Stratified charge engine
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A stratified charge engine is a type of internal combustion engine, used in automobiles, in which the fuel is injected into the cylinder just before ignition. This allows for higher compression ratios without "knock," and leaner air/fuel ratio than in conventional internal combustion engines.
Conventionally, a four-stroke (petrol or gasoline) Otto cycle engine is fueled 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, meaning it contains the exact amount of air necessary for a complete combustion of the fuel. This gives stable combustion, but it 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.
Direct fueling of petrol engines is rapidly becoming the norm, as it offers considerable advantages over port-fueling, a type of fuel injection in which the fuel injectors are placed in the intake ports, giving homogeneous charges. Powerful electronic management systems mean that there is no 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 fueling for the new petrol engines it is introducing. If the fuel is injected directly into the combustion chamber during the compression stroke, the petrol engine is liberated from a number of its limitations.
High compression ratio
First, a higher mechanical compression ratio (or, with supercharged engines, maximum combustion pressure) may be used 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.
The engine may also run on a much leaner overall air/fuel ratio, using stratified charge.
- Increased injector cost and complexity
- Higher fuel pressure requirements
- Carbon build-up on the back of the intake valve due to the lack of gasoline passing by the intake valve to act as a cleaning agent for the valve on traditional multi-port injection designs
Combustion can be problematic if a lean mixture is present at the spark plug. However, fueling a petrol engine directly allows more fuel to be directed 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.
Comparison with diesel engine
It is worth comparing contemporary directly-fuelled petrol engines with direct-injection diesel engines. 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.
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 fueling 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 were 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.
The Vespa ET2 scooter had a 50 cc two-stroke engine in which air was admitted through the transfer port and a rich fuel mixture was injected into the cylinder near the spark plug just before ignition. The injection system was purely mechanical, using a timed pumping cylinder and a non-return valve.
On its downward stroke it compresses the rich mixture to about 70 PSI at which time the rising pressure raises a spring loaded poppet valve off its seat and the charge is squirted into the cylinder. There it is aimed at the spark plug area and ignited. The combustion pressure immediately shuts the spring-loaded poppet valve and from then on its (sic) just a "regular" stratified-charge ignition process with the flame front igniting those lean mixture areas in the cylinder.
- "32 (17) strat" (PDF). Retrieved 2014-05-10.
- "Motorcycle Online: Vespa ET2". Web.archive.org. 2005-07-28. Retrieved 2014-05-10.
- "Browse Papers on Stratified charge engines : Topic Results - SAE International". Topics.sae.org. Retrieved 2014-05-10.