Engines

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.

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.