| MidWest AE series | |
|---|---|
| Type | Wankel aero engine |
| National origin | UK |
| Manufacturer | MidWest |
The MidWest AE series were lightweight liquid-cooled twin-rotor Wankel engines with dual ignition for light aircraft, formerly produced by MidWest at Staverton Airport, Gloucestershire, UK.
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This engine had its origins at the BSA Umberslade Hall research unit in Birmingham, where David Garside, a BSA engineer, designed an air-cooled twin-rotor motorcycle engine.[1][2] Wankel engines run very hot, so Garside gave this air-cooled motor additional cooling air that was filtered and drawn through the rotors and then through a large plenum before entering the combustion chambers via the carburreters.[3] The cooling plenum and the transfer of latent heat during the carburation process significantly reduced the temperature of this fuel-air mixture, but its volumetric efficiency was still somewhat impaired. On the motorcycle the cooling-air filter was sited to obtain a partial ram air effect, but the aero-engine used a centrifugal pump instead.
In the early 1970s, British motorcycle manufacturers were in difficulties. Under a government sponsored rescue, Norton-Villiers, a subsidiary of Manganese Bronze, merged with BSA-Triumph to form Norton Villiers Triumph (NVT). Norton became the "senior partner", despite being the smallest of the three companies and despite its motorcycle "range" being based on just one motorcycle, the pre-unit Commando. So the "BSA" engine went into production as the limited-edition Norton Classic. A watercooled version followed for a new roadster, the Norton Commander; and there were successful racing motorcycles too. When Norton ceased large-scale production of motorcycles, they retained the rights to the twin-rotor engine for motorcycle applications and also for military unmanned aerial vehicles, but MidWest acquired the rights for light aircraft applications.
MidWest then produced an aero engine version at Staverton, the twin rotor AE100/AE110. (The code number refers to the nominal bhp output). These engines were liquid-cooled but still used air to cool the rotors. Whereas in the motorcycle engine the rotors sucked cooling air into the chamber, the aero-engine forced cooling air through the rotors via a belt-driven centrifugal pump, and then dumped this very hot air overboard. This modification was because aero-engines use maximum power for longer periods than do motorcycles, and the loss in volumetric efficiency that was satisfactory on a motorcycle was not acceptable in aviation applications. The lubrication system was a semi-total-loss system: Silkolene 2-stroke oil was directly injected into the inlet tracts and onto the main roller bearings. The oil that fed the bearings became an oil-mist within the rotor-cooling air, and around 30%[4] of the oil was recovered and returned to the remote oil tank. Power was delivered to the propeller through a 2.95:1 reduction gearbox closely based on the design of the Hewland AE75 gearbox, the rights to which MidWest now owned. Although side ports enable a Wankel engine to run smoothly at part-throttle and idle, this engine's peripheral ports were ideal for aviation operation, where the engine runs at high power for most of the time.
The testbed aircraft for air-trials was an ARV Super2[5] that had its original Hewland engine removed. The MidWest engine suited this airframe well. Original-fit carburreters were Tillotson units, but these proved unsatisfactory and very difficult to "tune". This failing led Ivan Shaw to reject the MidWest engine as an option for the Europa XS, which proved a serious setback for MidWest. Some aircraft with MidWest engines were retro-fitted with bespoke fuel injection systems that worked well and avoided the complications of mixture controls, choke and "carb-heat". But although the MidWest engine was fully CAA-certified, it did not sell in large numbers.
MidWest had hoped to develop the engine to become a spark-ignition petrol-paraffin engine, which would start up on Avgas or Mogas, and then run on cheaper Jet fuel once warm; and trials were well-advanced. But before this work was completed, MidWest was bought by Diamond Aircraft Industries, near Vienna, Austria (although MidWest's rights to the Hewland AE75 engine were sold to a UK private buyer, an ARV enthusiast). Production ceased at Staverton; and Diamond, who were deeply committed to Mercedes Benz-derived Thielert diesel aero-engines, chose to discontinue the AE110 and produce only the AE50, a 50 hp (37 kW) single-rotor unit suitable for motor-gliders and microlights.
Diamond transferred the engine rights to Austro Engine, a company founded in 2007 and located in Wiener Neustadt. They produce two single rotor Wankel engines, the 55 hp (41 kW) AE50R & the 75 hp (56 kW) AE75R. These new designs feature liquid cooling with forced-air rotor cooling, metered oil lubrication pumped to the main bearings (directly) and to the rotor tips (via the inlet manifold), with partial oil recovery, dual ignition, and electric starting (all of which were features of the MidWest engines). They also have electronic fuel injection.[6][7][8]