Mirrlees National OP Medium Speed Diesel Engine

[JFC Fuller]

The Ruston AO series is reasonably well known because it entered production and service prior to anyone, including the Royal Navy, realising how awful it was. However, another British medium speed diesel received government funding at around the same time. The Department of Scientific and Industrial Research (D.S.I.R) funded Mirrlees National, who had provided the diesel engines for the Royal Navy Round Table class then part of Hawker Siddeley Group, to undertake research on a two-stroke apposed piston engine designed for 1250hp per cylinder. The total funding, split between Mirrlees and the D.S.I.R was £300,000 and the contract was to last five and a half years from mid-1964. The subsequent history of the development is a bit of a mystery but work does not appear to have extended beyond the end of the D.S.I.R contract. In addition to the description of the engine below I have attached a data table and graphic from a 1960s report entitled The Use of Medium-speed Geared Diesel Engines for Ocean-going Merchant Ship Propulsion by J. Neumann, B.Sc., C.Eng., A.M.I.Mech.E. (Associate Member)* and J. Carr, C.Eng., A.M.I.Mech.E. (Associate Member). The report is also attached.

The comparison size comparison with the Ruston AO series underscores the superficial attractiveness of that engine prior to its flaws being discovered. It had been intended to use it in a Royal Navy submarine tender for East of Suez service and AO16 was ultimately used in the Rover class tankers in the same sort of 16,000SHP configuration this report looks at. It was ultimately replaced in those ships due to its appalling unreliability. We have an existing thread on the Fairbanks Morse 38A20, referenced in the comparison table, here.

The OP engine is a through-scavenged opposed-piston, two-stroke cycle engine with the cylinders arranged in twin parallel vertical banks. For marine propulsion use the engine will normally be direct reversing. The upper pistons of each bank will control flow through the exhaust ports and the lower ones the flow through the inlet ports. The engine will have four crankshafts, rotating in the same direction, which may be coupled to the output shaft either by gearing or possibly a drive-plate arrangement. With the former method the output shaft will normally rotate at a lower speed than the crankshafts in a marine propulsion installation. Details of the design have now been settled and a single cylinder downward flow engine has been manufactured and is now being used to test the thermally-loaded components such as pistons and exhaust port bars. Other test rigs have also been manufactured to assess performance of fuel pumps, small-end bearings, air flow through ports etc. A half-scale back-to-back rig has also been manufactured to investigate the unique method of coupling the four cranks with a common drive plate to a central output shaft. The design of a twin cylinder test engine is now well advanced and this should be running by the end of 1967. The object is to produce anengine in which the maintenance load is comparatively light,due to long intervals between overhauls and designed accessibility. In particular, it will be possible to remove a complete cylinder liner sideways from the engine without disturbing the crankshafts and it will be possible to remove any of the four crankshafts without disturbing the engine frame or its alignment with the gearbox.The engine frame will be of cast-iron construction with hydraulically-tensioned through bolts carrying the firing loads.

There appears to be a more detailed diagram of the Mirrlees National OP, and a notional machinery layout, in a 1968 edition of the British Motorship but I don't have access to it.