In the pneumatic hybridization scheme, each cylinder is connected via a fully variable charge valve to a shared pressure tank. When braking, and with fuel cut-off, the engine can intake air and pump it into the pressure tank. Without the injection of fuel, the pressurized air can be used again for starting or driving. Shifting the operating point of the engine is also possible, using at least one cylinder in a conventional combustion manner at high load, and using at least one cylinder to pump air into the pressure tank.

Before realizing the design of a hybrid pneumatic engine (HPE) in hardware, the researchers evaluated the different pneumatic engine modes (motor mode and pump mode) associated with both two-stroke and four-stroke implementations of the pneumatic concept.

The HPE was long thought to be a system in which all engine valves had to be actuated in a fully variable manner to allow for two-stoke pneumatic modes and still enable the four-stroke combustion mode. However, this setup adds complexity and cost. [In sticking to] four-stroke cycles for both combustion and pneumatic operations...the intake and exhaust valves remain camshaft-driven and only the charge valve, which connects the cylinders to the pressure tank, is fully variable.

—Dönitz et al. (2009)

Although the HPE approach can in concept be applied to both spark ignition (SI) and compression ignition engines (CI), after an evaluation of engine types, the team decided that downsized SI engines offered the best target for an HPE application. CI engines (diesels) require costly exhaust gas treatment to deal with the high raw emissions—neither being problems that can be tackled with HPE.

In large naturally-aspirated SI engine, high-friction, pumping losses and part-load operation all contribute to poor fuel efficiency. In this case, they decided, fuel savings due to HPE might not be sufficient to meet emerging CO₂ targets. On the other hand, downsized SI engines are low-cost, and exhibit significantly improved efficiency. Their main disadvantage is poor driveability, which can be tackled with the HPE concept.

Test bench setup. Source: ETH Zürich. Click to enlarge.

The hardware concept.

The ETH Zürich team modified a two-cylinder, 0.75L multi-purpose engine to realize their HPE concept in hardware. The original engine was designed by Wenko Swissauto, and manufactured by Weber Automotive. The engine has a compression ratio of 9.0, and is port fuel injected.

For valve actuation, ETH Zürich used what it called a fixed camshaft configuration: the intake and exhaust valves remain camshaft-driven, and only the charge valve connecting the cylinders to the pressure tank is fully variable. The concept engine uses the Bosch electro-hydraulic valve system (EHVS).

The engine has two intake and two exhaust valves per cylinder. The team replaced one exhaust valve per cylinder with a charge valve. This compromise, they said, was the least restrictive solution for conventional combustion operation. Because the original engine connected the two exhaust channels to form a common exhaust port, the team welded channel separations into each exhaust channel. The EHVS actuators for the two charge valves are located on the outer part of the cylinder head.

Measurement: switching between all modes. Source: ETH Zürich. Click to enlarge.

The team was able to test all conventional and pneumatic engine modes on the concept engine. The most difficult task, the researchers noted, is the step from conventional combustion mode to the supercharged mode (injecting the air mass via the charge valve).

The concept demonstrates a low pneumatic regenerative efficiency (caused by the engine’s friction and heat losses, hence the pneumatic hybridization itself does not contribute much to the overall fuel savings.

Regenerative efficiency measurements and drive cycle simulations show that the idea of the pneumatic hybridization alone might not be effective enough. However, this new technology can provide the breakthrough for strong downsizing and supercharging of gasoline engines.

Compared to a naturally aspirated engine with the same rated power, the downsized and supercharged hybrid pneumatic engine can save as much as 32% fuel, as simulations show for the fixed-camshaft configuration. The major part of this gain is due to downsizing the engine.

The problem of “turbo-lag” usually associated with heavily downsized and supercharged engines is completely overcome by injecting additional air from a pressure tank and more fuel during transients. Experiments have verified the engine’s instantaneous torque response resulting from applying this “supercharged” mode.

—Dönitz et al. (2009)

The next step in the work is the implementation of a turbocharger, and the validation of fuel consumption of the HPE for complete drive cycles on the test bench. The final goal of the project is the integration of an HPE into a passenger car to demonstrate driving performance and fuel consumption.

Resources

• Dönitz et al. (2009) Realizing a concept for high efficiency and excellent driveability: The downsized and supercharged hybrid pneumatic engine. (SAE 2009-01-1326)

• Pneumatic Hybridization of Combustion Engines (ETH Zürich research project database)