Is it permissible for an engineer to take a commission from a supplier when selling a device to a client, without the client's knowledge of the commission, even if the engineer guarantees to advise the client on the appropriate specifications and price?

Your agreement with the merchant to receive a commission for directing buyers to him is permissible, but on condition of your trustworthiness and not deceiving or misleading the buyer. If you advise the buyer regarding his best interest, and direct him to a merchant who sells at or below market price, or whose appliances have better specifications, then there is no harm in you taking a commission from the merchant. You are not required to inform the buyer of this, because the one paying the commission is the merchant, not the buyer.