Operating Small Gas Turbine Engines by Ian F Bennett

Introduction

Since an early age I have been interested in electronics, radio and from time to time
mechanical devices as well. The electronics interest has resulted in a hobby and a career but more recently a number of events have inspired my interest in the last great prime-mover of the 20th century.......the gas turbine.

Back in 1986 a colleague at work gave me an old scrap diesel engine turbo-charger, this unit I dismantled to discover its internal workings and investigate a complete seizure. I expect that the unit was scraped due to this fault. Having taken the unit apart it reminded me of lessons taken at school which explained in simple terms how a jet engine worked. Looking at the turbo I couldn't help wondering if it could be made to work as a simple gas turbine, it wasn't until 1993 that I proved this theory possible.

Further gas turbine inspiration came in the form of a couple of foreign holidays travelling aboard the ubiquitous Boeing 737. When I got back I was browsing a technical book shop in London, I came across a fascinating publication called "Aircraft Gas Turbine Engine Technology" written by Erwin Treager. I had looked at aero-engine books before but had never found one as practical and as comprehensive as this one. Gas turbine books or gas turbine sections in books either seem to consist off kids drawings or pages of mathematics with hardly a diagram. The Treager title contained masses of information, diagrams, photos and examples, it was like a car workshop manual but for aircraft engines. I bought this book, it wasn't cheap at £40, but well worth it, I haven't come across a better title since. This book was the one of my primary sources of inspiration to build, collect and operate gas turbine engines

The Treager book contained many illustrations, one showed the original experimental turbo-jet built by Frank Whittle "The Whittle Unit". The "WU" consisted of a huge single combustion chamber joining a supercharger compressor to a turbine, this arrangement looked as if it could be reproduced with my diesel turbo and some sort of add on combustion chamber.

Over the years I tried some half hearted attempts to add very primitive combustion chamber systems to my turbo, but in 1993 the design began to come together and a working stationary engine started to emerge. More details of this pioneering engine can be found in chapter 8.

The Treager gas turbine book contained pictures of small stationary gas turbine engines called Auxiliary Power Units or APUs. An APU is normally mounted onboard an aircraft and is used to drive electrical generators, hydraulic pumps, air compressors or the unit itself is used as an air compressor. APUs may operate when the aircraft is standing on the ground or when it is in the air. APUs are normally required to provide starting power for the aircraft propulsion engines if some sort of external power supply is not available. Small stationary gas turbine engines are very rare in other applications, but can sometimes be found driving ground based generators and pumps. Since the end of the second world war many automotive manufacturers have experimented with installing gas turbines in cars and trucks. The most famous example of this must be the car built in the US by Crysler, the UK based Rover car company also built a series of prototypes.

During the 1970's gas turbines appeared briefly in motor sport. Formula One and the American Indycar series both made use of gas turbines. Several teams including Lotus fitted gas turbins to cars, the Pratt and Whitney PT6 and the General Electric T58 found applications here. The principal advantage of using a gas turbine in Motorsport was to get around various regulations as there is no equivalent CC capacity in a gas turbine. Specifying the air intake area was one way that regulations were eventually placed upon gas turbines which made them less competitive in terms of power output. Other problems with using gas turbines in Motorsport include huge throttle lag and high fuel consumption.

Today Gas Turbines are still used in motorsport, Drag Racing, Tractor Pulling and Powerboat racing find uses for ex-aircraft units.

In the 1960s a whole host of small engines were developed as starters and APUs for aircraft. These engines are magnificent works of mechanical art and very little has been written about them. I consider these units to be collectable in the same way that many stationary reciprocating engines are collected and restored by enthusiasts. Most of these units can be operated as stationary engines with some basic knowledge of electronics and fluids.

This document is based around my experiences in operating small gas turbine engines over the past few years. In addition to developing the turbo charger based unit I have restored and instrumented a number of small aircraft type engines. I have met and corresponded with a number of other like minded individuals which has provided further valuable experience and inspiration. From time to time I am asked for advice and information about small engines, this document seeks to condense my experience and provide technical information to assist others.

This document should never be used to substitute or override manufacturers recommendations. It should be confined to ground based stationary gas turbine activities and not applied to any aircraft installations. The author cannot except responsibility for any errors and the consequences of the information provided in this document. Gas turbine engines are potentially dangerous always take safety precautions when operating such units.

From my experience of "Hobby" gas turbine applications, the types of engines which can be found fall neatly into three basic categories-

1. Home or experimentally constructed stationary gas turbines based around a complete automotive turbo-charger. An engine is formed simply by adding a combustion chamber between the compressor outlet and turbine inlet of the turbo.

2. Home or commercially constructed miniature turbo-jet engines intended for the propulsion of model aircraft. These units can be constructed from the rotating parts of turbo chargers or can be completely fabricated from scratch. A number of manufacturers now offer ready to run gas turbine engines, prices start at about £1000.

3. Commercially built gas turbines intended for aircraft propulsion, aircraft starting or auxiliary services, these engines are obtained as surplus or life expired units. This document seeks to concentrate on these units and how to operate them.

I will apply the definition of "small" to a gas turbine of up to 250 horsepower. Most APUs and gas turbines intended as starters fall into this category. This document will concentrate on these engines but chapter 8 will detail how a turbo charger based stationary engine can be built.