|Basic design of a two-piston RoundEngine. Click to enlarge.|
Canada’s VGT Technologies, the developer of the RoundEngine, has started development of a “plug-in” compressed air hybrid vehicle using the RoundEngine technology. In this application, “plug-in” refers to connecting to an external air compressor to top off the storage tanks. The RoundEngine is a novel variable geometry toroidal (VGT) engine.
The gasoline/air hybrid vehicle is similar to a gasoline/electric hybrid vehicle but uses compressed air instead of electricity for an auxiliary drive. In the first phase of development VGT will focus on the air drive using the RoundEngine technology by modifying an AWD vehicle to incorporate the air drive on the rear axle. The front axle will be powered by the gasoline engine. The purpose is to prove the air drive in an automotive application and to demonstrate fuel and emissions savings. VGT is manufacturing a toroidal engine for this application.
In the first phase of the project, the air drive will be used mostly for acceleration and low speed, while the gasoline engine powers the car at higher speeds and recharges the air tanks. The system will support regenerative braking, and offer the optional plug-in capability to recharge airtanks in off-peak hours. The compressed air can be used to supercharge the combustion engine for short periods without any mechanical supercharger resulting in short power boosts.
In the second phase, VGT plans to apply waste heat from the exhaust and the engine cooling system to the compressed air and converting the expansion/pressure increase into power. Other options are to use fluids with phase changes to produce closed cycle high pressure steam for propulsion.
The third phase of development will focus on a dual-engined vehicle. There will be two RoundEngines, a combustion version and an air motor version. One engine will run on air only while the other RoundEngine will operate with a combustible fuel.
The RoundEngine. VGT was formed to commercialize the variable geometry toroidal engine developed by Rudy Pekau beginning in 1995. In 2002, VGT developed a 625 cc, single-cylinder prototype engine which used compressed air as a power source. In 2004/05, VGT converted the air-powered motor to a combustion prototype.
|The external combustion chamber. Click to enlarge.|
The RoundEngine is a toroidal rotary engine with two or more pistons fixed around a central disc that connects to the drive shaft, replacing the connecting rods and crankshaft. Combustion occurs in a external combustion chamber, with the gases venting into the toroidal chamber to move the pistons. Inlet, exhaust and combustion chamber valves are set in the base of the toroidal track.
A rotating timing disc, the shaft of which is tangential to the toroid, provides a means for the pistons to compress fresh air and is a backstop which prevents combusted air from moving back through the housing. As a piston passes an inlet port, the air is compressed between the piston and the timing disc until a valve opens upstream of the disc to allow the air charge to flow into the external combustion chamber. The combustion chamber is closed and a direct-injected fuel charge is ignited, then exhausted through a valve on the side of the timing disc.
The disc has a cutout to allow passage of the piston. By the time the combustion chamber exhaust valve opens, the piston has passed through the cutout, and the timing disc once again seals the track.
With two firings per revolution of the output shaft, a two-piston RoundEngine is equivalent to a conventional four cylinder, four-stroke design. VGT suggests that on large engines a second combustion chamber could be used, 180° away from the first.
The RoundEngine can vary two volumes. By using multiple inlet ports, the choice of which can be switching during operations, the engine can vary the volume—and hence displacement—of the compression stroke. Because the combustion chamber is external to the toroid, the actual working internal volume within the chamber can be increased, or decreased. This feature has more of an impact on fuel choice and compression ratios, but nonetheless can also be used to some degree to impact displacement.
VGT does note yet have empirical evidence for power from the engine, but has calculated a range of outputs. For example, a 2-piston, 2.4-liter displacement RoundEngine operating with a pressure ratio of 1.5 to 1 should deliver 88 bhp at 1,500 rpm and 134 bhp at 3,000 rpm, with 310 lb-ft and 236 lb-ft of torque, respectively.
VGT suggests that initially, the RoundEngine can achieve savings of 15-35% in fuel consumption over reciprocating combustion engines and smaller gas turbines.