The Pétrel was an upright V12 engine with two banks of six cylinders, arranged at 60° to each other, driving a common
crankshaft. The cylinder blocks were bolted onto the
crankcase, all light alloy parts. The crankcase came in two pieces, with seven crankshaft bearings in the upper section.
Roller bearings were used at the crankshaft ends; the remaining five were plain. The upper crankcase section also had integrally cast water channels as part of the cooling system. Steel cylinder liners were screwed into the heads, with their lower parts projecting into the crankcase. Steel seats for valves and sparking plugs were shrunk into the heads. The pistons were forged from
alugir, with three compression and one scraper ring and floating bronze bushes for the
gudgeon pins. The twelve pistons were connected to the six
crankpins in pairs, each with a master and an auxiliary
connecting rod. The master rods had forked big ends with white metal bearings; the auxiliary rod ends ran between the forks on bronze bushes. The Pétrel had four overhead valves per cylinder, two exhaust and two inlet, in bronze valve guides. Each bank had its own
overhead camshaft and each cam operated a pair of valves through T-shaped
tappets, the stem of the T moving in a guide to avoid sideways force on the valve stems. There were two
spark plugs per cylinder and twin magnetos. A carburetor fed the mixture into the intake of the
supercharger, at the rear of the engine. The Pétrel's output could be left or right handed; a Lorraine patent
planet gearset, with six satellite gears, provided an 11:17 reduction of propeller shaft speed. Engine lubrication was by forcing pressurized oil through the crankshaft, with sump scavenging. The supercharger had its own lubrication system. First run in 1932, the early Pétrels produced only 370 kW (500 hp), but by 1938 the engine had been developed into the 12Hars model which gave 640 kW (860 hp). ==Operational history==