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 5.2


Figure 7.3 Fuel injection and operation strategy for DISC engines

                       Table 7.2
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 14.7
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 12:1 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.

DISC Engine Performance and Emissions

The performance of a swirl dominated DISC engine is compared to that of a lean burn PFI engine on Fig. 7.4. Main differences observed are;

  • The lean limit of homogeneous charge PFI engine was about 25: 1 A/F ratio, while the DISC engine could be operated well beyond A/F ratio of 40:1.
  • The specific fuel consumption of DISC engine was lower by up to 20% compared to stoichiometric PFI engine. Other studies also showed 30 percent improvement in BSFC of DISC engines over the conventional PFI engine. Lower SFC results due to less throttling  or unthrottled engine operation, and lean  air-fuel ratio and higher compression ratio.
  • PFI engines exhibit a continuous decrease in NOx emissions as the mixture is leaned from near stoichiometric mixture. Very low NOx emissions without EGR are obtained at air-fuel ratios higher than 20:1. DISC engine is seen to give significantly high NOx levels even at overall air-fuel ratio of 45:1. In the DISC engine considerable amount of charge is contained in the combustion zones with stoichiometric or near stoichiometric mixture. The local burnt gas temperature in these zones is at higher level leading to higher NOx formation than when the engine is operating in lean homogeneous mode at the same overall air-fuel ratio.
  • The DISC engines have a higher tolerance of EGR before the combustion stability and fuel consumption becomes unacceptably poor as the mixture in the vicinity of spark plug in DISC engines is stoichiometric or richer and a stable combustion even with 40% or higher EGR rates is possible.