lunedì 2 febbraio 2009

COME FUNZIONA IL MOTORE A 4 TEMPI?

I nostri cari bicilindrici Ducati, i cosiddetti pomponi, appartengono alla categoria di motori più diffusi e affascinanti: i motori a quattro tempi, ovvero quelli che compiono un ciclo completo del loro funzionamento ogni quattro corse del pistone, cioè due giri completi dell'albero a gomito (albero motore).


E' utile ricordare che i normali motori sfruttano un cinematismo biella-manovella (simile alla gamba che aziona i pedali della bicicletta), così che a un giro completo dell'albero motore (vale a dire i pedali, nell'esempio della bicicletta), corrisponda il ciclo di salita e di discesa del pistone (che sarebbe il ginocchio del ciclista). Si ricorda inoltre che la valvola "di aspirazione" si trova tra cilindro e condotto di alimantazione, mentre quella "di scarico" fra cilindro e scappamento.

A cilindro
B pistone
C biella
D manovella



In questo articolo ne illustriamo in modo chiaro la sequenza, riferendoci ad un insieme pistone-cilindro di un generico motore a valvole in testa.

Fase di ASPIRAZIONE

Durante il funzionamento del motore, il pistone scorre verso l'alto e verso il basso all'interno del cilindro. Partendo dal Punto Morto Superiore (PMS), vale a dire il punto più alto della sua corsa, scendendo si allontana dalla testa del cilindro: la valvola di ammissione (valvola di aspirazione) è aperta e permette l'aspirazione all'interno del cilindro della miscela fresca aria/carburante.

Fase di COMPRESSIONE
Il pistone, giunto al Punto Morto Inferiore (PMI), vale a dire il punto più basso della sua corsa, inverte il moto spostandosi verso la testa del cilindro. In questo momento la valvola di aspirazione si chiude, perciò il movimento verso l'alto del pistone comprime la miscela appena aspirata.

ACCENSIONE
Quando il volume fra il pistone e il tetto del cilindro (camera di combustione) è al minimo (pistone al PMS), la miscela viene incendiata dalla scintilla prodotta dalla candela (che appositamente si affaccia dentro il cilindro). Ha inizio così la combustione (attenzione! Non si tratta assolutamente di scoppio, ma di veloce e graduale combustione).


Fase di ESPANSIONE
La combustione determina una rapida espansione del gas che esercita un'intensa pressione sulle pareti della camera e in particolare sul cielo del pistone che, essendo "mobile", si allontana dalla testa del cilindro spingendo sulla manovella dell'albero attraverso l'organo di collegamento, chiamato biella. Dunque con le valvole ancora tutte chiuse si compie la terza corsa. Si noti che questa è l'unica fase attiva, nel corso della quale il motore fornisce effettivamente il moto.

Fase di SCARICO


Terminata al PMI la fase attiva, inizia la risalita del pistone, questa volta però con la valvola di scarico aperta. In questo modo i gas combusti vengono spinti fuori dal cilindro e attraverso il condotto di scarico raggiungono l'esterno. al termine di questa corsa il motore è pronto per ripetere l'intero ciclo.


(tratto da Ducati Tech Cafè, Stefano Bianucci, rielaborato da Giovanni Savino)

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