Emphasis on uniformity
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for clean power by Paul Brockington
THE standard engine for Marathon — the TL 12 has been developed by AEC and will be built at the Southall plant. The engine has the same bore and stroke as the AV 760 because of production considerations but in all other respects differs so substantially from the AV 760 that it can be regarded as a completely new engine.
The main engineering objectives have been to produce a modestly turbocharged engine which has a long in-service life, good reliability and is satisfactory from an environmental point of view.
The TL 12 has a straight-six layout and a capacity of 12.47 litres (761 cu in.); bore is 135mm (5.37in.). stroke is 142mm (5.59in.). The engine's gross rating to BS AU 141a:1971 is 281.56 bhp (210 kW) at 220 rpm, maximum gross torque is 790 lb ft (109.3 kg /m) at 1300 rpm. Installed output is 273 bhp while the installed torque is 780 lb ft.
Environmentally, the engine gets good marks for both smoke and noise ratings, and it is notable that turbocharging and other detailed features reduce the noise level by 2 to 3 dBA compared with the AV 760 engine. At peak power. the Hartridge smoke rating is about 7 units, ie some 24 units belOw the BS AU 141a limit; at 1800 rpm the advantage is increased to 28 units and at 1200 rpm it is about 20 units. Production engines should operate at a smoke level of at least 10 HSU below the BS limit and give an external noise level of 88 dBA. Sound-deadening material in the neck of the sump and retarding the injection timing assist in reducing noise.
Modification of the combustion chambers of the Mahle pistons reduces thermal stressing, and spray from a low-level jet is distributed across the underside of each crown during each crankshaft revolution to give uniform piston cooling. Dry liners add to structural rigidity: they also reduce noise marginally and obviate the cavitation erosion of the liners or nearby surfaces.
Having ample surplus capacity, the lubrication system is priority conscious in distribution to the various systems and particular attention has been paid to the method of feeding oil to the main bearings to promote optimum lubrication and heat dissipation. No scraper ring is used below the gudgeon pin, which improves lubrication of the ring belt and is said to give freedom from scuffing. Other important areas of lubrication control include the rear main bearing of the nitrided crankshaft where a positive actionoil seat is fitted and it is notable than an oil/water heat exchanger is designed for rapid warming up of the oil as well as cooling at higher loads.
The compression ratio is 15 to I and a bmep at maximum torque of 159 psi (11.13 kg /sq cm) compared with a bmep at maximum. bhp of 134 psi gives a torque back-up of 19 per cent.