Variable valve timing systems and air hybrid engines

Simulation of an air hybrid engine with two storage tanks.

Simulation of an air hybrid engine with two storage tanks

Air hybrid

The overall objective of air hybrid research is to improve fuel efficiency of combustion engines through air hybrid engine and variable valve train system development. In air hybrid engines, the internal combustion engine is used as a compressor to capture the vehicle's kinetic energy while braking. The compressed air is stored in an air tank and used in the same engine to propel the vehicle via an air motor. This pneumatic hybridization significantly improves the engine’s efficiency and hence reduces emissions. We have achieved significant progress in addressing one of the most important barriers in bringing air-gas engines to the market: the size of the air tank. A novel air hybrid engine has been developed where the size of the air tank can be reduced using the new compression process performed at a much higher pressure, along with the development of a less complex variable valve timing system.

Diagram of air hybrid powertrain operation.

Air hybrid powertrain operation.

A novel compression strategy for air hybrid engines has been developed, designed and tested utilizing two storage tanks: one low pressure (LP) and one high pressure (HP) tank, which increases the efficiency of air hybrid vehicle regenerative braking significantly by increasing the stored air mass and consequently, storing pressure in the tank. In this method air is stored selectively in two tanks instead of a single tank in one revolution of the crank shaft. An experimental setup has also been designed and tested to evaluate the double-tank compression strategy in practice. Our research is currently focused on implementing the technology on a diesel engine.

Air hybrid engine prototype.

Air hybrid engine prototype

Air hybrid engine drive cycle efficiency results.

Air hybrid engine drive cycle efficiency results

Variable valve timing (VVT) systems

Engine valve systems have mainly been designed to accurately control the admission and rejection of intake and exhaust gases to the cylinder within each cycle. Throughout decades of engine development, conventional cam-follower mechanisms have been the primary means of controlling valve actuation and timing. In fixed valve systems, the engine valves open and close with fixed maximum lift and timings. Although this design provides reliable and accurate valve operation during various speed ranges, the engine cannot be operated at its most efficient manner over a broad range of speeds and load. In fact, because the dynamic behaviour of gas flow in a cylinder and through a valve varies significantly over different operating conditions, fixed valve timing is a compromised setting for a given design goal and, as a result, some desirable performance characteristics such as minimum emission or fuel consumption are sacrificed for other requirements like maximum power.

To comply with recent stringent emission regulations, the automotive powertrain has seen significant improvements over the past decades. Development of a variable valve timing system is one of the challenges that engine manufacturers are dealing with. Many designs and mechanisms including cam-based and camless systems have been already proposed for VVT systems. Due to problems such as lack of robustness, repeatability and high cost, only a few types of powertrains (mainly cam-based systems) with limited flexibility have been used in production vehicles. However, due to their low flexibility, they do not fulfill all the engine requirements.

Our team has developed a new fully hydraulic valve system, eliminating the reliability and time response issues in the existing VVT systems. Changing a valve lift is another important feature that could improve engine fuel efficiency and reduce emissions greatly. The new valve system can also change the lift accurately and swiftly. We are currently applying the design to a diesel engine for further evaluation, valve timing and lift optimization.

Experimental setup of the VVT system.

Experimental setup of the VVT system

Graph of experimental results of the VVT system.

Experimental results of the VVT system