2. LES EXPERIENCES DE P ERMIS NEGOCIABLES
5.4 Contribution de Voies Navigables de France
O atrito também foi mensurado durante todo o ensaio de desgaste, o qual durou 2 horas. O CoF representativo para todos os óleos nessa região é apresentado na Figura 64.
Figura 64 - Coeficiente de atrito FSA medido durante a sequência de desgaste de diferentes tipos de óleos. Condição: Carga = 200 N, frequência = 20 Hz, temperatura = 130 °C, velocidade máxima no meio do curso = 0,18 m/s.
0 0,02 0,04 0,06 0,08 0,1 0,12 0,14 0,16 0,18 0,2 0,5 1 1,5 2 CoF Tempo [h] BL A B C
108
Como pode ser observado, a resposta do CoF para os diferentes óleos nas condições do ensaio de desgaste varia significativamente. O óleo BL, sem FM, exibe valor de CoF mais alto, sobretudo no começo do teste. No entanto, o atrito decresce contínua e lentamente com o passar do tempo, devido ao desgaste do anel e da camisa. Já os óleos A e B apresentaram valores constantes de atrito relativamente baixos, com o óleo A obtendo melhor desempenho. De acordo com esse comportamento tanto o óleo A e B demonstraram estabilidade dos seus respectivos tribofilmes frente ao desgaste. Por fim, o comportamento do atrito representativo do óleo C foi o mais irregular de todos, decrescendo constantemente com o avanço do ensaio, se estabilizando apenas no final do ciclo. O óleo C contém etanol hidratado, assim sendo, água, a qual possui maior ponto de fusão que o etanol. Dessa forma, durante a fase inicial do desgaste, água ainda pode estar presente em maior quantidade e sendo eliminada lenta e gradativamente – podendo gerar cavitação, influenciando o CoF medido e provocando instabilidade do tribofilme. Em geral, os valores de CoF observados na sequência de desgaste seguiram a classificação do CoF das sequencias de atrito a altas temperaturas.
5 CONCLUSÕES
Neste estudo, uma investigação acerca do efeito da presença de etanol anidro e hidratado em um óleo de baixa viscosidade no comportamento tribológico do contato anel do pistão/camisa do cilindro foi conduzida, e os seguintes resultados foram obtidos:
1. A adição do modificador de atrito (FM) ao óleo de baixa viscosidade reduziu drasticamente o coeficiente de atrito e o desgaste da camisa. Desse modo, melhorando o desempenho do lubrificante em regimes de atrito misto e limítrofe, conferido pela formação de um tribofilme estável e durável. Assim, a formulação de um óleo de baixa viscosidade
aparenta demonstrar papel central quando a questão é a redução da perda de energia por atrito.
2. A diluição por etanol anidro e hidratado em óleo contendo FM aumentou consideravelmente o CoF na interface de deslizamento, sendo mais significativo em condições representativas do ciclo urbano. Porém, os níveis de CoF para esses óleos ainda foram mais baixos do que para o óleo base, ainda que sob efeito das diluições. A temperaturas mais elevadas, há tendência na diminuição desse efeito, contudo o impacto da presença da água persiste por mais tempo. 3. Tal aumento do CoF é provavelmente atribuído ao atraso da formação
de tribofilme e à sua instabilidade.
4. Como consequência da presença de etanol e água, maiores valores de desgaste foram reportados nas camisas. Esse comportamento envolve mecanismos complexos de mudanças de topografia dos pares de atrito, formação e remoção de tribofilme durante o deslizamento. 5. Após os ensaios conduzidos com óleos contendo FM, foi constatada a
presença de tribofilme em todas as superfícies de desgaste da camisa. Camisas mais desgastadas apresentaram maior área recoberta com tribofilme proveniente do FM.
110 Conclusões
Assim, espera-se, com este estudo, fornecer uma base de dados que possa amparar as companhias fabricantes de óleos a desenvolver lubrificantes que mitiguem os efeitos nocivos das presenças de etanol e da água durante o funcionamento de MCI’s da nova geração.
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