XM

Hydractive 1

The Hydractive I suspension system appeared with the XM. Unlike the simpler hydropneumatic suspension used on the DS, GS/GSA, CX, BX and some XMs, this one has two modes of operation, soft and hard. The suspension functions in soft mode but it will be switched to the hard mode when the computer deems this necessary for the sake of roadholding and safety.

To achieve this, the first hydractive system adds two spheres (one for each axle) and an electric valve to the struts and spheres of the standard hydropneumatic setup.

During normal driving, the computer keeps the suspension in soft mode most of the time but—based on the input provided by many sensors (steering wheel, accelerator pedal, body movement, road speed and brake), including the Sport/Comfort switch on the dashboard—the suspension ECU decides when to switch to hard mode; in other words, when to deactivate the additional spheres for extra roadholding and safety.

When the driver selects the Sport setting, the suspension is switched to hard mode constantly. This setting is not what any Citroën driver would call comfortable… The successor system, Hydractive II overcomes this limitation.

Hydropneumatic Suspension

As we saw, the ideal suspension would require elasticity decreasing with the load, constant ground clearance, shock absorbers integrated into the suspension—all these beyond the obvious independent suspension for all wheels. And this is exactly what Citroën’s unique hydropneumatic suspension offers.

According to the Boyle–Mariotte formula defined in the 17th century, the pressure and the volume of a mass of gas are inversely proportional at a constant temperature. Therefore, by keeping the mass of the gas constant and changing the volume of its container, its pressure can be controlled (the usual pneumatic suspensions operate on the opposite principle: air is admitted or withdrawn from the system by compressors and exhaust valves, modifying its mass while keeping the volume constant).

The volume changes are controlled by hydraulics, a technology in widespread use in every branch of the industry. As liquids are non-compressible, any amount of liquid introduced at one end of a hydraulic line will appear immediately at the other end (this phenomenon was first formulated by Blaise Pascal). Using this principle, motion can be transmitted, multiplied or divided (according to the relative sizes of the operation cylinders), with velocity increased or decreased (using varying cross sections in the tubing), to any distance desired, over lines routed freely.

Hydraulics are immensely useful, very efficient, reliable, simple to use, and—due to their widespread deployment—relatively cheap. It is no wonder that it is used for many purposes even in the most conventional vehicles: shock absorbers, brake circuit and power assisted steering being the most trivial examples; however, Citroën is the only one to use it for the suspension.

A Suspension Primer

From the early days of the automobile—and even before, in the time of horse-drawn carts—it was already well known that the body of the car, housing both the passengers and the load, has to be decoupled from the unevenness of the road surface.

This isolation is much more than a question of comfort. The vertical force of the jolts caused by the repeating bumps and holes of the road surface are proportional to the square of the vehicle speed. With the high speeds we drive at today, this would result in unbearable shock to both people and the mechanical parts of the car. Jolts in the body also make it more difficult to control the vehicle.

Consequently, there has to be an elastic medium between the body and the wheels, however, the elasticity and other features of this suspension medium are governed by many, mostly contradicting factors.

Electronic Diesel Control

Just like it is the case with gasoline engines and carburetors, a mechanical device—even one as complicated as a diesel injection pump—cannot match the versatility and sensibility of a microcomputer coupled with various sensors, applying sophisticated rules to regulate the whole process of fuel injection.

The only input a mechanical pump can measure is the engine speed. The amount of air entering into the engine, unfortunately, is far from being proportional to engine speed, and the turbo or the intercooler disturbs this relationship even further. As the injection always has to inject less fuel than the amount which would already generate smoke, the mechanical pump—capable only of a crude approximation of what is actually going on in the engine—wastes a significant amount of air, just to be of the safe side.

The satisfactory combustion in diesel engines relies on the exhaust as well—if this is plugged up, more of the exhaust gases stay in the cylinder, allowing less fresh air to enter. A mechanically controlled injection pump has no feedback from the engine (except for the engine speed)—it will simply pump too much fuel into the engine, resulting in black smoke. An electronically controlled injection pump, on the other hand, can tell how much air has actually entered by using a sensor (although only the latest systems use such a sensor).

Diesel engines

Diesel oil has been a contender to gasoline for many decades. Earlier diesel engines were not refined enough to win the hearts of many drivers but recent advances in technology made these engines not only worthy competitors in all areas but in some features—fuel economy or low end torque, to name just two—even exceeding the characteristics of their gasoline counterparts. And in addition to the general technological advantages, Citroën’s diesel engines have a widely accepted reputation—even among people blaming the quirkiness of its suspension or other features—of being excellent and robust.

As it is widely known, diesel engines have no ignition to initiate their internal combustion, they rely on the self-combustion of the diesel oil entering into a cylinder filled with hot air. Due to this principle of operation, the supply of the fuel has to comply with much more demanding requirements than it is necessary in the case of gasoline engines.

Unlike in the gasoline engine, not a mixture but air enters into the cylinders via the inlet valves. During the adiabatic compression all the energy absorbed is used to increase the temperature of the gas. The small droplets of fuel will be injected at high velocity near the end of the compression stroke into this heated gas still in motion. As they start to evaporate, they form a combustible mixture with the air present which self-ignites at around 800 °C.

This self-ignition, however, is not instantaneous. The longer the delay between the start of the injection and the actual ignition (which depends on the chemical quality of the diesel oil, indicated by the cetane number), the more fuel will enter the cylinder, leading to harsher combustion, with the characteristic knocking sound. Only with the careful harmonization of all aspects—beginning of injection, the distribution of the amount injected in time, the mixing of the fuel and air—can the combustion be kept at optimal level.

Electronic Fuel Injection

The Otto engine needs a mixture of fuel and air for its operation. It would be the task of the fuel supply—carburetor or injection—to provide the engine with the ideal mixture. Unfortunately, there is no such thing as an ideal mixture.

Perfect combustion, as chemistry calls it, would require air and fuel in proportion of 14.7 parts to 1 (this is the so-called stoechiometric ratio). While this might be satisfactory for the scientists, the real-life conditions of a vehicle call for slightly different characteristics.

We use the ratio of actual mixture to the stoechiometric mixture, called lambda (λ), to describe the composition of the mixture entering the engine: λ=1 denotes the chemically ideal mixture, λ<1 means rich, λ>1 is lean.

The best performance would require a slightly rich mixture, with the lambda around 0.9, while fuel economy would call for a slightly lean one, between 1.1 and 1.3. Some harmful components in the exhaust gas would reduce in quantity between lambda values of 1 to 1.2, others below 0.8 or above 1.4. And if this is not yet enough, a cold engine requires a very rich mixture to keep running. After warming up, the mixture can return to normal, but the temperature of the incoming air still plays a significant role: the cooler the air, the denser it becomes, and this influences the lambda ratio as well.

All these requirements are impossible to satisfy with simpler mechanical devices like carburetors. Electronic fuel injection provides a system that can measure the many circumstances the engine is operating in and decide on the amount of fuel (in other words, the lambda ratio) entering the engine. By carefully adjusting the internal rules of this device, manufacturers can adapt the characteristics of the fuel injection to the actual requirements: a sporty GTi would demand rather different settings than a city car; besides, catalytic converters have their own demands that, as we will later see, upset the applecart quite vehemently.

LEDs everywhere

A project I've been contemplating for quite some time now, and with the constant advances in LED technology, I thought the time had finally come: LEDs all around the car. Not in the external traffic lights, for sure (with one exception, I'll return to that) but inside, with the large collection (nearly a hundred, really!) of bulbs everywhere. Let's count: instrument panel, warning lights, LH and RH display, switches, A/C panel, ceiling and rear lights, gloveboox, boot, door sill courtesy, radio surround, ashtray, cigar lighters, sunvisor. Quite a lot.

The Circuit Diagram Project

The XM is a complicated beast when it comes to electrics and electronics: there is no denying that. It entered the executive market with practically all the toys of the era. There are billions of components, thick harnesses, a plethora of connectors, everywhere. Finding anything without the proper diagrams is nigh impossible.

Blower motor

There are two parts that might need replacing in the blower motor: the control module and the brushes.

You might suspect the control module when the blower doesn't want to run at all (although there might be other reasons for this as well). Brushes are likely to be worn if the motor doesn't start one day but runs fine on the other. In this case, a moderate whack on the motor from below starts it nicely.

Switches with LEDs

The XM Series 1 was born in the era of automotive bulbs. When it came to the Series 2, LEDs were already entering the scene. New switches like those on the dashboard started to come with LED illumination but earlier switches retained (and shared with other Citroëns of the time) remained with plain old bulbs in many switches of the door windows, electric seats or some center console switches. Retrofitting LEDs to these is a simple process with huge benefits: longevity (practically, eternity) of the LED, cool operation temperatures and lower power consumption are all welcome changes.

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