Editing Citroën 2CV (section)

== Engines ==
{{main|Production data of the Citroën 2CV}}
[[File:Citroën 2cv engine.jpg|thumb|right|upright|2CV6 engine compartment, post-1981 (with inboard disc brakes)]]
[[File:Allumage 2cv.svg|thumb|right|2CV ignition system diagram]]
[[File:Boxerengineanimation.gif|thumb|Movement of flat-twin engine pistons, connecting rods and crankshaft]]
[[File:Flat twin2CV.jpg|thumb|upright|2CV flat-twin engine halved with piston removed—showing connecting rod, crankshaft, crankcase, camshaft, spring-loaded split timing gear and engine oil pickup]]

The engine was designed by [[Walter Becchia]] and Lucien Gerard,<ref>{{Cite book|chapter-url=https://books.google.com/books?id=YIJ8AwAAQBAJ&q=Walter+Becchia+and+Lucien+Gerard&pg=PT198|title=Citroën: The Complete Story | last = Cole | first = Lance | publisher = Crowood | year = 2014 | isbn = 9781847976604 | location=Wiltshire|chapter=DS Details of design|access-date=4 March 2019}}</ref><ref name="TheTin" /> with a nod to the classic [[BMW]] [[Flat engine|boxer]] motorcycle engine. It was an air-cooled, [[flat-twin]], four-stroke, 375&nbsp;cc engine with pushrod operated [[overhead valve]]s and a hemispherical combustion chamber. The earliest model developed {{cvt|9|hp-metric|kW|1|adj=mid|DIN}}. A 425&nbsp;cc engine was introduced in 1955 with initially {{cvt|12|hp-metric|kW|1}}, later increased to {{cvt|18|hp-metric|kW|1}}, followed in 1970 by a 602&nbsp;cc one giving {{cvt|28|hp-metric|kW|1}} at 7000&nbsp;rpm. With the 602&nbsp;cc engine, the tax classification was bumped into the 3CV category, but the name remained unchanged. A 435&nbsp;cc engine was introduced at the same time to replace the 425&nbsp;cc unit; the 435&nbsp;cc engine car was named 2CV 4 while the 602&nbsp;cc took the name 2CV 6 (a variant in Argentina took the name 3CV). The 602&nbsp;cc engine evolved to the M28 with {{cvt|33|hp-metric|kW|1}} in 1970; this was the most powerful engine fitted to the 2CV. A new 602&nbsp;cc giving {{cvt|29|hp-metric|kW|1}} at a slower 5,750&nbsp;rpm was introduced in 1979, being the final version. This engine was less powerful, and 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.

The 2CV used the [[wasted spark]] [[ignition system]] for simplicity and reliability and had only speed-controlled ignition timing, no vacuum advance taking account of engine load.<ref>{{cite web |title=A Series Ignition System |url=http://www.2cvstuff.com/Files/ignition.pdf |access-date=25 December 2010 |archive-url=https://web.archive.org/web/20120316122922/http://www.2cvstuff.com/Files/ignition.pdf |archive-date=16 March 2012 |url-status=dead }}</ref> The [[Inlet manifold|inlet]] and [[exhaust manifold]]s were welded together into a single unit, with exhaust pipe and inlet tract abutting each other directly under the [[carburettor]] at an enlarged 'heat chamber'. Heat from the exhaust warmed both the metal and the air/fuel mixture inside the chamber, ensuring full [[vaporization]] of the fuel for greater combustion efficiency. The chamber also served as a reservoir of fuel/air mixture downstream of the carburettor body, allowing each cylinder to draw an equal and balanced amount of mixture for further efficiency and smooth-running. The heat chamber principle was especially suitable for an engine running at [[Wide open throttle]] and heavy loads for long periods of time, as was intended for the 2CV, when the throttle plate in the carburettor would be fully open, the [[Manifold vacuum]] would be low and the exhaust temperatures would be high.

Unlike other air-cooled cars (such as the [[Volkswagen Beetle]] and the [[Fiat 500]]) the 2CV's engine had no [[thermostat]] valve in its oil system. The engine needed more time for oil to reach normal operating temperature in cold weather. All the oil passed through an oil cooler 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 blind (canvas on early cars and a clip-on plastic item called a "muff" in the owner's handbook, on later ones) which blocked around half the aperture to reduce the flow of air to the engine.{{citation needed|date=October 2016}} Like many other air-cooled car engines, the 2CV's [[Sump|oil sump]] was wide and shallow, being formed from extensions to the crankcase castings in the form of an inverted 'T'. The exterior of the sump was formed with cooling fins and the underside protruded below the level of the chassis rails, exposing the sump to the flow of air as the car drove along. The shape of the sump ensured that as much of the oil within as possible lay close to the cooled metal on the underside, further helping to regulate the oil temperature. As with the oil cooler, this cooling effect was unregulated and varied greatly depending on air temperature, vehicle speed and engine load.

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 [[belt (mechanical)|drive belts]]. The use of [[gasket]]s, seen as another potential weak point for failure and leaks, was also kept to a minimum. The [[cylinder head]]s are mated to the cylinder barrels by lapped joints with extremely fine tolerances, as are the two halves of the [[crankcase]] and other surface-to-surface joints.{{citation needed|date=October 2016}}

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 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. The 2CV's engine has a combined engine "breather" and oil filler assembly which contains a series of rubber [[reed valve]]s. These allow positive pressure to escape the crankcase (to the engine air intake to be recirculated) but 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.<ref>{{cite web |url=https://discourse-cdn-sjc1.com/cartalk/uploads/default/optimized/2X/0/0e996467337b68f98a6b64844e16ba92610daec2_2_1035x567.gif |title=2CV engine animation |access-date=19 January 2019 |archive-url=https://web.archive.org/web/20190119174443/https://discourse-cdn-sjc1.com/cartalk/uploads/default/optimized/2X/0/0e996467337b68f98a6b64844e16ba92610daec2_2_1035x567.gif |archive-date=19 January 2019 |url-status=dead }}</ref>

As well as features intended for durability and efficiency, the early 2CV engines were also significantly under-tuned, making much less power than was theoretically possible. The original 375&nbsp;cc engine featured deliberately small-diameter inlet tracts and a small-diameter [[carburettor]] with conservative fuel jet sizes. This restricted both the engine's power output and its maximum rotational speed to far below the actual limits of its component parts, ensuring that however hard it was driven and despite extremes of temperature, it would not be close to its ultimate limits. The 375&nbsp;cc engine produced its {{cvt|9|hp-metric|kW|disp=flip}} at 3500rpm and peak torque at 2000rpm. Many of the improvements in power output made to the 2CV engine over its production life were merely the result of removing the original in-built restrictions with more efficient carburettors, manifolds and valve events. The power peak speed was raised to 4200rpm for the {{cvt|12.5|hp-metric|kW|disp=flip}} 425cc engine from 1955, 4500rpm from 1962 and 5000&nbsp;rpm - with {{cvt|18|hp-metric|kW|disp=flip}} - from 1963. The new 602&nbsp;cc and 435&nbsp;cc engines introduced in 1970 made their power at 6750rpm - nearly double the speed of the original engine from 1948 but with very few changes to the engine's internal design or components. If the original 375&nbsp;cc engine had the same [[power density]] as the original {{cvt|33|hp-metric|kW|disp=flip}} 602&nbsp;cc version it would have produced {{cvt|19|hp-metric|kW|disp=flip}}, more than double its actual rated output.  The original principle of deliberately restricting the engine's speed returned in 1979 for the revised M28 602&nbsp;cc engine, which had its carburettor and camshaft altered to make reduced power at a lower speed of 5750rpm in the interests of lower overall fuel consumption and better [[torque]] delivery. Even the most highly-tuned factory versions of the 2CV engine do not come close to the unit's actual upper limits - 2CVs used in the car's racing series use standard engines tuned to around {{cvt|45|hp-metric|kW|disp=flip}} which still prove reliable even in long 24-hour endurance competitions.

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 {{cvt|80000|km}} at full throttle. They also meant that the engine was "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. The engine is very under-stressed and long-lived, so this is not a major issue.{{citation needed|date=October 2016}}

If the starter motor or battery failed, the 2CV had the option of [[hand-crank#20th century|hand-cranking]], the jack handle serving as starting handle through [[Dog clutch|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.{{citation needed|date=October 2016}}