All aluminium aircraft engine

Design Engineering takes a look at a new lightweight diesel engine

In 1997, Bucks-based Diesel Air commenced the design of a lightweight diesel engine for aircraft use. Inspiration for the design and operation of the engine was drawn from the large piston aero engines of 50 years ago.

With the use of modern materials and configuring the engine in the horizontal plane, a light powerful 100hp engine has now been built that will retrofit into most existing aircraft engine cowlings requiring an engine of this power.

The 100hp engine is the first in a family of engines, using the same constructional techniques to cover the range from 100hp to 600hp. This first engine resembles a flat four or `boxer’ engine, and has two cylinders, each with two pistons. The engine has two outboard crankshafts connected by gears to the propeller-drive and accessories. This `flat plane’ layout ensures the engine can be easily retrofitted in place of the popular Continental or Lycoming engines, using the same mounting points in many cases.

The first prototype engine ran on a dynamometer in 1997 and in April 1999, the engine demonstrated 135hp at a propeller shaft speed of 2850rpm. The two crankshafts run at 3750rpm at this speed. Interestingly, the torque curve is still flat, indicating that more power can be achieved at a higher rating.

On 1 July, 1999, Diesel Air announced the DAIR-100 engine and work continues to refine its performance and bring it to production status. Flight trials in the company’s aircraft will follow before a small batch of engines are released under PFA regulations for customer evaluation.

OPERATING PRINCIPLES

The twin cylinder opposed piston engine has two pistons in each cylinder with the combustion chamber formed between the crowns of the pistons. It eliminates the need for a cylinder head, camshaft and associated valves and has in effect two half length crankshafts linked by a gear train driving a centrally mounted propeller.

The cylinders are ported at each end with one ring of ports opened by the air piston supplying the fresh air charge, and the other opened by the exhaust piston and connected to the exhaust system. Gas flow through the cylinder is in one direction (uniflow) and leads to more efficient scavenging – the gas exchange process – to that achieved in a normal loop scavenged engine.

The DAIR-100 has a centrifugal compressor supplying this scavenge air which also provides a degree of supercharging. Power can be increased further with the installation of a conventional exhaust gas driven turbocharger in series with the compressor. Lubrication is achieved by dry sump, high pressure oil and feeds plain main and connecting rod bearings. A hydraulically governed mechanical pump feeding injectors in each cylinder provides the means of fuel injection.

Engine management with single lever operation is greatly simplified with both carb heat and mixture controls eliminated. The DAIR-100 will operate on diesel fuel or jet A1 and this with the added reliability of liquid cooling (shock cooling avoided in the descent) contributes to the low operating costs. The `two stroke diesel’ has a power to weight ratio comparable with modern petrol engines.

Selection of materials for various elements of the engine has presented challenges to the designer, Chris Downton of Weslake Air Services. For example, the drive to the compressor or scavenge pump is via a flexible quill shaft designed to absorb torsional oscillations between the engine and the high effective inertia of the compressor wheel. The wheel, which is running at around 70,000rpm at full power, contributes around 25% of the overall flywheel effect required to ensure smooth running of the engine. The propeller contributes the rest. The quill shaft, which is running at 20,000rpm, is made in spring steel, to provide both flexibility and durability.

Carbon-carbon coated material is used in another area where torsional vibration damping is required, and with the coefficients of sliding and static friction being similar, it is ideal for the application, where a constant torque is required in all stages of transmission.

The piston rings and liner have also received a large amount of attention. The exhaust port ring is placed so that lubricating oil has difficulty in reaching the upper cylinder. Use is made of speciality coated high temperature steel rings to ensure not only survival, but longevity as well. One property of the coating, which is deposited by plasma spray coating, is that it collects and carries lubricant from the area of the liner wiped by the rings to the upper cylinder. As a result, oil carryover to the exhaust, a common fault with early piston-ported engines, is not measurable.

INTO AIRSHIPS

The company has attracted the interest of Airship Technologies who intend to use the engines in the `Vectored Thrust Propulsion’ units on its airships.

With the engine making its debut into this particularly demanding environment, applications in other areas, should be guaranteed.

Diesel Air Tel: 01234 711360