Citroën 2CV - Engines

Engines

The engine was designed by Walter Becchia and Lucien Gerard, with a nod to the classic "boxer" BMW motorcycle engine (it is reported that Becchia dismantled the engine of the BMW motorcycle of Flaminio Bertoni before designing the 2CV engine). It was an air-cooled, flat-twin, four-stroke, 375 cc engine with pushrod operated overhead valves and a hemispherical combustion chamber. The notoriously underpowered earliest model developed only 9 bhp DIN (6.5 kW). A 425 cc engine was introduced in 1955, followed in 1968 by a 602 cc one giving 28 bhp (21 kW) at 7,000 rpm. With the 602 cc engine, the tax classification of the car changed so that it became in fact a 3CV, but the commercial name remained unchanged. A 435 cc engine was introduced at the same time in replacement of the 425 cc; the 435 cc engine car was christened 2CV 4 while the 602 cc took the name 2CV 6 (although a variant did take the name 3CV in Argentina). The 602 cc engine evolved to the M28 33 bhp (25 kW) in 1970; this was the most powerful engine fitted to the 2CV. A new 602 cc giving only 29 bhp (22 kW) at a slower 5,750 rpm was introduced in 1979. Despite being less powerful, this engine was more efficient, allowing lower fuel consumption and better top speed, at the price of decreased acceleration. All 2CVs with the M28 engine can run on unleaded petrol, but attention is needed to ensure that valve clearances are maintained. Although there weren't any more powerful engines for this model, Citroen used the same engine design on other cars, like the AMI, the LN, the Dyane. The slightly increased capacity mapped electronic ignition version in the Visa was significantly different. They had a bit more power, and many 2CV owners installed those engines in their car for more flexibility. Cutaway drawings of the 2CV engine are pictured on the citroenet.org.uk website.(Pictured Here )

The 2CV used the wasted spark ignition system for both simplicity and reliability and had only speed-controlled ignition timing, no vacuum advance taking account of engine load.

Unlike other air-cooled cars (such as the Volkswagen Beetle and the Fiat 500) the 2CV's engine had (for simplicity and reliability) no thermostat valve fitted to its oil system to allow the oil to reach normal operating temperature quickly in cold weather. All the oil in the system passed through an oil cooler mounted behind the fan and received the full cooling effect regardless of the ambient temperature. This removes the risk of overheating from a jammed thermostat that can afflict water- and air-cooled engines and the engine can withstand many hours of running under heavy load at high engine speeds even in hot weather. To prevent the engine running cool in cold weather (and to improve the output of the cabin heater) all 2CVs were supplied with a grille blinds (canvas on early cars and a clip-on plastic item on later ones) which blocked around half the grille aperture to reduce the flow of cool air to the engine.

The engine's design concentrated on the reduction of moving parts. The cooling fan and dynamo were built integrally with the one-piece crankshaft, removing the need for drive belts. (Late models (shown in photo) used an alternator mounted high above the engine, to keep it dry, run with a drive belt). The crankshaft was a "built-up" design similar to that used in many motorcycle engines. In place of the split big ends and two-piece big-end bearings commonly used in car engines, the 2CV engine used connecting rods with one-piece big-ends and bearings, which were fitted to the crankpins before the crank was assembled. The crankpins were then hydraulically pressed into the webs after being chilled with liquid nitrogen to cause them to contract. The entire unit (crank, big-end bearings and connecting rods was then fitted to the engine. The camshaft drive gears incorporate a spring-loaded split gear, to reduce the effects of gear wear and backlash on valve timing and ignition timing. With the contact breaker in a housing on the end of the crankshaft there was no separate jackshaft to be affected by chain or gear wear and associated backlash.. The use of gaskets, seen as another potential weak point for failure and leaks, was also kept to a minimum. The cylinder heads are mated to the cylinder barrels by a lapped joints with extremely fine tolerances as are the two halves of the crankcase and other surface-to-surface joints.

As well as the close tolerances between parts the engine's lack of gaskets was made possible by a unique crankcase ventilation system. On any 2-cylinder boxer engine such as the 2CV's, the volume of the crankcase reduces by the cubic capacity of the engine (375 to 602cc in the Citroen's case) when the pistons move together. This, combined with the inevitable small amount of 'leakage' of combustion gases past the pistons leads to a positive pressure in the crankcase which must be removed in the interests of engine efficiency and to prevent oil and gas leaks as the pressure tries to escape. The 2CV's engine has a combined engine 'breather' and oil filler assembly which contains a series of rubber reed valves. These allow positive pressure to escape the crankcase (to the engine air intake to be recirculated) but which close when the pressure in the crankcase drops as the pistons move apart. Because gases are expelled but not admitted this creates a slight vacuum in the crankcase so that any weak joint or failed seal causes air to be sucked in rather than allowing oil to leak out. Since the oil serves both as the engine's lubricant and forms a vital part of the cooling system this 'anti leak' system was especially important.

These design features made the 2CV engine highly reliable; test engines were run at full speed for 1000 hours at a time, equivalent to driving 80,000 km (50,000 mi) at full throttle. They also meant that the engine was very much "sealed for life" - for example, replacing the big-end bearings required specialised equipment to dismantle and reassemble the built-up crankshaft, and as this was often not available the entire crankshaft had to be replaced. However, the engine is very under-stressed and long-lived, so this is not a major issue. Until the 1960s it was common for other car manufacturers' engines to need full strip downs and rebuilds at as little as 80,000 km (50,000 mi) intervals; un-rebuilt 2CV engines are still running that are passing 400,000 km (250,000 mi).

If the starter motor or battery failed, the 2CV had the option of hand-cranking, the jack handle serving as starting handle through dogs on the front of the crankshaft at the centre of the fan. This feature, once universal on cars and still common in 1948 when the 2CV was introduced, was kept until the end of production in 1990. The jack handle also served as the wheelbrace and could be used to remove the nuts that held the front wings on - part of the car's design to facilitate easy maintenance.