Napier Nomad II
Napier Nomad II Aircraft Engine Performance Verification
Using Numerical Modeling
J. David Kirk
Kirk Engines, Inc<br>Published 1 Apr 2021
Napier Nomad II<br>During the mid-1940s, D. Napier & Son Ltd. undertook a project to design a highly efficient, propeller driven piston aircraft engine that was considerably more fuel-efficient than other engines in use at that time [2,3,4]. Napier decided to use a “compound cycle” engine that made use of the normally wasted exhaust gas energy, the goal being to capture this energy, convert it to useful work, and feed back into the propeller shaft. The Napier engineering group settled on a CI (Compression Ignition, i.e. diesel) piston engine coupled with a gas turbine engine, less combustors, as the CI engine became the gas generator that supplied the exhaust energy to drive the turbine. The compressor, driven by the turbine, supplied all airflow to the piston engine. An infinitely variable, mechanical transmission coupled the turbomachine main shaft to the CI engine crankshaft to optimally combine the power outputs of each machine to drive the propeller. Additionally, some residual exhaust thrust was available for propulsion.
Summary
This study constructed a numerical computer model of the Napier Nomad II aircraft engine with the intent of verifying the rather remarkable performance claims made for this interesting machine. The model consists of the accepted equations and relationships describing the thermodynamic and fluid properties that are typically used for predicting various internal combustion engine performance parameters. Once the model was constructed, it was calibrated using stated sea level performance test data published by Napier. Various engine power and speed settings at differing altitudes were then run with excellent agreement between model predictions and published data. Results show that the Nomad performance claims are indeed legitimate, thus demonstrating that this form of compound-cycle engine deserves another look as a modern-day power source for certain aircraft applications.
Discussion
D. Napier & Son Ltd., a highly-respected British organization, produced some remarkable aircraft engines during its career as one of the premier British engine manufacturers [1]. Engines ranged from spark-ignition types to diesel; even gas turbine and rocket engines were briefly produced. Napier products always demonstrated cutting-edge technology for the time, with a high degree of novelty and a non-standard approach to engineering design. This methodology is clearly demonstrated in the engine that is the subject of this analysis.
Napier actually constructed two engines based on this concept, these being the Nomad I and Nomad II. The Nomad I was an overly-complicated engine [4] and is not considered in this analysis. Lessons learned from this first engine led to simplification, weight reduction and ultimately better performance of the Nomad II, which is the subject of this report. A simplified schematic of the Nomad II appears in Figure 1.
The engineering details are fascinating. Napier chose to use a two-stroke cycle, CI engine comprising a flat-12 (horizontally opposed) cylinder layout, incorporating a 6-throw crankshaft. Piston-controlled scavenge and exhaust ports were employed utilizing reverse loop-scavenging, thus no mechanical valve mechanism was required. The scavenge ports were located in a “Curtis” type arrangement thereby allowing a flat-domed piston to be used, as shown in Figure 2. A top-down sectional view appears in the upper left of Figure 2 and shows how the scavenge ports were nested within the cylinder spacing. A hemispherical combustion chamber was employed using a single, centrally-mounted fuel injector operating at 3,675 psi [4]. A single spark plug was oriented 45° from the injector with said spark plug required for engine starting and light load operation.
Right: Charles Gordon Curtis (1860-1953) invented the Curtis scavenging system in 1933. Kevin Cameron notes that the 'A' transfer ports on either side of the exhaust have relatively flat roof angles (little or no upflow). The 'B' transfers have maybe 30� upward roof angle, and the 'C' transfers yet more, as much as 60� upward. This produces a high-speed collective jet of fresh air that quickly travels the loop from transfer ports, across the piston to the non-exhaust cylinder wall, then up that wall to be deflected across the underside of the head, and finally down again to the exhaust port(s).
Fig. 1. Simplified Napier II Schematic<br>Fig. 2. Nomad II Cylinder Porting Arrangement
A liquid cooling system was employed, circulating coolant over the cylinder heads and around the cylinders. Mounted beneath the piston engine was a modified Napier Naiad turboprop machine, minus prop reduction system and combustors. The turbomachine consisted of a 12-stage axial flow compressor that supplied all discharge air to the piston engine by suitable...