A practical internal-combustion Stirling (ICS) engine configuration has been created, an analysis model developed, patents awarded, and a prototype engine designed (and almost ready to run), according to its creator, P&B Enterprises Inc. (PBEI). In addition, a test facility is nearing completion for troubleshooting the engine.
“No engine company offers an internal-combustion Stirling engine,” said Dr. Richard Patton, President, PBEI. “While ICS engines have been proposed in the past, none of them offers the patented features of the P&B ICS engine.” P&B Enterprises has a portfolio of four U.S. patents on the technology, plus patents in Europe and Canada.
The concept will be displayed at the SAE World Congress slated for April 12-14 in Detroit.
One goal for the engine is SFC (specific fuel consumption) of less than 200 g/kW·h in mixed city and highway driving. That number will help automakers meet the new, more stringent CAFE (corporate average fuel economy) requirements from the U.S. EPA.
Compared to a conventional IC engine, the ICS engine responds to higher peak temperatures with higher efficiency, according to PBEI. It operates most efficiently at low compression ratios, allowing for less friction and thus higher mechanical efficiency at part load. The ICS, which has no throttle, also has a more efficient air cycle.
An extensive simulator was developed to aid in the design of the prototype. It acts in one-degree crank increments and includes a friction model and a regenerator with calculated heat transfer and pressure drop. The simulator has been used to model starting, operating, and parameter sensitivity studies.
At part load, where most driving is done, the ICS engine is said to provide very high fuel economy. “Our calculations show that the engine will be substantially more efficient than either a spark-ignition engine or a diesel engine,” said Patton. Simulations have shown that the engine concept maintains high thermal efficiency over a much wider power demand range than conventional IC engines.
Company officials claim the ICS engine will provide a major boost to fuel economy, without redesigning a vehicle, “in the range of 50% for diesel engines and 100% for gasoline engines,” said Patton. “We expect the prototype to achieve an SFC of less than 200 g/kW·h at very low loads—2 bar BMEP at 1800 rpm.”
NOx emissions should be low due to nearly constant pressure and low-temperature combustion. The engine requires only a two-way catalyst to meet emissions regulations.
In a conventional IC engine, almost all of the waste heat is in exhaust, and many bottom cycles have been proposed to improve efficiency by using that heat. What the ICS engine does is capture the exhaust heat via a regenerator and recycle it back into the engine. “This is far simpler, cheaper, and more efficient than adding a complete new bottom cycle engine to a conventional IC engine,” said Patton.
The technology can also be retrofitted to existing engines. PBEI is in the process of retrofitting a small diesel engine as a prototype project. The goal with the prototype is to re-use the engine block and concentrate all of the changes in the head, crankshaft, and camshaft.
The ICS prototype engine has two cylinders, one called the compression cylinder and the other the power cylinder. The power-cylinder piston leads the compression-cylinder piston by 50°. As the compression-cylinder piston approaches top dead center, the power-cylinder piston is on its downstroke. Compressed gases are transferred from the compression cylinder to the power cylinder and pass through the regenerator, which heats the gases. Diesel fuel oil is sprayed into the power cylinder, which fires during its piston’s downstroke.
The regenerator and transfer passages constitute a dead space in which gases are pressurized; if the work to pressurize the dead space is not recovered, the engine would not be efficient. The concept’s patented spring-back process recovers this work during the compression-cylinder piston’s downstroke. That is a major difference between this engine and previous ICS engines.
After recovering the work, the intake valve opens and fresh air is drawn into the compression cylinder. Meanwhile, gases in the power cylinder expand. Near bottom dead center, the exhaust valve opens and the hot gases pass through the regenerator, heating it.
One ICS engine drawback is that it requires heavy turbocharging to increase power density. However, its low compression ratio means turbocharging is possible without high peak cylinder pressures greater than 69 bar (1000 psi).