Compression-ignition engine research is being undertaken by Pratt & Whitney Rocketdyne, which was commissioned by DARPA to develop a compression-ignition Wankel engine for use in a prototype VTOL flying car called the "Transformer". The engine, based on an earlier concept involving an unmanned aerial vehicle called "Endurocore", powered by a Wankel diesel. plans to utilize Wankel rotors of varying sizes on a shared eccentric shaft to increase efficiency. The engine is claimed to be a 'full-compression, full-expansion, compression-ignition-cycle engine'. An October 28, 2010 patent by Pratt & Whitney Rocketdyne, describes a Wankel engine superficially similar to Rolls-Royce's earlier prototype, that required an external air compressor to achieve high enough compression for compression-ignition-cycle combustion. The design differs from Rolls-Royce's compression-ignition rotary, mainly by proposing an injector both in the exhaust passage between the combustor rotor and expansion rotor stages, and an injector in the expansion rotor's expansion chamber, for 'afterburning'.