Geoffrey Freeman Allen wrote, "Whether it was in response to a BR specification or whether the builders off their own bat decided to go for a 50,000lb maximum tractive effort I can't discover. No other entrant in the Type B, mixed traffic category of BR's original 1955 Pilot Scheme had such starting punch. Despite the slight rating superiority of its original 1,250 hp Mirrlees engie, the Brush Type 2 could muster only 42,000lb.
That high peak effort and its corollary of a continuous 25,000lb were the primary reason for the type's quaint, though not worldwide unique, arrangement of odd bogies. Only by resorting to five powered axles, with no idle wheels, could the designers achieve their target and at the same time keep the locomotive within BR's Type B parameters. The 97 ton Co-Bo was in fact markedly lighter than the 104 ton Brush effort. Another distinguishing feature of the high tractive effort was the small diameter of its wheels - 3ft 3.5" against a Brush Type 2's 3ft 7".
BR invited the Co-Bo's candidacy in the Pilot Scheme chiefly to evaluate a two-stroke diesel engine against the four stroke models adopted for all the other types. Theoretically, a two-stroke promised higher output per pound of engine weight because in the two-stroke cycle, each cylinder generates power at every revolution of its crankshaft, whereas in a four-stroke, the power comes from alternate revolutions. The difference also minimises temperature variations within a two-stroke engine's cylinders. The two-stroke also dispenses with cylinder head valves and their operating gear, but on the other hand, the means by which a two-stoke "scavenges", or scours the burnt gasses of each ignition from a cylinder by injected air fom a blower are less efficient than the processes in a four-stroke cycle.
On the face of it, the Crossley HSTVee 8 cylinder power unit looked an engagingly simple alternative to four-stroke engines. The power on offer from each of its eight cylinders was high, yet was obtained without turbo-charging or the many other mechanical complications of four-stroke engines. This itself was the source of much mechanical problems in traffic. Spring metal air inlet 'reed' valves were a constant source of problems as the brittle metal soon shattered under load.
What follows are extracts from an instruction book that is the first of six volumes providing engineering data and information concerning the construction, operation and maintenance of the equipment supplied to the British Transport Commission by Associated Electrical Industries Ltd. Traction Division, Trafford Park, Manchester. The Volumes are dated 1962.
The first volume contains a brief general description of the complete locomotive, together with the essential operating data. The other volumes describe the methods of carrying out necessary maintenance on the equipment and are listed as follows purely for the sake of completeness;
1 : General
2 : Diesel Engine
3 : Main & Auxiliary Generators, and Traction Motors
4 : Auxiliaries
5 : Control Equipment
6 : Mechanical Parts, Brake Equipment, and Train Heating Boiler
The preface to Volume 1 states that "although every effort has been made to ensure that the details given are enough for the work to be done, this book is not meant to instruct untrained persons. It has been assumed that all maintenance will be done by experienced technicians, under the supervision of a qualified engineer.
The available extracts are :-
Locomotive Data & Performance
Driver's Controls, Instruments and Gauges
General Maintenance & Lubrication Schedules