Stratified charge engine

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 NO_x.

Advantages

Direct fueling of petrol engines 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,^[1] the motor industry in Europe and North America has now almost switched completely to direct fueling for the new petrol engines it is introducing.

High compression ratio

First, a higher mechanical compression ratio (or, with super-charged or turbo-charged 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.

Lean burn

The engine may also run on a much leaner overall air/fuel ratio, using stratified charge, in which a small charge of a rich fuel mixture is ignited first and used to improve combustion of a larger charge of a lean fuel mixture.

Disadvantages

Disadvantages include:

• 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
• Increased NO_x formation, due to the presenence of local extremely rich zones. These zones are not present in a gasoline engine, because the air and fuel is better mixed.

Combustion management

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.^[2] 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.

Charge stratification can also be achieved where there is no 'in cylinder' stratification: the inlet mixture can be so lean that it is unable to be ignited by the limited energy provided by a conventional spark plug. This exceptionally lean mixture can, however, be ignited by the use of a conventional mixture strength of 12-15:1, in the case of a petrol fuelled engine, being fed into a small combustion chamber adjacent to and connected to the main lean-mixture chamber. The large flame front from this burning mixture is sufficient to combust the charge. It can be seen from this method of charge stratification that the lean charge is 'burnt' and the engine utilising this form of stratification is no longer subject to ' knock' or uncontrolled combustion. The fuel being burnt in the lean charge is therefore not 'knock' or octane restricted. This type of stratification therefore can utilise a wide variety of fuels; the specific energy output being dependent only on the calorific value of the fuel.

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 fueled 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.

This comparison of 'burn' rates is a rather simplistic view. Although Petrol and Diesel engines appear similar in operation, the two types operate on entirely different principles. In earlier manufactured editions the external characteristics were obvious. Most petrol engines were carbureted, sucking the fuel/air mixture into the engine, while the diesel only sucked in air and the fuel was directly injected at high pressure into the cylinder. In the conventional four-stroke petrol engine the spark plug commences to ignite the mixture in the cylinder at up to forty degrees before top dead centre while the piston is still travelling up the bore. Within this movement of the piston up the bore, controlled combustion of the mixture takes place and the maximum pressure occurs just after top dead centre, with the pressure diminishing as the piston travels down the bore. i.e. the cylinder volume in relation to the cylinder pressure-time generation remains essentially constant over the combustion cycle. Diesel motor operation on the other hand inhales and compresses air only by the motion of the piston moving to top dead centre. At this point maximum cylinder pressure has been reached. The fuel is now injected into the cylinder and the fuel ' burn' or expansion is started at this point by the high temperature of the, now compressed, air. As the fuel burns it expands exerting pressure on the piston, which in turn develops torque at the crankshaft. It can be seen that the diesel motor is operating at constant pressure. As the gas expands the piston is also moving down the cylinder. By this process the piston and subsequently the crank experiences a greater torque, which is also exerted over a longer time interval than its petrol equivalent.

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.

Honda

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

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..

Vespa

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.

Quote

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.^[3]

Research

SAE International has published papers on experimental work with stratified charge engines.^[4]

See also

• Lean-burn
• Turbo fuel stratified injection

References