to that of hybrid electric powertrains while inducing much lower additional mass and cost penalties, according to the ETH Zürich researchers.
When required, e.g. when starting from rest or after changing gear, compressed air flows into the engine through an electronically controlled valve. If fuel is also injected, the engine responds quickly.
The compressed air supply also allows extreme downsizing. Conventional car engines can have peak powers of 150 hp or more, but usually need no more than 30 hp for everyday driving. Downsizing the engine in this instance halves the number of cylinders from four to two. This also halves frictional losses and increases the engine’s average efficiency. To keep the maximum power and thus satisfy the consumer’s drivability demands, the engine is highly supercharged by a turbocharger—which exploits the exhaust gas enthalpy as an energy source, and which boosts the to the desired levels.
Initial results from a prototype on the test stand in the ETH Zürich Machinery Laboratory show an increase in the engine’s average efficiency in the European Test Cycle from 18 to 24%. This corresponds to a fuel saving of one-third. Energy savings of up to 50% are achievable in purely urban traffic, because the engine can pump air into the compressed air tank during braking, thus recovering the kinetic energy.
The fuel saving achieved by the air hybrid is about 80% of that of a full electric hybrid, but the price-performance ratio is “distinctly better”, according to the research team.
Guzzella estimates the cost increment for the air hybrid over a conventional gasoline engine is approximately 20%, while the incremental costs for an electric hybrid are at least 200%, according to his calculations. With the cost advantage, Guzzella suggests that the air hybrid also would be suitable for use in poorer countries.
ETH Zürich says that the engine concept has interested several major motor companies and automotive suppliers.
Other papers on aspects of pneumatic hybrids and their control systems that will be presented at the SAE World Congress 2009 include contributions from Lund University, Sweden; Université D’Orléans, France; National Taipei University of Technology, Taiwan; the University of Waterloo, Canada; and Brunel University, UK.
Dönitz et al. (2009) Realizing a concept for high efficiency and excellent driveability: The downsized and supercharged hybrid pneumatic engine. (SAE 2009-01-1326, not yet published)
C Dönitz, I Vasile, C H Onder, L Guzzella (2009) Modelling and optimizing two- and four-stroke hybrid pneumatic engines. Journal Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering pp. 255-280 doi: 10.1243/09544070JAUTO97
Andrej Ivanco et al. (2009) Energy Management Strategies for Hybrid-Pneumatic Engine Studied on a Markov Chain Type Generated Cycles (SAE 2009-01-0145, not yet published)
Kuohsiu David Huang et al. (2009) Validation of Dynamic Model of Hybrid Pneumatic Power System. (SAE 2009-01-1304, not yet published)
Sasa Trajkovic et al. (2009) Simulation of a Pneumatic Hybrid Powertrain with VVT in GT-Power and Comparison with Experimental Data (SAE 2009-01-1323, not yet published)
Amir Fazeli et al. (2009) A New Air Hybrid Engine using Throttle Control (SAE 2009-01-1319, not yet published)
Cho-Yu Lee and Hua Zhao (2009) Analysis of a cost effective air hybrid concept (SAE 2009-01-1111, not yet published)