This article was taken from the May 2015 issue of WIRED magazine. Be the first to read WIRED's articles in print before they're posted online, and get your hands on loads of additional content by subscribing online.
Discreet, vast and very fast, Bentley's new Mulsanne Speed may look like the sort of automobiles you'd spot a Russian oligarch in the back seat of, but there's a secret under the hood: its born-again L-series engine first saw the light of day when Khrushchev was sitting in the Kremlin. You can have all the new technology in the world, but it takes an engine of character -- and 1,100Nm of torque -- to make a 2.8-tonne motor car travel at speeds of up to 296kph.
Now 47 years old and with no use-by date in sight, the Six-and-Three-Quarter Litre -- to give it its official registered name -- becomes this year the world's longest-serving passenger-car engine, passing Lamborghini's 60deg V12, which was first used in its Miura model in 1962 and only phased out when the Murciélago made way for the Aventador in 2010.
The original L-series -- the same design, the same construction, just a shorter stroke and a swept capacity of 6.25 litres -- first saw service in Rolls-Royce Phantoms and S-series Bentleys in 1959. Rolls-Royce had acquired Bentley in 1931 and the two companies' product lines mirrored each other's until Volkswagen finally acquired the Bentley name 57 years later. And, inevitably, rumours started circulating almost immediately that the L-series was being lined up for retirement.
The engine has, since 1959, seen constant development -- growing in displacement, the addition of fuel injection, gaining one and then two turbochargers -- but the arrival of Volkswagen heralded two threats: the internal peril of VW's W12 engine and the external challenges arising from European legislators pushing carmakers towards lower carbon emissions. European standards are set to steer carmakers to a goal of 95g of carbon dioxide per kilometre travelled by 2025 -- as of last year the L-Series was producing 394g per kilometre.
But the L-series team wasn't about to throw in the towel, and this latest iteration of the engine is EuroVI compliant -- the same standards new models making their debuts in 2015 must meet. Not that it is about to win any green awards -- it still belches out 342g per kilometre. Butthen again the 21gkm, 800cc twin in Volkswagen's featherweight XL1 eco-car would struggle to get the hefty Mulsanne Speed to move, never mind reach 98kph in 4.8 seconds.
Combustion
Revisions to the combustion chamber needed a new piston design. These created -- on the compression stroke -- an increased tumble motion. The V8's crankshaft -- channelling 1,100Nm of torque at less than 2,000rpm -- has been rebalanced and the engine is lubricated by low-viscosity 0W30 oil to reduce friction.
Emissions
The latest V8 has the classic, slow-revving, two-valve push-rod architecture. But its redesigned head now includes longer plugs to shift the spark position to the centre of the combustion chamber, as well as sodium-filled exhaust valves and a revised cooling system inside the cylinder head to deal with higher temperatures. Now, torque and power are up, and emissions down.
Modelling
Bentley uses computational fluid dynamics modelling, meaning multiple iterations of combustion chamber design were explored before settling on this design. This improved swirl (horizontal movement of air/fuel mix) and tumble (vertical) for increased combustion and faster burn.
Bosch unit
The Bosch unit in the Mulsanne Speed has 1,275 functions and runs at just 180MHz. However, there are no fewer than 22,720 "labels", each of which needs calibrating to help deliver the engine's driveability -- even when driven in four-cylinder "green" mode.
Pushrod valve
Old-school, push-rod valve control limits engineers' ability to influence combustion efficiency via valve timing. However, the latest V8 does have fresh cam profiles and valve spring rates. The next big step will be a hybrid version -- an example prototype of which was recently unveiled by Bentley.
This article was originally published by WIRED UK
