7. Anexos
7.3. Cuestionarios íntegros completados por los periodistas de datos
7.3.3. Cuestionario de Raúl Sánchez (eldiario.es)
M itsubishi have d evelop ed a novel barrel stratification engine that w as introduced into the Japanese market in 1996. The engine u ses the follow in g tech n o log ies (K um e et a!
1 ) Upright straight inlet ports to generate a tumbling m otion that is in the op posite direction to conventional tumble.
2) A spherical com pact piston cavity controlling the behaviour o f the fuel spray and vaporised ga seou s mixture,
3) A high pressure fuel feed pump with lo w driving loss.
4) An electrom agnetic swirl injector to realise optim ised spray dispersion and atom isation
Early injection produces a h om ogen ou s mixture which is required for high load conditions w hile late injection timing leads to a stratified charge mixture (figure 1.27). A FRs as lean as 100:1 can be achieved although in practice the engine is not operated with an APR in ex cess o f 40:1. A summary o f the con cep ts and their objectives is given in table 1 8
■
Compact & Atomized (a) Late Injection
Wide dispersion & No wall-wetting (b) Early Injection
Figure 1.27: Early and late injection timings [Kume et al (1996)]
Condition injectionTimina Mixing A/F Ratio Partial Load Later Stage of Compression Stroke Stratified 25-40 Higher
Load Early Stage of Intake Stroke geneousHomo Stoich./Rich or 20-25
Flow CQncePl J R e v e rs e tu m b le Injection Mixing H igher Load Partial Load Ignition E m ission Intake _i L ow er in), p r e s s u re -I E n a m e d riv e n Dump j H o m o g e n e o u s (early injection) J No w all-w etting J C h a rg e -a ir cooling J Stratified (late injection) J E n h a n c e d e v a p o ra tio n G oal J Fuel v a p o r t ra n s p o rt to plug
J H igher p o rt flow rate
j_LowerJuel_com2IG^jo^
j_Lowei_drivin2_loss_
J S o o t re d u c tio n in sto ich . & rich c o n d itio n s
J K nocking s u p p r e s s io n J H igher v o lu m e tric
efficiency_______________ J S ta b le le a n b u rn J L o w er-so o t-em issio n
a t high lo a d _________
_ _ _ M e a s u r e s _ _ Vertical in tak e port E n h a n c e d g a s - d y n a m ic s effect by stra ig h t p o rt J Swirl in jecto r J S tart by feed pum p
I S u p p re s s e d sp ra y tip p e n e tra tio n I W idely d is p e rs e d fuel s p ra y I E n h a n c e d re v e rse tu m b le J C o m p a c t c h a m b e r J S p ra y m o tio n by re v e rse tu m b le 3 Injection tim ing control J C o n v e n tio n a l ignition sy s te m w ith c o n firm e d reliability
3 High EGR 3 H ig h -sp e e d & a c c u ra te air co n tro l 3 NOx r e d u c tio n in
__^tratified^charge_
3 S m o o th o p e ra tio n 3 W ider EGR z o n e 3 E lectronically a c tu a te d EGR valve 3 E le ctro n ically a c tu a te d air co n tro l valveTable 1.8b: Concepts and objectives of Mitsubishi engine [Kume et al (1996)]
Although DI SC engines offer great potential, by their very nature, the fuel injection system is likely to be very expensive due to the high injection pressures involved A lso m ore research is required into the durability o f such system s as there is a tendency for fuel to carbonise in and on the injectors. A further concern is that the low -em ission perform ance o f these engines is very sensitive to ignition timing, and small changes in service could bring about a large increase in engine-out em issions
1.10.5.1.2
Port Injection Stratified Charge (Pi SC)
Axial Stratification
Axial stratification em ploys swirl as the mechanism for the enhancem ent o f com bustion The fuel is introduced in the latter half o f the intake stroke as the piston approaches B D C (K iyota et al ( 1992)) in order that it m oves w ith the swirling m otion and minimal mixing occurs b etw een the air in front o f and behind the fuel band (O hm et a l (1 9 9 3 )). The com bination o f swirling m otion and late injection allow s a rich mixture o f air at the top o f the cylinder and a leaner mixture beneath to remain until the end o f com pression. The use o f air, as op posed to E G R or a com bination o f EG R and air results in an overall lean mixture but on e in which the fuel distribution is not hom ogen eou s.
An exam ple o f a lean-burn axially SC engine that has achieved U S legislative limits is the H onda V T E C -E engine (H orie et a l (1 9 9 2 )). The V T E C -E engine u ses a variable swirl
mechanism (figure 1 2 8 ) such that at lo w speed and load conditions the valve lift o f one o f the inlet valves is reduced to 0 67 mm and axial swirl is established in the cylinder. B oth valves are fully activated at high speed and load conditions, thus preventing a decrease in volum etric efficiency. Straight ports with triangular cross sections achieve sufficient turbulence at lo w speed and load conditions w ithout significantly com prom ising high speed and load running (H orie et a l (1 9 9 2 )). In the Federal Test Procedure, an im provem ent in fuel econom y o f 8% w as achieved at the lo w speed and load conditions and 12% in the highway mode. The U S legislation limit on N O x w as met.
J
Figure 1.28: Honda VTEC-E mechanism [Horie et al (1992)]
There are three dominant factors to take account o f w hen attempting to realise the full potential o f axial stratification: swirl ratio injection timing and injector flo w rate (O hm et al (1 9 9 3 )). The swirl ratio is responsible for maintaining the rich mixture band up until the point o f com bustion, whilst the injection timing determ ines the position o f the band Bad injection timing will result in unfavourable stratification with the rich mixture band situated away from the spark-plug at the time o f ignition.
With a swirl ratio greater than 3 7, Ohm et a l (1 9 9 3 ) found that there existed a single optimum injection timing (1 1 0 ° -1 3 0 ° C A A T D C depending upon the test condition and the exact swirl ratio) in which the lean misfire limit (L M L ) w a s a maximum. The LM L taken here as being determined when the misfire frequency is greater than 1% The
higher the LM L, the greater the degree o f fuel enrichment around the spark-plug It w as clear that true axial stratification w as present w ith this arrangement, System s that had a swirl ratio less than 3.7, exhibited different variations o f LM L with injection timing (figure 1 29).
LML
2.6 3.3 4.2
Swirl Ratio 3.7
Figure 1.29: Variation of LM L with injection timing and swirl ratio [Ohm et a! (1993)1 In order to explain this, it w as postulated that at the low er swirl ratio, there existed a small tumbling m otion beneath the inlet valve during the early part o f the intake stroke, and this m otion is supported by the main swirling m otion The tumbling m otion is not as intense as the swirling m otion and tends to be destroyed relatively early (figure 1.30) The greater the swirl ratio, the low er the tendency for this tumble m otion to be created