Leyland L60

The Leyland L60 was a British vertical six-cylinder opposed-piston two-stroke multi-fuel diesel engine designed by Leyland Motors in the late 1950s/early 1960s for the Chieftain main battle tank (MBT). The engine was also used in the Vickers MBT and its Indian-built derivative, the Vijayanta.
Development.
The initial engine choice in 1954 for what was known at the time as "Medium Gun Tank No.2", later designated the "FV4201" and given the service name 'Chieftain', was a Rolls-Royce diesel V8, however during the Chieftain's design phase NATO introduced a policy in 1957 requiring all armoured fighting vehicles to have a multi-fuel capability. This left the Rolls-Royce engine an unsuitable option and so a new engine with this capability was required. 
Leyland Motors, under the direction of the Fighting Vehicles Research and Development Establishment (FVRDE) at Chertsey, was asked to develop an opposed-piston two-stroke diesel of similar design to those previously produced by Napier and Tilling-Stevens, the latter's Commer TS3 engine being particularly highly regarded.
This configuration, apart from being well-suited to multi-fuel use, also had the advantages of being of simple design with a low parts count, had low bearing loads, and possessed good cold-starting characteristics. Some technical assistance was provided to Leyland by Rolls-Royce, who by that time was a parent to the Napier aero-engine company, Napier itself remained a subsidiary of English Electric. Both Tilling-Stevens and Leyland produced single-cylinder prototype engines for the tank engine project and by 1959 the resulting complete engine design had become the Leyland 60, or L60, with the first engine running that same year.
Overview.
One of the reasons the L60's unusual configuration was chosen was so as to obtain as compact a power plant as possible so allowing the height of the vehicle to be kept as low as was practicable, a requirement for the Chieftain's design philosophy which was also seen in the recumbent driver's position.
The use of the two-stroke cycle allowed for a greater power for a given displacement, a 19- litre diesel engine being expected to be capable of around the same power as the 600 hp 27-litre petrol Meteor tank engine whilst taking up less room in the engine compartment. Scavenging, necessary in a large two-stroke diesel for evacuating the cylinders of exhaust gases, was performed by a Roots blower.
Maintenance.
The Chieftain's L60 engine and cooling system were designed into an integrated engine-pack which could be changed "in the field" using the crane of an FV434 Armoured Repair Vehicle, which had been designed for this purpose and a complete engine change took around one-and-a-half, to 2 hours.
The requirement for an easily changeable engine pack was the result of a British Army analysis of previous tank battles that concluded that a likely future tank battle would last no longer than two hours and so the most demanding requirement expected for any tank engine during wartime would be for it to be run at full power for this total amount of time only and so it would then be advantageous for it to be removed from the vehicle after the battle and exchanged for a fresh engine within a minimum of time.
This would also allow the engines to be worked on in properly equipped REME workshops rather than 'in the field', the engines being exchanged between vehicles and workshops as-and-when required. This philosophy was also applied to the contemporary FV430 series of vehicles.
Performance.
The initial production L60 units were, at 585 bhp at 2,100 rpm, down on the designed initial power of 600 bhp and were plagued with reliability problems. These problems were exacerbated during the Chieftain's introduction by initially an inadequate spares stock and an insufficient spare engine 'float' . The L60 reliability problem would have been far worse had it not been for the removable engine pack, which usually allowed a vehicle to be operational again with a replacement engine within a couple of hours of breaking down. A persistent source of trouble was the failure of the cylinder liner sealing resulting in coolant leakage into the cylinder bore. Fan drive belts overstressed fan bearing housings in the crankcase leading to cracking. Reliability did improve over time with modifications and improvement programmes, such as the "Sundance" programme which also improved power output. Sundance was carried out in five main phases between 1976 and 1979. Sundance had been preceded by "Dark Morn", "High Noon", and the initial "Fleetfoot" engine development programme -the person responsible for the choosing of code names apparently being an admirer of the Western film. Sundance programme was the subject parliamentary questions in the House of Commons in 1978. With the final rectification of most of the L60's previous reliability and power problems, vehicle availability levels rose to 80%. In the 1990 Gulf War Chieftain AARV and CHAVRE availability levels exceeded those of the Challenger 1 tank which had by that point replaced Chieftain.
Final production engines were . Modifications to engines in service under the various improvement programmes meant that final L60 engines were 
Initially, due to unfamiliarity with the two-stroke engine's different exhaust note and power band compared to a four-stroke engine, and with the resulting difficulty in choosing the correct gear required for the particular driving task, trainee drivers tended to under-rev the engines and use inappropriate gear selections, leading to great difficulty climbing gradients, and when the Chieftain Mk 1 was first introduced some drivers had difficulty climbing the vehicle onto the trailers of Thornycroft Antar tank transporters.
L60 variants.
In 1975 all British Chieftains were brought up to Chieftain Mark 5 standard as part of the "Totem Pole" programme which included the fitting of all vehicles with the 750 bhp L60 Mark 8A. On undergoing "Totem Pole" upgrades Chieftain Mk 2 vehicles were re-designated the Mark 6. Mk 3 vehicles became the Mk 7, and Mk 3/3 vehicles became the Mk 8.
Transmission.
The engine was mated with a Merritt-Brown TN12 triple-differential epicyclic gearbox providing "regenerative" steering, a derivative of the system first used on the Churchill tank. The gearbox was semi-automatic foot-operated and had six forward, and two reverse gears. Like the engine, it was designed to be quickly replaceable. The TN12 had originally been developed for the cancelled FV300 light tank series. A scaled down version of the TN12, the TN15, was used in the CVR(T) series of vehicles.