Binafsi ninavyofahamu, engine nyingi kama sio zote za Toyota za kuanzia 1997 kuendelea ni VVTi, weather ni D4 au sio D4. Za kabla ya hapo zilikua only VVT. So the letter 'i' added stands for vvt with intelligence, or simply intelligent. Kwamba engine inaweza kutengeneza variation ya air and fuel intake kwa mahitaji tofauti tofauti ya engine, tofauti na vvt ambapo variation zilikua fixed muda wote.
D4 ni kwenye fuel injection kama wengi walivyosema. Ila D4 engine bado ni vvti pia, sema yenyewe ina direct fuel enjection. Kiukweli D4 ziko poa tu, actually zina advantages nyingi kuliko zisizo na direct fuel injection (port injection delivery) ila utafiti unaonesha D4 engine nyingi zina matatizo ya reliability hasa kwa nchi zenye baridi kali huwa inasumbua kuwaka at times, ina kelele za engine saana ikiwa silent na wakati mwingine inapoteza nguvu hasa ukiwa kwenye mwendo mdogo, ndio utasikia yale maneno kuwa engine bado haijachanganya vizuri. N chanzo kikubwa ni kwamba huwa inajaza carbon kwenye intake valves.
Lengo la hiyo teknologia ilikua kuboresha utumiaji wa mafuta, na kupunguza emission, but ukilinganisha na engine nyingine za Toyota, D4 zimeprove less reliable especially kama hauko makini saana hasa kwa kuweka mafuta yaliyochakachuliwa. Na problems za D4 engines huwa zinahitaji higher maintenance and repair costs kuliko engines zingine. Toyota wamelikubali hilo tatizo, na sasa hivi wame fix problems za d4 kwa kutengeneza engine nyingine wanaita D-4S.
Hi D-4S in advantages za both engines, so ukipata hii ni bora zaidi kuliko ile D4 ya kawaida.
