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Module7:Advanced Combustion Systems and Alternative Powerplants
Lecture 32:Stratified Charge Engines
ADVANCED COMBUSTION SYSTEMS AND ALTERNATIVE POWERPLANTS
The Lecture Contains:
DIRECT INJECTION STRATIFIED CHARGE (DISC) ENGINES
Historical Overview
Potential Advantages of DISC Engines
DISC Engine Combustion Requirements
Methods of Charge Stratification and Combustion
Modes of DISC Engine Operation
DISC Engine Performance and Emissions
Module7:Advanced Combustion Systems and Alternative Powerplants
Lecture 32:Stratified Charge Engines
ADVANCED COMBUSTION SYSTEMS AND ALTERNATIVE
POWERPLANTS
DIRECT INJECTION STRATIFIED CHARGE (DISC) ENGINES
Historical Overview
For many decades the researchers have pursued development of direct injection stratified charge SI engines to have overall very lean engine operation for higher fuel efficiency. Charge stratification is a means of ensuring repeatable ignition without misfire and stable combustion while using overall very lean fuel-air ratios that is otherwise not possible with homogeneous mixtures. In the stratified charge engines, the mixture composition is varied within the combustion chamber such that stoichiometric or slightly richer mixture exits near spark plug to provide good ignition characteristics and the mixture gets progressively leaner away from the spark plug. Overall air-fuel ratio in the cylinder is significantly leaner than the stoichiometric. A typical configuration of DISC engine is shown on Fig 7.1. Liquid fuel is injected in the cylinder. The fuel spray is directed by air motion or by the geometry of piston crown or by combination of the both towards spark plug. By the time fuel spray reaches the spark plug electrodes some fuel gets vaporized and forms combustible mixture with air. The vaporized fuel in spray is then ignited by spark, combustion begins and the flame spreads in the combustion chamber.
Figure 7.
Schematic of a direct injection stratified
charge (DISC) engine combustion system
Module7:Advanced Combustion Systems and Alternative Powerplants
Lecture 32:Stratified Charge Engines
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Overall mixture being lean very low CO is produced as the CO produced early in combustion can be oxidized within the cylinder itself by the excess oxygen available. The mixture in piston ring crevice region being very lean the contribution of crevices to HC emissions would also be very low. The direct injection of fuel in the cylinder can decrease HC emissions during warm-up after cold start as liquid fuel film is not formed in the intake manifold and port. Also, a smaller fuel quantity needs to be injected during cold start compared to PFI engines. With direct injection of gasoline a faster dynamic response is possible hence a flatter air-fuel ratio curve during acceleration can be used that provides lower HC emissions. The DISC engines can tolerate higher EGR rates than the homogeneous charge SI engines and hence larger reductions in NO (^) x
DISC Engine Combustion Requirements
For stratified charge engine operation the following main requirements are to be met to obtain good combustion:
Combustible mixture must form quickly. The liquid fuel and fuel over-rich zones should be minimum at the time of ignition Suitable air motion is to be provided during compression stroke and at the time of fuel injection to accomplish charge stratification and transport mixture to the spark plug in a reproducible manner cycle after cycle. Air motion may be aided by a suitably designed cavity on the piston crown. Wetting of piston crown and spark plug by excessive liquid fuel deposition is to be prevented. Over mixed zones having excessively lean mixtures beyond flammability limits as well as under mixed over-rich zones are to be avoided.
Module7:Advanced Combustion Systems and Alternative Powerplants
Lecture 32:Stratified Charge Engines
Methods of Charge Stratification and Combustion
The methods of charge stratification and combustion that have been studied and some of these employed in production engines may be grouped in the following three types. These are also shown on Fig. 7.2.
Figure 7.
Spray, flow and wall controlled DISC
engine combustion.
Spray Controlled: The fuel spray characteristics primarily controls the charge stratification in this strategy. Ignitable mixture is formed at the boundaries of the fuel spray. The spark plug is placed close to the spray as it is there that the ignitable mixture is present at the time of ignition. Formation of good quality mixture becomes difficult at high engine loads. The combustion being highly sensitive to spray characteristics smoke formation is often observed at high loads. Wetting of the spark plug by liquid fuel causes frequent spark plug fouling.
Wall Controlled: In the wall-controlled concept, fuel injection is directed towards a specially designed piston cavity. The piston cavity is off centre. The spark plug is located away from the fuel injector on the side of combustion chamber. Fuel impinges on the piston cavity walls where it evaporates and mixes with air. An intense reverse tumble charge motion transports the mixture to spark plug electrodes.
Flow Controlled: Mixture is formed by interaction between fuel spray and suitably directed air motion like swirl or tumble. The spark plug and injector are generally widely spaced in these configurations. The air motion transports mixture to the spark plug such that the ignitable mixture is present at spark plug electrodes at the time of spark. When air motion is well organized, the combustion chamber walls do not get wetted by liquid fuel and a stable stratified charge operation is obtained over a wide range of engine operation.
The characteristics of combustion process obtained with the three charge stratification and combustion methods are compared in Table 7.
Homogeneous Lean Operation: Engine operates homogeneous lean in the mid-load range. This is the transition zone from stratified mode at low loads to stoichiometric operation at full engine load.
Module7:Advanced Combustion Systems and Alternative Powerplants
Lecture 32:Stratified Charge Engines
Injection strategy and typical operation regimes for a DISC engine are shown in Fig. 7.3 and their features are summarized in Table 7.2. The different features of DISC engine operation are summarized in Table 5.
Figure 7.3 Fuel injection and operation strategy for DISC engines
Table 7.
Features of DISC Engine
Operation
Stratified
Homogeneous
stoichiometric
Homogeneous lean
Injection timing Compression stroke Intake stroke Intake stroke Air-fuel ratio 24 -50 14.7 24 to 14. Intake Throttling Low High Medium
Mitsubishi and Toyota introduced 4-cylinder, DISC engine powered cars for the first time during 1996. Other manufacturers have also developed DISC engines. The DISC engines use 10 to 12: compression ratio and fuel is injected at 50 to 120 bar injection pressure. The leanest air-fuel ratio used is more than 40:1 reaching as lean as 55:1.
Module7:Advanced Combustion Systems and Alternative Powerplants
Lecture 32:Stratified Charge Engines
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Figure 7.
Comparison of performance and NO x emissions of DISC and lean
burn PFI engines.
The DISC engine gives much lower HC emissions than the PFI engine during engine cold start, engine warm-up phase and transient conditions. In the PFI engines during cold start and transient conditions, the amount of fuel entering the engine cylinder is not the same as being injected at the port as some fuel gets deposited at the port forming liquid fuel film. Hence, in the PFI engine very rich mixture is to be supplied to have quick cold start. In DISC engines, as the fuel is directly injected in the cylinder there is no delay of fuel being inducted into the cylinder. The DISC engines on the contrary can be started on stoichiometric or even slightly overall lean mixtures. After engine is switched on, the DISC engine achieves stable combustion in the very first or second cycle while the PFI engine requires about 10 cycles to attain stable combustion. The cold start HC emissions from a DISC engine are nearly 1/4th of that of PFI engine. CO emissions of DISC engines are very low due to overall very lean engine operation. For the success of DISC engines, catalytic reduction of NO (^) x under lean engine operation is required. Although with EGR large reductions in engine out NO (^) x emissions are obtained, but lean operating limit is narrowed with high EGR and during lean homogeneous engine operation high EGR cannot be used. Therefore, lean de-NO (^) x catalyst technology is essential to meet stringent emission standards in future while maintaining fuel economy benefits of the DISC engines. Mitsubishi and Toyota DISC engines employ a lean de-NO (^) x catalyst in addition to EGR to reduce nitrogen oxide emissions to the level of Euro 4 and beyond emission standards.
