|Cross-section of Pivotal’s two-stroke engine. Click to enlarge.|
A New Zealand company has redesigned the two-stroke combustion engine with the intention not only of overcoming the shortcomings of the conventional sliding-piston two-stroke engines, but also to produce a combustion engine that can better exploit hydrogen as a fuel.
The Pivotal engine replaces the conventional sliding piston with a pivoting piston, the pivot point of which is at the back of the piston. (See cutaway diagram to right.) The piston pin position connects to the crankshaft via a connecting rod. The side surface of the piston forms the inner wall of the combustion chamber when the unit pivots up in the compression cycle.
The result, according to the company, is the elimination of piston rock or slap, improved compression sealing and reduced lubrication requirements.
|Cutaway of a water-cooled pivotal piston. Click to enlarge.|
The engine is water-cooled via the center of the pivot shaft. This design provides an independent means of controlling piston temperature without the need to over-cool the engine.
With its high level of thermal control of the water-cooled piston and thermally smooth head surface, the Pivotal two-stroke engine is suited to running on a wide range of fuels including diesel, CNG, LPG and JP5—and, according to Pivotal, hydrogen.
Offers higher power density by weight than gasoline but lower power density by volume;
Is fast-burning, with a high combustion temperature and low ignition temperature;
Supports a broad range of air/fuel ratios with good ignition capability;
Has a high auto-ignition temperature; and
Offers no evaporative engine cooling and has no lubricating qualities.
Pivotal argues that its two-stroke engine is well-suited to exploit the potential of hydrogen-fueled internal combustion engines, particularly because it can avoid peak and localized high temperatures which cause pre-ignition or high NOx-forming conditions.
Pivotal won a 2005 AEI (Automotive Engineering International) Tech Award for the pivotal-piston engine. From the award statement:
The move to hydrogen fuel swings the balance in favor of a two-cycle engine, but only if the piston can be thermally controlled and required minimal lubrication. With the internal water cooling of the pivotal piston, a hydrogen-fueled IC engine can deliver higher power density than the modern gasoline automotive engine. The independent water cooling of the piston ensures uniform combustion chamber surface temperatures to control pre-ignition and excessive NOx emission.
|The four-chamber, 2.1-liter two-stroke pivotal-piston engine.|
Pivotal joined with Mace Engineering to develop a prototype 2.1-liter four-chamber gasoline pivotal-piston engine, which produced 170 kW of power while weighing 65 kg—a power density of 2.6 kW per kg of engine weight (1.5 hp per pound).
If optimized for hydrogen fuel at an air/fuel equivalency ratio of >.6, Pivotal estimates the output to be in the region of 120kW–130 kW. This hydrogen output represents about 1.8 kW per kg of engine weight—still an excellent level of power density when compared to one of the best gasoline, naturally aspirated, automotive engines: the V10 BMW at 1.55kW per kg.
Pivotal contends that this comparison suggests that it will be possible to develop hydrogen-fueled pivotal piston engines which are significantly smaller in size and lighter weight than current gasoline automotive engines.
In application, conventional two-stroke engines have proven to have a number of disadvantages—such as less efficient combustion, poor fuel efficiency and higher emissions—than modern four-stroke engines. Those limitations have largely limited two-stroke use to smaller displacement, off-road vehicles and scooters, where the higher power-to-weight ration and smaller size outweigh the problems.
Other engine-makers, notably Orbital in Australia, are applying modern technology such as direct injection to the two stroke, but cost-effectiveness remains an issue. Pivotal hopes that the combination of its new design and hydrogen fuel can alter the entire dynamic.
Pivotal Engine animation (requires Flash)