Stratified charge engine
In a stratified charge engine, the fuel is injected into the cylinder just before ignition. This allows higher compression ratios without "knock," and leaner air/fuel mixtures than in conventional internal combustion engines.
Conventionally, a four-stroke (petrol or gasoline) Otto cycle engine is fuelled by drawing a mixture of air and fuel into the combustion chamber during the intake stroke. This produces a homogeneous charge: a homogeneous mixture of air and fuel, which is ignited by a spark plug at a predetermined moment near the top of the compression stroke.
In a homogeneous charge system, the air/fuel ratio is kept very close to stoichiometric. A stoichiometric mixture contains the exact amount of air necessary for a complete combustion of the fuel. This gives stable combustion, but places an upper limit on the engine's efficiency: any attempt to improve fuel economy by running a lean mixture with a homogeneous charge results in unstable combustion; this impacts on power and emissions, notably of nitrogen oxides or NOx.
However, if we abandon the Otto cycle and inject fuel directly into the combustion-chamber during the compression stroke, we can liberate the petrol engine from a number of its limitations.
First, we can use a higher mechanical compression ratio (or, with supercharged engines, maximum combustion pressure) for better thermodynamic efficiency. Since fuel is not present in the combustion chamber until virtually the point at which combustion is required to begin, there is no risk of pre-ignition or engine knock.
We can also persuade the engine to run with a much leaner overall air/fuel ratio, using stratified charge.
As we have seen, combustion will be problematic if a lean mixture is present at the spark-plug. However, fueling a petrol engine directly allows us to direct more fuel towards the spark-plug than elsewhere in the combustion-chamber. This results in a stratified charge: one in which the air/fuel ratio is not homogeneous throughout the combustion-chamber, but varies in a controlled (and potentially quite complex) way across the volume of the cylinder.
A relatively rich air/fuel mixture is directed to the spark-plug using multi-hole injectors. This mixture is sparked, giving a strong, even and predictable flame-front. This in turn results in a high-quality combustion of the much weaker mixture elsewhere in the cylinder.
Direct fuelling of petrol engines is rapidly becoming the norm, as it offers considerable advantages over port-fuelling (in which the fuel injectors are placed in the intake ports, giving homogeneous charge), with no real drawbacks. Powerful electronic management systems mean that there is not even a significant cost penalty.
With the further impetus of tightening emissions legislation, the motor industry in Europe and north America has now switched completely to direct fuelling for the new petrol engines it is introducing.
It is worth comparing contemporary directly-fuelled petrol engines with direct-injection diesels. Petrol can burn faster than diesel fuel, allowing higher maximum engine speeds and thus greater maximum power for sporting engines. Diesel fuel, on the other hand, has a higher energy density, and in combination with higher combustion pressures can deliver very strong torque and high thermodynamic efficiency for more 'normal' road vehicles.
History
The principle of injecting fuel directly into the combustion-chamber at the moment at which combustion is required to start was invented by Rudolf Diesel, but it has been used to good effect in petrol engines for a long time. The Mercedes 300SL 'Gullwing' of 1952 used direct fuelling, though Mercedes-Benz subsequently switched to port fuelling for other models.
Honda's CVCC engine, released in the early 1970s models of Civic, then Accord and City later in the decade, is a form of stratified charge engine that had wide market acceptance for considerable time. The CVCC system had conventional inlet and exhaust valves and a third, supplementary, inlet valve that charged an area around the spark plug. The spark plug and CVCC inlet was isolated from the main cylinder by a perforated metal plate. At ignition a series of flame fronts shot into the very lean main charge, through the perforations, ensuring complete ignition. In the Honda City Turbo such engines produced a high power-to-weight ratio at engine speeds of 7,000 rpm and above.
Jaguar Cars in the 1980s developed the Jaguar V12 engine, H.E. (so called High Efficiency) version, which fit in the Jaguar XJ12 and Jaguar XJS models and used a stratified charge design called the 'May Fireball' in order to reduce the engine's very heavy fuel consumption.