The Duke sliding seal arrangement is analogous in function to a ported 2-stroke or Wankel engine port sealing, allowing similar technologies to be applied. The Duke uses sliding metallic seals operating on an oil film. Materials used are those found in production road vehicles. The V3i engine has seals which are of a simple design and operate at modest contact stress and sliding velocities. Oil film calculations conducted support good nominal lubricating film thickness.
All V3i gasoline and Jet A1 testing to date (March 2012) has been completed with a single set of prototype seals which remain in good condition. These seals will be reassembled into an engine for further testing without modification or repair.
Duke challenges in seal development are much less than in a 2-stroke or in the Wankel engine due to lower sliding velocity and a flat monoplane sealing surface (Wankel has 3 seal faces, 1 curved, that meet at a corner, seals). So far, our sliding seal challenges are proving to be modest in reality.
A Duke engine was tested with BSFC of 255g/kWhr in the load/speed region near maximum output. These tests were conducted at Mahle Powertrain in the UK in 2007. This result made it competitive with modern high end automotive engines operating a similar combustion cycle (gasoline, port injected, spark ignition).
It should be recognised that the Duke engine at an early and non optimized condition, being equal to conventional engines in their fully developed state, offers the very real prospect for a developed Duke engine to improve substantially in comparison to conventional engines.
We do not have calibrated BSFC numbers on Jet A1, as operating on this fuel is a recent development and the local test facility does not support BSFC measurement. We estimate the range 240g/kWhr to 275g/kWhr at rated condition for our current prototypes.
Since 2007 the Duke engine has been substantially redesigned with measures to reduce friction, improve performance and optimise heavy fuel operation. Brake specific fuel consumption on Gasoline and Jet A1 fuels will be confirmed in testing underway at Mahle Powertrain (Novi, MI, USA). Test results will be reported as available.
The current V3i Duke engine operates a simple port injected, homogeneous charge, spark ignition combustion system. This is specified in order to simplify the development process within current program resources. A production derivative would be able to benefit from direct injection, stratified charge, fully optimised compression ratio and other features to improve fuel efficiency.
In mobile platforms, especially air borne, mission benefits above and beyond simple BSFC comparison should be expected from the low mass and size of the Duke engine. A lighter more aerodynamic vehicle demands less propulsive effort and fuel burn to cruise and loiter, resulting in longer range, higher performance, lower fuel consumption or higher payload benefits to be chosen as the application requires.
Performance test status on gasoline:
Torque: 339 Nm / 250lbft @4500 rpm (lambda 0.9), safe map spark.
BMEP: 11.8 Bar / 171psi
Power: 160 kW / 215hp @4500 rpm (lambda 0.9), safe map spark.
Significance of current gasoline performance status:
11.8 Bar BMEP is competitive with conventional SI engines.
3.0L torque output of 339Nm / 250ft/lbs is above that typically achieved by comparable conventional engines - due to pistons reciprocating at 120% of output speed, allowing a 20% higher output to be achieved at any given speed.
160kW power output is competitive with conventional SI engines of equivalent displacement and is currently achieved at only 4500 rpm.
V3i gasoline performance reported is below its real potential due to the lower Jet A1 compression ratio used and interim precautionary limit of 4500 rpm in this test phase (design target 6000 rpm).
Performance test status on Jet A1:
Torque: 292Nm / 215lbft @3500 rpm (lambda 0.88) detonation detected spark advance – 5deg.
BMEP: 10.2 Bar / 148psi
Power: 126kW / 169hp @4500 rpm (lambda 0.88) safe map spark advance.
Significance of current Jet A1 performance status:
Torque on Jet A1 achieved between 79% and 100% of gasoline baseline between 2000 and 4500 rpm.
These Jet A1 full load test results are considered particularly encouraging. The 10.2 bar BMEP achieved on Jet A1 is marginally above that of a Lycoming O320E comparator at rated power, operating on AVGAS. This is above the BMEP considered feasible with Jet A1 in conventional naturally aspirated spark ignition engines.
V3i Performance on Jet A1 is expected to increase significantly from this initially encouraging level due to operating at higher speeds with further combustion system and calibration optimisation.
The centrifugal forces in the Duke engine have been thoroughly analysed. It should be considered that the cylinder block rotates at a fraction of crankshaft speed and as a result Duke radial inertial piston forces have been shown to be lower than the side forces created by the conrod angle in a conventional engine.
Like the Wankel rotary our combustion chamber is unencumbered with valves and such like. However unlike the rotary we approach a near optimal chamber shape far closer to a conventional 4-stroke engine. Therefore we achieve a much more complete burn than a rotary.
Charge motion development will further improve our combustion efficiency to approach that of a conventional 4-stroke.
There is currently some oil required for the seals to work and this can add to the emissions, however our consumption is far less than that required for a 2-stroke or rotary.
Independent tests have shown that the friction levels of our prototypes are very competitive with conventional engines in terms of over all engine friction.
We expect that some of our more recent developments have further lowered the friction and this will be confirmed in upcoming tests.
We know that the SI Duke engine runs on kerosene (JetA1) with few changes from an optimised petrol version. During our forth-coming test program we intend to try the engines on Diesel fuel as it is not greatly different (chemically speaking) from kerosene. We have high hopes that the Duke engine will become the the world's first internal combustion engine to run on such a wide variety of fuels. Using spark ignition Duke engines with heavy fuels will result in much smaller and lighter engines than comparative compression ignition Diesel engines. We will release our test results as they become available.
The axial layout is intrinsically more suited to variable compression ratio than in a conventional engine. We have some concepts for variable compression, but have not yet developed them.
To date most of our development and testing has been on gasoline of various octane ratings. Other commonly used SI fuels such as biogas CNG, ethanol, methanol, lpg could be easily tuned for with no physical changes to the engine.
We have done some SI testing on JP8 (AKA Jet A1 and kerosene). Initial results are very encouraging and we plan to do some more optimisation here as we learn more about the Duke's unique characteristics. We expect that JP5 will also be able to be used in Duke engines with few changes.