“平稳的”用英语怎么说？

他发动了汽车，车发出平稳的嗡嗡声。He started the car, which hummed smoothly.我们以平稳的速度开车.We drove at a steady rate.他的精神总是平稳的,并且他又是个循规蹈矩的人.His spirits were always equable and he was an observant person....