FEV Engine Technology is working on a new, two-stroke, light-weight, high-efficiency diesel concept engine that also promises to address the emissions issues associated with two-stroke designs. The approach could support a family of engines applied either as standalone engines in vehicles, as power sources for APUs, or as powerful and efficient engines in a hybrid configuration.
The OPOC (opposed-piston, opposed-cylinder) engine combines two engine designs pre-dating World War II: an opposed-piston, two-crankshaft diesel aircraft engine developed by Hugo Junkers and the opposed-cylinder boxer engine developed by Ferdinand Porsche. (Diagrams below left. The diagram below right depicts the design of an OPOC module. Click to enlarge.)
A two-stroke cycle engine differs from the more common four-stroke cycle by having only two linear movements of the piston instead of four, although the same four elements of the cycle (intake, compression, power, exhaust) still occur.
The new engine thus generates one power stroke per each crank revolution per cylinder. The OPOC configuration consists of two cylinders per module. Each cylinder has two pistons moving in opposite directions, and the crankshaft is placed between the two cylinders. (See diagram above, right.)
There are no valves or camshafts in this design; intake and exhaust ports are at opposite ends of the cylinder. The crankshaft controls the position of the pistons in such as way as to open the exhaust ports before the intake ports and then close the exhaust ports before the intake ports. This asymmetric timing is a must and provides for proper exhaust-blow-down and enables intake supercharging.
The combined intake and exhaust process in a two-stroke engine that clears the cylinder of exhaust gases and fills it with a fresh mixture of air and fuel is called scavenging. Managing combustion control and cylinder scavenging have proven problematic in the past, and have led to one of the major problems with two-stroke design: emissions control.
The OPOC engine optimizes its scavenging over the entire engine range through the use of uniflow scavenging controlled boost pressure, pulse turbocharging and the asymmetric intake and exhaust timing noted above.
An electrically-assisted turbocharger (both exhaust gas driven and electrically driven) allows the boost pressure to be independent of the engine operation. Thus, high pressure boost is available for acceleration at low engine loads and low rpm without any associated mechanical drag.
Without the turbocharger, according to the designers, the reduction of emissions along with other engine benefits are unachievable.
[Turbocharging] also helps to reduce the NOx emissions by enabling higher than normal EGR rates and enables monitoring and maintaining a constant fuel to air ratio. The electrically-assisted turbocharger can rapidly compress and recycle air in order to heat to 100ºC in less than one second to ensure easy start in cold weather without needing glow plugs, which are costly and add complexity. Compression ratios in the range of 15-16 could be achieved resulting in lower fuel consumption and reduced NOx emissions.
With this optimized scavenging process, the OPOC engine offers:
...a significant step towards the theoretical potential of the two-stroke engine having double the power density of a four-stroke engine. An estimated 90% scavenging efficiency has been achieved with unique gas exchange characteristics of the opoc engine and the use of APT’s electric assisted turbocharger.— Peter Hofbauer, FEV Engine Technology
The engine offers a number of advantages:
High power-to-weight ratio
High power-to-volume ration
Use of a variety of fuels including gasoline, diesel, natural gas, JP8 and hydrogen
Assisted HCCI combustion using proprietary diesel fuel system
Conventional parts, materials and maintenance
The original OPOC concept was invented by Prof. Peter Hofbauer, prior to his joining FEV as executive vice president for Research and Development. The intellectual property and trademarks relating to the original OPOC design have since been acquired by Advanced Propulsion Technologies (APT). FEV is working under contract to APT, DARPA and other interested parties to design and develop for-production variants of the basic OPOC concept. FEV has been selected by APT as their exclusive development partner for the OPOC engine.
It would be very interesting to see how this might work in an aggressive hybrid or plug-in hybrid application, as well as to get initial fuel efficiency and emissions data.
Opposed Piston Opposed Cylinder (opoc) for Military Ground Vehicles, SAE paper 2005-01-1548